Question 1

Metabolic versatility – Pseudomonas pseudoflava: Pseudomonas pseudoflava (historically described) is known to grow as which type(s) of chemotroph under appropriate conditions?

Accepted Answer

Introduction / Context: Certain non-fermentative Gram-negative rods display remarkable metabolic flexibility, allowing growth on either inorganic electron donors or organic substrates. Recognizing such facultative chemolithotrophy is useful in environmental and industrial microbiology. Given Data / Assumptions: The organism named has been reported with facultative lithotrophic capabilities. Chemo-lithotrophs derive energy from inorganic donors; chemo-organotrophs use organic donors. We assume availability of suitable electron acceptors (often oxygen) in lab culture. Concept / Approach: Pseudomonads and related genera can exhibit broad catabolic repertoires. Historical descriptions of Pseudomonas pseudoflava include growth on organic media (chemo-organotrophy) and the capacity to oxidize selected inorganic substrates (chemo-lithotrophy) under appropriate conditions, classifying it as facultatively lithotrophic. Step-by-Step Solution: Define chemo-lithotrophy vs chemo-organotrophy. Map reported capabilities of the organism to both modes. Select the inclusive option indicating dual capability. Verification / Alternative check: Environmental isolates capable of H2 or reduced sulfur oxidation alongside heterotrophic growth exemplify this flexibility; enrichment conditions dictate which metabolism is expressed. Why Other Options Are Wrong: Limiting to a single mode ignores documented versatility; “none/photoautotroph” contradict established chemo-based metabolism. Common Pitfalls: Assuming all Pseudomonas are strictly organotrophic; several species or reclassified strains show broader chemotrophic strategies. Final Answer: Both (a) and (b).

Question 2

Microbial interactions – synergy: A synergistic effect between microorganisms is observed under which circumstance(s)?

Accepted Answer

Introduction / Context: Microbial communities often show emergent behaviors that individual species cannot achieve alone. Synergy can boost growth rates, persistence, or pathogenicity, and is central to food spoilage ecology, biofilms, and polymicrobial infections. Given Data / Assumptions: The listed scenarios reflect common ecological facilitation mechanisms. We focus on positive interactions that enhance growth of at least one partner. Systems may include foods, natural environments, or medical contexts. Concept / Approach: Synergy encompasses cross-feeding (metabolite sharing), environmental modification (pH, Eh, aw), and detoxification (removing oxygen or inhibitory acids), allowing microbes to colonize niches otherwise inaccessible. For example, lactic acid bacteria reduce redox and produce growth factors for anaerobes; molds raise pH or release enzymes that liberate nutrients for bacteria; some bacteria consume oxygen, enabling strict anaerobes to thrive beneath biofilms. Step-by-Step Solution: Evaluate each scenario for potential growth enhancement of a partner. Recognize that each listed mechanism is a textbook example of synergy. Choose the inclusive “All of the above.” Verification / Alternative check: Mixed-culture studies in food spoilage (e.g., sausage, cheese rinds) show faster spoilage and higher counts when synergists co-occur compared with monocultures. Why Other Options Are Wrong: Selecting a single mechanism ignores the breadth of synergistic interactions; “None” contradicts extensive ecological evidence. Common Pitfalls: Confusing synergy with commensalism; in synergy, both partners may benefit or one benefits significantly due to the other’s activity. Final Answer: All of the above.

Question 3

Energy sources – chemotrophs vs others: In microbial nutrition, organisms that obtain their energy from chemical compounds (as opposed to light) are specifically called what?

Accepted Answer

Introduction / Context: Microbial nutritional classifications distinguish energy source (light vs chemicals), electron source (organic vs inorganic), and carbon source (CO2 vs organic carbon). Using the correct terms is essential for describing metabolic strategies in ecology and biotechnology. Given Data / Assumptions: We are asked specifically about the energy source category. Light-driven organisms are phototrophs; chemical-energy organisms are chemotrophs. Other terms in the options refer to different aspects (electron or carbon source, or auxotrophy). Concept / Approach: Chemotrophs derive energy by oxidizing chemical compounds. Depending on the electron source, chemotrophs are further divided into organotrophs (electrons from organic molecules) and lithotrophs (electrons from inorganic molecules). Carbon source is denoted independently: autotrophs fix CO2, heterotrophs use organic carbon. Prototrophs are organisms that can synthesize all required metabolites from minimal media, unrelated to energy sourcing. Step-by-Step Solution: Identify the requested attribute: energy source. Match “energy from chemicals” to “chemotrophs.” Differentiate organotroph/lithotroph (electron source) and autotroph (carbon source). Select “Chemotrophs.” Verification / Alternative check: Standard metabolic nomenclature constructs combinations like chemoorganoheterotroph or chemolithoautotroph by stacking energy/electron/carbon terms; the first term is energy, hence “chemo–”. Why Other Options Are Wrong: Prototroph concerns biosynthetic capability; organotrophs/lithotrophs denote electron donors; autotrophs denote carbon source, not energy. Common Pitfalls: Mixing electron source with energy source; while coupled, they label different dimensions of metabolism. Final Answer: Chemotrophs.

