Microbiology Quiz: Colony Morphology
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Colony MorphologyQuestion 1 of 20

A microbiologist performs a disk diffusion assay for ampicillin susceptibility on a lawn of E. coli. After incubation, a 20 mm zone of inhibition is observed around the disk. However, several small, distinct colonies are visible growing within this otherwise clear zone. What is the most likely identity of these small colonies?

They are spontaneous ampicillin-resistant mutants that arose during incubation and were selected for by the antibiotic.
They are a different, ampicillin-susceptible species that contaminated the plate before the lawn was made.
They are colonies of a satellite organism that requires growth factors released by the dying E. coli.
They are 'persister cells' that were dormant during antibiotic exposure and have now resumed growth.
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Microbiology Quiz

Microbiology Quiz: Colony Morphology

Practice Colony Morphology in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Colony Morphology, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

A microbiologist performs a disk diffusion assay for ampicillin susceptibility on a lawn of E. coli. After incubation, a 20 mm zone of inhibition is observed around the disk. However, several small, distinct colonies are visible growing within this otherwise clear zone. What is the most likely identity of these small colonies?

  1. They are spontaneous ampicillin-resistant mutants that arose during incubation and were selected for by the antibiotic. (correct answer)
  2. They are a different, ampicillin-susceptible species that contaminated the plate before the lawn was made.
  3. They are colonies of a satellite organism that requires growth factors released by the dying E. coli.
  4. They are 'persister cells' that were dormant during antibiotic exposure and have now resumed growth.
Explanation: The appearance of colonies within a zone of inhibition is a classic sign of antibiotic resistance. During the massive replication required to form the bacterial lawn, spontaneous mutations can occur at a low frequency. If a mutation confers resistance to the antibiotic (e.g., by producing beta-lactamase), that cell will be able to grow and form a colony despite the presence of the antibiotic. A susceptible contaminant (B) would not grow. Satellitism (C) does not confer resistance. While persister cells (D) can survive antibiotic treatment, they typically do not form robust colonies within the zone and represent a state of tolerance, not true genetic resistance that would be heritable and form a colony.

Question 2

A clinical isolate from a pneumonia patient is cultured on blood agar. The colonies are large, grey, and extremely mucoid, appearing 'wet' and tending to run together. Which of the following is the most likely clinical implication of this specific colony morphology?

  1. The organism is likely a facultative anaerobe capable of growth in diverse oxygen environments.
  2. The organism possesses a significant polysaccharide capsule that serves as an antiphagocytic virulence factor. (correct answer)
  3. The organism produces a potent beta-hemolysin, leading to complete lysis of red blood cells.
  4. The organism is an obligate thermophile, indicating the patient had a prolonged high-grade fever.
Explanation: A mucoid colony appearance is characteristic of bacteria that produce extensive extracellular polysaccharides, forming a capsule. In pathogenic bacteria, such as Klebsiella pneumoniae or Streptococcus pneumoniae, this capsule is a major virulence factor that helps the organism evade phagocytosis by host immune cells. The other options are not directly inferred from a mucoid morphology. Oxygen requirement (A) and temperature preference (D) are not determined by colony texture. Hemolysis (C) is a separate characteristic observed as clearing or discoloration of the blood agar itself.

Question 3

A bacterium isolated from a marine squid is streaked onto a seawater-based nutrient agar. After incubation, the plate is taken into a dark room, and the colonies are observed to be emitting a continuous, faint blue-green light. This colony characteristic depends on:

