Microbiology Quiz: Capsules Slime Layers And Biofilms
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Capsules Slime Layers And BiofilmsQuestion 1 of 20

Dispersal is an active, regulated stage of the biofilm life cycle that allows bacteria to colonize new surfaces. Which of the following conditions is a common and critical trigger for initiating the dispersal of sessile cells from a mature biofilm?

The accumulation of toxic metabolic byproducts and localized nutrient starvation within the biofilm.
A sudden increase in the concentration of essential nutrients in the surrounding environment.
The introduction of a new bacterial species into the biofilm community.
Exposure to a high concentration of the same quorum-sensing molecules that induced biofilm formation.
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Microbiology Quiz

Microbiology Quiz: Capsules Slime Layers And Biofilms

Practice Capsules Slime Layers And Biofilms in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Capsules Slime Layers And Biofilms, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

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Question 1

Dispersal is an active, regulated stage of the biofilm life cycle that allows bacteria to colonize new surfaces. Which of the following conditions is a common and critical trigger for initiating the dispersal of sessile cells from a mature biofilm?

  1. The accumulation of toxic metabolic byproducts and localized nutrient starvation within the biofilm. (correct answer)
  2. A sudden increase in the concentration of essential nutrients in the surrounding environment.
  3. The introduction of a new bacterial species into the biofilm community.
  4. Exposure to a high concentration of the same quorum-sensing molecules that induced biofilm formation.
Explanation: When you encounter questions about biofilm dispersal, think about the environmental pressures that would make it advantageous for bacteria to abandon their protective community and venture into planktonic (free-floating) life to find new locations. Biofilm dispersal is triggered primarily by stress conditions that make the current environment unsustainable. Option A correctly identifies the most common trigger: as biofilms mature and grow denser, cells in the interior experience nutrient depletion and accumulation of toxic waste products. This creates a survival pressure that activates dispersal mechanisms, prompting cells to leave and seek better conditions elsewhere. Option B is incorrect because abundant nutrients would actually promote biofilm growth and maintenance, not dispersal. Bacteria have no reason to leave a nutrient-rich environment. Option C misunderstands dispersal triggers - while new species might cause competitive stress, species introduction itself isn't a direct dispersal signal. The original community might actually benefit from the metabolic diversity. Option D reflects a common misconception about quorum sensing. High concentrations of the same signaling molecules that promote biofilm formation typically reinforce biofilm stability rather than trigger dispersal. Different signaling pathways control formation versus dispersal. Remember this pattern: biofilm dispersal questions often test whether you understand that bacteria make "decisions" based on survival advantage. Dispersal is essentially a stress response - bacteria leave when staying becomes more costly than the risks of planktonic life. Look for answer choices that describe deteriorating conditions rather than improving ones.

Question 2

The antiphagocytic capsules of most pathogenic bacteria, such as Streptococcus pneumoniae and Haemophilus influenzae, are composed of polysaccharides. The capsule of Bacillus anthracis is a notable exception because it is composed of which of the following?

  1. A polymer of hyaluronic acid that mimics host connective tissue.
  2. A polypeptide of poly-D-glutamic acid. (correct answer)
  3. A complex lipopolysaccharide structure similar to the outer membrane.
  4. A crystalline protein S-layer anchored to the cell wall.
Explanation: The capsule of Bacillus anthracis is unique among bacterial pathogens in that it is composed of a polypeptide, specifically a polymer of D-glutamic acid. The use of the D-isomer of the amino acid helps the capsule resist degradation by host proteases, which typically recognize L-isomers. This polypeptide capsule is a major virulence factor, protecting the bacterium from phagocytosis. (A) The hyaluronic acid capsule is found on Streptococcus pyogenes. (C) Lipopolysaccharide is a component of the Gram-negative outer membrane, not a capsule. (D) An S-layer is a different type of cell surface structure, composed of a monomolecular layer of protein.

Question 3

A microbiologist seeks to definitively establish that the polysaccharide capsule of a newly identified strain of Haemophilus influenzae is a critical virulence factor for causing pneumonia. Which of the following experimental approaches provides the most direct and rigorous evidence to support this claim?

