Question 1

Surface adhesion structures: Which organisms may possess fimbriae (short, hair-like attachment structures)?

Accepted Answer

Introduction / Context: Fimbriae (also called attachment pili) are short, filamentous surface structures that promote adhesion to host tissues, abiotic surfaces, and other cells. They are important in colonization, biofilm formation, and virulence. Given Data / Assumptions: Classic microbiology texts emphasize fimbriae as especially common in Gram-negative bacteria. Terminology varies: some Gram-positive species produce sortase-assembled pili, often discussed separately from classical Gram-negative fimbriae. The question likely expects the traditional association. Concept / Approach: In Gram-negative bacteria, fimbriae are assembled by dedicated secretion and assembly pathways and often recognize specific receptors on host cells (e.g., uropathogenic E. coli P fimbriae). While Gram-positive bacteria can have pili, the textbook convention associates “fimbriae” primarily with Gram-negative species, reflecting historical discovery and structural differences. Step-by-Step Solution: Identify the organism class most commonly linked to fimbriae in core curricula: Gram-negative bacteria.Confirm that the question likely targets this conventional teaching point.Select “Gram-negative bacteria.”Note that alternative wording might include Gram-positive pili in advanced contexts, but not for this basic question. Verification / Alternative check: Model organisms such as E. coli, Neisseria, and Salmonella illustrate fimbrial adhesins and their roles in disease. Laboratory assays show hemagglutination, mannose-sensitive binding, and biofilm formation driven by these structures. Why Other Options Are Wrong: Gram-positive only: understates the classical association. Both: would be acceptable in nuanced discussions of pili but diverges from the conventional “fimbriae = Gram-negative” emphasis likely intended here. None: incorrect; fimbriae are well documented. Common Pitfalls: Equating all surface fibers across bacteria; structural and assembly differences matter. Also, confusing long conjugative pili (involved in DNA transfer) with short fimbriae (adhesion). Final Answer: Gram-negative bacteria

Question 2

Outer membrane composition: In which organisms is lipopolysaccharide (LPS) a major component of the outer membrane?

Accepted Answer

Introduction / Context: Lipopolysaccharide (LPS) is a hallmark molecule of Gram-negative bacteria. It plays structural roles, affects membrane permeability, and acts as endotoxin (particularly the lipid A portion), driving potent immune responses in hosts. Given Data / Assumptions: The question asks where LPS is a major outer membrane component. Gram-positive bacteria lack an outer membrane. Fungi are eukaryotes and do not have LPS. Concept / Approach: Gram-negative envelopes have an inner membrane, a thin peptidoglycan layer in the periplasm, and an outer membrane. The outer leaflet of the outer membrane predominantly contains LPS, contributing to barrier function and immune recognition. Gram-positives possess a thick peptidoglycan wall without an outer membrane and therefore no LPS. Step-by-Step Solution: Identify organisms with an outer membrane: Gram-negative bacteria.Recall that LPS localizes to the outer leaflet of the outer membrane.Exclude Gram-positive bacteria and fungi based on envelope architecture.Select “Gram-negative bacteria.” Verification / Alternative check: Endotoxin tests in clinical labs detect LPS; antibiotics targeting outer-membrane function often have selective effects on Gram-negatives, reflecting the presence of LPS and porins. Why Other Options Are Wrong: Gram-positive bacteria do not have an outer membrane; LPS is absent. Fungi have chitinous cell walls and sterol-containing membranes, not LPS. None of these is false because Gram-negatives clearly have LPS. Common Pitfalls: Confusing LPS with lipoteichoic acid (Gram-positive), or assuming all microbes share similar envelope chemistry. Envelope differences are pivotal in antibiotic susceptibility and immune detection. Final Answer: Gram-negative bacteria

Question 3

Architecture of the bacterial flagellum: Which of the following are the major structural components of a bacterial flagellum?

Accepted Answer

Introduction / Context: The bacterial flagellum is a rotary nanomachine that enables swimming motility. Understanding its modular architecture is essential for appreciating how rotation is transmitted from the motor to the helical propeller and how direction changes govern runs and tumbles during chemotaxis. Given Data / Assumptions: The question lists three candidate parts. We need to determine whether each is part of the flagellum core. We focus on the classic Gram-negative model, which also generalizes to many Gram-positives with differences in rings. Concept / Approach: The basal body houses the rotary motor and rings that span the membranes and cell wall. The hook functions as a flexible universal joint coupling the motor to the filament. The filament is the long, helical propeller composed mainly of flagellin. Together, these elements convert ion motive force into rotation and thrust to propel the cell through liquid environments. Step-by-Step Solution: Confirm that the basal body includes rotor and stator complexes with rings appropriate to the envelope type.Identify the hook as a curved connector transmitting torque while allowing bending.Recognize the filament as the extracellular helical structure visible under microscopy.Since all three are essential modules, select “All of these.” Verification / Alternative check: Mutations eliminating any single module abolish motility, confirming functional necessity. Electron cryotomography images clearly resolve these parts and their spatial arrangement. Why Other Options Are Wrong: Picking any single element alone ignores the integrated machine. Excluding any component is inconsistent with established structural and genetic data. Common Pitfalls: Confusing pili with flagella; pili do not rotate and are used for adhesion or DNA transfer. Also, mixing up Gram-negative ring composition with Gram-positive architecture does not change the three-module concept. Final Answer: All of these

Question 4

Peptidoglycan abundance: Approximately what fraction of the dry weight of the cell wall does peptidoglycan account for in many Gram-positive bacteria?