  1. The accumulation of a fluorescent pigment that absorbs and re-emits ambient light.
  2. A symbiotic relationship with bioluminescent dinoflagellates that were co-isolated with the bacteria.
  3. The anaerobic degradation of medium components, which releases phosphine gas that glows upon contact with air.
  4. The enzymatic reaction catalyzed by luciferase, the expression of which is often regulated by quorum sensing. (correct answer)
Explanation: When you encounter bacteria emitting light in a marine environment, you're dealing with bioluminescence—a fascinating biochemical phenomenon common in ocean ecosystems. The key clue here is "continuous, faint blue-green light" from bacterial colonies in a dark room. Bacterial bioluminescence occurs through a specific enzymatic reaction where luciferase catalyzes the oxidation of luciferin (a substrate) in the presence of oxygen, producing light as a byproduct. This system is particularly well-studied in marine bacteria like Vibrio fischeri, which forms symbiotic relationships with marine animals including squid. Importantly, luciferase expression is typically controlled by quorum sensing—a density-dependent regulatory mechanism where bacteria communicate through chemical signals to coordinate group behaviors like light production. Option A is incorrect because fluorescent pigments require an external light source to absorb and re-emit photons; they cannot produce light in complete darkness. Option B misidentifies the light source—while dinoflagellates can be bioluminescent, the scenario describes bacterial colonies producing light independently after isolation and culture. Option C presents an implausible mechanism; phosphine gas production from anaerobic degradation doesn't occur under these culture conditions and wouldn't produce the described steady bioluminescence. The correct answer is D, as it accurately describes the luciferase-mediated biochemical process and mentions quorum sensing regulation, which explains why the light production becomes apparent once bacterial density reaches sufficient levels during colony growth. Remember: Marine bioluminescence questions often test your understanding of enzyme-mediated light production versus other light-related phenomena like fluorescence or phosphorescence.

Question 4

A bacteriophage suspension is mixed with a susceptible host bacterium and plated using the soft agar overlay technique. After incubation, the plate shows a confluent lawn of bacterial growth, but it is covered with numerous small, circular, clear areas. What is the most accurate description of these clear areas?

  1. They are satellite colonies of a phage-resistant contaminant.
  2. They are zones of inhibition caused by an antibiotic produced by the bacteriophage.
  3. They are colonies of L-form bacteria that have lost their cell walls due to stress.
  4. They are plaques, representing zones of bacterial lysis caused by phage replication. (correct answer)
Explanation: When you encounter questions about bacteriophage experiments using soft agar overlay techniques, you're being tested on your understanding of phage biology and how viral replication appears visually on bacterial lawns. The soft agar overlay technique creates a thin layer where bacteria can grow into a confluent lawn while allowing phages to move through and infect nearby cells. When a single phage infects a bacterium, it replicates inside the cell, causes lysis, and releases progeny phages that infect neighboring bacteria. This creates an expanding zone of bacterial death that appears as a clear, circular area called a plaque. Each plaque represents the descendants of one original phage particle, making this a standard method for counting and studying bacteriophages. Option A is incorrect because satellite colonies would appear as small bacterial growths, not clear areas, and they wouldn't form the uniform circular pattern described. Option B misinterprets the mechanism - bacteriophages kill bacteria through lysis during replication, not by producing antibiotics that create zones of inhibition. Option C confuses the scenario with L-form bacteria, which are bacterial variants that have lost their cell walls but still grow as visible colonies, not clear zones. The key visual clue here is "small, circular, clear areas" on a bacterial lawn - this classic description always points to plaques formed by bacteriophage activity. Study tip: Remember that clear zones on bacterial lawns typically indicate cell death or lysis, while visible growth indicates living bacteria. In phage experiments, plaques are always described as clear, circular zones where bacteria have been lysed.

Question 5

A culture of Proteus vulgaris is treated with penicillin in an isotonic medium containing horse serum. When this culture is plated on a penicillin-free agar medium, the resulting colonies have a distinct 'fried egg' appearance, with a dense center and a lighter peripheral zone. This morphology is characteristic of:

  1. The normal, swarming colony form of Proteus vulgaris.
  2. A yeast contaminant that was resistant to the penicillin treatment.
  3. Cell wall-deficient L-form variants induced by the penicillin. (correct answer)
  4. The formation of endospores triggered by the stress of antibiotic exposure.
Explanation: Penicillin inhibits cell wall synthesis. In an osmotically protective (isotonic) medium, this can lead to the formation of L-forms, which are bacteria that lack a cell wall. When L-forms grow on agar, they can grow down into the medium as well as on the surface. This growth pattern, with cells embedded in the agar at the center and growing outwards on the surface, creates the characteristic 'fried egg' colony morphology. This is distinct from the normal swarming of Proteus (A). Proteus is a bacterium, not a yeast (B), and it is non-spore-forming (D).

Question 6

A strain of Pseudomonas aeruginosa is grown on two different agar plates. On standard nutrient agar, the colonies are flat with irregular margins. On an agar medium containing a high concentration of alginate, the colonies become highly convex and mucoid. What is the best explanation for this morphological change?