  1. Visualizing the capsule on the wild-type strain using transmission electron microscopy and negative staining.
  2. Generating antibodies against the purified capsule and demonstrating they can opsonize the bacteria for phagocytosis in vitro.
  3. Identifying and sequencing the capsule biosynthesis operon (cap) in the wild-type strain's genome.
  4. Creating a specific, isogenic capsule-deficient mutant and showing it is significantly less virulent than the wild-type strain in a mouse pneumonia model. (correct answer)
Explanation: This question tests the application of molecular Koch's postulates. The most direct way to prove a gene (or the structure it encodes, like a capsule) is a virulence factor is to create a specific knockout mutant that lacks only that factor. By comparing this mutant to the wild-type (isogenic) parent strain in a relevant infection model, one can directly attribute any loss of virulence (attenuation) to the absence of the capsule. (A) only shows the structure exists. (C) only shows the genetic potential to make it. (B) provides strong correlational evidence for its function but is less direct than showing that its removal eliminates virulence.

Question 4

The bacterium Streptococcus mutans is a primary causative agent of dental caries. Its virulence is highly dependent on its ability to produce an extracellular glucan polymer from dietary sucrose, a reaction catalyzed by the enzyme glucosyltransferase (Gtf). A mutation rendering the Gtf enzyme non-functional would most directly impair which of the following processes?

  1. The fermentation of sucrose to produce lactic acid.
  2. The bacterium's ability to survive in the acidic environment of a dental plaque.
  3. The synthesis of peptidoglycan for cell division.
  4. The adherence of the bacterium to tooth enamel and the formation of a stable biofilm. (correct answer)
Explanation: The glucan polymer produced by glucosyltransferase is a sticky exopolysaccharide that functions as the primary adhesive for S. mutans, allowing it to cling to the tooth surface and to other bacteria. This glucan forms the bulk of the early dental plaque matrix. Without Gtf, the bacterium cannot produce this essential 'glue', severely impairing its ability to adhere and form a biofilm (plaque). (A) Fermentation of sucrose to lactic acid is a separate metabolic pathway; Gtf uses sucrose as a substrate for polymerization, not for energy production via fermentation. (B) The glucan biofilm actually helps create and maintain the acidic microenvironment; without the biofilm, acid can diffuse away more easily. (C) Peptidoglycan synthesis is unrelated to extracellular glucan production.

Question 5

A student proposes that the high level of antibiotic resistance observed in biofilms is primarily due to an elevated rate of spontaneous mutation within the protected environment of the EPS matrix, which allows for the rapid selection of genetically resistant mutants. Why is this hypothesis an incomplete explanation for the phenomenon of biofilm tolerance?

  1. The dense EPS matrix actively prevents horizontal gene transfer, which is the sole source of new resistance genes.
  2. Biofilm tolerance is largely a transient, physiological state of the cells rather than a result of stable genetic changes. (correct answer)
  3. Bacteria within a biofilm are post-mitotic and do not replicate, thus no mutations can occur.
  4. Antibiotics are completely unable to penetrate the EPS matrix, so there is no selective pressure within the biofilm.
Explanation: While mutations can and do occur in biofilms, the primary reason for their remarkable antibiotic tolerance is the physiological state of the cells. This includes the presence of slow-growing or non-growing 'persister' cells that are phenotypically tolerant (but genetically susceptible), as well as reduced antibiotic penetration and altered chemical microenvironments. The fact that biofilm cells can often be cultured in the lab and show full antibiotic susceptibility proves that the resistance is a feature of the biofilm community lifestyle, not necessarily a stable genetic change in most cells. (A) and (C) are false; HGT can be enhanced in biofilms and cells do replicate. (D) is an overstatement; penetration is often slowed or reduced, but not completely blocked.