Accepted Answer

Introduction / Context: Peptidoglycan is the rigid polymer that gives bacterial cells shape and osmotic protection. Gram-positive bacteria characteristically have thick peptidoglycan layers, whereas Gram-negative bacteria have thinner layers between membranes. Estimating the proportion of peptidoglycan highlights envelope differences relevant to staining and antibiotic action. Given Data / Assumptions: We focus on Gram-positive cell walls, not entire cells. Typical values in many Gram-positives are high relative to Gram-negatives. The question asks for an approximate fraction by dry weight of the wall. Concept / Approach: In Gram-positive bacteria, peptidoglycan can constitute the majority of the cell wall mass, often exceeding 50% of the dry weight and in some species reaching much higher values. This abundance correlates with strong retention of crystal violet during the Gram stain and susceptibility to antibiotics targeting peptidoglycan synthesis (e.g., beta-lactams, glycopeptides). Step-by-Step Solution: Recall that Gram-positives have thick peptidoglycan compared with Gram-negatives.Identify the answer choice representing a majority fraction.“50% or more” best matches common textbook descriptions for many Gram-positive walls.Select that option. Verification / Alternative check: Biochemical analyses of purified walls show high peptidoglycan content along with teichoic acids. In contrast, Gram-negatives have additional outer-membrane constituents such as LPS that reduce the relative peptidoglycan proportion. Why Other Options Are Wrong: About 10% or 20% is too low for many Gram-positive walls. The oddly specific 11% + 0.22% is not a standard value and is inconsistent with typical references. Common Pitfalls: Confusing percentage of the whole cell with percentage of the isolated wall; assuming uniformity across all species rather than recognizing typical ranges used in teaching. Final Answer: 50% or more

Question 5

Motility and flagella: Bacteria that lack flagella are generally unable to perform which function?

Accepted Answer

Introduction / Context: Flagella are rotary appendages that allow many bacteria to swim through liquid environments. While not all bacteria possess flagella, those that lack them generally cannot perform swimming motility and must rely on other mechanisms (if any) to relocate, such as twitching, gliding, or passive transport by fluid flow. Given Data / Assumptions: The question targets the functional consequence of having no flagella. Other cellular processes like reproduction and growth do not require flagella. Adhesion to tissues typically involves adhesins, fimbriae, or extracellular polymers rather than flagella. Concept / Approach: Flagella enable active swimming and, in some cases, swarming across semi-solid surfaces. Without flagella, a bacterium loses this type of motility. However, cell division (binary fission) proceeds independently, growth on media is possible if nutrients and conditions are suitable, and adhesion is mediated by specific adhesins or surface polymers unrelated to flagella. Step-by-Step Solution: List potential functions: movement, reproduction, adhesion, growth.Identify which is directly dependent on flagella: swimming motility.Confirm that cell division, adhesion, and growth can occur without flagella.Select “Move.” Verification / Alternative check: Motility tests (e.g., hanging drop, semi-solid agar) readily distinguish flagellated swimmers from non-motile strains. Non-flagellated organisms can still form colonies and reproduce, validating that movement is the specific deficit. Why Other Options Are Wrong: Reproduction: many non-motile bacteria replicate normally by binary fission. Stick to tissues: often mediated by fimbriae, pili, or extracellular polysaccharides. Grow on nutrient agar: non-motile bacteria commonly grow as colonies in vitro. Common Pitfalls: Equating motility with viability; a bacterium can be entirely viable yet non-motile. Also, confusing flagella (motility) with fimbriae (adhesion) is a typical early error. Final Answer: Move

Question 6

Bacterial cell membranes — identifying organisms with sterols In microbiology, only certain prokaryotes incorporate sterols (such as cholesterol) into their cytoplasmic membrane to increase rigidity and protect against osmotic stress. Among the foll…

