Question 1

Preserving pure cultures: Which practice(s) are used for proper maintenance and long-term preservation of pure microbial cultures?

Accepted Answer

Introduction / Context:Maintaining pure cultures without genetic drift or contamination is central to reproducible microbiology. Different time horizons (short-term vs long-term) require different preservation strategies to maintain viability and phenotype.Given Data / Assumptions:The options list common maintenance and preservation techniques.Goal: identify whether each is valid.Organisms vary in survivability with each method.Concept / Approach:Periodic transfer to fresh medium maintains active cultures for short-term work but risks contamination and selection for fast growers. Mineral oil overlay reduces oxygen diffusion and evaporation, extending shelf life of slants at ambient or refrigerated temperatures. Lyophilization (freeze-drying) provides long-term storage by removing water under vacuum after freezing; cultures remain stable for years when stored properly.Step-by-Step Solution:Match each listed technique to a recognized preservation method.Confirm all three are accepted practices in culture collections.Recognize differing time frames and organism-specific considerations.Select “All of the above.”Verification / Alternative check:Culture collections (e.g., ATCC protocols) document these methods among others (cryopreservation at -80°C or in liquid nitrogen). Mineral oil overlays and lyophilization are classic, widely taught approaches.Why Other Options Are Wrong:Choosing any single method disregards the others’ validity and complementary uses.Rejecting all contradicts standard laboratory preservation practices.Common Pitfalls:Too-frequent subculturing causing genetic drift; improper sealing of lyophilized ampoules; using mineral oil with obligate aerobes that require higher oxygen transfer.Final Answer:All of the above

Question 2

Acid tolerance: Which of the following bacteria are capable of growth at acidic pH and are therefore associated with acid-tolerant or acidophilic behavior?

Accepted Answer

Introduction / Context:pH preference is a key ecological and diagnostic trait. While many pathogens prefer neutral pH, some bacteria thrive in acidic environments, influencing their habitats, roles in food fermentation, and survival in the host.Given Data / Assumptions:We are contrasting acid-tolerant organisms with those that prefer neutral or alkaline pH.Lactobacilli are well-known for acid production and tolerance.Other listed genera are enteric or aquatic pathogens with different pH preferences.Concept / Approach:Lactobacillus species are lactic acid bacteria used in yogurt, sauerkraut, and other fermented foods; they tolerate and generate acidic conditions. In contrast, Vibrio cholerae prefers alkaline environments, and many Shigella and Salmonella strains are neutrophiles even though they can survive transient acid stress.Step-by-Step Solution:Identify the group known for acid tolerance and production: lactobacilli.Exclude vibrios due to alkaline preference (e.g., alkaline peptone water).Exclude Shigella and Salmonella as primarily neutrophiles.Select “Lactobacilli.”Verification / Alternative check:Food microbiology texts document lactobacilli thriving at lower pH; they both produce lactic acid and tolerate acidic environments better than many pathogens.Why Other Options Are Wrong:Vibrio cholerae: alkaline-tolerant; selected at pH around 8.5.Shigella, Salmonella: generally neutrophilic growth optima; acid exposure is a stressor, not a preferred condition.Common Pitfalls:Confusing survival under acid stress with growth preference; assuming all pathogens thrive across broad pH ranges without selective advantage.Final Answer:Lactobacilli

Question 3

Alkaline tolerance in medical microbiology Among common enteric and gastrointestinal pathogens, which bacterium is well known to grow optimally in alkaline conditions (for example, pH about 8.5–9.5) and is deliberately enriched on alkaline peptone w…

