MLT GlobeBacteriology: Gram-positive bacilli – page 2
58 Bacteriology MCQs on Gram-positive bacilli with answers and explanations.
Besides antibiotics, which treatment must be given promptly in respiratory diphtheria?
Antitoxin neutralises circulating toxin before it binds to cells; antibiotics alone do not neutralise toxin already formed.
A pregnant woman eats soft cheese; her neonate later develops meningitis. The CSF isolate is most likely a:
Listeria monocytogenes is the classic food-borne cause of meningitis in neonates, the elderly and immunocompromised; it is a Gram-positive, non-sporing, tumbling-motile rod.
Which of these Gram-positive organisms is catalase-positive?
Aerobic spore-forming Bacillus produces catalase. Clostridium (anaerobic), Enterococcus and Streptococcus are catalase-negative.
Sputum from a transplant patient grows chalky colonies on BCYE agar in air after 6 days. Gram stain shows delicate branching, beaded Gram-positive rods that are partially acid-fast. The organism is most likely:
Aerobic growth, fine branching and modified acid-fast positivity point to Nocardia. Actinomyces is anaerobic and not acid-fast; Mycobacterium does not branch; Streptomyces is not acid-fast.
Both Nocardia and Actinomyces form branching Gram-positive rods. Which feature points to Actinomyces?
Actinomyces is anaerobic/microaerophilic and not acid-fast, producing sulphur granules in pus. Nocardia is aerobic and partially acid-fast.
Which genus is NOT an actinomycete (high-G+C Gram-positive group)?
Mycoplasma lacks a cell wall and belongs to the class Mollicutes. Mycobacterium, Nocardia and Actinomyces are actinomycetes.
Which bacterium shows characteristic tumbling motility at 25 °C but is poorly motile at 37 °C?
Listeria expresses flagella at room temperature, giving tumbling motility and an umbrella pattern in semi-solid agar. Y. pestis, Nocardia and Shigella are non-motile.
A wool-sorter develops severe pneumonia and mediastinitis after handling imported fleece. 'Wool-sorter's disease' is which form of anthrax?
Inhaling B. anthracis spores from animal fibres causes haemorrhagic mediastinitis. The 'malignant pustule' is the cutaneous form.
Ascoli's thermoprecipitation test on animal tissue extracts is used to diagnose:
Heat-stable B. anthracis antigen from tissues forms a ring precipitate with specific antiserum, useful when the carcass is decomposed.
A laboratory sets up an Elek immunodiffusion plate using a filter-paper strip soaked in antitoxin. What is being demonstrated?
The Elek test detects diphtheria toxin: toxigenic C. diphtheriae forms precipitin lines where toxin meets antitoxin. Salmonella Typhi antibodies are detected by the Widal test, not the Elek test.
Throat and skin 'diphtheroids' such as Corynebacterium xerosis and C. pseudodiphtheriticum (formerly C. hoffmannii) are distinguished from pathogenic C. diphtheriae mainly because they:
Commensal corynebacteria are gram-positive, non-acid-fast, non-sporing rods like C. diphtheriae; the key difference is lack of toxin production (Elek-negative). 'C. acnes' is now Cutibacterium acnes.
Branching, beaded gram-positive rods from a lung abscess stain red with a modified (weak acid) Kinyoun stain. Most likely:
Nocardia is partially acid-fast when decolourised with 1% sulphuric acid. Actinomyces is also branching but not acid-fast.
A throat isolate forms black colonies surrounded by a brown halo on Tinsdale agar. This appearance suggests:
Tinsdale agar contains potassium tellurite and cystine; C. diphtheriae reduces tellurite (black colony) and has cystinase (brown halo). Most other corynebacteria lack the brown halo.
A neutropenic patient with a central line has several positive blood cultures with a catalase-positive diphtheroid that grows better when lipid is added. It is resistant to most antibiotics except vancomycin. The most likely organism is:
C. jeikeium is lipophilic, multidrug resistant and causes line-related bacteraemia in immunocompromised patients; vancomycin is the drug of choice. Repeated isolation from several sets makes contamination unlikely.
A newborn's CSF grows small beta-haemolytic colonies that look like group B streptococci. The Gram stain shows short gram-positive rods and coccobacilli. Which quick test best separates Listeria from Streptococcus agalactiae?
Listeria is catalase positive, while streptococci are catalase negative. Both organisms can be CAMP positive, hippurate positive and beta-haemolytic, so those tests do not separate them.
A large gram-positive rod from a wool worker's skin lesion forms flat, ground-glass colonies with 'Medusa head' edges. Which pair of results best suggests Bacillus anthracis rather than B. cereus?
B. anthracis is non-haemolytic on sheep blood agar and non-motile, whereas B. cereus is beta-haemolytic and motile. A suspected isolate should be referred to a reference laboratory.
Sputum from a renal transplant patient shows thin, beaded, branching gram-positive filaments. They stain red with a modified Kinyoun stain using 1% sulfuric acid but not with the standard acid-fast stain. The most likely genus is:
Nocardia is partially acid-fast: it keeps carbol fuchsin when a weak acid decolourizer is used. Actinomyces and Streptomyces are not acid-fast, and Mycobacterium resists strong acid-alcohol and does not branch.
A fish handler has a painful purple swelling on the finger. The culture grows a catalase-negative, non-motile gram-positive rod that produces H2S in the butt of a TSI slant. The most likely organism is:
Erysipelothrix causes erysipeloid in people who handle fish or meat and is the only common gram-positive rod that produces H2S in TSI. Listeria is catalase positive and H2S negative.
Empiric treatment of neonatal meningitis includes ampicillin because Listeria monocytogenes is intrinsically resistant to:
Listeria is intrinsically resistant to cephalosporins, so cephalosporin-only regimens fail. Ampicillin (often with gentamicin) is the treatment of choice.
Small colonies grow on bile esculin agar with blackening. The isolate is catalase positive, and the Gram stain shows short gram-positive rods. The most likely organism is:
Listeria hydrolyzes esculin in the presence of bile, like enterococci, but it is a catalase-positive rod. Enterococci are catalase-negative cocci, and group B streptococci are esculin negative.