Question 6

In the development of dental plaque, a multi-species biofilm, bacteria such as Streptococcus gordonii act as early colonizers by binding directly to the salivary pellicle on the tooth enamel. Subsequently, late colonizers like Porphyromonas gingivalis, which cannot bind directly to the pellicle, adhere specifically to the surface of the already-attached streptococci. This phenomenon is best described as:

  1. Competitive exclusion
  2. Conjugation
  3. Quorum sensing
  4. Coaggregation (correct answer)
Explanation: Coaggregation is the process by which genetically distinct bacteria become attached to one another through highly specific molecular interactions between adhesin molecules on one cell and receptor polysaccharides or proteins on another. This process is fundamental to the development of complex, multi-species biofilms like dental plaque, creating a specific, ordered community structure. (A) Competitive exclusion is the opposite, where one species outcompetes another for resources or space. (B) Conjugation is a mechanism of horizontal gene transfer. (C) Quorum sensing is cell-to-cell communication, which regulates coaggregation and other processes but is not the binding itself.

Question 7

A patient with a long-term indwelling urinary catheter develops a persistent infection with Escherichia coli. The planktonic form of the isolate is found to be fully susceptible to gentamicin in laboratory tests. Despite treatment with intravenous gentamicin at concentrations well above the minimum inhibitory concentration (MIC), the infection fails to resolve. What is the most likely primary reason for this treatment failure?

  1. The catheter is colonized by a biofilm containing metabolically dormant persister cells that are phenotypically tolerant to gentamicin. (correct answer)
  2. The biofilm's exopolysaccharide matrix binds to and chemically neutralizes the gentamicin molecules.
  3. The bacteria rapidly acquired a plasmid encoding a gentamicin-modifying enzyme via conjugation within the biofilm.
  4. The E. coli formed a thick capsule, in addition to the biofilm, that is impermeable to the aminoglycoside antibiotic.
Explanation: When you encounter questions about catheter-associated infections that persist despite appropriate antibiotic therapy, think about biofilm formation and the unique challenges it presents for antimicrobial treatment. The key insight here is understanding how bacteria behave differently in biofilms versus their planktonic (free-floating) state. While the planktonic E. coli shows full susceptibility to gentamicin in standard laboratory tests, the biofilm environment creates a protected community where bacteria can survive antibiotic exposure through multiple mechanisms. Answer A correctly identifies the primary mechanism: biofilms contain metabolically dormant persister cells that are phenotypically tolerant to antibiotics. These cells aren't genetically resistant—they're simply in a slow-growing or dormant state that makes them less susceptible to antibiotics like gentamicin, which primarily target actively dividing bacteria. This explains why the infection persists despite adequate drug concentrations. Answer B is incorrect because while biofilm matrices can impede antibiotic penetration, they don't chemically neutralize gentamicin. Answer C describes acquired resistance, but this would show up in susceptibility testing and doesn't explain the discrepancy between lab results and clinical failure. Answer D incorrectly suggests capsule formation as the primary mechanism—capsules don't significantly impair gentamicin penetration, and this wouldn't explain the biofilm-specific treatment failure. Remember: biofilm-associated infections often require combination therapy, longer treatment courses, or device removal because standard susceptibility testing doesn't predict biofilm behavior. Always consider biofilm formation when indwelling devices and persistent infections are mentioned together.

Question 8

A bacterium isolated from a submerged rock in a fast-flowing stream is found to produce a copious, loose, and easily deformable glycocalyx that is not tightly bound to the cell surface. This structure is best classified as a slime layer, and its most important function in this specific environment is likely which of the following?

  1. Protection against phagocytosis by protozoa.
  2. Formation of a durable, shear-resistant biofilm.
  3. Adherence to the rock surface, preventing washout. (correct answer)
  4. Storage of carbon and energy reserves for starvation periods.
Explanation: A loose, non-uniform glycocalyx is characteristic of a slime layer. In a high-shear environment like a fast-flowing stream, the primary challenge for a bacterium is to remain attached to a surface. The sticky nature of the slime layer is crucial for this initial and continued adherence, preventing the cells from being washed away. (A) While a glycocalyx can offer some protection, this is a more prominent feature of well-organized capsules in pathogenic contexts. (B) A slime layer is by definition not durable or highly shear-resistant; it facilitates attachment but not necessarily a robust, mature biofilm on its own. (D) While it can serve as a nutrient reserve, its primary role in this context is physical attachment.