Question 4

Microbial nutrition classification: Organisms such as Lactobacillus spp. that require complex, specific nutrients (e.g., vitamins, amino acids, and other growth-promoting substances) are generally referred to as what type of heterotrophs?

Accepted Answer

Introduction / Context: In clinical and food microbiology, some bacteria grow only when the medium provides preformed growth factors such as vitamins, amino acids, nucleotides, or heme. These microbes are described as “fastidious,” and recognizing this term guides media selection (e.g., enriched or selective-enriched media) and interpretation of growth failures. Given Data / Assumptions: Lactobacillus and related genera often need complex nutrients and growth factors. Question asks for the correct descriptor for such nutrient-demanding organisms. We contrast this with terms describing energy sources or carbon sources (chemo-, litho-, photo-). Concept / Approach: “Fastidious” refers to organisms with complex or specific nutritional requirements beyond basic mineral salts and a carbon source. They commonly require enriched media (e.g., blood, serum, yeast extract). The other listed categories describe how organisms obtain energy or electrons, not the complexity of nutritional needs. Step-by-Step Solution: Identify that the stem emphasizes “elaborate requirements” for vitamins and growth factors. Recall the standard term: fastidious organisms (here, fastidious heterotrophs). Differentiate from chemo-/photo- terms, which relate to energy metabolism rather than nutritional complexity. Select the option that directly matches the definition. Verification / Alternative check: Classic examples include Neisseria, Haemophilus, and Lactobacillus requiring factors like NAD, hemin, or complex peptides; they grow best on enriched media (e.g., chocolate agar, tomato juice agar for lactobacilli). Why Other Options Are Wrong: Chemo-lithotrophs: Use inorganic electron donors (e.g., NH3, Fe2+); says nothing about fastidiousness. Chemotrophs: Very broad; includes fastidious and non-fastidious organisms alike. Photo-organotrophs: Light-driven energy with organic carbon; again, not about complex requirements. Oligotrophs: Thrive at very low nutrient concentrations, not necessarily complex supplements. Common Pitfalls: Confusing energy/electron-source terminology with nutritional complexity; these describe different dimensions of microbial ecology. Final Answer: Fastidious heterotrophs.

Question 5

Turbidimetric growth measurement: In routine spectrophotometric monitoring of a liquid culture, growth is normally expressed as which parameter?

Accepted Answer

Introduction / Context: Turbidimetry estimates biomass in a broth culture by measuring light scattering rather than counting cells directly. It is fast, non-destructive, and widely used for growth curves, enzyme induction timing, and fermentation control. Given Data / Assumptions: A spectrophotometer or colorimeter is used at a standard wavelength (commonly ~600 nm). The readout is a unitless optical density (OD) or absorbance. Direct counts or CFU/mL are separate methods requiring microscopy or plating. Concept / Approach: Optical density reflects the proportion of light scattered by cells in suspension. While OD can be correlated with CFU/mL or dry weight using a calibration curve, the immediate measurement reported by the instrument is OD at the chosen wavelength, not absolute cell numbers. Step-by-Step Solution: Blank the instrument with uninoculated medium. Measure sample absorbance at the chosen wavelength (e.g., OD600). Record growth as optical density versus time to generate a growth curve. Optionally convert OD values to CFU/mL using a pre-determined calibration. Verification / Alternative check: Parallel plate counts at several ODs allow construction of a standard curve for a given organism and path length. Why Other Options Are Wrong: Cells per mL / CFU/mL: Derived from counting methods, not direct turbidimetric outputs. mg N per mL / Dry weight only: Biochemical or gravimetric determinations, not the standard turbidimetric readout. Common Pitfalls: Using OD outside the linear range leads to underestimation; dilute dense samples to maintain linearity. Final Answer: Growth by turbidimetry is reported as optical density.