Accepted Answer

Introduction / Context:Recognizing the preferred pH ranges of clinically important bacteria is essential for specimen processing and selecting the correct culture media. Vibrio cholerae, the cholera pathogen, has a notable ability to tolerate and even prefer alkaline conditions. Laboratories exploit this property to selectively enrich and recover the organism from fecal samples that contain mixed flora.Given Data / Assumptions:We compare enteric organisms routinely encountered in clinical microbiology.Alkaline peptone water (approximately pH 8.5–9.5) is used as an enrichment broth for suspected Vibrio species.Typical intestinal commensals and many pathogens prefer near-neutral conditions.Concept / Approach:The key is to match organism physiology to laboratory methods. Vibrio cholerae grows rapidly in alkaline peptone water, outcompeting neutrophilic organisms. In contrast, Shigella and Salmonella are neutrophiles; Lactobacilli favor acidic environments (they produce lactic acid and are used in fermented foods). Therefore, the species that “thrives in alkaline pH” is Vibrio cholerae.Step-by-Step Solution:Identify the organism associated with alkaline enrichment: Vibrio cholerae.Recall selective/enrichment methods: alkaline peptone water followed by plating on thiosulfate–citrate–bile salts–sucrose agar.Exclude Shigella/Salmonella (neutrophiles) and Lactobacilli (acid-tolerant).Select Vibrio cholerae as the only option aligned with alkaline growth.Verification / Alternative check:Standard diagnostic algorithms instruct inoculating feces into alkaline peptone water for several hours and then subculturing; recovery rates for Vibrio increase because competing flora are suppressed at high pH.Why Other Options Are Wrong:Shigella: prefers neutral pH; no alkaline enrichment advantage.Lactobacilli: acidophilic; thrive in acidic, not alkaline, media.Salmonella: grows best near neutral pH.None of the above: incorrect because Vibrio cholerae fits.Common Pitfalls:Confusing “selective” with “enrichment.” Alkaline peptone water is an enrichment that leverages pH preference; TCBS is the selective plate.Final Answer:Vibrio cholerae

Question 4

Clues to anaerobic infections in clinical specimens Which of the following observations strongly suggest that a specimen likely contains anaerobic bacteria and warrants anaerobic culture techniques?

Accepted Answer

Introduction / Context:Prompt recognition of anaerobic infections helps clinicians choose the right transport systems, culture media, and empiric therapy. Certain bedside or laboratory clues consistently indicate an anaerobic process, guiding the microbiologist to use oxygen-free handling and appropriate media such as anaerobic blood agar and enrichment broths.Given Data / Assumptions:The specimen may show gas production, malodor, or sulfur granules.Anaerobes are fastidious and die upon oxygen exposure; proper collection and transport are critical.Mixed infections with aerobes and anaerobes are common in deep abscesses and necrotic tissue.Concept / Approach:Three classic clues are emphasized in textbooks: (1) gas in tissue or specimens, (2) a foul, putrid smell due to volatile fatty acids, and (3) sulfur granules, classically associated with Actinomyces infections (anaerobic, branching gram-positive rods). When any of these are present, laboratories should prioritize anaerobic culture and susceptibility testing as indicated.Step-by-Step Solution:Assess the listed clues for specificity to anaerobes.Gas in tissues (emphysematous infections) frequently implicates anaerobes such as Clostridium.Putrid odor results from anaerobic metabolism (short-chain fatty acids, amines).Sulfur granules indicate actinomycetoma; crushing reveals gram-positive filaments within.Conclude that all listed clues support anaerobiosis and choose “All of these.”Verification / Alternative check:Clinical guidance recommends using anaerobic transport vials and avoiding swabs for deep collections when such features are present.Why Other Options Are Wrong:A/B/C alone do suggest anaerobes, but the question asks which hints suggest anaerobes; all apply.None of these: contradicted by established diagnostic signs.Common Pitfalls:Submitting superficial swabs exposed to air; improper transport can yield false-negative anaerobic cultures.Final Answer:All of these

Question 5

Selecting a pathogen from a mixed culture A disease-producing (pathogenic) species present within a mixed culture can often be selected based on which distinguishing property when using suitable laboratory models or host systems?