Question 9

The structural integrity of many bacterial biofilms depends on the cross-linking of anionic exopolysaccharides within the extracellular polymeric substance (EPS). A novel therapeutic agent is found to strongly chelate extracellular divalent cations, particularly Ca²⁺. Which aspect of biofilm physiology would this agent most directly inhibit?

  1. Synthesis of peptidoglycan for the bacterial cell wall.
  2. Activity of ATP synthase in the bacterial cell membrane.
  3. Intermolecular bridging that stabilizes the EPS matrix. (correct answer)
  4. Signaling through N-acyl homoserine lactone (AHL) molecules.
Explanation: Divalent cations like Ca²⁺ act as ionic bridges, cross-linking negatively charged polymer strands (like alginate or other anionic polysaccharides) in the EPS. This cross-linking is crucial for the mechanical stability and structural integrity of the biofilm matrix. A chelating agent would sequester these cations, preventing the bridging and thus destabilizing the matrix. (A) Peptidoglycan synthesis is an intracellular process and is not dependent on extracellular calcium. (B) ATP synthase relies on a proton gradient, not extracellular calcium. (D) AHL signaling is a common quorum sensing mechanism but does not directly depend on divalent cations for signal integrity.

Question 10

A researcher is studying biofilm formation by Staphylococcus epidermidis on a titanium surface. They discover that a secreted bacterial protease is essential for processing surface-adhesion proteins. If this specific protease inhibitor is added to the culture medium at the time of inoculation, which stage of biofilm development would be most directly disrupted?

  1. Initial reversible attachment of planktonic cells to the surface. (correct answer)
  2. Production of polysaccharide intercellular adhesin (PIA) to form the mature matrix.
  3. Cell-to-cell signaling via the autoinducing peptide (AIP) quorum sensing system.
  4. Release of extracellular DNA (eDNA) following localized cell lysis.
Explanation: The question states that the protease processes surface-adhesion proteins. These proteins are critical for the first step of biofilm formation: the initial attachment of planktonic bacteria to a surface. Inhibiting the protease would prevent the proper function of these adhesins, thereby disrupting initial attachment. (B) PIA production occurs after initial attachment, during matrix formation. (C) While some quorum sensing signals are peptides, the inhibitor is specific to a protease for adhesion proteins, not the AIP system. (D) eDNA release is part of matrix maturation, which occurs after initial attachment.

Question 11

A mucoid strain of Pseudomonas aeruginosa, isolated from a cystic fibrosis patient, is genetically engineered to have a null mutation in algC, a gene essential for alginate biosynthesis. When this mutant strain is used in a chronic lung infection animal model, what is the most probable outcome compared to the wild-type mucoid strain?

  1. The mutant strain will be hyper-motile and cause a more rapidly spreading, acute infection.
  2. The mutant strain will be unable to form a mature, protective biofilm and will be rapidly cleared by host immune cells. (correct answer)
  3. The mutant strain will form a biofilm composed primarily of proteins and eDNA, resulting in equivalent persistence.
  4. The mutant strain will exhibit increased susceptibility specifically to cell wall synthesis inhibitors like β-lactams.
Explanation: Alginate is the primary exopolysaccharide component of the mucoid biofilm matrix produced by P. aeruginosa in the cystic fibrosis lung. This biofilm is critical for protecting the bacteria from antibiotics and host immune responses (e.g., phagocytosis). A mutant unable to synthesize alginate cannot form this protective structure and would be highly susceptible to clearance by the immune system, failing to establish a chronic infection. (A) The switch between motility and biofilm formation is complex, but the primary outcome of losing the main protective shield is clearance, not enhanced acute infection. (C) While proteins and eDNA are part of the matrix, alginate is the dominant structural component in this context; its absence cannot be fully compensated for. (D) Alginate provides a physical barrier but doesn't directly affect the susceptibility of the cell wall target itself; the primary effect is protection from immune clearance.

