General Microbiology

       Which organism below, if present in only one of three blood cultures from a single patient, is most likely to represent contamination caused by poor antiseptic technique, rather than genuine infection?
A Staphylococcus epidermidis.  
B Streptococcus pneumoniae.  
C Escherichia coli.  
D Pseudomonas aeruginosa.  
E Haemophilus influenzae.

       Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis all colonize the upper respiratory tract and can spread from there to the bloodstream and/or meninges. What essential virulence factor, that aids such spread, is produced by all three organisms?
A IgA protease.  
B Polysaccharide capsule.   
C Thick peptidoglycan.  
D Endotoxin.  
E Exotoxin.

       Three days after an appendectomy, the patient develops high fever. The patient's vital signs include a dangerously low blood pressure. The patient's erythrocyte sedimentation rate (ESR) is above-normal. Which of the cytokines below most directly contributed to development of these symptoms?
A IL-4  
B IFN-gamma  
C IL-8  
D IL-10  
E TNF-alpha 

       A patient began to receive intravenous antibiotics dissolved in sterile saline solution developed fever within an hour. (The solution in which antibiotics were dissolved had been sterilized by autoclaving.) The IV infusion was stopped and the fluid cultured, but cultures on a variety of media were sterile.  Which of the following bacterial components would have been most effective in producing the patient's fever?
A Spores  
B Lipopolysaccharide.  
C Capsule polysaccharide.  
D Outer membrane proteins.  
E Siderophores.  

       Cells of pathogenic Gram-negative bacteria often have 'common pili' extending from their surfaces. What process is promoted by these pili?
A Adherence of bacteria to mammalian cells.  
B Inhibition of Complement activation.  
C Resistance to opsonization by antibodies.  
D Transfer of DNA between bacterial cells.  
E Transport of nutrients into bacterial cells.  

       Water used to prepare solutions for intravenous use must not only be sterile, but also be free of bacterial components. After sterilization, which bacterial component is it most important to remove?
A Lipids of the plasma (inner) membrane.  
B Capsular polysaccharide.   
C Spores.  
D Lipopolysaccharide.  
E Proteases which activate complement.

       A patient with bacterial infection of the bloodstream developed fever and shock, produced by lipopolysaccharide (LPS). What part of LPS was responsible for its toxic effect?
A Long repeating polysaccharide (O-antigen).  
B Complex phosphorylated polysaccharide core.  
C Unsaturated fatty acids, released by hydrolysis.  
D Disaccharide substituted with saturated fatty acids (Lipid A).  
E Protein, freed from covalent linkage to polysaccharide by plasma proteases.

       Which compound is found in the envelopes of Gram-positive bacteria, but not in Gram-negatives?
A Peptidoglycan.  
B Porin proteins.  
C Lipid A.  
D Teichoic acid.  
E Capsule

       Streptococcus pneumoniae is cultured from a patient's sputum. Clinical isolates of this species typically have thick polysaccharide capsules; varieties without capsules seldom produce serious disease. How do capsules increase the ability of S. pneumoniae to cause disease?
A Inhibit binding of antibodies.  
B Directly toxic to phagocytic cells.  
C Inhibit activation of complement.  
D Inhibit phagocytosis by neutrophils.  
E Inhibit antigen presentation by MHC.  
F Not immunogenic, but mask immunogenic surface structure

       In the immune response to invasive infections by Streptococcus pneumoniae, antibodies are required for effective defense. Antibodies to which antigen are most important?
A Teichoic acid.  
B Peptidoglycan.  
C C-carbohydrate.  
D Type III secretion system.  
E Polysaccharide capsule.

       A child develops severe pharyngitis with purulent exudate and high fever, consistent with Group A streptococcal infection. He is treated with Penicillin G, but his parents discontinue the antibiotic after a few days because he feels much better and does not like to take pills. Some time later he develops acute rheumatic fever (ARF). Which bacterial antigen provoked the immune response which caused ARF?
A Peptidoglycan.  
B Teichoic acid.  
C Group A carbohydrate  
D A superantigen protein exotoxin.   
E M protein.

       An important virulence factor of coagulase-positive staphylococci is Protein A. How does Protein A aid in virulence?
A Binds the Fc region of IgG, decreases opsonization (Phagocytosis).  
B Hydrolyzes secretory IgA.  
C Binds Factor H, prevents activation of complement.  
D Extracts iron from plasma proteins.  
E Promotes tight binding of bacteria to extracellular matrix.

       A notable virulence factor of pathogenic Yersinia is the ability to paralyze phagocytes and so prevent ingestion of bacteria. What mechanism enables Yersinia to do this?
A Quorum-sensing system.  
B Two component regulatory system.  
C Type III secretion system.  
D Iron-acquisition system.  
E Phase variation for adhesive pili.

       One week after surgery to remove an inflamed appendix the patient develops fever, chills, and hypotension. Anaerobic blood cultures grow Bacteroides fragilis. What was the most likely source of this infection?