  1. The high viscosity of the alginate medium physically prevents the colony from spreading, forcing a convex shape.
  2. The alginate in the medium induces the expression of genes for alginate exopolysaccharide biosynthesis. (correct answer)
  3. The alginate is a toxic compound that selects for a pre-existing, resistant mucoid subpopulation.
  4. The organism is displaying pleomorphism, where individual cell shape changes in response to the nutrient source.
Explanation: This phenomenon demonstrates phenotypic plasticity in response to environmental cues. In P. aeruginosa, particularly in isolates from cystic fibrosis patients, the presence of certain substrates or stress conditions can induce the expression of the alg operon, leading to the overproduction of the exopolysaccharide alginate. This results in the characteristic mucoid phenotype. While medium viscosity (A) might play a minor role in colony shape, it does not explain the biochemical switch to produce a mucoid matrix. Alginate is a carbohydrate, not a toxin (C). Pleomorphism (D) refers to variation in the shape of individual cells, not the macroscopic colony morphology.

Question 7

An isolate of Pseudomonas aeruginosa from a patient with a chronic lung infection yields two colony variants on agar: a small, smooth, motile variant and a large, wrinkled, non-motile variant. The wrinkled ('rugose') variant is most likely associated with which of the following phenotypes in vivo?

  1. Increased susceptibility to antibiotics and enhanced clearance by phagocytes.
  2. Enhanced planktonic growth and a higher likelihood of systemic dissemination.
  3. Overproduction of exopolysaccharides, such as Pel or Psl, leading to robust biofilm formation. (correct answer)
  4. A switch to an obligately anaerobic metabolism to adapt to the lung environment.
Explanation: The transition from a smooth to a wrinkled colony morphology in P. aeruginosa is a well-studied adaptation associated with chronic infections and biofilm formation. The wrinkled phenotype is typically caused by the overproduction of exopolysaccharides (like Pel and Psl) that act as a biofilm matrix. This matrix encases the bacteria, promotes strong adherence, and confers increased resistance to antibiotics and host immune responses. The smooth, motile phenotype is associated with acute infection and dissemination (planktonic growth). Thus, the wrinkled variant is a key indicator of a biofilm-forming, persistent phenotype.

Question 8

A Tryptic Soy Agar (TSA) plate is accidentally left open on a lab bench for an hour before being incubated. After 48 hours, two distinct colony types are visible. Type 1 colonies are small, circular, white, and opaque with a glistening texture. Type 2 colonies are large, irregular, fuzzy with a cottony texture, and greenish-grey. What is the most logical conclusion from these observations?

  1. The plate contains a pure culture of a pleomorphic bacterium that expresses different morphologies.
  2. The plate is contaminated; Type 2 colonies are likely fungi (mold) and Type 1 are likely bacteria. (correct answer)
  3. The two colony types represent the rough and smooth variants of a single bacterial species.
  4. Type 1 colonies are facultative anaerobes and Type 2 colonies are obligate aerobes from the air.
Explanation: The colony descriptions strongly suggest a mixed culture of bacteria and fungi, a common result of airborne contamination. Bacterial colonies (Type 1) are typically smooth or rough, with defined margins, and are opaque or translucent. Fungal colonies, specifically molds (Type 2), are characterized by a fuzzy, cottony, or hairy appearance due to the growth of mycelia (hyphae). Pleomorphism (A) refers to variation in cell shape, not such drastically different colony types. Rough/smooth variation (C) involves changes in surface texture but doesn't produce a fuzzy, mold-like appearance. Oxygen requirements (D) cannot be determined from colony morphology alone on a plate incubated aerobically.

Question 9

A pure culture of Serratia marcescens is streaked onto two Tryptic Soy Agar plates. Plate A is incubated at 25°C, and Plate B is incubated at 37°C. After 48 hours, Plate A shows colonies with intense red pigmentation, while Plate B shows off-white, non-pigmented colonies. If a single white colony from Plate B is subcultured onto a new plate and incubated at 25°C, what is the most probable outcome?

  1. The colonies will remain white, as the 37°C incubation selected for a permanent, non-pigmented mutant.
  2. The colonies will be red, as the gene for prodigiosin pigment production is regulated by temperature. (correct answer)
  3. The colonies will show sectoring, with red and white patches, due to high rates of reversion mutation.
  4. The colonies will fail to grow, as the shift from 37°C to 25°C induces a viable but nonculturable state.
Explanation: The production of the red pigment prodigiosin by Serratia marcescens is a classic example of temperature-dependent gene expression. The enzymes in the synthesis pathway are optimally produced at lower temperatures (e.g., 25-30°C) and are often repressed at human body temperature (37°C). This is a phenotypic change, not a permanent genotypic one. Therefore, subculturing the non-pigmented cells back to a permissive temperature (25°C) will restore pigment production. While a mutation (A) is possible, it is far less likely than the known temperature regulation.

