Bacteriology MCQs: Virulence Factors, Resistance and Toxin Mechanisms

1. The survival of Mycobacteria after ingestion by macrophages is attributed to
   a. Bacterial inhibition of complement activation via the alternative pathway.
   b. Bacterial inhibition of phagolysosome formation and interference with endosomal acidification
   c. The poor immunogenicity of the cell wall glycolipids.
   d. The bacterium’s rapid escape from the endosome into the cytoplasm of infected cells.
   e. The bacterium’s resistance to oxygen-active radicals released into the phagolysosome.
2. A 21 year newly married woman developed a urinary tract infection (UTI). At the time she sought medical advice, she was febrile and complained of painful urination and flank pain. Her urine appeared “cloudy”. Urine culture yields a lactose-fermenting, indole-positive, Gram-negative bacillus.
   
   The infectiveness of the organism responsible for this urinary tract infection is associated with specific,
   a. Exotoxins
   b. K antigens
   c. Metabolic properties
   d. P fimbriae
   e. Plasmids

3. The role of bacterial capsules as virulence factors is usually related to their ability to interfere with
a. Antibody binding
b. B lymphocyte activation
c. Antibacterial penetration of bacterial cells
d. Phagocytosis
e. The release of interferon-gamma and other macrophage activating cytokines

4. A mutation in DNA gyrase is likely to result in resistance to which one of the following antibiotics?
a. Amphotericin B
b. Ciprofloxacin
c. Penicillin
d. Rifampin
e. Streptomycin

5. Resistance of Staphylococcus aureus to methicillin is most often caused by
a. alteration of the major target for the drug
b. cell membrane impermeability
c. decreased uptake of the antibiotic
d. Inactivation of autolysins
e. synthesis of a potent Beta-Lactamase

6. The molecular basis for the effect of cholera toxin on duodenal mucosal cells is
a. activation of adenylate Cyclase
b. inactivation of a G1 protein
c. increased activity of potassium pumps
d. increased generation of cyclic adenosine monophosphate (cAMP)
e. ribosylation of guanosine triphosphate (GTP) binding protein.

7. The synthesis of erythrogenic toxin by specific strains of group A Streptococcus is determined by a
a. bacterial chromosomal gene
b. gene carried by a lysogenic phage
c. specific virulence plasmid
d. Transposon

8. Which one of the following factors, released by heating a suspension of sheep erythrocytes, is required for the growth of Haemophilus Influenzae in chocolate agar?
a. Coagulase
b. Nicotinamide adenine dinucleotide (NAD)
c. Hemoglobin
d. Hemolysin
e. Protein A

9. Which one of the following bacteria is most likely to be relatively resistant to antibiotics as a result of the relative impermeability of its cell wall?
a. Haemophilus influenzae
b. Pseudomonas aeruginosa
c. Staphylococcus aureus
d. Streptococcus pneumoniae
e. Streptococcus pyogenes

10. A patient develops explosive, watery diarrhea 24 hours after eating seafood. What bacterium is most likely involved?
a. Campylobacter fetus
b. Salmonella typhimurium
c. Shigella flexneri
d. Vibrio cholerae
e. Vibrio parahaemolyticus

Answer key

1. b. Bacterial inhibition of phagolysosome formation and interference with endosomal acidification
2. d. P fimbriae
3. d. Phagocytosis
4. b. Ciprofloxacin
5. a. Alteration of the major target for the drug (penicillin-binding proteins)
6. d. Increased generation of cyclic adenosine monophosphate (cAMP)
7. b. Gene carried by a lysogenic phage
8. b. Nicotinamide adenine dinucleotide (NAD), that is, factor V
9. b. Pseudomonas aeruginosa
10. e. Vibrio parahaemolyticus

