MLT GlobeBacteriology: Neisseria & fastidious – page 3
68 Bacteriology MCQs on Neisseria & fastidious with answers and explanations.
A rapid nitrocefin (chromogenic cephalosporin) test is routinely used on which isolate to predict ampicillin resistance?
Beta-lactamase detection by nitrocefin predicts ampicillin resistance in H. influenzae. Pneumococcal penicillin resistance is due to altered PBPs, not beta-lactamase.
A gram-negative diplococcus from a urethral culture gives immediate, strong bubbling when a drop of 30% hydrogen peroxide is added. This superoxol result is most typical of:
N. gonorrhoeae gives a strong (4+), immediate superoxol reaction. N. lactamica and N. cinerea give weak reactions, and Kingella is catalase negative. (Moraxella catarrhalis can also react strongly, so superoxol is only presumptive.)
An oxidase-positive gram-negative diplococcus from a middle-ear aspirate is asaccharolytic, DNase positive and butyrate esterase positive. Colonies slide intact across the agar when pushed. It is:
M. catarrhalis produces no acid from sugars, is DNase and butyrate esterase positive, and forms 'hockey puck' colonies. N. meningitidis produces acid from glucose and maltose.
A man has a painful, soft genital ulcer with tender inguinal lymph nodes. A Gram stain of the ulcer base shows small gram-negative coccobacilli in parallel chains ('school of fish'). The most likely organism is:
H. ducreyi causes chancroid, a painful ulcer, and shows chains of coccobacilli. Syphilitic chancres are painless, and K. granulomatis shows Donovan bodies inside macrophages.
A wound from a human bite grows an oxidase-positive, catalase-negative gram-negative rod that pits the agar and smells of bleach after 48 hours in CO2. It is most likely:
Eikenella corrodens, a HACEK member of oral flora, pits agar and has a bleach-like odour. Pasteurella and Capnocytophaga are linked to animal bites.
Buffered charcoal yeast extract (BCYE) agar supports Legionella mainly because it supplies:
Legionella needs L-cysteine and is helped by soluble iron; charcoal removes toxic oxygen radicals. It does not grow on standard blood agar, which lacks enough cysteine.
A patient with severe pneumonia has a negative Legionella urinary antigen test, but Legionella is later grown on BCYE. The best explanation is that the urinary antigen test:
Most urinary antigen kits detect only L. pneumophila serogroup 1. Infections due to other serogroups or species are missed, so culture on BCYE remains important.
A shepherd who drinks unpasteurized goat milk has undulating fever for weeks. After 4 days, blood cultures grow tiny gram-negative coccobacilli that are oxidase, catalase and urease positive. The most likely organism is:
Brucella is a slow-growing, tiny coccobacillus that is oxidase, catalase and urease positive; B. melitensis is linked to goat and sheep milk products and causes undulant fever. It is a major cause of laboratory-acquired infection, so work must move to a biosafety cabinet. Francisella is oxidase negative.
Modified Thayer-Martin agar contains vancomycin, colistin, nystatin and trimethoprim. Trimethoprim is added to inhibit:
Trimethoprim lactate inhibits the swarming of Proteus. Vancomycin inhibits gram-positive bacteria, colistin inhibits most gram-negative rods and commensal Neisseria, and nystatin inhibits yeasts.
A patient has persistent gonococcal urethritis after ceftriaxone treatment. The initial diagnosis was made by NAAT. What should the laboratory recommend?
NAATs do not provide an isolate for susceptibility testing. Suspected treatment failure needs culture so that antimicrobial resistance, including to ceftriaxone, can be tested and reported.
A nasopharyngeal isolate is an oxidase-positive gram-negative diplococcus that grows on modified Thayer-Martin agar. It produces acid from glucose and maltose and is ONPG positive. The most likely organism is:
N. lactamica resembles N. meningitidis but is beta-galactosidase (ONPG) positive and uses lactose. N. meningitidis is ONPG negative; M. catarrhalis is asaccharolytic.
A 20-year-old has his third episode of invasive meningococcal disease. Which laboratory test is most useful to look for an underlying cause?
Deficiency of terminal complement components (C5–C9) greatly increases the risk of recurrent Neisseria infections. CH50 screens the whole classical pathway, including these components.
A Haemophilus isolate on a medium without X or V factors grows around the V disk and the XV disk, but not around the X disk. The most likely species is:
Growth with V alone shows a V-only requirement, as in H. parainfluenzae. H. influenzae and H. haemolyticus need both X and V factors; H. ducreyi needs X only.
Nasopharyngeal specimens for Bordetella pertussis culture are plated on Regan-Lowe charcoal agar. After 3–5 days, typical colonies appear:
B. pertussis forms tiny, domed, shiny colonies resembling mercury drops or bisected pearls on Regan-Lowe or Bordet-Gengou agar. Black colonies with a brown halo describe C. diphtheriae on Tinsdale agar.
A rabbit hunter has a skin ulcer and tender axillary nodes. Tiny, faintly staining gram-negative coccobacilli grow after 3 days on chocolate agar and BCYE, but not on sheep blood agar. The most likely organism is:
Francisella tularensis causes ulceroglandular tularemia after handling rabbits and needs cysteine, so it grows on chocolate agar or BCYE. It is highly infectious, so work must be done in a biosafety cabinet.
A child has had tender axillary lymphadenopathy for 3 weeks after scratches from a new kitten. Which laboratory test is most useful to confirm the likely cause?
Cat-scratch disease is caused by Bartonella henselae, which grows very slowly and is rarely cultured. Serology (IgG/IgM) or PCR of node tissue is used for diagnosis.
When Brucella is suspected in a blood culture, what should the clinician do?
Brucella is one of the most common causes of laboratory-acquired infection. Early warning lets staff use a biosafety cabinet and avoid aerosol-producing steps such as sniffing plates.
In carbohydrate utilisation tests, Neisseria meningitidis produces acid from:
N. meningitidis uses glucose and maltose; N. gonorrhoeae uses glucose only, and N. lactamica also uses lactose.
Which Neisseria meningitidis serogroup has historically caused the large epidemics of the African 'meningitis belt'?
Serogroup A caused the major meningitis-belt epidemics, which fell sharply after mass conjugate vaccination.
Which CSF pattern is typical of acute bacterial meningitis?
Bacteria and neutrophils consume glucose and inflammation raises protein; viral meningitis usually shows lymphocytes with normal glucose.