Accepted Answer

Introduction / Context: Sterols (for example, cholesterol) are membrane-stiffening lipids that are abundant in eukaryotic membranes but typically absent from the membranes of most bacteria. An important exception exists among wall-less bacteria such as Mycoplasma, which scavenge sterols from the host to stabilize their plasma membrane. This question checks your ability to recall that distinctive feature and to contrast Mycoplasma with common Gram-positive and Gram-negative genera that do not rely on sterols. Given Data / Assumptions: We compare representative bacterial groups: Bacillus, Clostridium, Proteus, Pseudomonas, and Mycoplasma. Most eubacteria synthesize membranes rich in phosphoglycerides without sterols. Mycoplasma lack a peptidoglycan cell wall and depend on sterols for membrane integrity. Concept / Approach: Bacterial membrane composition is taxon-specific. Typical eubacteria do not produce or require sterols. Mycoplasma, however, have no cell wall; their single lipid bilayer would be too fragile without sterols. Incorporating host-derived sterols increases membrane rigidity, reduces permeability, and supports survival in variable osmotic conditions. This also explains intrinsic resistance to beta-lactam antibiotics, which target peptidoglycan synthesis that Mycoplasma lack entirely. Step-by-Step Solution: Identify the group known to be wall-less: Mycoplasma.Recall that Mycoplasma acquire sterols from the host and embed them in the cytoplasmic membrane.Exclude Bacillus and Clostridium (Gram-positive spore formers) and Proteus or Pseudomonas (Gram-negative rods), none of which require sterols in their cytoplasmic membranes.Select Mycoplasma as the only correct match. Verification / Alternative check: Laboratory culture media for Mycoplasma commonly include serum (a source of cholesterol), further evidencing the requirement for sterols. In contrast, standard bacterial media for Bacillus, Clostridium, Proteus, and Pseudomonas do not need sterol supplementation. Why Other Options Are Wrong: Bacillus: possesses a rigid peptidoglycan wall; membranes lack sterols. Proteus: Gram-negative enteric; outer membrane has LPS but no cytoplasmic membrane sterols. Clostridium: anaerobic Gram-positive spore former; no sterols in the cytoplasmic membrane. Pseudomonas: Gram-negative nonfermenter; membranes do not contain sterols. Common Pitfalls: Confusing Mycoplasma (wall-less, sterol-requiring) with Mycobacterium (has a complex wall with mycolic acids but not membrane sterols). Also, do not conflate eukaryotic sterol-rich membranes with typical bacterial membranes. Final Answer: Mycoplasma (wall-less bacteria)

Question 7

Bacterial surface structures — identifying the component that does not contain protein Consider common bacterial envelope appendages and polymers. Which of the following structures is not composed of protein as its primary macromolecule?

Accepted Answer

Introduction / Context: Bacterial cell envelopes include a variety of macromolecular structures. Many appendages (pili, flagella, porins) are proteinaceous, while certain cell-wall polymers in Gram-positive bacteria are predominantly carbohydrate-phosphate polymers. Recognizing which components are proteins versus carbohydrate or lipid-linked polymers is essential for understanding staining, antigenicity, and antibiotic targets. Given Data / Assumptions: Pili (fimbriae) are built from pilin protein subunits. Flagellar filaments are composed of flagellin protein. Porins are trimeric beta-barrel proteins in Gram-negative outer membranes. Lipoteichoic acid is a glycerol/ribitol phosphate polymer covalently linked to a membrane lipid anchor in Gram-positive bacteria. Concept / Approach: Differentiate structural classes: protein appendages vs. carbohydrate-phosphate polymers. Lipoteichoic acid (LTA) is not a protein; it is a polymer of glycerol/ribitol phosphate with D-alanine and sugar substitutions, tethered to the cytoplasmic membrane by a glycolipid. This composition contrasts with the purely protein nature of pili, flagella, and porins. Correctly identifying the non-protein structure reinforces understanding of Gram-positive envelope chemistry and immune recognition (LTA can act as a pathogen-associated molecular pattern). Step-by-Step Solution: List the options and recall their primary macromolecules.Mark pili, flagella, and porins as proteinaceous.Recognize lipoteichoic acid as a non-protein polymer linked to lipids.Choose lipoteichoic acid as the only non-protein option. Verification / Alternative check: Biochemistry references show LTA localization in Gram-positive membranes and isolation as a polymer distinct from proteins. SDS-PAGE of pili/flagella yields protein bands, whereas LTA purification requires carbohydrate-specific staining or phosphate assays. Why Other Options Are Wrong: Pili: protein pilin subunits form the filament. Flagellum: flagellin protein composes the filament; hook and basal body also contain proteins. Porin: outer-membrane channel proteins enabling small solute diffusion. S-layer: made of protein or glycoprotein, not carbohydrate polymer. Common Pitfalls: Confusing teichoic acids (polymers) with teichuronic acids or mistaking lipid anchors as “proteins.” The presence of a lipid anchor does not make the whole polymer a protein. Final Answer: Lipoteichoic acid (teichoic acid anchored to membrane lipids)

Question 8

Bacterial motility — terminology for flagellar arrangements In prokaryote morphology, what is the correct term for a cluster (tuft) of flagella emerging from one pole of a bacterial cell?