Question 10

An isolate of Mycobacterium tuberculosis is grown on Löwenstein-Jensen medium. The colonies are observed to be rough, dry, and wrinkled (rugose), and are difficult to emulsify. This characteristic 'corded' or 'cording' appearance at the microscopic level and the rugose colony morphology are primarily attributed to which component of the mycobacterial cell envelope?

  1. The thick layer of peptidoglycan, which provides osmotic stability and a rigid structure.
  2. The presence of a loose, polysaccharide slime layer that promotes diffuse colony formation.
  3. The high content of mycolic acids and other lipids, especially trehalose dimycolate (cord factor). (correct answer)
  4. The production of numerous pili and fimbriae that mediate strong attachment to the agar surface.
Explanation: The characteristic rough, wrinkled, and hydrophobic colonies of M. tuberculosis are due to its unique, lipid-rich cell wall. Specifically, trehalose dimycolate, also known as 'cord factor', is a glycolipid on the cell surface that causes the cells to adhere to each other in serpentine cords and is a major contributor to the rugose colony morphology. This hydrophobic nature makes the colonies difficult to emulsify. While peptidoglycan (A) provides rigidity, it does not cause this specific morphology. A slime layer (B) would result in a mucoid, not dry, appearance. Pili/fimbriae (D) are involved in attachment but are not the primary cause of the overall colony texture and structure.

Question 11

A bacterium isolated from a marine squid is streaked onto a seawater-based nutrient agar. After incubation, the plate is taken into a dark room, and the colonies are observed to be emitting a continuous, faint blue-green light. This colony characteristic depends on:

  1. The accumulation of a fluorescent pigment that absorbs and re-emits ambient light.
  2. A symbiotic relationship with bioluminescent dinoflagellates that were co-isolated with the bacteria.
  3. The anaerobic degradation of medium components, which releases phosphine gas that glows upon contact with air.
  4. The enzymatic reaction catalyzed by luciferase, the expression of which is often regulated by quorum sensing. (correct answer)
Explanation: When you encounter bacteria emitting light in a marine environment, you're dealing with bioluminescence—a fascinating biochemical phenomenon common in ocean ecosystems. The key clue here is "continuous, faint blue-green light" from bacterial colonies in a dark room. Bacterial bioluminescence occurs through a specific enzymatic reaction where luciferase catalyzes the oxidation of luciferin (a substrate) in the presence of oxygen, producing light as a byproduct. This system is particularly well-studied in marine bacteria like Vibrio fischeri, which forms symbiotic relationships with marine animals including squid. Importantly, luciferase expression is typically controlled by quorum sensing—a density-dependent regulatory mechanism where bacteria communicate through chemical signals to coordinate group behaviors like light production. Option A is incorrect because fluorescent pigments require an external light source to absorb and re-emit photons; they cannot produce light in complete darkness. Option B misidentifies the light source—while dinoflagellates can be bioluminescent, the scenario describes bacterial colonies producing light independently after isolation and culture. Option C presents an implausible mechanism; phosphine gas production from anaerobic degradation doesn't occur under these culture conditions and wouldn't produce the described steady bioluminescence. The correct answer is D, as it accurately describes the luciferase-mediated biochemical process and mentions quorum sensing regulation, which explains why the light production becomes apparent once bacterial density reaches sufficient levels during colony growth. Remember: Marine bioluminescence questions often test your understanding of enzyme-mediated light production versus other light-related phenomena like fluorescence or phosphorescence.

Question 12

A swab from a skin lesion is plated on Mannitol Salt Agar (MSA). After 24 hours of incubation at 35°C, two distinct colony types are observed. Type 1 colonies are approximately 1 mm, circular, and pink, and the surrounding medium remains pinkish-red. Type 2 colonies are 2-3 mm, circular, and bright yellow, and they are surrounded by a distinct yellow halo in the agar. Which interpretation is most accurate?