Question 6

Bacterial population growth mathematics: Which of the following equivalent equations correctly relate initial population (N0), final population (N), and the number of generations (n) during binary fission?

Accepted Answer

Introduction / Context: Microbial growth in the exponential phase can be modeled mathematically. For binary fission, each generation doubles the population, leading to simple relationships between the number of generations and population size. Given Data / Assumptions: Binary fission: each generation doubles cell number. Use base-10 logarithms where log10(2) ≈ 0.3010. We compare algebraically equivalent forms. Concept / Approach: The fundamental expression is N = N0 * 2^n. Taking log10 on both sides yields log10(N) − log10(N0) = n * log10(2) ≈ 0.3010 * n. Therefore, options (a), (b), and (c) are mathematically equivalent descriptions of the same growth model; any other form that deviates from this equivalence is incorrect. Step-by-Step Solution: Start with binary fission model: N = N0 * 2^n. Take log10: log10(N/N0) = n * log10(2). Insert log10(2) ≈ 0.3010 to get a convenient linear form. Recognize all three expressions describe the same growth relationship. Verification / Alternative check: Plot log10(N) versus time; the slope equals log10(2)/g, where g is generation time. This confirms consistency among the formulas. Why Other Options Are Wrong: N = N0 * n^2: Quadratic growth, not doubling; does not model binary fission. Common Pitfalls: Mixing natural logs and base-10 logs without converting the constant log(2) appropriately. Final Answer: All of these (a–c) are correct and equivalent forms.

Question 7

Exponential (log) phase dynamics: During the exponential phase of batch culture, how do cell number and cell mass change with time?

Accepted Answer

Introduction / Context: The exponential (log) phase is characterized by balanced growth in which cellular components and cell numbers increase proportionally. Understanding this phase is essential for physiological experiments, enzyme production, and estimating generation times. Given Data / Assumptions: Nutrients are not yet limiting and waste products have not accumulated to inhibitory levels. Environmental conditions (temperature, pH, aeration) remain constant. Population is genetically uniform and adapted to the medium (post-lag phase). Concept / Approach: In exponential phase, each cell divides after a roughly constant generation time, so both total cell number and biomass increase geometrically. On a semi-log plot, growth appears as a straight line; on a linear plot, it appears as an accelerating curve. Biochemical composition per cell remains relatively constant (balanced growth). Step-by-Step Solution: Recognize that exponential phase implies constant generation time. Conclude that population doubles by a fixed factor over equal intervals. Therefore, both cell number and mass increase at a constant rate of doubling. Select the option describing doubling at a constant rate. Verification / Alternative check: Calculate generation time g from slope: log10(N) = log10(N0) + (t/g)*log10(2). Consistent slopes confirm balanced growth. Why Other Options Are Wrong: Increase-then-decrease, decrease, or constant mass describe lag, stationary, or death phases, not exponential. Unpredictable oscillations do not occur in simple batch culture under stable conditions. Common Pitfalls: Confusing “constant rate” with “linear increase”; exponential growth doubles over equal time intervals but is not linear in absolute numbers. Final Answer: In exponential phase, cells double at a constant rate.

Question 8

Temperature preferences: Under ordinary food and clinical conditions, bacteria and fungi generally multiply most rapidly within which temperature range?

Accepted Answer

Introduction / Context: Most pathogens and food spoilage organisms are mesophiles, thriving at moderate temperatures that overlap with ambient and body temperatures. Recognizing this range informs food safety controls and incubation choices in the laboratory. Given Data / Assumptions: We consider “general” microbial growth of common bacteria and many yeasts/molds. Specialists like psychrophiles or thermophiles are excluded from the general statement. Concept / Approach: Mesophiles grow best approximately between 20–45 °C, with many pathogens optimized near 35–37 °C. For a broad practical range, 16–38 °C captures conditions where multiplication is typically efficient in foods and clinical specimens before selective temperature controls are applied. Step-by-Step Solution: Identify the category: mesophiles dominate foodborne and clinical contexts. Compare ranges: sub-16 °C slows growth (refrigeration), >38 °C selects thermotolerant species. Choose the range that best aligns with rapid, general multiplication. Verification / Alternative check: Food codes and lab protocols commonly specify incubation at 30–37 °C for many tests, consistent with mesophilic preferences. Why Other Options Are Wrong: Below 16 °C or near-freezing: slows metabolism; mainly psychrotrophs grow. Above 38 °C: favors thermotolerant/thermophilic species; not “general.” None of these: incorrect; mesophilic range is well established. Common Pitfalls: Assuming all fungi are psychrophiles; many molds also grow well at moderate temperatures. Final Answer: Between 16–38 °C (mesophilic range).