Accepted Answer

Introduction / Context: The spatial arrangement of flagella on bacterial cells provides diagnostic clues to identification and influences motility patterns. Textbook terms distinguish between single and multiple flagella and whether they occur at one or both poles or around the entire cell surface. Given Data / Assumptions: Monotrichous: one flagellum at a pole. Amphitrichous: a single flagellum (or tuft) at each of two poles. Lophotrichous: a tuft (cluster) of flagella at one pole. Peritrichous: flagella distributed around the cell surface. Concept / Approach: Precisely match the definition in the prompt (“cluster of polar flagella”) with established terminology. A tuft at a single pole defines lophotrichous arrangement. This differs from amphitrichous (poles on both ends), monotrichous (single), and peritrichous (all around). Correct usage aids in interpreting motility tests (for example, wet mount, semisolid agar) and species descriptions (for example, Pseudomonas often monotrichous; Spirillum species may be lophotrichous). Step-by-Step Solution: Note the keywords: “cluster” and “polar.”Map “cluster at one pole” to lophotrichous.Exclude amphitrichous (both poles) and monotrichous (single).Exclude peritrichous (flagella all around) and atrichous (no flagella). Verification / Alternative check: Illustrations in microbiology atlases consistently label tufts at one pole as lophotrichous; amphitrichous depicts opposite-end placement. Why Other Options Are Wrong: Amphitrichous: implies poles at both ends, not a single polar tuft. Monotrichous: only one flagellum, not a cluster. Peritrichous: distributed over the entire surface. Atrichous: no flagella; not a tuft. Common Pitfalls: Interchanging lophotrichous with amphitrichous; remember “lopho-” suggests a tuft or crest at one location. Final Answer: Lophotrichous

Question 9

Peptidoglycan assembly — identifying the next-to-last step of biosynthesis During bacterial cell-wall synthesis, which process occurs just before the final transpeptidation (cross-linking) step?

Accepted Answer

Introduction / Context: Bacterial peptidoglycan is built by a well-ordered sequence: cytosolic precursor synthesis, membrane linkage to a lipid carrier, flipping to the periplasmic side, polymerization of glycan chains, and cross-linking of peptide stems. Knowing the order clarifies how antibiotics (for example, glycopeptides, beta-lactams) disrupt wall assembly. Given Data / Assumptions: Lipid II (MurNAc-peptide–GlcNAc) is flipped across the membrane on bactoprenol. Transglycosylases catalyze glycan-strand elongation. Transpeptidases (penicillin-binding proteins) make peptide cross-links as the final step. Concept / Approach: The final event is transpeptidation (peptide cross-linking). The step immediately prior is transglycosylation, in which the disaccharide-peptide unit is transferred from lipid II to the non-reducing end of the growing glycan chain, releasing bactoprenol for recycling. This sequencing explains why vancomycin (binding D-Ala-D-Ala) blocks transglycosylation and indirectly prevents subsequent cross-linking, whereas beta-lactams directly inhibit transpeptidation. Step-by-Step Solution: Arrange steps: precursor synthesis → lipid carrier linkage → flipping → transglycosylation → transpeptidation.Identify “next-to-last” as the step before cross-linking.Recognize transglycosylation as that penultimate step.Select the option describing addition of the disaccharide-peptide to the growing chain. Verification / Alternative check: Mechanistic diagrams of cell-wall synthesis show PBPs with bifunctional domains; the glycosyltransferase activity precedes the transpeptidase reaction in strand maturation. Why Other Options Are Wrong: A: Cytosolic precursor synthesis occurs much earlier. B: “Removal” wording is imprecise; lipid II donates its unit during transglycosylation. D: This is the last step, not the next-to-last. E: Autolysin activity facilitates insertion but is not the ordered penultimate biosynthetic step. Common Pitfalls: Confusing transglycosylation (glycan elongation) with transpeptidation (peptide cross-linking). Remember: sugars first, peptides second. Final Answer: Transglycosylation: adding the disaccharide-peptide unit to the growing peptidoglycan chain

Question 10

Bacterial capsules — identifying the typical chemical composition In pathogenic bacteriology, “capsules” and very thin “microcapsules” are surface layers that contribute to immune evasion. What is the predominant chemical composition of these bacter…