  1. Type 1 is a salt-tolerant mannitol fermenter, likely S. aureus, while Type 2 is salt-intolerant.
  2. The plate is contaminated; Type 1 is a non-pathogenic Micrococcus, and Type 2 is a pathogenic Streptococcus.
  3. Both colony types are salt-tolerant, but Type 1 is a weak mannitol fermenter and Type 2 is a strong mannitol fermenter.
  4. Type 2 is a salt-tolerant mannitol fermenter, likely S. aureus, while Type 1 is salt-tolerant but does not ferment mannitol. (correct answer)
Explanation: When you encounter Mannitol Salt Agar (MSA) questions, remember that MSA serves two functions: it's both selective (high salt concentration inhibits most non-staphylococci) and differential (mannitol fermentation changes the pH indicator from red to yellow). Let's interpret these results systematically. Both colony types grew on MSA, indicating both organisms are salt-tolerant and likely staphylococci. The key differentiator is mannitol fermentation. Type 1 colonies remained pink with no color change in the surrounding medium, indicating they cannot ferment mannitol. Type 2 colonies turned bright yellow with yellow halos, showing strong acid production from mannitol fermentation - this classic pattern strongly suggests Staphylococcus aureus. Option A incorrectly identifies Type 1 as the mannitol fermenter and assumes Type 2 is salt-intolerant, but Type 2 clearly grew well on the high-salt medium. Option B misidentifies the organisms entirely - Micrococcus can be salt-tolerant, and Streptococcus typically cannot grow on MSA due to salt sensitivity. Option C incorrectly suggests both organisms ferment mannitol, but Type 1 showed no acid production (no color change). Option D correctly identifies Type 2 as the mannitol-fermenting, salt-tolerant organism (likely S. aureus) and Type 1 as salt-tolerant but mannitol-negative (possibly S. epidermidis or another coagulase-negative staphylococcus). Study tip: For MSA interpretation, focus on two questions: "Did it grow?" (salt tolerance) and "Did the medium turn yellow?" (mannitol fermentation). Yellow colonies with yellow halos typically indicate pathogenic S. aureus.

Question 13

A urine sample is plated on a chromogenic agar designed for UTI pathogen identification. The plate reveals two colony types after incubation: large, smooth, deep pink colonies and smaller, turquoise-blue colonies. According to the manufacturer's guide, pink indicates E. coli and blue indicates Enterococcus. What is the primary mechanism that allows this color differentiation?

  1. The bacteria produce unique, species-specific intracellular pigments that become visible as the colony grows.
  2. The agar contains multiple pH indicators that produce different colors in response to the specific metabolic end products of each species.
  3. The agar contains multiple colorless chromogenic substrates that, when cleaved by an enzyme specific to a species, release a distinctively colored molecule. (correct answer)
  4. The agar is impregnated with species-specific antibodies linked to a color-changing compound that binds to the colony surface.
Explanation: Chromogenic agars work by incorporating specific enzyme substrates linked to a chromophore (a colored molecule). Each target species possesses a characteristic enzyme or set of enzymes. For example, the agar might contain a substrate for β-glucuronidase (an enzyme common in E. coli) linked to a pink chromophore, and a different substrate for β-glucosidase (common in Enterococcus) linked to a blue chromophore. When E. coli grows, it cleaves its specific substrate, releasing the pink molecule and coloring the colony pink. Enterococcus cleaves its substrate, coloring its colony blue. This enzyme-substrate principle is distinct from pH changes (B) or natural pigmentation (A).

Question 14

An isolate of Pseudomonas aeruginosa from a patient with a chronic lung infection yields two colony variants on agar: a small, smooth, motile variant and a large, wrinkled, non-motile variant. The wrinkled ('rugose') variant is most likely associated with which of the following phenotypes in vivo?

  1. Increased susceptibility to antibiotics and enhanced clearance by phagocytes.
  2. Enhanced planktonic growth and a higher likelihood of systemic dissemination.
  3. Overproduction of exopolysaccharides, such as Pel or Psl, leading to robust biofilm formation. (correct answer)
  4. A switch to an obligately anaerobic metabolism to adapt to the lung environment.
Explanation: The transition from a smooth to a wrinkled colony morphology in P. aeruginosa is a well-studied adaptation associated with chronic infections and biofilm formation. The wrinkled phenotype is typically caused by the overproduction of exopolysaccharides (like Pel and Psl) that act as a biofilm matrix. This matrix encases the bacteria, promotes strong adherence, and confers increased resistance to antibiotics and host immune responses. The smooth, motile phenotype is associated with acute infection and dissemination (planktonic growth). Thus, the wrinkled variant is a key indicator of a biofilm-forming, persistent phenotype.