Question 12

A cerebrospinal fluid sample from a patient with suspected bacterial meningitis is stained with India ink. Microscopic examination reveals spherical bacteria surrounded by distinct, clear halos against a dark background. This observation strongly suggests the presence of which structure and associated function critical for this type of infection?

  1. A slime layer, which aids in adherence to the meninges.
  2. A thick peptidoglycan cell wall, which resists osmotic lysis.
  3. An endospore, which allows for survival in harsh conditions.
  4. A polysaccharide capsule, which protects against phagocytosis. (correct answer)
Explanation: India ink is a negative stain; its particles cannot penetrate the bacterial capsule, resulting in a clear halo around the cell against a dark background. The presence of a capsule is a key virulence factor for major meningitis pathogens like Streptococcus pneumoniae and Neisseria meningitidis. Its primary function is to inhibit phagocytosis by immune cells like neutrophils and macrophages, allowing the bacteria to survive and replicate in the host. (A) A slime layer is less organized and would not produce such a distinct halo. (B) The cell wall would be adjacent to the cell and not form a large halo. (C) Endospores are intracellular structures and are not characteristic of the typical meningitis pathogens.

Question 13

Researchers observe that adding DNase I to a 6-hour-old Staphylococcus aureus biofilm leads to significant dissolution of the structure. However, adding the same concentration of DNase I to a 48-hour-old biofilm has a much less pronounced effect. What is the most accurate explanation for this differential sensitivity?

  1. The eDNA in mature biofilms becomes extensively cross-linked and shielded by other matrix components, limiting enzyme access. (correct answer)
  2. eDNA serves as a primary scaffold in early biofilms but is later replaced by polysaccharides and proteins.
  3. The mature biofilm produces DNase inhibitors that protect the eDNA from degradation.
  4. Cells in the mature biofilm have a lower rate of autolysis, reducing the amount of eDNA released into the matrix.
Explanation: When you encounter questions about biofilm development and enzyme sensitivity, focus on how biofilm architecture and composition change over time, affecting molecular accessibility. The key insight here is that biofilm maturation involves progressive structural complexity that protects matrix components from enzymatic attack. In young biofilms, extracellular DNA (eDNA) exists in a relatively exposed state where DNase I can readily access and cleave it. However, as biofilms mature, the eDNA becomes increasingly protected through cross-linking with other matrix molecules like proteins and polysaccharides, while the overall matrix density increases. This creates physical barriers that prevent DNase I from reaching its target sites, explaining why the 48-hour biofilm shows reduced sensitivity despite containing eDNA. Option A correctly identifies this protective mechanism - mature biofilms develop extensive cross-linking and shielding that limits enzyme access to eDNA. Option B incorrectly suggests eDNA is replaced over time; while polysaccharides and proteins do increase, eDNA remains present and functional throughout biofilm development. Option C proposes DNase inhibitor production, but S. aureus biofilms don't typically employ this defensive strategy against exogenous DNases. Option D focuses on reduced autolysis decreasing eDNA content, but this doesn't explain why existing eDNA becomes less susceptible to enzymatic degradation. Remember that biofilm maturation questions often test your understanding of how structural complexity affects molecular interactions. The older and more developed the biofilm, the more protected its internal components become from external enzymatic attacks.

Question 14

Dispersal is an active, regulated stage of the biofilm life cycle that allows bacteria to colonize new surfaces. Which of the following conditions is a common and critical trigger for initiating the dispersal of sessile cells from a mature biofilm?

