Microbiology Lab Results: Case Study of Mrs. Septic's Infection

USMLE Board Review Questions

You are paged by the Microbiology Lab and told that blood 

cultures drawn from your patient, Mrs. I. M. Septic, are growing 

gram-positive cocci.  Mrs. Septic is a 42-year-old woman who was

admitted with a fever and malaise.  Which of the following is NOT 

a possible cause of your patient's infection?

A. 

Enterococcus faecium

B. 

Streptococcus pyogenes

C. 

Staphylococcus aureus

D. 

Staphylococcus epidermidis

E. 

Listeria monocytogenes

E. 

Listeria 

is a Gram-positive rod, not coccus.

Anxious to narrow the differential diagnosis on Mrs. Septic, 

you ask the Microbiology Lab for more information.  They state 

that they have not yet identified the organism, but they have 

shown that it is catalase positive.  Which two of the following 

organisms are possible?

A. 

Enterococcus faecium

B. 

Streptococcus pyogenes

C. 

Staphylococcus aureus

D. 

Staphylococcus epidermidis

C. and D.  Streptococci and enterococci are catalase

negative. 

Staphylococci are catalase positive.  

The next morning you again call the microbiology laboratory.  

They now tell you that Mrs. Septic's bacterium is also coagulase 

positive.  Which organism is the cause of Mrs. Septic's infection?

C. Staphylococcus aureus

D. Staphylococcus epidermidis

C.  

Staphylococcus aureus 

is coagulase positive whereas

Staphylococcus epidermidis 

is coagulase negative. 

When you return to tell Mrs. Septic that you have identified the 

organism that is making her ill, you notice that she is febrile, 

hypotensive, is vomiting and has diarrhea and an erythematous 

rash.  She states that she is currently menstruating.  Which of the 

following toxins is most likely causing Mrs. Septic's symptoms?

A. exfoliatin

B. TSST-1

C. alpha-toxin

D. coagulase

E. pyrogenic exotoxin A

B.  Mrs. Septic has the signs and symptoms of 

toxic shock syndrome.  Thus TSST-1

is the most likely causative toxin.  Note that although 

bacteremia is not necessary in the pathogenesis of 

toxic shock syndrome, bacteremia may still be present

in these patients.

Which of the following organisms is NOT a common cause 

of food-poisoning?

A. 

Staphylococcus aureus

B.

 Yersinia enterocolitica

C

. Pseudomonas aeruginosa

D

. Salmonella enterica

E

. Bacillus cereus

C.  

Pseudomonas aeruginosa

 is a frequent cause of

hospital-acquired infections, but usually does not cause

food poisoning.

All of the following organisms are associated with cardiac 

damage and abnormalities EXCEPT:

A.

 Clostridium difficile

B. 

Staphylococcus aureus

C. 

Corynebacterium diphtheriae

D. 

Borrelia burgdorferi

E. 

Streptococcus pyogenes

A.  

Clostridium difficile

 causes antibiotic-associated

diarrhea and colitis, but not cardiac damage.

Staphyloccocus aureus 

is a frequent cause of

endocarditis, 

Corynebacteria diphtheriae 

can cause

cardiac damage through the action of diphtheria

toxin, 

Borrelia burgdorferi

 is associated with

arrhythmias, and 

Streptococcus pyogenes

 can cause

rheumatic fever.

Which of the following is a strict anaerobe?

A. 

Corynebacterium diphtheriae

B. 

Actinomyces israelii

C. 

Bordetella pertussis

D. 

Yersinia pestis

E. 

Nocardia asteroides

B. 

Actinomyces israelii

A 34-year-old man presents to the Emergency Room 

complaining of fever, a stiff neck, and pain upon looking 

at bright lights.  You suspect meningitis.  All of the following 

are likely causes of his illness EXCEPT:

A. 

Streptococcus pneumoniae

B. 

Listeria monocytogenes

C. 

Bordetella pertussis

D. 

Neisseria meningitidis

E. 

Haemophilis influenzae

C. 

Bordetella pertussis 

usually causes whooping cough,

whereas the other listed organisms are frequent causes of

meningitis.

Upon further questioning, you determine that he works on a 

dairy farm and recently drank some unpasteurized milk.  This 

information supports which of the following as a cause of 

his symptoms?

