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. Bacillus spp.
Note: 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. Motility and β-hemolysis on a blood agar plate
Note: 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. Food
Note: 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. Penicillin (10-unit) susceptibility test
Note: 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. Gram stain, motility at room temperature, catalase
Note: 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.
Culture of a finger wound specimen from a meat packer produced short gram-positive bacilli on a blood agar plate with no hemolysis. Given the
following test results at 48 hours, what is the most likely identification?
Catalase = Neg H2S/TSI = +
Motility (wet prep) = Neg
Motility (media) = Neg (bottle-brush growth in stab culture)
A. Bacillus cereus
B. Listeria monocytogenes
C. Erysipelothrix rhusiopathiae
D. Bacillus subtilis
C. Erysipelothrix rhusiopathiae
Note: E. rhusiopathiae is catalase negative, whereas the other three organisms are catalase positive.
E. rhusiopathiae is seen primarily as a skin infection on the fingers of meat and poultry workers. Colonies
growing on blood agar are small and transparent, may be either smooth or rough, and are often surrounded by a green tinge. E. rhusiopathiae is
characterized by H2S production in the butt of a TSI slant, which differentiates it from other catalase-negative, gram-positive rods.
A non–spore-forming, slender gram-positive rod forming palisades and chains was recovered from a
vaginal culture and grew well on tomato juice agar. The most likely identification is:
A. Lactobacillus spp.
B. Bacillus spp.
C. Neisseria spp.
D. Streptococcus spp.
A. Lactobacillus spp.
Note: Lactobacillus spp. produce both long, slender rods or short coccobacilli that form chains. Lactobacillus spp.
are part of the normal flora of the vagina (are not considered a pathogen) and are sometimes confused with the streptococci.
A Corynebacterium species recovered from a throat culture is considered a pathogen when it produces:
A. A pseudomembrane of the oropharynx
B. An exotoxin
C. Gray-black colonies with a brown halo on Tinsdale’s agar
D. All of these options
D. All of these options
Note: Corynebacterium species recovered from a throat culture are usually considered part of the normal
throat flora. C. diphtheriae is an exception and should be suspected when one of the conditions described
occurs. In this event, direct inoculation on Loeffler serum medium or tellurite medium and the following biochemical tests should be performed to confirm the identification of C. diphtheriae.
Gelatin hydrolysis = Neg Catalase = +
Motility = + Urease = +
Acid from glucose = + Carbohydrate
fermentation = +
A presumptive diagnosis of Gardnerella vaginalis can be made using which of the following findings?
A. Oxidase and catalase tests
B. Pleomorphic bacilli heavily colonized on vaginal epithelium
C. Hippurate hydrolysis test
D. All of these options
D. All of these options
Note: A Gram stain smear from a vaginal secretion showing many squamous epithelial cells loaded with
pleomorphic gram-variable (positive and negative) bacilli is considered presumptive for G. vaginalis. Such
cells are called clue cells. Other important findings are:
β-Hemolysis on BAP = +
Catalase = Neg Oxidase = Neg
Hippurate hydrolysis = +
A gram-positive branching filamentous organism recovered from a sputum specimen was found to
be positive with a modified acid-fast stain method. What is the most likely presumptive identification?
A. Bacillus spp.
B. Nocardia spp.
C. Corynebacterium spp.
D. Listeria spp.
B. Nocardia spp.
Note: Nocardia spp. should be suspected if colonies that are
partially acid fast by the traditional method are positive with the modified acid-fast method using Kinyoun stain and 1% sulfuric acid as the decolorizing agent. The other organisms listed are negative for
acid-fast stain.
Routine laboratory testing for Treponema pallidum involves:
A. Culturing
B. Serological analysis
C. Acid-fast staining
D. Gram staining
B. Serological analysis
Serological tests of the patient’s serum for evidence of syphilis are routinely performed, but culturing is
not because research animals must be used for inoculation of the suspected spirochete. T. pallidum
does not stain by either the Gram or acid-fast technique. Darkfield microscopy for direct visualization or indirect immunofluorescence using
fluorescein-conjugated antihuman globulin (the fluorescent treponemal antibody-absorption test, FTA-ABS) may be used to identify syphilis. Newer tests for specific antibodies to T. pallidum are available in a wide range of immunoassay formats
including chemiluminescence and point-of-care immunochromatography. T. pallidum infection can be diagnosed by PCR. Because the bacterium is fastidious, blood samples should be collected in
EDTA, CSF should be frozen and sent on dry ice, and samples analyzed as soon as possible.
Spirochetes often detected in the hematology laboratory, even before the physician suspects the infection, are:
A. Borrelia spp.
B. Treponema spp.
C. Campylobacter spp.
D. Leptospira spp.
A. Borrelia spp.
Note: Borrelia spp. are often seen on Wright’s-stained smears of peripheral blood as helical bacteria with 3–10 loose coils. They are gram negative but stain well with Giemsa’s stain.
Which of the following organisms is the cause of Lyme disease?
A. Treponema pallidum
B. Neisseria meningitidis
C. Babesia microti
D. Borrelia burgdorferi
D. Borrelia burgdorferi
Note: Lyme disease may result in acute arthritis and meningitis and is caused by B. burgdorferi. This
spirochete is carried by the deer tick belonging to the Ixodes genus (I. dammini in the Eastern and
North-central United States and I. pacificus in the Northwest United States). The life cycle of the tick
involves small rodents such as the white-footed mouse and the white-tailed deer.
The diagnostic method most commonly used for the identification of Lyme disease is:
A. Serology
B. Culture
C. Gram stain
D. Acid-fast stain
A. Serology
Note: Serological analysis using immunofluorescence or an enzyme immunoassay is the method of choice
for diagnosis of Lyme disease. Titers of IgM remain high throughout the infection. B. burgdorferi can be cultured directly from lesions, and darkfield microscopy can be used for detection of spirochetes in blood cultures after 2–3 weeks of incubation
at 34°C–37°C.
Primary atypical pneumonia is caused by:
A. Streptococcus pneumoniae
B. Mycoplasma pneumoniae
C. Klebsiella pneumoniae
D. Mycobacterium tuberculosis
B. Mycoplasma pneumoniae
Note: A common cause of respiratory tract illness, M. pneumoniae, generally causes a self-limited infection (3–10 days) and usually does not require
antibiotic therapy. M. pneumoniae can be cultured from the upper and lower respiratory tracts onto specially enriched (diphasic) media, but is most
frequently diagnosed by the change in antibody titer from acute to convalescent serum using enzyme
immunoassay or other serological methods.
