MLT GlobeBacteriology: Gram-positive bacilli – page 3
58 Bacteriology MCQs on Gram-positive bacilli with answers and explanations.
In a blood smear stained with polychrome methylene blue (M'Fadyean reaction), Bacillus anthracis shows:
The M'Fadyean stain shows the poly-D-glutamic acid capsule of B. anthracis as pink-purple material around blue, square-ended rods. Capsule can also be seen with India ink.
A sentinel laboratory isolates a large, non-hemolytic, non-motile gram-positive rod from blood that looks like Bacillus anthracis. What should the laboratory do?
Sentinel laboratories must not try to fully identify possible bioterrorism agents. Further work is done in a biosafety cabinet and the isolate is referred to a Laboratory Response Network reference laboratory, with public health notified.
A patient with advanced HIV has cavitary pneumonia. Sputum grows salmon-pink, mucoid colonies after 48 hours. The organisms are partially acid-fast gram-positive coccobacilli. The most likely organism is:
Rhodococcus equi forms salmon-pink mucoid colonies, is partially acid-fast and causes cavitary pneumonia in immunocompromised hosts, often after contact with horses or soil. Nocardia forms dry, chalky colonies with branching filaments.
A catalase-negative, non-motile gram-positive rod in long chains grows from the blood of a patient with a central line. It has a very high vancomycin MIC. The most likely genus is:
Lactobacillus is catalase negative, and many clinical species (such as L. rhamnosus) are intrinsically vancomycin resistant. Corynebacterium, Listeria and Bacillus are catalase positive.
A man has weight loss, malabsorption and joint pain. Small-bowel biopsy shows foamy macrophages full of PAS-positive material, and an acid-fast stain is negative. PCR is most likely to detect:
Whipple disease is caused by Tropheryma whipplei, an actinobacterium seen as PAS-positive material in macrophages and confirmed by PCR. Mycobacterium avium complex also stains PAS positive but is acid-fast.
The gene for diphtheria toxin (tox) in Corynebacterium diphtheriae is carried on:
Only strains lysogenized by a beta-corynephage carrying the tox gene produce diphtheria toxin. This is why toxin production must be tested, for example by Elek test or PCR.
A bronchoalveolar lavage is sent with a request to rule out nocardiosis. How long should routine culture plates be kept?
Nocardia grows slowly and may take a week or longer to form chalky colonies, so plates should be kept for up to 2–3 weeks. Selective media such as BCYE also help.
For culture of Corynebacterium diphtheriae in suspected respiratory diphtheria, the best specimen is:
Organisms are most numerous under the edge of the pseudomembrane, so a swab from there, together with a nasopharyngeal swab, gives the best yield. Blood cultures are rarely positive.
After an eye injury with a soil-covered metal fragment, a patient develops rapidly progressive endophthalmitis. Vitreous fluid grows a large, beta-hemolytic, motile, spore-forming gram-positive rod that is lecithinase positive. The most likely organism is:
Bacillus cereus causes severe post-traumatic endophthalmitis. It is motile, beta-hemolytic and lecithinase positive. B. anthracis is non-motile and non-hemolytic; C. perfringens is anaerobic and non-motile.
The capsule of Bacillus anthracis is unusual because it is made of:
Unlike most bacterial capsules, which are polysaccharide, the anthrax capsule is a polypeptide of D-glutamic acid.
Diphtheria toxin damages cells by:
Diphtheria toxin ADP-ribosylates elongation factor 2, halting host protein synthesis; blocking acetylcholine release is the action of botulinum toxin.
A single blood culture bottle grows a non-diphtheria Corynebacterium species in a patient with no line or implant. This most likely represents:
Diphtheroids are common skin flora; growth in only one bottle without a risk factor usually indicates contamination.
Gram stain of pus from a brain abscess shows beaded, branching Gram-positive filaments that are partially acid-fast with modified Kinyoun stain. Which aerobic actinomycete is likely?
Nocardia is an aerobic branching actinomycete that is partially acid-fast and spreads from lung to brain. Actinomyces is anaerobic and not acid-fast; Streptomyces is non-acid-fast; M. tuberculosis does not branch.
A patient has cystitis with alkaline urine and struvite encrustation of the bladder wall. Urine grows a slow-growing, lipophilic diphtheroid that is strongly urease positive within minutes. The most likely organism is:
C. urealyticum is a strong urease producer that makes urine alkaline and causes encrusted cystitis. It grows slowly and can be missed if urine cultures are read only at 24 hours.
A partially acid-fast aerobic actinomycete from a lung biopsy grows in lysozyme broth, while a Streptomyces control strain does not. This result supports identification as:
Nocardia is lysozyme resistant and keeps growing in lysozyme broth, while Streptomyces is inhibited. This, with partial acid-fastness, helps separate Nocardia from other aerobic actinomycetes.
A teenager has pharyngitis with a scarlet fever-like rash. Throat culture grows small beta-hemolytic colonies of a catalase-negative gram-positive rod that inhibits the hemolysis of S. aureus beta-lysin (reverse CAMP positive). The most likely organism is:
Arcanobacterium haemolyticum causes pharyngitis with rash in teenagers and young adults. It is a catalase-negative rod whose phospholipase D blocks S. aureus beta-lysin. S. pyogenes is a coccus.
Which result helps separate Corynebacterium diphtheriae from most other Corynebacterium species?
C. diphtheriae is pyrazinamidase negative (and cystinase positive), while most other corynebacteria are pyrazinamidase positive. Catalase, lack of motility and club shape are shared by most corynebacteria.
A farmer in Latin America has a swollen foot with draining sinuses that discharge small granules. The granules contain branching, partially acid-fast gram-positive filaments that grow aerobically. The most likely cause is:
Nocardia brasiliensis is a leading cause of actinomycetoma in the Americas. It is aerobic and partially acid-fast. Actinomyces is anaerobic and not acid-fast; Madurella is a fungus causing eumycetoma.