A. 

Streptococcus pneumoniae

B. 

Listeria monocytogenes

D. 

Neisseria meningitidis

E. 

Haemophilis influenzae

B. 

Listeria monocytogenes 

is a cause of meningitis that

may be obtained by drinking unpasteurized mild.

You perform a lumbar puncture.  A gram-stain of the 

cerebral spinal fluid shows gram-negative diplococci.  Which 

of the following is the most likely cause of the man's illness?

A. 

Streptococcus pneumoniae

B. 

Listeria monocytogenes

D. 

Neisseria meningitidis

E. 

Haemophilis influenzae

D. 

Neisseria meningitidis 

is a frequent cause of 

meningitis that is a Gram-negative diplococci (cocci

that are often paired).

The man states that he had an anaphylactic response to 

penicillin in the past.  What should you use to treat him?

A. penicillin

B. ceftriaxone

C. trimethroprim/sulfamethoxazole

D. gentamicin

E. chloramphenicol

Extra Credit Question

E. Chloramphenicol is the treatment of choice for patients

with 

Neisseria meningitidis 

meningitis who have a severe

allergy to penicillin.

Regarding this case, which of the following is true?

A. Household members should receive meningococcal vaccine.

B. Household members should receive ciprofloxacin.

C. Household members should receive penicillin.

D. A lumbar puncture should be performed on all household 

members to determine who should receive treatment.

E. Throat cultures should be performed on all household 

members to determine who should receive antibiotics.

B. Household members exposed to a patient with

Neisseria meningitidis 

meningitis should receive

ciprofloxacin prophylaxis, since they are at a

significantly increased risk of acquiring the disease.

Rifampin and ceftriaxone are also acceptable options.

A throat culture is mistakenly ordered on a patient on your 

service who is hospitalized for a diabetic foot ulcer.  Her throat 

looks normal and she denies any oropharyngeal symptoms.  You 

are surprised when the Clinical Microbiology Laboratory calls you 

to say that the culture is growing 

Neisseria meningitidis.  

Which 

of the following would be appropriate:

A. Start treatment immediately with penicillin and place the 

patient in respiratory isolation.

B. Immunize her with the meningococcus vaccine.

C. Immunize the patient's family members as well as the physicians 

and nurses who have cared for her with the meningococcus vaccine.

D. Administer ciprofloxacin to the patient's family members as 

well as the physicians and nurses who have cared for her.

E.  Take no action.

E.  Take no action.  The patient

’

s throat is transiently

colonized with 

Neisseria meningitidis, 

but he is not

currently experiencing disease.  Approximately 10% of

the population will be colonized with this bacterium at

any given time.  Therefore no treatment

or other measures is necessary.

Which of the following is true regarding 

Neisseria

meningitidis?

A. This organism secretes siderophores to obtain iron from

host tissues.

B. This organism has a capsule which is the target of a

vaccine.

C. This organism causes a sexually transmitted disease.

D. This organism is a Gram-negative rod.

E. A single commercially available vaccine is effective

against all strains of this organism.

B.  

Neisseria 

spp. do not secrete siderophores.  

Neisseria

gonorrhoeae 

causes a sexually transmitted disease, not

Neisseria meningitidis.  

This organism is a Gram-negative

diplococcus, not rod.  A vaccine against the polysaccharide

capsule is used, but is only effective against capsular types

A, C, and Y.  A separate vaccine is effective against capsular

type B.

Which of the following is NOT a spirochete?

A.

 Treponema pallidum

B

. Borrelia burgdorferi

C

. Borrelia recurrentis

D.

Leptospira interrogans

E.

 Helicobacter pylori

E. 

Helicobacter pylori

 is a Gram-negative spiral-

shaped organism, but is not considered a spirochete.

The bacterium that causes Lyme Disease makes which bacterial

protein?

A. VacA

B. ActA

C. OspA

D. OmpA

E. 

dot

C. OspA is an surface protein of 

Borrelia burgdorferi

.  VacA

is a 

Helicobacter pylori 

protein, ActA is a 

Listeria

monocytogenes 

protein, and 

dot 

is a genetic locus in

Legionella pneumophila.

Which of the following is NOT a Rickettsia species

important in human disease?