Accepted Answer

Introduction / Context:When a clinical or environmental sample contains a mixture of microbes, the objective is to recover the causative pathogen. Beyond nutritional selection, microbiologists may exploit the organism’s pathogenic properties using in vivo or ex vivo models to enrich or identify the disease-causing species.Given Data / Assumptions:The culture is mixed and includes at least one pathogen.Some pathogens exhibit virulence in susceptible hosts (for example, laboratory animals, cell cultures, embryonated eggs).Nutritional selection alone may not uniquely identify a pathogen in complex flora.Concept / Approach:Selective recovery can be achieved by leveraging virulence. Historically, inoculation into a susceptible host (animal passage) enriches for pathogenic organisms because they multiply in vivo while commensals fail to persist. Modern analogs include invasion assays on cell lines, toxin detection, or bacteriophage susceptibility tests that reflect pathogenic traits. While specialized media (carbon or nitrogen sources) can be helpful, many pathogens share nutritional profiles with commensals, making pathogenic properties more discriminating.Step-by-Step Solution:Define the goal: select the disease producer from a mixed population.Identify a discriminant that commensals lack: virulence in a host or model system.Recognize that special carbon/nitrogen requirements are neither necessary nor sufficient in many cases.Choose pathogenic properties as the most reliable basis for selection when appropriate facilities and ethics permit.Verification / Alternative check:Classical methods such as Koch’s postulates employed animal inoculation to isolate and re-isolate the pathogen, demonstrating disease causation and effectively selecting the disease-producing agent.Why Other Options Are Wrong:B/C: Nutritional idiosyncrasies occur but are not universal markers of pathogenicity.D: Not correct because pathogenic properties can be used.E: Pigment can be characteristic (for example, Pseudomonas), but pigments are not general selectors for disease producers.Common Pitfalls:Assuming one medium will always select the pathogen. In practice, combined strategies (selective media plus virulence or toxin assays) yield the best results.Final Answer:Pathogenic properties (for example, virulence in a susceptible host)

Question 6

Media definitions in bacteriology When a substance is added to a liquid culture medium that inhibits unwanted bacteria while favoring the growth of the target organism, this medium is best described as:

Accepted Answer

Introduction / Context:Precise terminology for culture media guides correct laboratory choices. “Selective,” “enrichment,” and “differential” are commonly confused. The wording of the question emphasizes inhibition of unwanted flora plus relative advantage to the target organism in a liquid medium.Given Data / Assumptions:A substance is added that suppresses competitors.The medium is liquid.The goal is to recover a “wanted” organism from a mixed specimen.Concept / Approach:Selective media contain agents (for example, bile salts, dyes, antibiotics, high salt) that inhibit unwanted microbes while permitting growth of the desired group. Differential media allow visual distinction (for example, pH indicators) without necessarily inhibiting others. “Enrichment medium” usually refers to a liquid formulation that favors a target by providing conditions or nutrients (for example, alkaline peptone water for Vibrio) and may not always rely on direct inhibitors; however, the explicit mention of inhibition aligns squarely with “selective.”Step-by-Step Solution:Read the key phrase: “inhibits the growth of unwanted bacteria.”Map this to the definition of selective medium.Confirm that differential and enriched media do not primarily act via inhibition.Choose “Selective medium.”Verification / Alternative check:Examples include MacConkey agar (bile salts and crystal violet inhibit Gram-positives) and mannitol salt agar (7.5% NaCl selects staphylococci). Liquid analogs can also be selective when they include inhibitors.Why Other Options Are Wrong:A: Differential media reveal biochemical traits but are not primarily inhibitory.B: Enriched media add nutrients (for example, blood, serum) to support fastidious organisms without inhibiting others.C: Enrichment media favor a target organism but the question emphasizes inhibition, making “selective” the precise term.E: Transport media preserve viability, not selection.Common Pitfalls:Confusing enrichment with selective. Some enrichment broths can be selective, but the defining feature in this prompt is inhibition.Final Answer:Selective medium

Question 7

Classifying routine bacteriologic media Peptone water and nutrient broth are best categorized as which type of culture media used as general-purpose foundations for microbial growth and biochemical testing?

