MLT GlobeBacteriology: Neisseria & fastidious
68 Bacteriology MCQs on Neisseria & fastidious with answers and explanations.
A man has purulent urethral discharge. Neisseria gonorrhoeae most commonly gains entry to the body through which route?
Gonococci are sexually transmitted and attach to columnar epithelium of the urethra and cervix; pharyngeal and rectal sites follow specific exposures.
Haemophilus influenzae needs both X factor (haemin) and V factor (NAD). Which medium supports its growth?
Heating blood lyses red cells, releasing X and V factors and inactivating V-factor-degrading enzymes. Sheep blood agar only supports growth as satellite colonies near S. aureus.
A child with meningitis has CSF showing small pleomorphic Gram-negative coccobacilli that grow on chocolate agar but not sheep blood agar, needing X and V factors. The organism is:
H. influenzae needs haemin (X) and NAD (V), both available in chocolate agar. Moraxella is a diplococcus; the others are Gram-positive.
On sheep blood agar, pinpoint colonies grow only close to a streak of Staphylococcus aureus. Which organism typically shows this pattern?
Satellitism: S. aureus releases V factor (NAD) and its haemolysis frees X factor (haemin) from red cells, supporting growth of H. influenzae nearby.
Gram-negative diplococci from a genital swab are screened first with which rapid test, since all Neisseria are positive?
Neisseria species are oxidase-positive (purple within seconds). Catalase is also positive but less discriminating from other genera.
Tiny pleomorphic gram-negative coccobacilli from CSF grow on chocolate agar but on nutrient agar only around an X+V factor disc. Identify them.
H. influenzae requires both haemin (X) and NAD (V). Listeria and Bacillus are gram-positive; E. coli needs no factors.
A painless genital chancre shows motile spirochaetes on dark-ground microscopy. The causative organism causes:
Treponema pallidum causes syphilis and cannot be seen on Gram stain; dark-ground microscopy or serology is used. Chancroid ulcers are painful.
Chocolate agar, made by heating blood agar, is an example of an:
Heating lyses red cells, releasing X and V factors for fastidious organisms like Haemophilus and Neisseria.
Borrelia recurrentis, transmitted by lice, causes:
Antigenic variation produces recurring fever episodes, and spirochaetes are seen in blood films during fever. Q fever is caused by Coxiella burnetii.
For a woman with suspected gonorrhoea, which specimen is preferred for culture of Neisseria gonorrhoeae?
N. gonorrhoeae infects columnar epithelium of the endocervix, so an endocervical swab is best for culture; NAAT also accepts vaginal swabs. Clue cells indicate bacterial vaginosis.
For which specimen is a Gram stain showing intracellular gram-negative diplococci in neutrophils considered sufficient for a presumptive diagnosis of gonorrhoea?
In symptomatic men, urethral Gram stain is highly sensitive and specific. Endocervical, rectal and pharyngeal sites contain commensal Neisseria and other gram-negative flora, so stain alone is not reliable there.
Which handling best preserves Neisseria gonorrhoeae in a genital swab for culture?
Gonococci are very sensitive to cold and drying. Direct inoculation onto pre-warmed selective medium with 3–7% CO2 gives the best recovery; refrigeration kills the organism.
Why should subcultures and suspensions of Neisseria meningitidis be handled inside a biological safety cabinet?
Laboratory workers exposed to aerosols of N. meningitidis have developed fatal infection. Manipulating isolates in a class II cabinet, and offering vaccination to staff, reduces this risk.
The Haemophilus influenzae type b (Hib) vaccine is directed against which capsular antigen?
The type b capsule is made of polyribosylribitol phosphate, and conjugated PRP is the Hib vaccine antigen. Lipo-oligosaccharide is an outer membrane component, not capsule.
The preferred specimen for laboratory diagnosis of pertussis by culture or PCR is:
Bordetella pertussis attaches to ciliated cells of the nasopharynx, so a posterior nasopharyngeal swab or aspirate is best. Cotton is toxic to the organism and calcium alginate inhibits PCR.
For nucleic acid amplification testing (NAAT) for gonorrhoea and chlamydia in men, the preferred non-invasive specimen is:
The first 20–30 mL of voided urine contains organisms washed from the urethra and is the recommended NAAT specimen in men. Midstream urine dilutes urethral material.
A CSF specimen for suspected meningococcal meningitis arrives in the laboratory at night. How should it be held until processing?
N. meningitidis and H. influenzae are sensitive to cold, so CSF for bacterial culture must never be refrigerated. It should be kept at room temperature or incubated at 35 °C.
The HACEK organisms are clinically important mainly because they are a cause of:
HACEK organisms (Haemophilus, Aggregatibacter, Cardiobacterium, Eikenella, Kingella) are oral flora that cause endocarditis and may need longer incubation to grow in blood cultures.
Plates for Neisseria gonorrhoeae culture should be incubated at:
N. gonorrhoeae is capnophilic and sensitive to drying and temperature changes. It grows best at 35–37 °C in 3–7% CO2 with high humidity.
On a Gram stain, Neisseria species typically appear as:
Neisseria are Gram-negative cocci in pairs with their flat sides facing, giving a kidney-bean shape.