Question 15

A bacteriophage suspension is mixed with a susceptible host bacterium and plated using the soft agar overlay technique. After incubation, the plate shows a confluent lawn of bacterial growth, but it is covered with numerous small, circular, clear areas. What is the most accurate description of these clear areas?

  1. They are satellite colonies of a phage-resistant contaminant.
  2. They are zones of inhibition caused by an antibiotic produced by the bacteriophage.
  3. They are colonies of L-form bacteria that have lost their cell walls due to stress.
  4. They are plaques, representing zones of bacterial lysis caused by phage replication. (correct answer)
Explanation: When you encounter questions about bacteriophage experiments using soft agar overlay techniques, you're being tested on your understanding of phage biology and how viral replication appears visually on bacterial lawns. The soft agar overlay technique creates a thin layer where bacteria can grow into a confluent lawn while allowing phages to move through and infect nearby cells. When a single phage infects a bacterium, it replicates inside the cell, causes lysis, and releases progeny phages that infect neighboring bacteria. This creates an expanding zone of bacterial death that appears as a clear, circular area called a plaque. Each plaque represents the descendants of one original phage particle, making this a standard method for counting and studying bacteriophages. Option A is incorrect because satellite colonies would appear as small bacterial growths, not clear areas, and they wouldn't form the uniform circular pattern described. Option B misinterprets the mechanism - bacteriophages kill bacteria through lysis during replication, not by producing antibiotics that create zones of inhibition. Option C confuses the scenario with L-form bacteria, which are bacterial variants that have lost their cell walls but still grow as visible colonies, not clear zones. The key visual clue here is "small, circular, clear areas" on a bacterial lawn - this classic description always points to plaques formed by bacteriophage activity. Study tip: Remember that clear zones on bacterial lawns typically indicate cell death or lysis, while visible growth indicates living bacteria. In phage experiments, plaques are always described as clear, circular zones where bacteria have been lysed.

Question 16

A pure culture of Serratia marcescens is streaked onto two Tryptic Soy Agar plates. Plate A is incubated at 25°C, and Plate B is incubated at 37°C. After 48 hours, Plate A shows colonies with intense red pigmentation, while Plate B shows off-white, non-pigmented colonies. If a single white colony from Plate B is subcultured onto a new plate and incubated at 25°C, what is the most probable outcome?

  1. The colonies will remain white, as the 37°C incubation selected for a permanent, non-pigmented mutant.
  2. The colonies will be red, as the gene for prodigiosin pigment production is regulated by temperature. (correct answer)
  3. The colonies will show sectoring, with red and white patches, due to high rates of reversion mutation.
  4. The colonies will fail to grow, as the shift from 37°C to 25°C induces a viable but nonculturable state.
Explanation: The production of the red pigment prodigiosin by Serratia marcescens is a classic example of temperature-dependent gene expression. The enzymes in the synthesis pathway are optimally produced at lower temperatures (e.g., 25-30°C) and are often repressed at human body temperature (37°C). This is a phenotypic change, not a permanent genotypic one. Therefore, subculturing the non-pigmented cells back to a permissive temperature (25°C) will restore pigment production. While a mutation (A) is possible, it is far less likely than the known temperature regulation.

Question 17

A urine sample is plated on a chromogenic agar designed for UTI pathogen identification. The plate reveals two colony types after incubation: large, smooth, deep pink colonies and smaller, turquoise-blue colonies. According to the manufacturer's guide, pink indicates E. coli and blue indicates Enterococcus. What is the primary mechanism that allows this color differentiation?