Accepted Answer

Introduction / Context:Understanding media categories helps ensure appropriate lab workflows. Basal (or basic) media supply the minimum nutrients to support the growth of many nonfastidious organisms and serve as the base for preparing more complex formulations.Given Data / Assumptions:Peptone water and nutrient broth are simple formulations of peptones, beef extract, and salts.No inhibitors, selective agents, or special growth factors are explicitly included.They can be supplemented to become enriched or selective media.Concept / Approach:Basal media function as general-purpose culture media suitable for maintaining stock cultures, preparing inocula, and performing simple biochemical reactions. Enriched media add blood, serum, or growth factors for fastidious organisms (for example, blood agar, chocolate agar). Selective media include inhibitory substances. Differential media contain indicators to distinguish groups. Peptone water and nutrient broth fit the basal definition.Step-by-Step Solution:Identify the composition: simple digests and extracts without inhibitors.Match to media class: basal.Exclude enriched (would require added blood/serum) and selective/differential (would require inhibitors/indicators).Select “Basal media.”Verification / Alternative check:Standard manuals list peptone water for indole testing and nutrient broth for culture maintenance—classic basal uses.Why Other Options Are Wrong:A: No selective agents are present.B: Not enriched; lacks blood/serum or special factors.D: Inapplicable because a correct category exists.E: No color indicators or substrate/indicator pairs to differentiate organisms.Common Pitfalls:Assuming “broth” equals “enriched.” Many broths are basal unless supplemented.Final Answer:Basal media

Question 8

Selecting oral Treponema species When attempting to select Treponema species from the human oral cavity, which practical criteria can be used to enrich and recognize these spirochetes during laboratory work?

Accepted Answer

Introduction / Context:Treonemes in the oral cavity are delicate, thin spirochetes that can be difficult to culture. When recovery is required for research or advanced diagnostics, laboratories combine environmental controls, microscopic screening, and tailored media to enrich treponemes from polymicrobial oral specimens.Given Data / Assumptions:Treponema are slender, highly motile spirochetes.They prefer near-neutral pH, reduced oxygen tension, and specific temperatures.Dark-field microscopy allows rapid recognition by size and motility.Concept / Approach:Enrichment of oral Treponema typically includes microaerophilic or anaerobic incubation and temperature near human physiological range, together with media components that do not suppress spirochetes. Screening by dark-field detects their distinctive corkscrew motility and small diameter compared with commensal rods and cocci. pH adjustment, while not extreme, supports viability and can suppress some competitors.Step-by-Step Solution:Control pH to conditions compatible with treponemal survival (near neutral, avoiding extremes).Incubate at appropriate temperature (about 35–37 °C) under microaerophilic/anaerobic conditions.Use dark-field to identify thin, actively motile spirochetes by size and movement pattern.Combine these measures to enrich and select oral Treponema from mixed flora.Verification / Alternative check:Classic periodontal research protocols employ microaerophilic chambers and dark-field motility counts to track Treponema abundance.Why Other Options Are Wrong:A/B/C individually help, but optimal recovery uses the combination; hence “All of these.”E is incorrect because each listed factor can contribute to selection.Common Pitfalls:Over-oxidizing environments or using routine aerobic incubation will kill or severely reduce treponemes, leading to false-negative results.Final Answer:All of these

Question 9

Nutritional classification of human-associated bacteria Considering metabolic requirements, all bacteria that regularly inhabit the human body are best described as:

Accepted Answer

Introduction / Context:Microbial ecology categorizes organisms by energy and carbon sources. Understanding the trophic types of human-associated bacteria clarifies why certain nutrients and environments support or inhibit their growth and informs strategies for probiotics, infection control, and culture media formulation.Given Data / Assumptions:Human-associated bacteria colonize nutrient-rich, organic environments (skin, gut, mucosa).Host tissues and secretions provide organic carbon and energy sources.Sunlight is not directly harnessed by human commensals or pathogens for photosynthesis.Concept / Approach:Heterotrophs use organic compounds as carbon sources and usually as energy sources. This describes essentially all bacteria that inhabit humans, regardless of oxygen preference. Chemolithotrophs oxidize inorganic compounds (for example, ammonia, hydrogen sulfide) and are characteristic of environmental niches such as soil or hydrothermal vents. Autotrophs fix carbon dioxide; phototrophs use light as an energy source—neither category represents typical human-associated microbiota.Step-by-Step Solution:Identify the host environment: abundant organic substrates (amino acids, sugars, fatty acids).Match to trophic category: heterotrophy.Exclude chemolithotrophy (inorganic donors), autotrophy (CO2 fixation), and phototrophy (light as energy source).Select “Heterotrophs.”Verification / Alternative check:Standard culture media for human pathogens are formulated with organic nutrients (peptones, extracts, blood), reflecting heterotrophic growth needs.Why Other Options Are Wrong:B: Chemolithotrophs are rare in the human body and not typical colonizers.C: Autotrophs are not a feature of human microbiota.D: Phototrophs require light and photosynthetic machinery absent in human-associated bacteria.E: Methanotrophy is a specialized metabolism not characteristic of the human microbiome.Common Pitfalls:Confusing oxygen requirement (aerobe/anaerobe) with nutritional class (heterotroph/autotroph).Final Answer:Heterotrophs

Question 10

Selective agents and Campylobacter jejuni In selective media for isolating Campylobacter jejuni from stool, which of the following chemotherapeutic agents are typically included at low levels to suppress competing flora without inhibiting Campylobac…

Accepted Answer

Introduction / Context:Campylobacter jejuni is a fastidious, microaerophilic pathogen frequently isolated from diarrheal stool. Because stool contains abundant competing flora, selective media incorporate antimicrobial agents that inhibit commensals while allowing C. jejuni to grow.Given Data / Assumptions:Common selective formulations include Skirrow, Butzler, and Campy-BAP media.These media often contain vancomycin, polymyxin B (or colistin), and trimethoprim among other agents (for example, amphotericin B, cephalothin depending on recipe).Campylobacter jejuni exhibits relative resistance to these agents at the concentrations used in the media.Concept / Approach:The question asks which agents “may affect” recovery. In practice, they are deliberately added to affect the mixed microbiota by suppressing them, yet they do not significantly inhibit Campylobacter at selective doses. Therefore, all three named agents are part of standard Campylobacter selective supplements. The best match is “All of these.”Step-by-Step Solution:Recall standard selective supplement components for Campylobacter media.Match listed drugs: vancomycin (targets Gram-positives), polymyxin (targets many Gram-negatives), trimethoprim (blocks folate pathway; low impact on Campylobacter at used doses).Conclude that all are used to shape the flora and support Campylobacter recovery.Choose “All of these.”Verification / Alternative check:Commercial dehydrated media inserts list these antimicrobials in Campylobacter selective supplements; recovery is improved versus nonselective plates.Why Other Options Are Wrong:D: “None of these” is false; these agents are intentionally included.Any single-agent choice is incomplete because typical recipes use a combination.Common Pitfalls:Assuming every antimicrobial in a medium targets the pathogen; in selective media, additives primarily suppress competitors without harming the target organism.Final Answer:All of these

Question 11

Quantifying bacteria in a specimen Among the following routine methods, which is most likely to yield a quantitative result for viable bacterial numbers in a sample?

Accepted Answer

Introduction / Context:Clinicians and microbiologists often need quantitative data, such as colony-forming units per milliliter, to interpret urinary tract infections, food contamination, or water quality. Different methods provide either qualitative or quantitative information; knowing which method is designed for quantitation is fundamental.Given Data / Assumptions:Serial dilution and plating can enumerate viable organisms.Gram stain provides morphology and approximate bacterial load but is not reliably quantitative.Wet mounts demonstrate motility or presence but not accurate counts.Concept / Approach:The viable plate count method involves preparing tenfold dilutions of a sample, plating measured aliquots onto agar, incubating, and counting colonies. Because each colony theoretically arises from a single viable cell (or clump), the result can be expressed as CFU per mL or per gram using the formula: CFU/mL = colony count * (1 / plated volume) * dilution factor.Step-by-Step Solution:Perform serial dilutions (for example, 10^-1 to 10^-6).Plate a known volume (for example, 0.1 mL) onto appropriate agar.After incubation, count plates with 30–300 colonies for accuracy.Compute CFU/mL using dilution factor and plated volume.Verification / Alternative check:Results are reproducible within accepted variance and correlate with standard curves; other quantitative methods include Most Probable Number and flow cytometry, but classic dilution plating remains common.Why Other Options Are Wrong:B: Gram stain is semi-quantitative at best.C: Wet mounts reveal motility but not standardized counts.D: Incorrect because option A is quantitative.E: DFA can identify organisms but is not a routine quantitative method.Common Pitfalls:Counting plates outside the 30–300 range without adjusting dilutions; ignoring clumping which may underestimate cell numbers.Final Answer:Serial dilution followed by plating (viable count)