Question 9

Bacterial survival strategy: Compared with an actively replicating vegetative cell, a bacterial endospore differs in which of the following ways?

Accepted Answer

Introduction / Context: Bacterial endospores (e.g., Bacillus, Clostridium) represent a survival form adapted to harsh environments. Recognizing how spores differ from vegetative cells underpins sterilization, infection control, and food-processing validations. Given Data / Assumptions: Sporulation is a stress-induced developmental program, not a reproductive division. Endospores contain dipicolinic acid, small acid-soluble proteins, low water content, and specialized coats. They are dormant and highly resistant. Concept / Approach: Sporulation begins with an asymmetric septum, forming a forespore and mother cell. Protective layers and dehydrated core confer resistance to environmental extremes. Endospores resume metabolism only upon germination when conditions improve. Step-by-Step Solution: Confirm sporulation is asymmetric and developmental (option a). Recognize resistance traits (heat, drying, chemicals) exceed vegetative cells (option b). Note dormancy and metabolic inactivity until germination (option c). Conclude that all listed statements are true. Verification / Alternative check: Autoclaving parameters target spore inactivation (e.g., 121 °C saturated steam), highlighting their enhanced resistance. Why Other Options Are Wrong: None of the above: Contradicted by well-known spore biology. Common Pitfalls: Confusing endospores (survival) with exospores or reproductive spores in fungi; bacterial endospores are not reproductive. Final Answer: All of the above differences are correct.

Question 10

Carbon source in metabolism: Which organisms typically obtain their carbon for biosynthesis from organic compounds (i.e., are chemoorganotrophs/heterotrophs)?

Accepted Answer

Introduction / Context: Microbes are classified by both energy source (light vs chemical) and carbon source (CO2 vs organic carbon). Chemoorganotrophs (heterotrophs) use organic compounds as both energy and carbon sources, while autotrophs fix CO2. Given Data / Assumptions: We compare three archetypes: aerobic glucose respirers, chemolithoautotrophs, and photoautotrophs. Identify which typically build biomass from organic carbon. Concept / Approach: Aerobic, glucose-respiring bacteria oxidize organic substrates for energy and derive carbon skeletons from the same organics, thus they are chemoorganotrophic heterotrophs. Ammonia oxidizers derive energy from NH3 but usually fix CO2 (autotrophy). Cyanobacteria use light for energy and fix CO2 (photoautotrophy). Methanogens use CO2 as an electron acceptor/substrate in methanogenesis, not as an organic carbon source. Step-by-Step Solution: Identify carbon source for each group. Chemoorganotrophs use organic carbon → matches aerobic glucose respirers. Rule out chemolithoautotrophs and photoautotrophs (CO2 fixation). Select the heterotrophic option. Verification / Alternative check: Textbook metabolic classifications place most pathogens and many environmental heterotrophs (e.g., Pseudomonas, E. coli) in the chemoorganotroph category. Why Other Options Are Wrong: Ammonia oxidizers / Cyanobacteria / Methanogens: Rely on CO2 as the primary carbon source; they are not typical organic-carbon heterotrophs. None of the above: Incorrect because aerobic glucose respirers are classic heterotrophs. Common Pitfalls: Equating “uses glucose for energy” with “autotrophy”; autotrophy refers to carbon source (CO2), not energy source. Final Answer: Aerobic, glucose-respiring bacteria obtain carbon from organic compounds.

Question 11

Marine ecology basics: What forms the primary source of food (the main producers at the base of the marine food web) in ocean ecosystems?