  1. The accumulation of toxic metabolic byproducts and localized nutrient starvation within the biofilm. (correct answer)
  2. A sudden increase in the concentration of essential nutrients in the surrounding environment.
  3. The introduction of a new bacterial species into the biofilm community.
  4. Exposure to a high concentration of the same quorum-sensing molecules that induced biofilm formation.
Explanation: When you encounter questions about biofilm dispersal, think about the environmental pressures that would make it advantageous for bacteria to abandon their protective community and venture into planktonic (free-floating) life to find new locations. Biofilm dispersal is triggered primarily by stress conditions that make the current environment unsustainable. Option A correctly identifies the most common trigger: as biofilms mature and grow denser, cells in the interior experience nutrient depletion and accumulation of toxic waste products. This creates a survival pressure that activates dispersal mechanisms, prompting cells to leave and seek better conditions elsewhere. Option B is incorrect because abundant nutrients would actually promote biofilm growth and maintenance, not dispersal. Bacteria have no reason to leave a nutrient-rich environment. Option C misunderstands dispersal triggers - while new species might cause competitive stress, species introduction itself isn't a direct dispersal signal. The original community might actually benefit from the metabolic diversity. Option D reflects a common misconception about quorum sensing. High concentrations of the same signaling molecules that promote biofilm formation typically reinforce biofilm stability rather than trigger dispersal. Different signaling pathways control formation versus dispersal. Remember this pattern: biofilm dispersal questions often test whether you understand that bacteria make "decisions" based on survival advantage. Dispersal is essentially a stress response - bacteria leave when staying becomes more costly than the risks of planktonic life. Look for answer choices that describe deteriorating conditions rather than improving ones.

Question 15

In the development of dental plaque, a multi-species biofilm, bacteria such as Streptococcus gordonii act as early colonizers by binding directly to the salivary pellicle on the tooth enamel. Subsequently, late colonizers like Porphyromonas gingivalis, which cannot bind directly to the pellicle, adhere specifically to the surface of the already-attached streptococci. This phenomenon is best described as:

  1. Competitive exclusion
  2. Conjugation
  3. Quorum sensing
  4. Coaggregation (correct answer)
Explanation: Coaggregation is the process by which genetically distinct bacteria become attached to one another through highly specific molecular interactions between adhesin molecules on one cell and receptor polysaccharides or proteins on another. This process is fundamental to the development of complex, multi-species biofilms like dental plaque, creating a specific, ordered community structure. (A) Competitive exclusion is the opposite, where one species outcompetes another for resources or space. (B) Conjugation is a mechanism of horizontal gene transfer. (C) Quorum sensing is cell-to-cell communication, which regulates coaggregation and other processes but is not the binding itself.

Question 16

The establishment of steep chemical and metabolic gradients is a defining feature of mature biofilms. Which of the following is a direct and significant consequence of this structural and physiological heterogeneity?

  1. A uniform rate of horizontal gene transfer throughout all layers of the biofilm.
  2. The development of diverse micro-niches supporting distinct bacterial subpopulations, including dormant persister cells. (correct answer)
  3. Synchronization of cell division cycles across the entire biofilm population to optimize growth.
  4. An equal distribution of mechanical stress, ensuring the structural stability of the entire community.
Explanation: The dense nature of biofilms creates gradients of nutrients, oxygen, pH, and waste products. Cells in different locations experience vastly different microenvironments. This heterogeneity leads to physiological diversification, where distinct subpopulations arise. For example, cells deep within the biofilm may be in an anaerobic, low-nutrient state, causing them to enter a dormant, slow-growing phase characteristic of antibiotic-tolerant persister cells. (A), (C), and (D) are incorrect because they describe uniformity, which is the opposite of the heterogeneity that characterizes mature biofilms.

Question 17

A student proposes that the high level of antibiotic resistance observed in biofilms is primarily due to an elevated rate of spontaneous mutation within the protected environment of the EPS matrix, which allows for the rapid selection of genetically resistant mutants. Why is this hypothesis an incomplete explanation for the phenomenon of biofilm tolerance?

  1. The dense EPS matrix actively prevents horizontal gene transfer, which is the sole source of new resistance genes.
  2. Biofilm tolerance is largely a transient, physiological state of the cells rather than a result of stable genetic changes. (correct answer)
  3. Bacteria within a biofilm are post-mitotic and do not replicate, thus no mutations can occur.
  4. Antibiotics are completely unable to penetrate the EPS matrix, so there is no selective pressure within the biofilm.
Explanation: While mutations can and do occur in biofilms, the primary reason for their remarkable antibiotic tolerance is the physiological state of the cells. This includes the presence of slow-growing or non-growing 'persister' cells that are phenotypically tolerant (but genetically susceptible), as well as reduced antibiotic penetration and altered chemical microenvironments. The fact that biofilm cells can often be cultured in the lab and show full antibiotic susceptibility proves that the resistance is a feature of the biofilm community lifestyle, not necessarily a stable genetic change in most cells. (A) and (C) are false; HGT can be enhanced in biofilms and cells do replicate. (D) is an overstatement; penetration is often slowed or reduced, but not completely blocked.