Question 12

Reporting viable plate counts In bacteriology reports, colony counts from viable plate methods are most commonly expressed in which unit?

Accepted Answer

Introduction / Context:Standardized reporting ensures clinicians can interpret results consistently across laboratories. Viable counts typically quantify organisms relative to sample volume to allow comparisons across patients and time.Given Data / Assumptions:Plate counts originate from a measured volume plated after serial dilution.Clinical thresholds (for example, urinary infection cutoffs) are defined in CFU per milliliter.Results should allow direct comparison independent of plating volume.Concept / Approach:The conventional unit is CFU per mL (cfu/mL). Reporting only “cfu” lacks context because colony numbers depend on the volume plated and dilution factor. Expressing thousands/mL is nonstandard and may obscure significant digits; percent growth is meaningless for plate counts.Step-by-Step Solution:Count colonies on a plate within 30–300 colonies.Compute CFU/mL = colonies * (1 / plated volume in mL) * dilution factor.Report the final value as cfu/mL (for example, 1.6 × 10^5 cfu/mL).Round and flag according to clinical thresholds if applicable.Verification / Alternative check:Laboratory accreditation guidelines and clinical microbiology texts use cfu/mL for urine, respiratory specimens, and quantitative wound cultures.Why Other Options Are Wrong:A: “cfu” alone omits the critical volume context.C: Using both simultaneously for the same test would be inconsistent; cfu/mL is preferred.D: “thousands/mL” is an informal expression and not standard.E: “Percent growth” is not a recognized unit for plate counts.Common Pitfalls:Forgetting to include the dilution factor or plated volume; misreporting significant figures beyond the method’s precision.Final Answer:cfu/mL (colony-forming units per milliliter)

Question 13

In microbiology culture techniques, if you add a substance to a solid growth medium that inhibits the growth of unwanted bacteria while allowing the desired bacteria to grow, what is this type of culture medium called?

Accepted Answer

Introduction / Context:Selecting the correct culture medium is fundamental in diagnostic microbiology and research. When mixed samples contain both target and non-target organisms, we often rely on media that either suppress competitors or help visually differentiate similar colonies. Understanding the definition of a selective medium versus enriched, enrichment, and differential media helps you choose the right tool for isolation and identification.Given Data / Assumptions:The scenario explicitly states that a substance is added to a solid medium.This additive inhibits unwanted bacteria.The desired (wanted) bacteria are permitted to grow.We are classifying the medium by function.Concept / Approach:A selective medium contains inhibitory agents (for example, bile salts, dyes, antibiotics, high salt) that suppress specific groups while allowing the target group to grow. This differs from an enriched medium (extra nutrients for fastidious organisms, no inhibition), an enrichment medium (usually liquid, conditions that favor a target’s proliferation before plating), and a differential medium (indicators that distinguish colonies by metabolic traits without necessarily inhibiting others).Step-by-Step Solution:Identify key requirement: inhibit unwanted organisms and allow desired ones.Match requirement to medium type: selective media include inhibitors (for example, phenylethyl alcohol agar inhibits Gram-negatives; mannitol salt agar’s 7.5% NaCl selects for staphylococci).Confirm that the medium is solid and the function is selective, not merely differential or nutrient-rich.Choose the term “selective medium.”Verification / Alternative check:Classic examples: MacConkey agar is both selective (bile salts, crystal violet inhibit Gram-positives) and differential (lactose fermentation indicator). The selective aspect directly addresses inhibition, validating the choice.Why Other Options Are Wrong:Differential medium: distinguishes organisms (for example, color change) but does not necessarily inhibit others.Enriched medium: supplies additional nutrients; no inhibition intended.Enrichment medium: commonly liquid and designed to favor target growth pre-isolation; not merely about inhibition on solid plates.Common Pitfalls:Confusing “enrichment” with “selective.” Enrichment favors a target’s growth but is not defined strictly by inhibiting non-targets on a solid plate. Also, assuming “differential” implies selection; it does not.Final Answer:selective medium

