Exam Prep
Stains, culture media, Gram-positive bacteria, Enterobacterales, non-fermenters, mycobacteria and susceptibility testing.
Independent practice questions written by Labologics. Not affiliated with DHA, DOH (formerly HAAD), SCFHS, Prometric or any exam body. Not recalled exam questions.
Which step of the Gram stain differentiates Gram-positive from Gram-negative bacteria?
Answer: A — Decolorization with alcohol or acetone
Over-decolorizing can make Gram-positive organisms look Gram-negative. Gram-positive cells stain purple; Gram-negative cells stain pink-red.
Reference: Bailey & Scott's Diagnostic Microbiology
Which stain is used to detect Mycobacterium tuberculosis in sputum?
Answer: B — Ziehl-Neelsen (or auramine-rhodamine fluorescent) acid-fast stain
Nocardia is weakly (partially) acid-fast with the modified Kinyoun method.
Reference: Bailey & Scott's Diagnostic Microbiology
Metachromatic (volutin) granules of Corynebacterium diphtheriae are shown with:
Answer: C — Albert or Neisser stain (or methylene blue)
C. diphtheriae appears in 'Chinese letter' arrangements. Malachite green (Schaeffer-Fulton) stains endospores.
Reference: Bailey & Scott's Diagnostic Microbiology
Dark-field microscopy is used for organisms that are:
Answer: D — Too thin to see with ordinary stains, e.g., Treponema pallidum and Leptospira
Cryptococcus capsules are shown with India ink, not dark-field.
Reference: Bailey & Scott's Diagnostic Microbiology
During which growth phase do bacteria divide most rapidly?
Answer: A — Log (exponential) phase
Many antibiotics that act on cell wall synthesis work best against actively dividing bacteria.
Reference: Bailey & Scott's Diagnostic Microbiology
MacConkey agar is both selective and differential because it:
Answer: B — Inhibits Gram-positive bacteria (bile salts, crystal violet) and shows lactose fermentation (pink colonies)
Lactose fermenters (E. coli, Klebsiella, Enterobacter) are pink; non-fermenters (Salmonella, Shigella, Proteus, Pseudomonas) are colorless.
Reference: Bailey & Scott's Diagnostic Microbiology
Chocolate agar is mainly used to grow:
Answer: C — Fastidious organisms such as Haemophilus and Neisseria
Heating blood releases X factor (hemin) and V factor (NAD).
Reference: Bailey & Scott's Diagnostic Microbiology
Thayer-Martin agar contains antibiotics (e.g., vancomycin, colistin, nystatin) to selectively isolate:
Answer: D — Neisseria gonorrhoeae and N. meningitidis
Colistin inhibits most Gram-negative bacteria but not pathogenic Neisseria. Incubate in 5% CO2.
Reference: Bailey & Scott's Diagnostic Microbiology
On XLD agar, Salmonella typically appears as:
Answer: A — Red colonies with black centers (H2S)
Shigella forms red colonies without black centers; lactose/sucrose fermenters such as E. coli are yellow.
Reference: Bailey & Scott's Diagnostic Microbiology
Which medium is used to isolate Vibrio cholerae, giving yellow colonies?
Answer: B — TCBS agar
Yellow colonies result from sucrose fermentation. Cholera causes 'rice-water' stool with loss of fluid, sodium and potassium.
Reference: Bailey & Scott's Diagnostic Microbiology
Buffered charcoal yeast extract (BCYE) agar is used for:
Answer: C — Legionella pneumophila
Legionella needs cysteine and iron and does not grow on routine blood agar. Urine antigen testing is also used.
Reference: Bailey & Scott's Diagnostic Microbiology
Lowenstein-Jensen medium is used for:
Answer: D — Mycobacteria
LJ is an egg-based medium with malachite green. Liquid media (e.g., MGIT) give faster results; best recovery uses both solid and liquid media.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which medium is used for Bordetella pertussis?
Answer: A — Bordet-Gengou or Regan-Lowe (charcoal) agar
PCR on a nasopharyngeal swab is now the main diagnostic test for whooping cough.
Reference: Bailey & Scott's Diagnostic Microbiology
Loeffler serum and tellurite blood agar are used for:
Answer: B — Corynebacterium diphtheriae
Toxin production is confirmed by the Elek test or PCR for the tox gene.
Reference: Bailey & Scott's Diagnostic Microbiology
An important quality check for every new batch of culture media is:
Answer: C — Sterility and growth performance with control strains
Uninoculated plates are incubated to check sterility; known strains check growth and expected reactions.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which test separates Staphylococcus from Streptococcus?
Answer: D — Catalase
Staphylococci are catalase positive; streptococci and enterococci are catalase negative.