Accepted Answer

Introduction / Context: In aquatic ecosystems, primary producers convert inorganic carbon (CO2) into organic matter using light energy. Identifying the primary producers in the open ocean clarifies nutrient cycles, fisheries productivity, and climate interactions via carbon sequestration. Given Data / Assumptions: Primary producers perform photosynthesis and sit at the base of the food web. Marine systems include both microscopic (phyto/zooplankton) and macroscopic (seaweeds, seagrasses) organisms. Concept / Approach: Phytoplankton (photosynthetic microalgae and cyanobacteria) dominate primary production in the pelagic ocean, fueling higher trophic levels. Zooplankton are primary consumers that graze on phytoplankton. Macroalgae and seagrasses contribute significantly in coastal habitats but do not dominate global open-ocean primary production. Step-by-Step Solution: Define primary producers: organisms that fix CO2 using light. Recognize phytoplankton as the most abundant and widespread marine producers. Differentiate consumers (zooplankton) from producers. Select phytoplankton as the base of the marine food web. Verification / Alternative check: Global carbon models attribute a large fraction of annual net primary production to oceanic phytoplankton, supporting major fisheries and biogeochemical cycles. Why Other Options Are Wrong: Zooplankton: Consumers of phytoplankton, not primary producers. Seaweed/Seagrass only: Important locally but not the primary base across the open ocean. Detritus from land: Organic input, not new primary production. Common Pitfalls: Assuming visible macroalgae dominate because they are conspicuous; in the open ocean, microscopic phytoplankton are the main producers. Final Answer: Phytoplankton are the primary food source at the base of marine food webs.

Question 12

Food microbiology factors: In predictive microbiology, “nutrient content” and “biological structures” (e.g., rinds, shells, peels) are classified as which type of factors affecting microbial growth in foods?

Accepted Answer

Introduction / Context: Microbial growth in foods depends on intrinsic factors (properties of the food itself) and extrinsic factors (storage environment). Correctly categorizing variables helps design preservation strategies using the “hurdle” concept. Given Data / Assumptions: Nutrient content is an inherent property of a food matrix. Biological structures (skins, shells, membranes) are also inherent barriers. We must choose the correct classification. Concept / Approach: Intrinsic factors include pH, water activity (aw), oxidation-reduction potential (Eh), nutrient content, natural antimicrobials, and biological structures. Extrinsic factors include temperature, relative humidity, and gas atmosphere during storage. Processing factors are interventions (heating, packaging). Therefore, nutrient content and biological structures are intrinsic. Step-by-Step Solution: List intrinsic vs extrinsic vs processing factors. Place nutrient content and protective structures under intrinsic. Select “Intrinsic factors.” Verification / Alternative check: Food microbiology texts consistently group these variables as intrinsic hurdles that influence what organisms can grow. Why Other Options Are Wrong: Implicit: Not a standard category in predictive microbiology. Processing: Refers to applied treatments (e.g., pasteurization), not inherent properties. Extrinsic: Environmental conditions surrounding the food, not the food itself. Common Pitfalls: Confusing packaging atmosphere (extrinsic) with pre-existing internal barriers (intrinsic). Final Answer: They are intrinsic factors for microbial growth.

Question 13

Modes of bacterial multiplication: Besides the usual binary fission, which additional processes can also serve as modes of cell reproduction in certain bacteria?

Accepted Answer

Introduction / Context: While binary fission is the canonical mode of bacterial multiplication, diversity in bacterial cell biology allows alternative strategies in specific taxa. Appreciating these exceptions prevents overgeneralization and deepens understanding of microbial life cycles. Given Data / Assumptions: Question asks whether processes other than binary fission also contribute to reproduction. Examples include budding and filament fragmentation in particular groups. Concept / Approach: Most bacteria divide by binary fission. However, budding has been documented (e.g., Planctomycetes, some prosthecate bacteria), producing daughter cells from a polar growth site. Filamentous bacteria (actinomycetes, cyanobacteria) may reproduce via fragmentation of hyphae or trichomes. Note that endospore formation is a survival mechanism, not a reproductive one, but does not negate the fact that budding and fragmentation are valid reproduction modes alongside binary fission; hence “All of these” is correct when options include fission, budding, and fragmentation. Step-by-Step Solution: Confirm binary fission as the primary mechanism. Identify taxa where budding occurs (polar growth with daughter release). Recognize filament fragmentation as generating viable offspring units. Select the option that includes all valid reproduction modes listed. Verification / Alternative check: Microbiology references describe budding bacteria (e.g., Planctomyces) and filament fragmentation in actinomycetes (e.g., Streptomyces fragments forming new colonies). Why Other Options Are Wrong: Binary fission only: Overly restrictive; overlooks documented alternatives. Single-mode answers (b or c): Incomplete relative to the question. Endospore formation as reproductive: Misconception; endospores are for survival, not an increase in cell number. Common Pitfalls: Assuming endospores represent reproduction; one cell yields one spore and later one vegetative cell—no net multiplication. Final Answer: All of these (binary fission, budding, and fragmentation) can serve as reproductive modes across different bacteria.