Accepted Answer

Introduction / Context: Bacterial capsules are hydrated, gelatinous layers external to the cell wall that help resist phagocytosis, desiccation, and complement-mediated killing. They are major virulence factors and important vaccine targets (for example, pneumococcal polysaccharide vaccines). Knowing their chemistry guides staining methods and immunology. Given Data / Assumptions: Most classic capsules are polysaccharides composed of repeating oligo- or polysaccharide units. Some exceptions exist (for example, Bacillus anthracis has a polypeptide capsule of poly-D-glutamic acid). “Microcapsule” refers to a thin or loosely adherent polysaccharide layer that may be harder to visualize. Concept / Approach: Although rare proteinaceous exceptions occur, the overwhelming majority of bacterial capsules are polysaccharide based. This explains why capsule detection often relies on negative staining (for example, India ink), Quellung reaction (capsular swelling with specific antisera), and why purified polysaccharides can be used as antigens in conjugate vaccines. Lipids do not form capsules; peptidoglycan is part of the rigid cell wall, not the capsule. Step-by-Step Solution: Recall that “capsule” generally means sugar polymers external to the cell wall.Consider notable exception (poly-D-glutamate of B. anthracis) but recognize it is not the rule.Eliminate options proposing lipids or peptidoglycan as capsule material.Select polysaccharides as the predominant composition. Verification / Alternative check: Laboratory manuals describe capsular serotyping based on polysaccharide antigenic variation (for example, K antigens in Enterobacteriaceae, pneumococcal serotypes), confirming sugar composition. Why Other Options Are Wrong: A: S-layers are distinct protein lattices, not capsules. C: Lipids organize into membranes, not hydrated capsules. D: Not equally common; polysaccharides dominate. E: Peptidoglycan forms the rigid sacculus, not the capsule. Common Pitfalls: Assuming all extracellular layers are the same. Differentiate capsules (polysaccharide), S-layers (protein), and slime layers (loosely adherent exopolysaccharide). Final Answer: Polysaccharides (for example, repeating sugar polymers)

Question 11

Bacterial locomotion — naming the structure that confers motility In prokaryotes, which cellular structure is primarily responsible for swimming motility in liquid media and swarming on semisolid surfaces?

Accepted Answer

Introduction / Context: Directional movement allows bacteria to navigate toward nutrients and away from hazards. The machinery that generates motion is a classic structure–function example: the flagellum is a helical filament connected to a rotary motor embedded in the cell envelope. Understanding which structure drives motility is fundamental for interpreting motility tests and virulence mechanisms (for example, chemotaxis toward host tissues). Given Data / Assumptions: Flagella rotate to produce propulsion; their distribution (mono-, lophotrichous, peritrichous) affects swimming behavior. Pili/fimbriae mediate adhesion, conjugation (sex pili), or twitching motility (type IV pili) on surfaces, not classic swimming. Capsules and sheaths do not generate motility. Concept / Approach: Match function (active propulsion in liquids) to structure (flagellum). The flagellar motor uses proton (or sodium) motive force to rotate a basal body, transmitting torque via a hook to the filament. Reversing rotation causes “tumble” events that reorient cells during chemotaxis. While type IV pili enable twitching (jerky surface movement), the canonical answer for bacterial motility in fluids is flagella. Step-by-Step Solution: Identify the movement type: swimming/swarming.Recall the rotary engine apparatus unique to flagella.Exclude pili (adhesion/twitching), capsules (protection), and generic “sheath.”Select flagella as the correct structure. Verification / Alternative check: Motility agar assays show characteristic diffuse growth for flagellated bacteria; electron micrographs reveal flagellar filaments extending from the cell surface. Why Other Options Are Wrong: Pili: enable attachment or twitching, not swimming propulsion. Sheath: not a motility organelle. Capsule: protective, anti-phagocytic, non-motile. Axial filaments: associated with spirochetes (modified periplasmic flagella), not eukaryotic cilia. Common Pitfalls: Confusing twitching (type IV pili) with swimming; the question specifically targets classic motility. Final Answer: Flagella (helical filaments powered by rotary motors)

Question 12

Bacterial shapes — common term for helically curved rods In standard morphological terminology, what is the common name for bacteria that appear as helically curved rods under the microscope?

Accepted Answer

Introduction / Context: Bacterial morphology aids preliminary identification. Among common shapes, cocci are spherical, bacilli are straight rods, spirilla are rigid helically curved rods, and spirochetes are flexible helical organisms with periplasmic flagella. Distinguishing spirilla from spirochetes helps avoid taxonomic confusion and informs motility expectations. Given Data / Assumptions: “Helically curved rods” indicates a spiral shape. Rigidity versus flexibility differentiates spirilla from spirochetes. “Pleomorphic” describes variability, not a single defined shape. Concept / Approach: Match descriptive terms with standard categories. Spirilla (for example, Spirillum volutans) possess external flagella and rigid helical forms; spirochetes (for example, Treponema, Borrelia) have internal axial filaments, are more flexible, and move with corkscrew motion. The prompt’s wording best aligns with “spirilla.” Step-by-Step Solution: Identify “helically curved rods.”Distinguish rigid (spirilla) from flexible (spirochetes).Exclude cocci (spherical), bacilli (straight rods), and pleomorphic (variable forms).Select “Spirilla.” Verification / Alternative check: Atlas images show spiral rigid rods labeled “spirilla,” whereas spirochetes appear as thin, highly flexible helices visible with dark-field or special stains. Why Other Options Are Wrong: Cocci: not rod-like. Pleomorphic: describes variability across shapes. Bacilli: straight rods, not helical. Spirochetes: flexible helices with unique motility; not simply “helically curved rods.” Common Pitfalls: Using “spirochete” as a catch-all for any spiral bacterium; morphology and flagellar arrangement matter. Final Answer: Spirilla (helically curved rigid rods)

Question 13

Bacterial behavior — term for swimming toward an attractant In microbial ecology, what is the correct term for directed movement of bacteria toward a higher concentration of a beneficial chemical attractant?