A. 

Rickettsia typhi 

B. 

Rickettsia tsutsugamushi 

C. 

Rickettsia prowazekii 

D. 

Rickettsia burnetti

E. 

Rickettsia rickettsia

D. 

burnetti 

is a species of 

Coxiella

 not 

Rickettsia.

Rickettsia typhi 

causes murine typhus, 

R. tsutsugamushi

causes scrub typhus, 

R. prowazekii 

causes epidemic

typhus, and 

R. rickettsia 

causes Rocky Mountain

Spotted Fever.

Which of the following media/agars is routinely used

to grow 

Legionella pneumophila?

A. Buffered charcoal-yeast extract agar

B. Sheep red blood cell agar

C. MacConkey

’

s agar

D. Bordet-Gengou media

E. Tellurite selective media

A. Buffered charcoal yeast extract agar is commonly used

to grow 

Legionella pneumophila.  

This agar is supplemented

with cysteine and antibiotics, which aid in the growth and

selection of 

Legionella.

Which of the following organisms causes an illness

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A. 

Listeria monocytogenes

B. 

Bordetella pertussis

C. 

Legionella pneumophila

D. 

Bacillus anthracis

E. 

Coxiella burnetti

B. 

 Bordetella pertussis 

causes whooping cough, a disease

consisting of three stages: (1) an incubation stage, (2) a 

catarrhal stage, and (3) a paroxysmal stage, during which

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Your patient is admitted to the surgical intensive care unit

following a car accident leading to abdominal trauma.

He undergoes abdominal surgery for removal of his spleen,

which has ruptured.  He remains intubated following the

surgery, and on post-op day 3 develops a fever and infiltrate

on his chest XRay.  Sputum and blood cultures grow an

organism that is a rod by Gram-stain.  Which of the

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A. 

Klebsiella pneumoniae

B. 

Pseudomonas aeruginosa

C. 

Citrobacter freundii

D. 

Enterbacter cloacae

E. 

Legionella pneumophila

E. All have a Gram-negative structure to their 

cell walls, but 

Legionella pneumophila 

usually does not take

up Gram-stain.  It is visualized using other stains, such

as silver stain or DFA.

Preliminary analysis of the organism by the Microbiology

Laboratory indicates that it is oxidase positive.  Which of 

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A. 

Klebsiella pneumoniae

B. 

Pseudomonas aeruginosa

C. 

Citrobacter freundii

D. 

Enterbacter cloacae

B.  Only 

Pseudomonas aeruginosa 

is a Gram-negative

rod that is oxidase positive.

The laboratory says that antibiotic susceptibilities on the 

P.

aeruginosa 

isolate are pending.  Which of the following is an

appropriate treatment regimen for this patient?

A. Ampicillin

B. Ceftazidime

C. Azithromycin

D. Cefazolin

E. Trimethoprim-Sulfamethoxazole

B.  Of the antibiotics listed, only ceftazidime has activity

against 

P. aeruginosa.

Which of the following is not associated with the cell

wall of Gram-positive bacteria?

A. Lipopolysaccharide

B. Murein

C. Teichoic acid

D. Peptidoglycan

E. Penicillin-binding proteins

A. Lipopolysaccharide is associated with the outer

membrane of Gram-negative bacteria.

Which of the following is the toxic portion of 

lipopolysaccharide?

A. O-side chain

B. Core polysaccharide

C. Lipid A

D. KDO

E. ribotol

C. Lipid A is responsible for the toxic activity of LPS, 

which can lead to Gram-negative shock.

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A. Pili

B. Capsule

C. Flagella

D. LPS

E. Peptidoglycan

C. Flagellum serotype

Which of the following genes are routinely sequenced

to determine phylogenetic relationships between

bacteria?

A. 18s rRNA 

B. 50s ribosomal subunit

C. 23s rRNA

D. 16s rRNA

E. 5s rRNA

D.  The 16s rRNA gene is often sequenced and compared

to known sequences to determine evolutionary 

relationships between bacteria.

Which of the following does NOT involve the 

proton-motive force?

A. Respiration

B. Electron transport chain

C. Chemiosmosis

D. ATP synthase

E. Fermentation

E. Fermentation.  Energy generation by fermentation

does not require an electron transport chain and does

not involve the generation of a proton-motive force.  

