MLT GlobeBacteriology: Neisseria & fastidious – page 4
68 Bacteriology MCQs on Neisseria & fastidious with answers and explanations.
Rat-bite fever (sodoku) in Asia, with a short, rigid spiral organism seen by dark-ground microscopy, is caused by:
Spirillum minus causes sodoku and is not a Borrelia. B. duttonii and B. recurrentis cause tick- and louse-borne relapsing fever.
A Haemophilus isolate produces red fluorescence under UV light in the porphyrin (delta-aminolevulinic acid) test. This result means the isolate:
A positive porphyrin test shows the organism can make porphyrins and heme from ALA, so it does not need X factor (e.g. H. parainfluenzae). H. influenzae is porphyrin negative because it requires X factor.
A 2-year-old has septic arthritis of the knee. Routine plating of the joint fluid shows no growth. Which step most improves recovery of the most likely fastidious organism?
Kingella kingae is a major cause of bone and joint infection in children under 4 and grows poorly on plates. Inoculating joint fluid into blood culture bottles (or using PCR) greatly improves detection.
A patient with endocarditis has blood cultures that grow a fastidious gram-negative rod. After 48 hours, colonies stick to the agar and show a star-shaped internal structure. It is catalase positive. The most likely organism is:
A. actinomycetemcomitans, a HACEK organism, forms adherent colonies with a star-shaped centre and is catalase positive. Cardiobacterium, Eikenella and Kingella are catalase negative.
A patient with fever, rash and arthritis after a rat bite has negative blood cultures in bottles containing sodium polyanethol sulfonate (SPS). The best explanation is that:
Streptobacillus moniliformis, the main cause of rat-bite fever, is inhibited by SPS in most blood culture bottles. Culture needs SPS-free media enriched with serum or blood.
Before plating sputum on BCYE agar for Legionella, a brief acid treatment (pH about 2.2) may be used. Its purpose is to:
Legionella tolerates short exposure to acid better than most respiratory flora, so acid pretreatment reduces overgrowth by contaminants. Selective BCYE with antibiotics serves the same aim.
A Haemophilus influenzae isolate is beta-lactamase negative but ampicillin resistant. The most likely mechanism is:
Beta-lactamase-negative ampicillin-resistant (BLNAR) H. influenzae have mutations in the ftsI gene encoding PBP3. TEM-1 and ROB-1 are beta-lactamases and would give a positive test.
Which enzyme test separates Neisseria meningitidis (positive) from N. gonorrhoeae and N. lactamica?
In chromogenic enzyme substrate tests, gamma-glutamyl aminopeptidase is positive for N. meningitidis. N. gonorrhoeae is prolyl aminopeptidase positive and N. lactamica is beta-galactosidase positive.