Accepted Answer

Introduction / Context: Chemotaxis enables bacteria to sense chemical gradients and bias their random walks to locate nutrients or escape toxins. It is central to survival in changing environments and is mediated by chemoreceptors, signal transduction pathways, and flagellar motor control. Terms differentiate movement toward attractants versus away from repellents. Given Data / Assumptions: Attractant concentration increases along the path of movement. Bacteria modulate “run” and “tumble” frequencies by altering flagellar rotation. Alternative taxis behaviors exist (photo-, magneto-, aero-), but the prompt specifies chemicals. Concept / Approach: Movement toward a desirable chemical gradient is defined as positive chemotaxis; movement away from harmful substances is negative chemotaxis. Phototaxis responds to light, magnetotaxis to magnetic fields, and aerotaxis to oxygen gradients. Therefore, the correct term directly follows from the sign of the response to a chemical cue. Step-by-Step Solution: Identify the stimulus type: chemical (not light, magnetic field, or oxygen specifically).Determine the direction: toward higher attractant concentration.Select “positive chemotaxis.” Verification / Alternative check: Capillary assays and microfluidic gradient chambers demonstrate accumulation of motile bacteria near attractants, confirming positive chemotaxis behavior experimentally. Why Other Options Are Wrong: B: Negative chemotaxis is movement away from a chemical, not toward it. C: Phototaxis refers to light stimuli, not chemicals. D: Magnetotaxis aligns with geomagnetic fields via magnetosomes. E: Aerotaxis is movement along oxygen gradients, a subset distinct from generic chemotaxis to other chemicals. Common Pitfalls: Equating any directional movement with chemotaxis; the stimulus modality must match the term used. Final Answer: Positive chemotaxis

Question 14

Bacterial cell structure — eukaryotic analog of the (artifact) mesosome Classical electron micrographs once described “mesosomes” in bacteria (now considered artifacts of chemical fixation). Which eukaryotic organelle was historically viewed as func…

Accepted Answer

Introduction / Context: Early electron microscopy of bacteria revealed membrane invaginations termed “mesosomes,” thought to be involved in respiration, cell wall synthesis, or DNA segregation. Later work showed these structures were artifacts of preparation. Nonetheless, historically, mesosomes were compared to eukaryotic organelles based on presumed function. Given Data / Assumptions: The question references the historical analogy grounded in respiratory function. Bacterial aerobic respiration occurs on the cytoplasmic membrane. In eukaryotes, oxidative phosphorylation occurs in mitochondria. Concept / Approach: If mesosomes were thought to compartmentalize respiratory enzymes in bacteria, the closest eukaryotic analogy would be mitochondria, which house the electron transport chain and ATP synthase. Golgi and lysosomes handle protein processing/trafficking and degradation, not primary ATP generation. Modern consensus emphasizes that “mesosomes” are not genuine organelles, but the analogy question still maps to mitochondria by function. Step-by-Step Solution: Identify the presumed function: respiration/energy generation.Match to the eukaryotic organelle with that function: mitochondria.Exclude Golgi (secretion/processing) and lysosomes (degradation).Answer with mitochondria as the historical analog. Verification / Alternative check: Textbook sidebars discuss the artifact status of mesosomes, noting their erstwhile comparison to mitochondria owing to respiratory enzyme localization. Why Other Options Are Wrong: Golgi apparatus: involved in post-translational modification and sorting. Lysosomes: catabolic organelles with hydrolases, not oxidative phosphorylation. Peroxisomes: fatty acid oxidation and detoxification; not the main site of ATP generation. None of these: incorrect because mitochondria fit the historical analogy. Common Pitfalls: Assuming mesosomes are real functional organelles; they are artifacts, but the exam item focuses on the traditional analogy. Final Answer: Mitochondria

Question 15

Gram-negative envelope — structure most exposed on the cell surface Considering the outer membrane architecture of Gram-negative bacteria, which listed component is most likely exposed at the cell surface and directly contacts the environment?

