ken b, waites, m.d. f(aam) professor of pathology director of clinical microbiology gram-positive...
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Ken B, Waites, M.D. F(AAM)
Professor of Pathology
Director of Clinical Microbiology
Gram-positive Bacilli
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Objectives• To review and discuss:
• microbiological characteristics• epidemiology• virulence factors• associated diseases • laboratory detection
of clinically important gram-positive bacilli. Organism groups to be discussed include:
– Bacillus– Listeria– Erysipelothrix– Corynebacterium– Rhodococcus– Nocardia– Tropheryma
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Bacillus
• Ubiquitous in soil worldwide• Endospore-forming• Aerobic/facultative anaerobic • Gram-positive bacilli• Can be gram-variable• Often hemolytic & motile
– (except B. anthracis)
• Usually catalase positive
B. cereus - Gray, spreading -hemolytic colonies on SBA
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Bacillus Endospores
• Thick walled structures formed by vegetative cells
• Resistant to radiation, chemicals, heat, desiccation (dipicolinic acid)
• Steam autoclaving necessary for destruction
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Bacillus: Clinical Significance
• Common environmental commensals
• Occasional opportunistic pathogens & culture contaminants
• B. cereus group
• B. anthracis
• Others
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Bacillus cereus Group
• Gastroenteritis– Heat-stable enterotoxin (emetic form)– Heat-labile enterotoxin (diarrheal form)
• Similar to V. cholera – stimulates cAMP watery diarrhea
• Contaminated rice, meat, vegetables
• Other Clinical conditions– Ocular infections after trauma
• Necrotic toxin, hemolysin, phospholipase C
– IV-catheter-related sepsis, endocarditis, meningitis• Immunocompromised persons and drug abusers
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B. anthracis: Anthrax• Rare in US due to control in animals• Enzootic in middle east• Transmitted by contact with animal products• Spores remain infectious for years• Usually cutaneous inoculation - slow healing ulcer, bacilli
spread to lymphatics and bloodstream– 20% mortality if untreated– Respiratory anthrax is usually fatal even if treated– GI anthrax also occurs in some Asian countries
• Excellent biological warfare agent
Gram stain with spores
eschar
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Anthrax Pathogenesis
• Poly-D-glutamic acid (protein) capsule – antiphagocytic• 3 component exotoxin
Protective antigen (PA) - binds to cells, forms channel that permits EF and LF to enter
Edema factor (EF) – adenyl cyclase causes fluid to accumulate at the site of infection and inhibit immune function
Lethal factor (LF)
-disrupts cell's functions, stimulates TNF-alpha and IL-1-beta, kills infected cells
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Anthrax Vaccine
• Made from avirulent, nonencapsulated B. anthracis strain
• Requires series of injections and annual boosters
• Used in military
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Listeria monocytogenes
•Gram-positive bacilli/coccobacilli•Catalase-positive•Motile•Esculin positive hemolytic•Multiplies at 4 o C Umbrella
motility
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• Habitat & Sources of Infection – Intestines of mammals, birds – Cheese, other dairy products– Vegetables – Undercooked meat
• Epidemiology & Disease– Asymptomatic carriage– Meningitis/sepsis in immunocompromised
host– Foodborne illness– Neonatal infections (transplacental)
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Listeria: Pathogenesis
• Organism adapted to grow at low temperatures
• Multiplies in cytoplasm of macrophages and epithelial cells
• Listeriolysin O (hemolysin)
• Asymptomatic carriage reservoir
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Erisipelothrix rhusopathiae
• Gram-positive bacillus• Common in animals• Transmitted to humans by
skin wound• Occupational hazard in
butchers, farmers, veterinarians
• Self-limited skin lesions with erythema & eruption
• May spread to bloodstream
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Aerobic Actinomycetes
• Gram-positive, catalase-positive rods
• Occur in soil and decaying vegetation
+ mycolic acid - mycolic acid
MycobacteriumCorynebacterium
NocardiaStreptomycesRhodococcusTropherymaOthers
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Corynebacterium• Gram-positive, curved pleomorphic rods• “Chinese Letters”• Aerobic/facultatively anaerobic• Grow readily on Sheep blood agar• Catalase & oxidase positive• Usually non-motile• Commensals on skin• Some species pathogenic
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Diphtheria (C. diphtheriae)• Spreads by droplet
• Phospholipase D increases vascular permeability & promotes spread
• Phage-coded exotoxin (A&B subunits) acts on respiratory mucous membranes interfering with protein synthesis by inactivating EF-2
• Pseudomembrane of fibrin, bacteria, epithelial & phagocytic cells impairs breathing
• Toxin spreads to heart, CNS, & adrenals
• Selective media (cysteine-tellurite)
• Identify in Public Health Labs
• Protect by toxoid vaccination - DaPT
• < 5 cases/yr in US
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Other Pathogenic Corynebacteria
• C. jeikeium (JK)– Opportunistic bloodstream infections in bone
marrow transplant recipients
– Multiple antibiotic resistance - except vancomycin and tetracycline
– Commonly carried on skin of hospitalized persons
• C. urealyticum– Occasional cause of UTI & stones (splits urea)
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Nocardia • Epidemiology
– Ubiquitous in environment• Inhalation• Cutameous inoculation
• Pathogenesis– Cord factor – prevents
phagolysosome fusion & intracellular killing in phagocytes
– Catalase– Superoxide dismutase
Filamentous gram-positive bacilli
Partial Acidfast Smear
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Nocardia Diseases• Opportunist (HIV, Malignancy)
– Pneumonia– Lung Abscess– Brain Abscess/Meningitis– Cellulitis/ulceration– Mycetoma
Suppurative granulomasFibrosisNecrosisSinus tracts
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Nocardia
6 year-old boy with HIV with 2-month-history of fever & cough.
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Nocardia Lab Diagnosis
• Stained smears• Culture - slow
growth ~ 7 days in 5% CO2
• Several species associated with human disease
• Identify by PCRNocardia colonies Middlebook 7H11 agarAerial hyphae
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Rhodococcus equi• Gram-positive weakly
acidfast bacilli that sometimes revert to coccoid forms
• Common in animals & environment
• Opportunistic lung infections in AIDS & transplant patients
• Facultative intracellular pathogen that survives in macrophages, leading to granuloma & abscess
Slow-growing, mucoid salmon-colored colonies of R. equi on chocolate agar
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Tropheryma whippeli
• Actinomycete etiologic agent of Whipple’s Disease – a malabsorptioin syndrome affecting the small bowel
• Organism cannot be cultured except in tissue culture
• Diagnose by histopathology & PCR