Which of the following is generally NOT true of 

facultative anaerobes?

A. They contain superoxide dismutase.

B. They contain catalase.

C. They often ferment in the absence of oxygen.

D. They often respire in the presence of oxygen

E. They are unable to grow in the absence

of oxygen.

E. Facultative bacteria by definition can grow in the

presence and absence of oxygen.

A sex pilus is involved in which of the following?

A. Transformation

B. Conjugation

C. Transduction

D. Electroporation

E. Adherence to eukaryotic cells

B. Conjugation

Which of the following statements about phages is

true?

A. Virulent phages may cause a lysogenic infection.

B. Temperate phages may cause a lytic infection.

C. Bacteriophages are viruses that infect eukaryotic cells.

D. Genetic exchange mediated by phages is referred to

as transformation.

E. There are two types of transduction: natural and

artificial.

B. Temperate phages may cause either a lytic or a

lysogenic infection.  Virulent phages may cause only

a lytic infection.

Treatment of 

Clostridioides difficile 

infections may include the

use of fecal transplants to prevent recurrence.

A. True

B. False

A. True.  Fecal transplants restore the gastrointestinal

microflora, which tends to prevent relapse of 

C. difficile

disease.

Which of the following organisms does NOT use a type III

secretion system to cause disease?

A. 

Yersinia pestis

B. 

Pseudomonas aeruginosa

C. 

Staphylococcus aureus

D. 

Salmonella enterica

E. 

Shigella flexneri

C. 

Staphylococcus aureus 

is a Gram-positive organism.

Gram-positive bacteria do not harbor type III secretion

systems. Type III secretion systems are embedded in both

the inner and outer bacterial membranes.  Gram-positive

bacteria don

’

t have an outer membrane.

You see in clinic a 23 year-old-woman who recently immigrated

from Vietnam.  She states that she was referred to you because

the PPD skin test she received during her immigration

screening was positive.  She has never received the BCG

vaccine.  She denies coughing, fever, night sweats, and weight

loss.  After taking a complete history and doing a physical exam,

which of the following would be most appropriate?

A. Start isoniazid.

B. Start isoniazid and order a chest X-ray.

C. Start isoniazid, rifampin, ethambutol, and pyrazinamide.

D. Order three sputums to be collected and examined for acid-

fast bacilli

E. Order a chest X-ray.

E. Order a chest X-ray.  You must first determine whether the

patient has latent tuberculosis or active disease. You would do

this by obtaining a chest X-ray even if the patient were not

complaining of symptoms. A chest X-ray is the accepted method

of excluding active pulmonary tuberculosis in someone who is

asymptomatic.  It is important to discriminate between latent

tuberculosis and active disease because the former could be

treated with a single drug, but active disease should be initially

treated with four drugs.  Treatment of active disease, even

transiently, with a single drug has the potential to lead to

resistance.

Which of the following bacteria exhibit coiling phagocytosis?

A. 

Legionella pneumophila

B. 

Coxiella burnetti

C. 

Francisella tularensis

D. 

Bartonella quintana

E. 

Salmonella enterica

A. 

Legionella pneumophila

Which of the following bacteria is a cause of cancer?

A. 

Pseudomonas aeruginosa

B. 

Streptococcus pyogenes

C. 

Bartonella quintana

D.  

Francisella tularensis

E.  

Helicobacter pylori

E.  

Helicobacter pylori 

has been linked to

adenocarcinoma,

non-Hodgkin's lymphoma, and low-grade B cell mucosa-

associated lymphoid tissue (MALT) lymphoma of  the

stomach

Which of the following is NOT true of 

Leptospira interrogans

?

A. It can be transmitted by ingestion of contaminated water

B. It can be transmitted by exposure of cuts in the skin to

contaminated water.

C. It can be transmitted by exposure of the conjunctiva to

contaminated water

D.

It can be transmitted by inhalation of bacteria aerosolized

during the birth of animals

E.

It also commonly infects rats, cattle, and dogs

D. 

Leptospira

 cannot be transmitted by inhalation of bacteria

aerosolized during the birth of animals.  This route of infection

is typical of 

Coxiella burnetti.