Question 14

Microbial nutrition – carbon sources and classifications All organisms require at least small amounts of carbon dioxide (CO₂). However, some microorganisms can use CO₂ as their sole source of carbon for biosynthesis. Such organisms are termed:

Accepted Answer

Introduction / Context: Microorganisms are classified by how they obtain energy and carbon. Understanding these terms is fundamental for environmental microbiology, biogeochemical cycles, and bioprocess design. The question asks for the specific term used for organisms that fix carbon dioxide (CO₂) as their sole carbon source to build cellular material. Given Data / Assumptions: All living cells require a carbon source to synthesize biomolecules. Some microbes assimilate inorganic CO₂; others require preformed organic carbon such as sugars or organic acids. The classification focuses on the carbon source (not necessarily the energy source). Concept / Approach: Organisms that use CO₂ as their exclusive or principal carbon source are called autotrophs. They reduce CO₂ to organic carbon via pathways such as the Calvin–Benson–Bassham cycle, the reverse TCA cycle, or the Wood–Ljungdahl pathway. Autotrophs can be phototrophic (using light as energy) or chemolithotrophic (using energy from oxidation of inorganic compounds). The key feature is CO₂ fixation, independent of whether energy comes from light or chemicals. Step-by-Step Solution: Identify the property: ability to use CO₂ as sole carbon source.Map property to term: organisms with this trait are autotrophs.Differentiate from energy terms: phototroph and chemotroph describe energy, not carbon source.Conclude the correct classification is 'Autotrophs'. Verification / Alternative check: Examples include cyanobacteria (photoautotrophs) and nitrifiers like Nitrosomonas (chemolithoautotrophs). These organisms fix CO₂ and support global primary production in diverse habitats. Why Other Options Are Wrong: Phototrophs: use light for energy; may be auto- or heterotrophic for carbon. Chemotrophs: use chemical compounds for energy; carbon source may be CO₂ or organic. Photo-organotrophs: not a standard primary category; suggests light energy with organic carbon (photoheterotrophy) rather than CO₂ fixation. Mixotrophs: combine strategies and do not necessarily rely solely on CO₂. Common Pitfalls: Confusing energy source (photo vs chemo) with carbon source (auto vs hetero). Always parse both axes when classifying metabolism. Final Answer: Autotrophs.

Question 15

Temperature classes of microbes Which of the following temperature ranges best represents mesophiles (microorganisms with moderate temperature preference)?

Accepted Answer

Introduction / Context: Microbial growth strongly depends on temperature. For process control, food safety, and clinical microbiology, it is important to recognize typical ranges for psychrophiles (cold-loving), mesophiles (moderate), and thermophiles (heat-loving). The question asks for the temperature range typical of mesophiles. Given Data / Assumptions: Mesophiles inhabit environments such as the human body, soils, and freshwater. They generally show optimal growth around body/ambient temperatures. Provided ranges include moderate, cold, and high-temperature values. Concept / Approach: Mesophiles usually grow between about 20°C and 45°C, with optima commonly near 30–37°C. A concise range that captures the heart of mesophilic growth and typical optima is 25–40°C. Values above 40–45°C start to favor thermophiles or thermotolerant organisms, while values below ~20°C suit psychrophiles or psychrotrophs. Step-by-Step Solution: Recall standard ranges: psychrophiles (<20°C), mesophiles (~20–45°C), thermophiles (>45°C).Check the options for the best mesophile window.Select 25–40°C as the most appropriate concise range in the list. Verification / Alternative check: Human pathogens (e.g., Escherichia coli, Staphylococcus aureus) are mesophiles with optima near 37°C, supporting the 25–40°C range as suitable for mesophilic growth. Why Other Options Are Wrong: 20–30°C: too narrow; excludes many mesophiles that peak near 37°C. > 40°C: favors thermophiles rather than mesophiles. None of these: incorrect because 25–40°C fits standard definitions. 5–15°C: psychrophilic/psychrotrophic range. Common Pitfalls: Confusing optimum with range; many mesophiles survive up to ~45°C but do not necessarily thrive beyond ~40°C. Final Answer: 25–40°C.

Question 16

Carbon sources for biosynthesis Which organisms typically obtain their cell carbon primarily from carbon dioxide (CO₂) rather than from organic substrates?