Why these are the answers

1. Mycobacterium tuberculosis survives inside macrophages by blocking phagolysosome formation and preventing the phagosome from acidifying. Normally a phagosome fuses with a lysosome and the merged compartment turns acidic to kill the microbe. M. tuberculosis arrests this fusion and keeps the compartment near neutral pH, so it persists in the very cell meant to destroy it. Option d describes a different strategy (escape into the cytoplasm, used by Listeria and Shigella, not mycobacteria), and option e is wrong because resisting oxygen radicals alone would not explain survival; the key is the fusion block.
2. P fimbriae. The organism described (lactose-fermenting, indole-positive, gram-negative bacillus from urine) is Escherichia coli, and uropathogenic E. coli attaches to the urinary tract using P fimbriae, which bind the P blood-group glycolipid on uroepithelial cells. This adhesion lets the organism ascend the tract and cause pyelonephritis (the flank pain and fever here point to kidney involvement). K antigens contribute, but the specific virulence structure tied to this ascending infection is the P fimbria.
3. Phagocytosis. A bacterial capsule is antiphagocytic: it masks the surface molecules that phagocytes and complement would otherwise grab, so the organism resists engulfment. This is why encapsulated organisms (S. pneumoniae, H. influenzae type b, N. meningitidis, K. pneumoniae) are more virulent, and why capsular polysaccharide vaccines work: antibody against the capsule restores phagocytosis (opsonization). The capsule does not block antibody binding or B-cell activation directly; its job is to evade being eaten.
4. Ciprofloxacin. DNA gyrase (topoisomerase II) is the direct target of the fluoroquinolones, so a gyrase mutation reduces quinolone binding and confers resistance. The other drugs act elsewhere: penicillin on cell-wall synthesis, rifampin on RNA polymerase, streptomycin on the 30S ribosome, and amphotericin B on fungal ergosterol (not even antibacterial). Link the enzyme to the drug class: gyrase means quinolones.
5. Alteration of the drug target. Methicillin resistance in S. aureus (MRSA) comes from the mecA gene, which encodes an altered penicillin-binding protein, PBP2a, that beta-lactams bind poorly. Because the target itself is changed, the whole beta-lactam class fails, not just methicillin. Note the trap in option e: beta-lactamase (penicillinase) is the mechanism of ordinary penicillin resistance in S. aureus, but it is not how methicillin resistance works, methicillin was designed to resist that enzyme. The distinction between "makes an enzyme that destroys the drug" and "changes the target so the drug cannot bind" is the whole point of the question.
6. Increased generation of cAMP. Cholera toxin ADP-ribosylates the Gs regulatory protein, locking adenylate cyclase in the "on" state. Adenylate cyclase then pours out cyclic AMP, and the sustained high cAMP drives massive secretion of chloride and water into the gut lumen, producing the watery diarrhea of cholera. Options a and e describe steps in the pathway, but the molecular basis of the effect on the mucosal cell is the rise in cAMP itself; that is the second messenger doing the damage.
7. A gene carried by a lysogenic phage. The erythrogenic (pyrogenic) toxin of group A Streptococcus, which produces the rash of scarlet fever, is encoded not on the bacterial chromosome but on a bacteriophage that has integrated into the genome. Only strains lysogenized by that phage make the toxin. This phenomenon, where a phage gives its host a new virulence trait, is called lysogenic (phage) conversion, and the same mechanism explains diphtheria toxin and botulinum toxin.
8. NAD (factor V). Haemophilus influenzae needs two accessory growth factors: factor X (hemin, heat-stable) and factor V (NAD, heat-labile). On chocolate agar the red cells are gently heated until they lyse, which both releases intracellular NAD and destroys the NADase enzymes that would otherwise degrade it. That is why H. influenzae grows on chocolate agar but not on plain blood agar, where the factor V stays locked inside intact red cells. The question hinges on the word "released by heating": that points specifically at factor V.
9. Pseudomonas aeruginosa. Its outer membrane is strikingly impermeable, roughly a tenth as permeable as that of E. coli, because it lacks large general-diffusion porins and relies on selective channels instead. This low permeability, working together with efflux pumps, gives P. aeruginosa its high intrinsic resistance to many antibiotics. The distractor to reject is S. aureus: it is gram-positive and has no outer membrane at all, so impermeability of an outer membrane cannot explain its resistance. Whenever a question ties resistance to cell-envelope impermeability, think gram-negative outer membrane, and Pseudomonas above all.
10. Vibrio parahaemolyticus. Explosive watery diarrhea within about a day of eating seafood, especially raw or undercooked shellfish, is the classic presentation of V. parahaemolyticus, a halophilic (salt-loving) vibrio found in warm coastal and brackish seawater. V. cholerae also causes watery diarrhea but is classically waterborne and epidemic rather than tied to a seafood meal; the seafood exposure plus short incubation points to V. parahaemolyticus.