Reference: Bailey & Scott's Diagnostic Microbiology
Which test separates Staphylococcus aureus from other staphylococci?
Answer: A — Coagulase
S. aureus is coagulase positive and ferments mannitol on mannitol salt agar.
Reference: Bailey & Scott's Diagnostic Microbiology
A coagulase-negative staphylococcus from the urine of a young sexually active woman is resistant to novobiocin. It is most likely:
Answer: B — S. saprophyticus
S. epidermidis is novobiocin susceptible and is a common contaminant and cause of catheter and prosthetic device infections.
Reference: Bailey & Scott's Diagnostic Microbiology
Methicillin (oxacillin) resistance in S. aureus is detected with:
Answer: C — Cefoxitin disk or mecA/mecC PCR
Cefoxitin is a better inducer of mecA. Vancomycin is a treatment option for serious MRSA infections.
Reference: CLSI M100: Performance Standards for Antimicrobial Susceptibility Testing
How can Micrococcus be distinguished from Staphylococcus?
Answer: D — Micrococcus is modified oxidase positive and bacitracin susceptible; it often forms tetrads
Micrococcus is usually a skin contaminant.
Reference: Bailey & Scott's Diagnostic Microbiology
A beta-hemolytic, catalase-negative coccus causing pharyngitis that is bacitracin susceptible and PYR positive is:
Answer: A — Streptococcus pyogenes (group A)
Group A strep can lead to rheumatic fever and post-streptococcal glomerulonephritis.
Reference: Bailey & Scott's Diagnostic Microbiology
Group B streptococcus (S. agalactiae) is identified by:
Answer: B — Positive CAMP test and hippurate hydrolysis
GBS is a major cause of neonatal sepsis and meningitis; pregnant women are screened at 36-37 weeks.
Reference: Bailey & Scott's Diagnostic Microbiology
An alpha-hemolytic, Gram-positive diplococcus that is optochin susceptible and bile soluble is:
Answer: C — Streptococcus pneumoniae
Viridans streptococci are optochin resistant. S. pneumoniae causes pneumonia, meningitis and otitis media; its capsule resists phagocytosis.
Reference: Bailey & Scott's Diagnostic Microbiology
A catalase-negative coccus that grows in 6.5% NaCl and is bile esculin positive and PYR positive is:
Answer: D — Enterococcus
Enterococci cause UTIs and hospital infections; vancomycin-resistant enterococci (VRE) are an important concern. E. faecalis is pyruvate positive; E. faecium is negative.
Reference: Bailey & Scott's Diagnostic Microbiology
A Gram-positive rod with tumbling motility that grows at 4 C and causes neonatal meningitis is:
Answer: A — Listeria monocytogenes
It is catalase positive with narrow beta-hemolysis on blood agar. Sources include soft cheese, deli meats and refrigerated food; risk groups include pregnant women.
Reference: Bailey & Scott's Diagnostic Microbiology
Vomiting a few hours after eating reheated fried rice suggests food poisoning by:
Answer: B — Bacillus cereus
B. cereus produces a heat-stable emetic toxin.
Reference: Bailey & Scott's Diagnostic Microbiology
Large, non-motile, non-hemolytic Gram-positive rods with 'medusa head' colonies from a wool worker suggest:
Answer: C — Bacillus anthracis
Anthrax is a potential bioterrorism agent; notify the reference laboratory and handle with biosafety precautions.
Reference: Bailey & Scott's Diagnostic Microbiology
Gas gangrene with a double zone of beta-hemolysis and a positive Nagler reaction (lecithinase) indicates:
Answer: D — Clostridium perfringens
C. perfringens also causes a common type of food poisoning.
Reference: Bailey & Scott's Diagnostic Microbiology
Diarrhea and pseudomembranous colitis after antibiotic therapy is most often caused by:
Answer: A — Clostridioides (Clostridium) difficile
Test unformed stool for toxin and/or the toxin gene (GDH/toxin EIA, PCR).
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Lockjaw after a deep puncture wound is caused by a neurotoxin from:
Answer: B — Clostridium tetani
C. tetani has 'drumstick' terminal spores. It is prevented with tetanus toxoid vaccine (DTaP, Td).
Reference: Bailey & Scott's Diagnostic Microbiology
Branching, beaded Gram-positive filaments that are partially (weakly) acid-fast, from the lung of an immunocompromised patient, suggest:
Answer: C — Nocardia
Nocardia lives in soil and is inhaled; it is aerobic. Actinomyces is anaerobic, non-acid-fast and forms sulfur granules.