Accepted Answer

Introduction / Context: Microbes differ in both energy and carbon acquisition. While heterotrophs require organic carbon, autotrophs fix CO₂ into biomass. The question asks which organisms usually derive their biomass carbon from CO₂. Given Data / Assumptions: Fermenters and respirers of glucose use organic carbon both for energy and biomass. Chemolithotrophs oxidize inorganic electron donors and often fix CO₂. Ammonia-oxidizing bacteria are classic chemolithoautotrophs. Concept / Approach: Chemolithoautotrophs (e.g., nitrifiers) oxidize reduced inorganic compounds for energy and use that energy to fix CO₂ as their carbon source. Ammonia-oxidizing bacteria such as Nitrosomonas oxidize NH₃ to nitrite and assimilate CO₂, fitting the definition precisely. In contrast, organisms using glucose—whether fermenting or respiring—are heterotrophs for carbon, not CO₂ fixers. Step-by-Step Solution: Identify which group is chemolithoautotrophic.Recognize ammonia oxidizers as archetypal CO₂ fixers among bacteria.Select the option naming ammonia-oxidizing (chemolithotrophic) bacteria. Verification / Alternative check: Nitrifiers are central to nitrogen cycling and primary production in some ecosystems due to their CO₂ fixation capability. Why Other Options Are Wrong: Glucose users (aerobic/anaerobic; fermenters): use organic carbon; do not rely on CO₂ as sole carbon source. Lactic acid bacteria: also heterotrophs, using sugars from milk. Common Pitfalls: Mixing up energy source (inorganic vs organic) with carbon source (CO₂ vs organic). Chemolithotrophy commonly pairs with autotrophy. Final Answer: Ammonia-oxidizing bacteria (chemolithotrophic bacteria).

Question 17

Nitrifiers and electron donors Bacteria of the genus Nitrosomonas obtain the electrons used for energy conservation by oxidizing which compound?

Accepted Answer

Introduction / Context: Nitrifying bacteria drive the first step of nitrification in the nitrogen cycle. Nitrosomonas species are chemolithoautotrophs that oxidize reduced nitrogen compounds to harvest energy while fixing CO₂ for biomass. Knowing their electron donor is crucial in wastewater treatment and environmental microbiology. Given Data / Assumptions: Nitrosomonas perform ammonia oxidation. Electron donors define chemolithotrophic niches. Alternative donors in the options represent other metabolisms. Concept / Approach: Nitrosomonas oxidize ammonia (NH₃/NH₄⁺) to nitrite (NO₂⁻), transferring electrons through their respiratory chain to generate proton motive force and ATP. They are not phototrophs and do not use typical organic donors like succinate; those are common in chemoorganoheterotrophs. H₂S is used by sulfur-oxidizers, not by classic ammonia oxidizers. Step-by-Step Solution: Identify organismal role: ammonia-oxidizing bacterium.Match role to electron donor: ammonia provides electrons upon oxidation to nitrite.Select 'Ammonia' as the correct electron source. Verification / Alternative check: Nitrification engineering models (e.g., activated sludge) explicitly allocate ammonia as the substrate for Nitrosomonas, confirming this choice. Why Other Options Are Wrong: H₂S: substrate for sulfur oxidizers like Acidithiobacillus. Succinate: organic donor for heterotrophs. Light: used by phototrophs. H₂: donor for hydrogen-oxidizing bacteria, not canonical Nitrosomonas. Common Pitfalls: Confusing different chemolithotroph groups (nitrifiers vs sulfur oxidizers vs hydrogen oxidizers). Final Answer: Ammonia.

Question 18

Thermal classes of microbes Organisms that grow best at temperatures above 45°C are referred to as:

Accepted Answer

Introduction / Context: Temperature tolerance categories help predict microbial habitats and guide industrial bioprocesses. Recognizing which organisms thrive at elevated temperatures supports bioreactor design, enzyme selection, and food sterilization strategies. Given Data / Assumptions: >45°C indicates a heat-loving preference. Common categories: psychrophiles, mesophiles, thermophiles, hyperthermophiles. Terminology is standardized in microbiology texts. Concept / Approach: Thermophiles prefer growth above ~45°C, often displaying optima between 50°C and 65°C. Hyperthermophiles extend this range, thriving at ≥80°C, while mesophiles grow best at moderate temperatures (about 25–40°C) and psychrophiles at cold temperatures (≤20°C). Therefore, the appropriate label for organisms growing best above 45°C is thermophiles. Step-by-Step Solution: Identify threshold (>45°C) as the defining clue.Relate threshold to standard categories.Select 'Thermophiles' as the correct class. Verification / Alternative check: Examples include Thermus aquaticus (source of Taq polymerase) and certain Bacillus species adapted to hot springs or compost heaps. Why Other Options Are Wrong: Psychrophiles: prefer cold, not >45°C. Mesophiles: prefer moderate temperatures. Any of these: incorrect—only thermophiles fit. Hyperthermophiles: typically require even higher temperatures (~80–110°C). Common Pitfalls: Confusing thermophiles with hyperthermophiles; overlooking that thermotolerant mesophiles are not the same as thermophiles. Final Answer: Thermophiles.

