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6 Cards in this Set

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Listeria monocytogenes
gram stain
lab notes
lab diag
1 .Colonizes many animals and presumably humans, but usually a pathogen when isolated from patients.

2. Short, gram-positive rods, sometimes in pairs, may look like corynebacteria or streptococci.
3. Significance:
A major cause of bacteremia and meningitis in newborns, occasional cause of the same in imrnunocompromised adults. In both of these groups this organism may not be correctly identified: patients may have normal appearing spinal fluid and physicians may think they just have a viral syndrome.
4. The laboratory may misidentify the organism as a Corynebacterium (usually nonpathogenic) or as Group B Streptococcus (another cause of meningitis in newborn) by gram stain and colony morphology.

Laboratory Diagnosis:
Culture of blood, CSF. Should see small zone of beta-hemo lysis on sheep blood agar (different than corynebacteria).
Catalase positive (different than streptococci) tumbling, end-over-end motility in broth at 25 degrees C (different than both)
corynebacterium species

location and #


referred to as


C. jeikeium

interretation of positive cultures
Corynebacterium species:
1 . Present in large numbers on skin as normal flora
2. Small pleomorphic gram-positive rods forming palisades or "Chinese characters"
3. commonly referred to as diphtheroids
4. Significance:
(a) Most often contaminants when present in cultures
(b) Can cause endocarditis on prosthetic heart valves
(c) C. jeikeium causes lymphadenitis, abscesses, meningitis, skin infections and bacteremia in immunosuppressed patients, especially neutropenic cancer patients.
(d) C. diphtheriae causes diphtheria (see below)
5. Interpretation of positive cultures:
When multiple cultures of same site are positive and patient is a compromised host, assume is causing disease


clinical disease

1 . Childhood immunization usually adequate protection
2. Clinical disease
Pharyngitis with thick, leathery, gray membrane with surrounding edema. May extend down to larynx, trachea, bronchi. May have cutaneous ulcers instead of pharyngitis. Organism produces a protein exotoxin that affects predominantly myocardium and peripheral nervous system. Exotoxin produced only when organism contains a bacteriophage. Death caused by either asphyxiation or myocarditis.

3. Diagnosis
Initially clinical. Therapy must be started immediately.
Confirmatory diagnosis: Throat culture on special laboratory media.
Lab must be notified if diphtheria suspected for lab to set up appropriate media.
Lab diagnosis: methylene blue stain shows metachromatic granules; culture
shows growth on Loeffler or Tinsdale agar. Growth must be confirmed as
clinically significant by toxicity assay.
Erysipelothrix rhusiopathiae


gram stains


laboratory diagnosis
1 . Colonizes domestic animals and causes infections in humans, but rarely in U.S.
2. Small gram-positive rods that form long filaments
3. Significance
Causes ulcerating, erythematous skin infections called erysipeloid on exposed, abraided skin in fisherman, butchers, veterinarians.
4. Laboratory diagnosis: cultures of skin lesions, blood
Clinical diagnosis may be more helpful in right setting
Alpha hemolytic colonies on sheep blood agar with characteristic gram-stain Produces hydrogen sulfide gas (unique among gram-positive bacilli)
bacillus species




laboratory diagnosis

Bacillus species

1. Found everywhere in the environment and often contaminate cultures
2. Large, gram-positive rods that form spores (unique among aerobic gram-positive rods)
3. Significance
Can cause bacteremia and endocarditis in drug abusers and immunocompromised

B. anthracis causes anthrax

B. cereus very commonly causes food poisoning, usually related to ingestion of Trndefcooked?rtc^)

4. Laboratory diagnosis (except anthrax): blood cultures
Large, often beta hemolytic, irregular colonies on sheep blood agar, characteristic gram stain

5. Anthrax
Rare in U.S., but common in other parts of the world. Is a possible bioterrorist agent. Can be in 3 forms: cutaneous (most common natural form), inhalation (the type that would occur with terrorist release of spores), and gastrointestinal. Cutaneous form is a nonpainful ulcer at a site of inoculation characterized by the formation of a black eschar in the ulcer. Inhalation disease would present with severe shortness of breath and an overall appearance of extreme toxicity with a widenedjnediastinum. GI is extremely rare, would present with abdominal pain arid bloody diarrhea
lactobacillus species




Lactobacillus species
1. Major normal flora of lower gastrointestinal tract, and genital tract. Ingredient in yogurt
2. long, slender, gram-positive rods
3. Significance
Virtually always protective—infections are associated with failure to see or grow lactobacilli. Rare case reports of endocarditis with lactobacilli exist
4. Diagnosis: Small, alpha hemolytic colonies on blood agar. Typical gram-stain appearance and negative catalase.