Anaerobic Bacteria Flashcards
Obligate anaerobes, facultative anaerobes, and microaerophiles are terms referring to bacteria that require:
A. Increased nitrogen
B. Decreased CO2
C. Increased O2
D. Decreased O2
D. Decreased O2
Note: The anaerobic bacteria are subdivided according to their requirement for O2. Obligate anaerobes are killed by exposure to atmospheric O2 for 10 min or longer. Facultative anaerobes grow under aerobic or anaerobic conditions. Microaerophilic organisms do not grow in an aerobic incubator on solid media and only minimally under anaerobic conditions. However, they will grow in minimal oxygen (5% O2). Superoxide dismutase (SOD) is produced by many anaerobes, which catalyzes the conversion of superoxide radicals to less toxic H2O2 and molecular O2.
Which of the following most affects the oxidation–reduction potential (Eh or redox potential) of media for anaerobic bacteria?
A. O2
B. Nitrogen
C. pH
D. Glucose
C. pH
Note: The Eh is most affected by pH and is expressed at pH 7.0. In cultivating anaerobic bacteria, reducing agents such as thioglycollate and L-cysteine are added to anaerobic transport and culture media in
order to maintain a low Eh. Certain anaerobes do not grow in the media above a specific critical Eh level.
Which of the following is the medium of choice for the selective recovery of gram-negative anaerobes?
A. Kanamycin–vancomycin (KV) agar
B. Phenylethyl alcohol (PEA) agar
C. Cycloserine–cefoxitin–fructose agar (CCFA)
D. THIO broth
A. Kanamycin–vancomycin (KV) agar
Note: KV allows the growth of Bacteroides spp., Prevotella spp., and Fusobacterium spp. and inhibits most facultative anaerobic gram-negative rods and gram-positive bacteria (both aerobic and anaerobic). PEA inhibits facultative gram-negative bacteria but
will support gram-positive aerobes and anaerobes and gram-negative obligate anaerobes. CCFA is selective for C. difficile from stool, while THIO broth supports gram-positive and gram-negative aerobes and anaerobes.
Anaerobic bacteria are routinely isolated from all of the following types of infections except:
A. Lung abscesses
B. Brain abscesses
C. Dental infections
D. Urinary tract infections
D. Urinary tract infections
Note: The incidence of anaerobic bacteria recovered from the urine is approximately 1% of isolates. The
other three types of infection are associated with a 60%–93% incidence of anaerobic recovery. Urine is not cultured routinely under anaerobic conditions unless obtained surgically (e.g., suprapubic aspiration).
Methods other than packaged microsystems used to identify anaerobes include:
A. Antimicrobial susceptibility testing
B. Gas–liquid chromatography (GLC)
C. Special staining
D. Enzyme immunoassay
B. Gas–liquid chromatography (GLC)
Note: Anaerobic bacteria can be identified by analysis of metabolic products using gas–liquid
chromatography. Results are evaluated along with Gram staining characteristics, spore formation,
and cellular morphology in order to make the identification.
Which broth is used for the cultivation of anaerobic bacteria in order to detect volatile fatty acids as an aid to identification?
A. Prereduced peptone–yeast extract–glucose (PYG)
B. THIO broth
C. Gram-negative (GN) broth
D. Selenite (SEL) broth
A. Prereduced peptone–yeast extract–glucose (PYG)
Note: Peptone yeast and chopped meat with carbohydrates support the growth of anaerobic bacteria. The end products from the metabolism of
the peptone and carbohydrates are volatile fatty acids that help to identify the bacteria. After incubation, the broth is centrifuged, and the supernatant injected into a gas–liquid chromatograph. Peaks for acetic,
butyric, or formic acid, for example, can be identified by comparison to the elution time of volatile organic acid standards.
A gram-positive spore-forming bacillus growing on sheep-blood agar anaerobically produces a double
zone of β-hemolysis and is positive for lecithinase. What is the presumptive identification?
A. Bacteroides ureolyticus
B. Bacteroides fragilis
C. Clostridium perfringens
D. Clostridium difficile
C. Clostridium perfringens
Note: C. perfringens produces a double zone of β-hemolysis on blood agar, which makes identification relatively easy. The inner zone of
complete hemolysis is caused by a θ-toxin and the outer zone of incomplete hemolysis is caused
by an α-toxin (lecithinase activity). The Bacteroides spp. are gram-negative bacilli, and C. difficile is lecithinase negative and does not produce a
double zone of β-hemolysis.