Question 19

Definition check – autotrophic bacteria Autotrophic bacteria are those which:

Accepted Answer

Introduction / Context: Autotrophy is a core metabolic strategy in which organisms synthesize organic biomass from inorganic carbon (CO₂). In bacteria, this can be coupled to light (photoautotrophy) or to oxidation of reduced inorganic compounds (chemolithoautotrophy). The question asks for the accurate definition of autotrophic bacteria. Given Data / Assumptions: Autotrophs rely on CO₂ as principal carbon source. Energy can come from light or chemical oxidation. Other listed statements refer to unrelated traits (capsules, pigments, drug susceptibility, oxygen requirement). Concept / Approach: The hallmark of autotrophy is CO₂ fixation into organic molecules via pathways such as the Calvin cycle or reverse TCA, powered by photonic or chemical energy. This distinguishes autotrophs from heterotrophs, which must assimilate preformed organic carbon. Many ecologically important bacteria, including nitrifiers and sulfur oxidizers, are autotrophs. Step-by-Step Solution: Identify defining feature: CO₂ as carbon source.Include energy source note: light or inorganic chemicals.Select the option that explicitly states CO₂ fixation as “making their own food.” Verification / Alternative check: Textbook definitions (microbial physiology) consistently define autotrophs by their carbon source, not by morphology, pigmentation, or antibiotic response. Why Other Options Are Wrong: Glycocalyx formation: relates to surface structures and biofilms, not carbon metabolism. Penicillin susceptibility: depends on cell wall traits; not a defining metabolic feature. Blue–green pigment: describes some cyanobacteria but is not a defining property of autotrophy. Oxygen absence: conflates metabolism with aerotolerance; many autotrophs are aerobic. Common Pitfalls: Equating autotrophy with photosynthesis only; chemolithoautotrophs also fix CO₂ without light. Final Answer: Make their own food by fixing inorganic carbon (CO₂) using an energy source.

Question 20

Microbial growth curves – identifying a false statement Which of the following is NOT a characteristic of a standard batch-culture microbial growth curve?

Accepted Answer

Introduction / Context: The batch-culture growth curve is a foundational concept in microbiology. It describes how a closed microbial population changes over time in a fixed volume of medium without input or removal, producing the classic lag, exponential, stationary, and death phases. One of the statements given is not characteristic of such a curve. Given Data / Assumptions: Batch cultures evolve over time as nutrients deplete and wastes accumulate. Growth curves are typically plotted as log(cell number) versus time. Four-phase pattern is a standard teaching model. Concept / Approach: In a batch system, environmental conditions are not stable; they continuously change as the culture grows. Nutrients decline, pH may drift, oxygen may drop, and inhibitory by-products accumulate. Therefore, describing the environment as “relatively stable” is inaccurate for a batch growth curve. The other statements correctly describe plotting and interpretation conventions for growth curves. Step-by-Step Solution: Identify the system type: closed (batch) with no fresh medium added.Note that environmental parameters change significantly through time.Confirm remaining statements (log plotting, time axis, four phases) as correct.Select the option asserting “relatively stable conditions” as the incorrect one. Verification / Alternative check: Continuous culture (chemostat) is the system designed to maintain near-steady conditions; batch culture is not. This contrast validates the incorrectness of the “stable conditions” claim in batch growth curves. Why Other Options Are Wrong: Log plotting: standard to linearize exponential growth. Numbers vs time: definitionally correct. Four phases: classic model for batch culture. Estimating generation time: routine from the exponential phase slope. Common Pitfalls: Confusing batch and continuous cultures; assuming nutrient and waste levels remain steady during batch growth. Final Answer: Shows development of a microbial population under relatively stable environmental conditions.

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