Aerobic Gram-Positive Rods, Spirochetes, Mycoplasmas and Ureaplasmas, and Chlamydia Flashcards
Large gram-positive spore-forming rods growing on blood agar as large, raised, β-hemolytic colonies that spread and appear as frosted green-gray glass are most likely:
A. Pseudomonas spp.
B. Bacillus spp.
C. Corynebacterium spp.
D. Listeria spp.
B
The only spore former listed is the Bacillus spp., which grow as large, spreading colonies on blood agar plates. Pseudomonas spp. are gram-negative rods; Corynebacterium spp. appear as small, very dry colonies on BAP; Listeria spp. appear as very small β-hemolytic colonies on BAP, resembling Streptococcus species.
Bacillus anthracis and Bacillus cereus can best be differentiated by which tests?
A. Motility and β-hemolysis on a blood agar plate
B. Oxidase and β-hemolysis on a blood agar plate
C. Lecithinase and glucose
D. Lecithinase and catalase
A
Both species of Bacillus are catalase and lecithinase positive and produce acid from glucose. B. cereus is β-hemolytic and motile, but B. anthracis is neither.
Which is the specimen of choice for proof of food poisoning by Bacillus cereus?
A. Sputum
B. Blood
C. Stool
D. Food
D
The best specimen is the suspected food itself. Stool cultures are not useful because B. cereus is part of the normal fecal flora. The suspected food can be the source of food poisoning by B. cereus if 100,000 or greater organisms per gram of infected food are demonstrated.
A suspected Bacillus anthracis culture obtained from a wound specimen produced colonies that had many outgrowths (Medusa-head appearance), but were not β-hemolytic on sheep blood agar. Which test should be performed next?
A. Penicillin (10-unit) susceptibility test
B. Lecithinase test
C. Glucose test
D. Motility test
A
The best differentiating test to perform on a suspected B. anthracis culture is the 10-unit
penicillin disk test. B. anthracis is susceptible but other Bacillus spp. are not. Organisms suspected to be B. anthracis should be sent to a reference laboratory for final confirmation. All tests should be performed in a biological safety hood, and personnel should wear protective clothing to reduce risk from possible production of aerosols.
Which of the following tests should be performed for initial differentiation of Listeria monocytogenes from group B streptococci?
A. Gram stain, motility at room temperature,
catalase
B. Gram stain, CAMP test, H2S/TSI
C. Oxidase, CAMP test, glucose
D. Oxidase, bacitracin
A
Streptococcus spp. are catalase negative and
L. monocytogenes is catalase positive. L. monocytogenes appears on the Gram stain smear as gram-positive short, thin, diphtheroidal shapes, whereas streptococci usually appear as short, gram-positive chains. The reactions shown in the following chart differentiate L. monocytogenes from the group B streptococci.