Accepted Answer

Introduction / Context: Gram-negative bacteria possess a dual-membrane envelope: an inner (cytoplasmic) membrane and an outer membrane containing lipopolysaccharide (LPS) in its outer leaflet. Surface exposure determines interactions with host tissues, immune recognition, and antibiotic permeability. Identifying the components directly facing the environment is key to understanding nutrient uptake and immune evasion. Given Data / Assumptions: Outer membrane: asymmetric bilayer with LPS outward and phospholipids mainly inward. Porins span the outer membrane and present extracellular loops at the surface. Lipoteichoic and wall teichoic acids are hallmarks of Gram-positive bacteria, not Gram-negative. Concept / Approach: Among the choices, the structure that is truly surface-exposed in Gram-negative bacteria is porin proteins, which form water-filled channels for small hydrophilic solutes and have extracellular loops contacting the environment. Inner-membrane energy proteins are buried and not surface-exposed. Phospholipids exist in membranes, but in the outer membrane the external leaflet is largely LPS, not phospholipid; thus phospholipids are not predominantly exposed at the surface. Step-by-Step Solution: Focus on Gram-negative outer membrane composition.Recognize porins traverse and protrude from the outer membrane, presenting external domains.Eliminate Gram-positive polymers (lipoteichoic/teichoic acids) as inapplicable.Eliminate inner-membrane proteins as non-surface. Verification / Alternative check: Structural studies of OmpF/OmpC show extracellular loops that serve as antigenic determinants; immunoblots using surface labeling confirm their exposure. Why Other Options Are Wrong: B: Inner-membrane proteins are not exposed to the exterior environment. C/E: Teichoic polymers are Gram-positive features, absent from Gram-negatives. D: The outer leaflet is LPS; phospholipids are not the predominant surface-exposed lipids. Common Pitfalls: Assuming any membrane lipid is exposed; in Gram-negative outer membranes, LPS dominates the external leaflet, and proteins like porins are major exposed structures. Final Answer: Porin proteins (outer-membrane channel proteins)

Question 16

In bacteriology, peptidoglycan (also called murein) is a hallmark structural polymer. In which bacterial component is peptidoglycan uniquely found (that is, not in eukaryotes or archaea)?

Accepted Answer

Introduction / Context: Peptidoglycan is the mesh-like exoskeleton that gives bacteria their shape and resists osmotic pressure. Recognizing where peptidoglycan resides is fundamental for understanding Gram staining, antibiotic action (for example, beta-lactams), and differences between Gram-positive and Gram-negative organisms. Given Data / Assumptions: The question asks for the specific bacterial structure that uniquely contains peptidoglycan. Consider common bacterial surface components: cell wall, cell membrane (plasma membrane), glycocalyx (capsule/slime), and spore layers. Focus on what is universal across most bacteria and unique compared with eukaryotes and archaea. Concept / Approach: The bacterial cell wall contains peptidoglycan (also called murein/mucopeptide). In Gram-positive bacteria, thick peptidoglycan forms the bulk of the wall. In Gram-negative bacteria, a thinner peptidoglycan layer sits within the periplasm between the inner and outer membranes. Eukaryotic cells lack peptidoglycan, and archaeal walls use different polymers (for example, pseudopeptidoglycan or S-layers), making peptidoglycan a bacterial signature. Step-by-Step Solution: List candidate locations: membrane, glycocalyx, cell wall, spore.Recall: peptidoglycan polymer consists of N-acetylglucosamine and N-acetylmuramic acid cross-linked by short peptides.Associate that polymer with the structural wall external to the membrane.Select the cell wall as the correct site. Verification / Alternative check: Gram staining outcomes (retention of crystal violet by thick peptidoglycan) and the mechanism of action of penicillins (inhibiting transpeptidation in peptidoglycan) both confirm that peptidoglycan is integral to the bacterial cell wall. Why Other Options Are Wrong: Cell membrane: phospholipid bilayer; peptidoglycan is not part of the membrane. Glycocalyx: capsule/slime composed of polysaccharide or polypeptide, not peptidoglycan. Spore: cortex and coats are specialized layers; their composition and function differ and are not simply peptidoglycan wall. Common Pitfalls: Confusing the capsule (glycocalyx) with the cell wall; the capsule is outside the wall and chemically distinct. Final Answer: cell wall

Question 17

In Gram-negative bacteria, which component is exposed on the outer surface and serves as a key antigenic marker for serotyping?

Accepted Answer

Introduction / Context: Gram-negative envelopes have an outer membrane containing lipopolysaccharide (LPS). The most external, variable sugar chain of LPS—the O-antigen—is a major serological marker and an important virulence determinant. Identifying which moiety is surface-exposed guides diagnostics and vaccine design. Given Data / Assumptions: The organism is Gram-negative. We compare LPS O-antigen, LTA, Braun lipoprotein, and electron transport components. We want the structure exposed on the exterior face of the outer membrane. Concept / Approach: LPS spans the outer membrane: lipid A anchors, core polysaccharide connects, and the O-antigen extends outward into the environment. LTA is a Gram-positive cell wall/ membrane-associated polymer. Braun lipoprotein tethers the outer membrane to peptidoglycan but is not the distal, exposed polysaccharide. Electron transport proteins are typically in the inner membrane/periplasm, not the outermost surface. Step-by-Step Solution: Identify the Gram-negative outer membrane components.Recall that O-antigen projects outward and varies by strain (serotype).Eliminate LTA (Gram-positive), Braun lipoprotein (periplasmic linkage), and ETS components (inner membrane).Choose O-antigen of LPS as the exposed surface marker. Verification / Alternative check: Serotyping of Enterobacteriaceae (for example, E. coli O157) uses O-antigen, demonstrating its surface exposure and immunogenicity. Why Other Options Are Wrong: LTA: associated with Gram-positive envelopes. Braun lipoprotein: structural linker, not the outward polysaccharide chain. Electron transport system: localized to inner membrane; not exposed externally. Common Pitfalls: Confusing teichoic/lipoteichoic acids (Gram-positive) with LPS (Gram-negative). Final Answer: O-antigen of lipopolysaccharide (LPS)

Question 18

Cyanobacteria (blue–green bacteria) exhibit which overall Gram-type architecture of the cell wall?