A. Home-grown tomatoes brought to the patient by his family.
B. Direct contact with a health care worker.
C. Inhalation of airborne spores.
D. Fecal contamination of food or water.
E. The patient’s own intestinal flora.

       A febrile patient with E. coli sepsis develops ‘septic shock’.  Which of the following is likely to have played largest role in this reaction?

A. A humoral immune response to bacterial surface antigens.
B. A cell-mediated immune response to bacteria.
C. Secretion of cytokines by monocytes in response to stimulation by LPS [endotoxin].
D. Release of granule contents by mast cells and basophils.
E. Secretion of a bacterial superantigen toxin that activates TH cells.

       E. coli and Neisseria meningitidis isolated from blood cultures often have transport systems (for acquiring metal ions from the environment) lacking in their less-virulent relatives. Which metal is acquired by the majority of these systems?

A. Copper
B. Sodium
C. Potassium
D. Magnesium
E. Iron

       A bacillus cultured from stool produces dark pink colonies on MacConkey agar.  Which of the following organisms is most likely?

A. Escherichia coli
B. Salmonella enterica
C. Listeria monocytogenes
D. Shigella sonnei
E. Shigella dysenteriae

       Fresh liquid cultures of Pseudomonas aeruginosa and Streptococcus bovis were mixed and Gram-stained. However, the safranin counter-stain was accidentally omitted. If you looked at this preparation under the microscope, which line below best describes what you should expect to see?

Cocci                           Bacilli
A. Colorless               Colorless
B. Blue                        Pink/red
C. Blue                        Colorless
D. Pink/red                Colorless
E. Pink/red                 Pink/red

       Bacteroides fragilisis isolated from an abdominal abscess. Which phrase below best describes infections like this one?

A. Other species of bacteria also often present.
B. Initiated by ingestion of spores.
C. Can be successfully treated with aminoglycosides alone.
D. Contracted by the respiratory route.
E. Sexually-transmitted.

       A young woman has a urinary tract infection caused by E. coli. By what route are such infections most commonly acquired?

A. Respiratory route, followed by spread from the anterior nares.
B. Respiratory route, followed by spread from the oro-pharyngeal region.
C. Fecal-oral route, followed by colonization of the intestine and then the perineum.
D. Intestinal infection, followed by spread of bacteria to the urethra via the bloodstream.

       Ability of E. coli and related Gram-negative rods to cause intestinal and urinary-tract infections is promoted by ability of bacteria to adhere tightly to epithelial cells. Which of the following bacterial structures is primarily an organelle of adhesion?

A. Outer membrane.
B. Inner membrane.
C. Peptidoglycan.
D. Flagella.
E. Common pili.

       A 4-year-old child is brought to the emergency room, stuperous and unresponsive, with a temperature of 39oC. A lumbar puncture is performed and a smearof cerebrospinal fluid is Gram-stained. Each high-power field contains many neutrophils, red cells, and Gram-positive cocci. What does the blue color of the cocci indicate about the structure of their envelopes?

A. Thick peptidoglycan.
B. Thin peptidoglycan.
C. Thick capsule.
D. Many adhesive pili.
E. Long O-antigen chains.

       Sputum is obtained from a patient believed to have tuberculosis, produced by infection with Mycobacterium tuberculosis, an acid-fast bacterium. A sputumsmear is stained using the same technique used on the demonstration slides in the lab. If the patient indeed has Tuberculosis, and the stain is done with proper procedure, how should the bacteria appear?

A. Cocci, which stain dark red.
B. Cocci, which stain dark green.
C. Cocci, which stain dark blue.
D. Bacilli, which stain dark red.
E. Bacilli, which stain dark green.
F. Bacilli, which stain dark blue.

       A six-month old boy develops bloodstream infection with a Gram-negative bacterium and rapidly develops high fever and hypotension. There is no pasthistory of similar infections. Which of the following was most greatly responsible for the fever and hypotension?

A. Capsule polysaccharide, which activated Complement by the Alternate Pathway.
B. Binding of IgG antibodies to bacteria, which activated Complement by the Classical Pathway.
C. Lipopolysaccharide, which bound to Toll-like receptors.
D. Common pili, which allowed bacteria to bind tightly to endothelial cells.
E. Porin proteins, which allowed bacterial toxins to exit the periplasm.

       Patients with bloodstream infections with Gram-positive bacteria can develop septic shock.  Which inflammatory components are present inthe envelopes of Gram-positive bacteria?

A. Core and Lipid A of Lipopolysaccharide.
B. O-antigen chains of Lipopolysaccharide.
C. Porin proteins and periplasmic lipoproteins.
D. Peptidoglycan and teichoic acids.

       Bacteria of the genus Mycoplasmalack all envelope components external to the plasma membrane.  How would you expect this unusual structural feature to affect their biology?

A. Their Lipid A should produce intense inflammation.
B. They should be highly-resistant to osmotic lysis.
C. They should be resistant to penicillin and structurally-related drugs.
D. They should stain Gram-positive.
E. Absence of an outer membrane means that they must lack a respiratory chain.


  1. Great work sir. No doubt these are some of the mcqs that are frequently asked in papers.

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