Question 14

Measuring cell populations: which instrument can measure both cell number and approximate cell size (volume) in a suspension with high throughput?

Accepted Answer

Introduction / Context:Quantifying microbial or cell suspensions is essential for growth studies, vaccine production, and bioprocess control. Several tools exist, ranging from manual counting chambers to automated electronic systems. Knowing which tool provides both counts and size information guides instrument selection in labs and fermentation suites.Given Data / Assumptions:The task is to measure both the number of cells and an estimate of cell size (volume).Suspension is amenable to electronic sizing/counting.We compare manual and automated approaches.Concept / Approach:A Coulter counter measures particles passing through a small aperture with an applied electric field. Each particle displaces electrolyte, causing a change in electrical resistance that is proportional to particle volume. This produces both a count and an approximate size distribution. In contrast, a hemocytometer or a Petroff-Hausser chamber enables manual counting under a microscope, providing counts but not precise, automated size distributions.Step-by-Step Solution:Identify requirement: obtain counts and size (volume) simultaneously.Recall operation of Coulter principle: resistance pulses scale with particle volume.Eliminate manual chambers that do not directly yield size data.Select “coulter counter.”Verification / Alternative check:Instrument specifications from Coulter devices show histograms for particle size and total counts per unit volume, confirming dual capability.Why Other Options Are Wrong:Hemocytometer: manual counting grid; no direct electronic sizing output.Petroff-Hausser chamber: similar to hemocytometer for bacteria; provides counts only.None of these: incorrect because the Coulter counter satisfies the requirement.Common Pitfalls:Assuming microscope-based methods easily provide size; while you can estimate dimensions microscopically, it is not equivalent to automated volume histograms produced by Coulter systems.Final Answer:coulter counter

Question 15

In routine preparation of solid microbiological media, which solidifying agent is most commonly used because it is largely inert to most microbes and melts/solidifies at convenient temperatures?

Accepted Answer

Introduction / Context:Solid media are indispensable for isolating colonies and assessing colony morphology. The choice of solidifying agent affects melting point, clarity, and microbial compatibility. A widely adopted gelling agent enables robust, routine culturing across diverse organisms.Given Data / Assumptions:The question asks for the commonly used solidifying agent.Agent should be generally non-nutritive and stable across incubation temperatures.It should not be degraded by most microbes.Concept / Approach:Agar, a polysaccharide from red algae, is the standard. It melts near 85–95 °C and solidifies around 40–45 °C, remaining solid at typical incubation temperatures (for example, 25–37 °C). Most bacteria cannot metabolize agar, preserving plate integrity. While silica gel has niche uses (for example, some specialized chemoautotroph cultures), it is not the common, general-purpose choice.Step-by-Step Solution:List candidates: agar and silica gel.Evaluate suitability: agar provides ideal melting/solidification and inertness for most microbes.Recognize that silica gel is uncommon and technically specialized.Choose “agar.”Verification / Alternative check:Standard media recipes (Nutrient agar, LB agar, MacConkey agar) universally employ agar at about 1.5–2.0% w/v, supporting the answer.Why Other Options Are Wrong:Silica gel: rarely used in routine plate media.Both (a) and (b): overbroad; only agar is the common standard.None: incorrect; agar is widely used.Common Pitfalls:Confusing “common” with “possible.” Although alternatives exist, the question focuses on typical microbiology practice.Final Answer:agar

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