  1. The bacteria produce unique, species-specific intracellular pigments that become visible as the colony grows.
  2. The agar contains multiple pH indicators that produce different colors in response to the specific metabolic end products of each species.
  3. The agar contains multiple colorless chromogenic substrates that, when cleaved by an enzyme specific to a species, release a distinctively colored molecule. (correct answer)
  4. The agar is impregnated with species-specific antibodies linked to a color-changing compound that binds to the colony surface.
Explanation: Chromogenic agars work by incorporating specific enzyme substrates linked to a chromophore (a colored molecule). Each target species possesses a characteristic enzyme or set of enzymes. For example, the agar might contain a substrate for β-glucuronidase (an enzyme common in E. coli) linked to a pink chromophore, and a different substrate for β-glucosidase (common in Enterococcus) linked to a blue chromophore. When E. coli grows, it cleaves its specific substrate, releasing the pink molecule and coloring the colony pink. Enterococcus cleaves its substrate, coloring its colony blue. This enzyme-substrate principle is distinct from pH changes (B) or natural pigmentation (A).

Question 18

A microbiologist inoculates a motile bacterium onto two nutrient agar plates with different agar concentrations: Plate 1 contains 0.4% agar, and Plate 2 contains 1.5% agar. Both plates are inoculated with a single stab to the center. After identical incubation, which outcome is most likely?

  1. Extensive swarming will occur on Plate 2, while growth on Plate 1 will be restricted to the stab line.
  2. Growth will be inhibited on Plate 1 due to the low solidity of the medium, which is osmotically stressful.
  3. Colony morphology will be identical on both plates, as agar concentration only affects handling, not bacterial growth.
  4. Diffuse, spreading growth will be seen on Plate 1, while on Plate 2, growth will be confined to discrete colonies at the inoculation site. (correct answer)
Explanation: The concentration of agar is critical for observing motility. A low concentration (0.4%) creates a semi-solid medium that allows motile bacteria to swim through the agar, resulting in diffuse, spreading growth or swarming away from the inoculation point. A standard concentration (1.5%) creates a solid gel that prevents translocation of bacteria, so non-spreading, discrete colonies will form only where the bacteria were initially placed. Therefore, motility will be evident on Plate 1, but not on Plate 2. Option A has the roles reversed. Option B is incorrect as agar is inert and its concentration in this range doesn't cause osmotic stress. Option C is incorrect as the physical properties of the medium directly influence the macroscopic appearance of motile organisms.

Question 19

A single, large colony of Neisseria gonorrhoeae is growing on a chocolate agar plate. After several days of incubation, very small, new colonies are observed to have appeared on top of and immediately adjacent to the original 'mother' colony. This phenomenon is most accurately explained by:

  1. The release of essential nutrients and growth factors from the autolysing center of the mother colony, which supports new growth. (correct answer)
  2. A high rate of phase variation leading to the production of a more motile phenotype that spreads from the original colony.
  3. The germination of endospores that were produced by the mother colony in response to nutrient limitation.
  4. Contamination by a satellite organism that requires the NAD (V factor) produced by the mother colony to grow.
Explanation: This describes the formation of 'daughter' colonies, a phenomenon observed with fastidious organisms like Neisseria. As the primary (mother) colony ages, cells in the center begin to die and lyse due to nutrient depletion and accumulation of toxic waste. This autolysis releases intracellular contents, including nutrients and growth factors, into the immediate vicinity. These released factors can then support the growth of new, smaller colonies from the viable cells at the periphery of the original colony. Neisseria are non-motile and non-spore-forming, ruling out B and C. This is not satellitism (D) because the new colonies are the same species, not a different one.

Question 20

A single, large, circular yeast colony growing on a YPD agar plate is observed to have a distinct wedge-shaped red section within an otherwise cream-colored colony. This phenomenon, known as sectoring, is most likely caused by:

  1. A spontaneous mutation in a pigment synthesis gene that occurred in a single cell during colonial growth. (correct answer)
  2. Cross-contamination of the plate with a second, red-pigmented yeast species that grew into the first colony.
  3. Differential gene expression in response to a nutrient gradient, with cells at the edge expressing a red pigment.
  4. A lytic viral infection (mycovirus) that has spread in a wedge pattern through the colony.
Explanation: Sectoring is a visual manifestation of a genetic event occurring in a single cell during the growth of a colony. If a spontaneous mutation (e.g., a forward mutation in a gene in the adenine biosynthesis pathway in Saccharomyces cerevisiae, which leads to a red color) occurs in one cell, all of its descendants will inherit this mutation. As the colony expands, these descendants will form a distinct lineage, appearing as a wedge or sector. Contamination (B) is unlikely to form such a perfect, integrated sector. A nutrient gradient (C) would produce concentric rings, not a radial sector. Viral infection (D) would cause lysis, not a change in colony color.