Reference: Bailey & Scott's Diagnostic Microbiology
A catalase-negative Gram-positive rod that produces H2S in TSI, from a skin infection in a butcher or fish handler, is:
Answer: D — Erysipelothrix rhusiopathiae
It causes erysipeloid. Listeria is catalase positive and H2S negative.
Reference: Bailey & Scott's Diagnostic Microbiology
The most common cause of urinary tract infection is:
Answer: A — Escherichia coli
E. coli is indole positive and lactose positive; IMViC ++--.
Reference: Bailey & Scott's Diagnostic Microbiology
Bloody diarrhea followed by hemolytic uremic syndrome in a child is classically caused by:
Answer: B — Shiga toxin-producing E. coli (e.g., O157:H7)
O157:H7 does not ferment sorbitol (colorless on sorbitol-MacConkey). Shigella dysenteriae type 1 can also cause HUS.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
An IMViC pattern of - - + + (indole -, MR -, VP +, citrate +) with mucoid lactose-fermenting colonies suggests:
Answer: C — Klebsiella pneumoniae
Klebsiella is non-motile with a large capsule.
Reference: Bailey & Scott's Diagnostic Microbiology
A urease-positive, H2S-positive Gram-negative rod that swarms over blood agar is:
Answer: D — Proteus
Proteus raises urine pH and causes struvite stones. Swarming does not occur on MacConkey agar.
Reference: Bailey & Scott's Diagnostic Microbiology
TSI shows an alkaline slant, acid butt and H2S (K/A, H2S+) from a stool culture of a patient with diarrhea. The likely pathogen is:
Answer: A — Salmonella
Shigella is K/A without H2S and is non-motile. Proteus mirabilis is also K/A with H2S, but it is urease positive.
Reference: Bailey & Scott's Diagnostic Microbiology
An alkaline slant and alkaline butt (K/K) in TSI indicates:
Answer: B — A non-fermenter such as Pseudomonas
Non-fermenters do not produce acid from any of the sugars in TSI.
Reference: Bailey & Scott's Diagnostic Microbiology
The best test to diagnose typhoid fever in the first week of illness is:
Answer: C — Blood culture
Widal is unreliable. Blood culture also gives antibiotic susceptibility, which is essential with XDR typhoid in Pakistan.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which feature fits Shigella?
Answer: D — Non-motile, non-lactose fermenter, H2S negative
A small infective dose causes bacillary dysentery.
Reference: Bailey & Scott's Diagnostic Microbiology
An oxidase-positive, non-fermenting Gram-negative rod with blue-green pigment and a grape-like odor that grows at 42 C is:
Answer: A — Pseudomonas aeruginosa
It is a major hospital pathogen (burns, ventilators, cystic fibrosis, where mucoid strains are common).
Reference: Bailey & Scott's Diagnostic Microbiology
An oxidase-negative, non-fermenting Gram-negative coccobacillus causing multidrug-resistant ICU infections is:
Answer: B — Acinetobacter baumannii
Stenotrophomonas maltophilia is another oxidase-negative non-fermenter.
Reference: Bailey & Scott's Diagnostic Microbiology
Haemophilus influenzae requires which growth factors?
Answer: C — X factor (hemin) and V factor (NAD)
It grows on chocolate agar or around S. aureus on blood agar (satellitism). Type b (Hib) is the most invasive and is prevented by the Hib conjugate vaccine.
Reference: Bailey & Scott's Diagnostic Microbiology
Gram-negative intracellular diplococci in a urethral discharge from a man indicate:
Answer: D — Neisseria gonorrhoeae
N. gonorrhoeae ferments glucose only; N. meningitidis ferments glucose and maltose. NAAT is used for most genital specimens.
Reference: Bailey & Scott's Diagnostic Microbiology
Patients with deficiency of terminal complement components (C5-C9) are especially prone to:
Answer: A — Neisseria meningitidis infections
The membrane attack complex is needed to kill Neisseria.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
A curved, microaerophilic, strongly urease-positive rod causing peptic ulcer is:
Answer: B — Helicobacter pylori
Diagnose with urea breath test, stool antigen or biopsy. H. pylori is linked to gastric cancer and MALT lymphoma.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
A curved Gram-negative rod that grows best at 42 C in microaerophilic conditions and causes diarrhea is:
Answer: C — Campylobacter jejuni
It is often linked to poultry and can be followed by Guillain-Barre syndrome.
Reference: Bailey & Scott's Diagnostic Microbiology
Undulant fever after drinking raw camel or goat milk suggests:
Answer: D — Brucella
Brucella is a major laboratory-acquired infection risk; warn the lab when it is suspected.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
A rapidly spreading wound infection after a cat or dog bite is classically caused by:
Answer: A — Pasteurella multocida
Bartonella henselae causes cat-scratch disease.