Egg yolk agar is used to detect which enzyme produced by Clostridium species?
A. Lecithinase
B. β-Lactamase
C. Catalase
D. Oxidase
A. Lecithinase
Note: Egg yolk agar (modified McClung’s or neomycin egg yolk agar) is used to determine the presence of
lecithinase activity, which causes an insoluble, opaque, whitish precipitate within the agar. Lipase activity is indicated by an iridescent sheen or pearly layer on the surface of the agar.
Which of the following organisms will display lipase activity on egg yolk agar?
A. Clostridium botulinum
B. Clostridium sporogenes
C. Clostridium novyi (A)
D. All of these options
D. All of these options
Note: Lipase is produced by some Clostridium spp. and is seen as an iridescent pearly layer on the surface of the colonies that extends onto the surface of the egg yolk agar medium surrounding them. C. perfringens, the
most frequently isolated Clostridium species, is negative for lipase production.
Which spore type and location is found on Clostridium tetani?
A. Round, terminal spores
B. Round, subterminal spores
C. Ovoid, subterminal spores
D. Ovoid, terminal spores
A. Round, terminal spores
Note: Spore appearance and location, along with Gram stain morphology, aids in distinguishing the Clostridium spp. Round, terminal spores (drumstick spores) are demonstrated when C. tetani is grown in chopped meat with glucose broth. Recognition of spores is particularly important because C. tetani sometimes appears as gram negative.
Gram-positive bacilli recovered from two blood cultures from a 60-year-old diabetic patient gave the following results:
Spores seen = Neg
Hemolysis = + (double zone)
Motility = Neg Lecithinase = +
Volatile acids by GLC (PYG) = acetic acid (A) and butyric acid (B)
What is the most likely identification?
A. Clostridium tetani
B. Clostridium perfringens
C. Clostridium novyi (B)
D. Clostridium sporogenes
B. Clostridium perfringens
Note: Spores are generally not demonstrated from clinical specimens containing C. perfringens, which is the only species producing a double zone of hemolysis. The reactions in the chart above distinguish the four
species listed.
Which mechanism is responsible for botulism in infants caused by Clostridium botulinum?
A. Ingestion of spores in food or liquid
B. Ingestion of preformed toxin in food
C. Virulence of the organism
D. Lipase activity of the organism
A. Ingestion of spores in food or liquid
Note: Infant botulism is the most frequent form occurring in the United States. Epidemiological studies have
demonstrated that infant botulism results from the ingestion of spores via breastfeeding or exposure to
honey. Preformed toxin has not been detected in food or liquids taken by the infants. C. botulinum multiplies in the gut of the infant and produces the
neurotoxin in situ.
The classic form of foodborne botulism is characterized by the ingestion of:
A. Spores in food
B. Preformed toxin in food
C. Toxin H
D. All of these options
B. Preformed toxin in food
Note: Foodborne botulism in adults and children is caused by ingestion of the preformed toxin (botulinum
toxins A, B, E, and F) in food. The neurotoxins of C. botulinum are protoplastic proteins made during
the growing phase and released during lysis of the organisms. Confirmation of botulism is made by
demonstration of the toxin in serum, gastric, or stool specimens.
Which test is performed in order to confirm an infection with Clostridium botulinum?
A. Toxin neutralization
B. Spore-forming test
C. Lipase test
D. Gelatin hydrolysis test
A. Toxin neutralization
Note: C. botulinum and C. sporogenes have similar characteristics biochemically (see the following chart), and definitive identification of C. botulinum is made by the toxin neutralization test for its neurotoxins in serum or feces. Specimens should be kept at 4°C and sent to the CDC for culture and toxin assays.
Which Clostridium spp. causes pseudomembranous colitis or antibiotic-associated colitis?
A. C. ramosum
B. C. difficile
C. C. perfringens
D. C. sporogenes
B. C. difficile
Note: C. difficile is also implicated in hospital acquired diarrhea and colitis. Clinical testing for C. difficile includes culture and cytotoxin testing. Because
culture takes 3 days and will detect nontoxigenic strains that do not cause diarrheal disease, immunoassays using antibodies against either the
A toxin or both the A and B toxins are most frequently employed. Assays detecting both toxins are only slightly more sensitive, since infections producing only B toxin are infrequent. The cytotoxin assay requires that specimens be shipped to a reference
laboratory on dry ice or kept at 4°C–6°C if done in-house.