Accepted Answer

Introduction / Context: Cyanobacteria are oxygenic photosynthetic prokaryotes. Although their staining can vary, their envelope architecture is classically Gram-negative, which has implications for nutrient transport, susceptibility to antibiotics, and environmental resilience. Given Data / Assumptions: Classify cyanobacterial cell-wall type in the Gram sense. Recognize ultrastructure rather than staining alone as the key criterion. Concept / Approach: Gram-negative architecture features a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide-like molecules. Cyanobacteria possess this overall architecture, including a periplasmic space, making option “a gram-negative cell wall” correct, even if Gram stain outcomes may be atypical in practice. Step-by-Step Solution: Recall cyanobacterial envelope: inner membrane, periplasm with thin peptidoglycan, and outer membrane.Map this to Gram-negative organization rather than thick peptidoglycan of Gram-positives.Choose the Gram-negative cell wall option. Verification / Alternative check: Electron microscopy and biochemical analyses demonstrate outer membranes and thin peptidoglycan in cyanobacteria, aligning with Gram-negative architecture. Why Other Options Are Wrong: Gram-positive wall: would require a very thick peptidoglycan without an outer membrane. Neither/No wall: cyanobacteria clearly have peptidoglycan and an outer membrane. Common Pitfalls: Equating Gram staining color strictly with envelope type; some organisms stain irregularly yet have definable architecture. Final Answer: a gram-negative cell wall

Question 19

The classic “Chinese letter” (palisade) arrangement on microscopy is characteristic of which bacterium in clinical microbiology?

Accepted Answer

Introduction / Context: Arrangement and morphology are rapid bench clues in bacteriology. “Chinese letter” or cuneiform arrangements refer to angular groupings of club-shaped rods that help presumptively identify certain organisms before definitive tests return. Given Data / Assumptions: The pattern described is “Chinese letter” (palisading, V or L shapes). We compare several Gram-positive rods of clinical relevance. Concept / Approach: Corynebacterium diphtheriae (and related coryneforms) show club-shaped, pleomorphic rods that align at sharp angles due to snapping division. This yields characteristic V, L, and “Chinese letter” patterns on smears, especially with methylene blue or Albert stain. Step-by-Step Solution: Recall key morphologies: Bacillus anthracis forms long chains; Clostridium species are anaerobic rods with spores; Mycobacterium tuberculosis are slender acid-fast bacilli.Match the angular, palisading arrangement to Corynebacterium diphtheriae.Choose Corynebacterium diphtheriae as the answer. Verification / Alternative check: Classic lab teaching emphasizes C. diphtheriae’s metachromatic granules (volutin) and angular arrangements on direct smears or low-complexity cultures. Why Other Options Are Wrong: Bacillus anthracis: forms chains of boxcar rods, not “Chinese letters.” Mycobacterium tuberculosis: acid-fast straight/slender rods without palisading. Clostridium tetani: terminal spores (“drumstick”), not angular groupings. Common Pitfalls: Over-reliance on morphology alone; confirm with culture, toxin assays, or PCR for diphtheria. Final Answer: Corynebacterium diphtheriae

Question 20

Peptidoglycan is also known by which alternative names in microbiology terminology?

Accepted Answer

Introduction / Context: Microbiology texts and older literature use several synonymous terms for the bacterial cell wall polymer. Recognizing these synonyms avoids confusion when reading diverse sources or exam questions. Given Data / Assumptions: The polymer of interest is peptidoglycan, composed of alternating N-acetylglucosamine and N-acetylmuramic acid with peptide cross-links. Common synonyms must be identified among options. Concept / Approach: Both “mucopeptide” and “murein” historically refer to peptidoglycan. The terms emphasize its mixed carbohydrate (muco-) and peptide nature. Modern usage favors “peptidoglycan,” but older names appear in classic references and test banks. Step-by-Step Solution: Map each term to peptidoglycan synonyms: mucopeptide = yes; murein = yes.Confirm there is no alternative conflicting meaning in bacterial envelopes.Select the combined option acknowledging both as correct. Verification / Alternative check: Standard bacteriology glossaries list murein/mucopeptide as synonyms for peptidoglycan. Why Other Options Are Wrong: A or B alone: incomplete because both are valid synonyms. None of these: incorrect since both are correct synonyms. Common Pitfalls: Assuming each option is exclusive; in this case, the inclusive choice is correct. Final Answer: both (a) & (b)

Bacteria Morphology Questions