Reference: Bailey & Scott's Diagnostic Microbiology
The most common Gram-negative anaerobic rod in intra-abdominal abscesses is:
Answer: B — Bacteroides fragilis
It is bile resistant and grows on Bacteroides bile esculin (BBE) agar.
Reference: Bailey & Scott's Diagnostic Microbiology
Which specimen is suitable for anaerobic culture?
Answer: C — Pleural fluid or tissue aspirate collected with a needle
Specimens contaminated with normal flora (throat, voided urine, surface swabs, stool) are not suitable.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Tuberculosis is transmitted mainly by:
Answer: D — Inhalation of airborne droplet nuclei
TB specimens are processed in a biosafety cabinet in a BSL-3 (or enhanced BSL-2) laboratory.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which test is positive for M. tuberculosis and helps separate it from most other mycobacteria?
Answer: A — Niacin accumulation
M. bovis needs pyruvate for growth on LJ medium. Rapid molecular tests (e.g., Xpert MTB/RIF) also detect rifampicin resistance.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
A slow-growing mycobacterium whose colonies turn yellow after light exposure (photochromogen) is:
Answer: B — M. kansasii
M. avium complex causes disseminated disease in advanced HIV; M. fortuitum is a rapid grower linked to catheters and wounds.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Syphilis is caused by Treponema pallidum, which:
Answer: C — Cannot be cultured on artificial media; diagnosis uses serology and dark-field microscopy
Secondary syphilis gives a rash including the palms and soles.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Lyme disease is caused by:
Answer: D — Borrelia burgdorferi, transmitted by Ixodes ticks
Borrelia are loosely coiled. Tick-borne and louse-borne relapsing fever are caused by other Borrelia species.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
A tightly coiled spirochete with hooked ends, from contact with water contaminated by rat or animal urine, is:
Answer: A — Leptospira interrogans
Leptospirosis is diagnosed mainly by serology (MAT, IgM) or PCR.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Rocky Mountain spotted fever is caused by Rickettsia rickettsii, transmitted by:
Answer: B — Ticks
Serology (IFA) is the reference method; PCR on skin biopsy helps early. In the Middle East and Mediterranean, R. conorii causes spotted fever.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which organism cannot be grown on routine culture media and is best diagnosed by NAAT?
Answer: C — Chlamydia trachomatis
Chlamydia is an obligate intracellular bacterium. C. psittaci is acquired from birds.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Why is Mycoplasma pneumoniae resistant to penicillins?
Answer: D — It has no cell wall
Macrolides (e.g., azithromycin) are used. It causes 'walking' (atypical) pneumonia and does not Gram stain.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
The lowest antibiotic concentration that inhibits visible bacterial growth is the:
Answer: A — MIC
MIC values are interpreted against CLSI or EUCAST breakpoints. MBC is the lowest concentration that kills 99.9% of the inoculum.
Reference: CLSI M100: Performance Standards for Antimicrobial Susceptibility Testing
In a checkerboard test, a fractional inhibitory concentration (FIC) index of 0.4 indicates:
Answer: B — Synergy
FIC index 0.5 or less = synergy; above 4 = antagonism.
Reference: Bailey & Scott's Diagnostic Microbiology
Rifampicin acts by inhibiting:
Answer: C — Bacterial RNA polymerase
Aminoglycosides (e.g., streptomycin) inhibit protein synthesis at the 30S ribosome; vancomycin inhibits cell wall synthesis in Gram-positive bacteria.
Reference: Bailey & Scott's Diagnostic Microbiology
A calibrated loop of 0.001 mL is used to inoculate urine. 100 colonies grow. The count is:
Answer: C — 100,000 CFU/mL
100 colonies / 0.001 mL = 100,000 (10^5) CFU/mL.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Blood culture bottles contain SPS (sodium polyanethol sulfonate) because it:
Answer: A — Acts as an anticoagulant and inhibits complement, phagocytosis and some antibiotics
Adults should have about 8-10 mL per bottle (20-30 mL per set), with two sets from separate sites.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Coagulase-negative staphylococci in one of four blood culture bottles most likely represent:
Answer: B — Skin contamination during collection
Repeated isolation from several sets, especially with a line or prosthesis, suggests true infection.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology
Which disinfectant is suitable for routine microbiology bench surfaces?
Answer: C — 70% ethanol or freshly prepared sodium hypochlorite (bleach) solution
Hypochlorite is preferred for blood spills; follow the laboratory's biosafety SOP.
Reference: Mahon & Lehman: Textbook of Diagnostic Microbiology