Question 18

The bacterium Streptococcus mutans is a primary causative agent of dental caries. Its virulence is highly dependent on its ability to produce an extracellular glucan polymer from dietary sucrose, a reaction catalyzed by the enzyme glucosyltransferase (Gtf). A mutation rendering the Gtf enzyme non-functional would most directly impair which of the following processes?

  1. The fermentation of sucrose to produce lactic acid.
  2. The bacterium's ability to survive in the acidic environment of a dental plaque.
  3. The synthesis of peptidoglycan for cell division.
  4. The adherence of the bacterium to tooth enamel and the formation of a stable biofilm. (correct answer)
Explanation: The glucan polymer produced by glucosyltransferase is a sticky exopolysaccharide that functions as the primary adhesive for S. mutans, allowing it to cling to the tooth surface and to other bacteria. This glucan forms the bulk of the early dental plaque matrix. Without Gtf, the bacterium cannot produce this essential 'glue', severely impairing its ability to adhere and form a biofilm (plaque). (A) Fermentation of sucrose to lactic acid is a separate metabolic pathway; Gtf uses sucrose as a substrate for polymerization, not for energy production via fermentation. (B) The glucan biofilm actually helps create and maintain the acidic microenvironment; without the biofilm, acid can diffuse away more easily. (C) Peptidoglycan synthesis is unrelated to extracellular glucan production.

Question 19

A bacterium isolated from a submerged rock in a fast-flowing stream is found to produce a copious, loose, and easily deformable glycocalyx that is not tightly bound to the cell surface. This structure is best classified as a slime layer, and its most important function in this specific environment is likely which of the following?

  1. Protection against phagocytosis by protozoa.
  2. Formation of a durable, shear-resistant biofilm.
  3. Adherence to the rock surface, preventing washout. (correct answer)
  4. Storage of carbon and energy reserves for starvation periods.
Explanation: A loose, non-uniform glycocalyx is characteristic of a slime layer. In a high-shear environment like a fast-flowing stream, the primary challenge for a bacterium is to remain attached to a surface. The sticky nature of the slime layer is crucial for this initial and continued adherence, preventing the cells from being washed away. (A) While a glycocalyx can offer some protection, this is a more prominent feature of well-organized capsules in pathogenic contexts. (B) A slime layer is by definition not durable or highly shear-resistant; it facilitates attachment but not necessarily a robust, mature biofilm on its own. (D) While it can serve as a nutrient reserve, its primary role in this context is physical attachment.

Question 20

In Bacillus subtilis, a master regulator protein called SinR acts as a repressor for genes involved in exopolysaccharide (EPS) synthesis and matrix production. Another protein, SinI, antagonizes SinR activity. A mutation in the sinR gene that results in a truncated, non-functional protein would most likely lead to which of the following phenotypes?

  1. A defect in motility and an inability to form biofilms.
  2. A hyper-biofilm phenotype characterized by excessive, constitutive matrix production. (correct answer)
  3. Formation of morphologically normal biofilms that are hypersensitive to antibiotics.
  4. A failure to enter the sporulation pathway under nutrient-limiting conditions.
Explanation: SinR is a repressor of biofilm genes. If SinR is non-functional due to a mutation, its repressive effect is lost. This means the genes for EPS synthesis and matrix production, which are normally repressed by SinR, will be expressed constitutively (all the time). This leads to a phenotype of excessive biofilm formation, often described as 'hyper-biofilm' or 'hyper-rugose'. (A) This is the opposite of the expected outcome. Loss of the repressor turns biofilm formation on. (C) The biofilm would likely be more, not less, resistant to antibiotics due to the excess matrix. (D) While biofilm formation and sporulation are linked developmental pathways in B. subtilis, the direct effect of losing this specific repressor is on matrix production.