Gram-positive bacilli

Gram-positive bacilli

A. Bacillus sp.1. Characteristics

a. Aerobic, spore-forming, gram-positive rod

2. Epidemiologya. B. anthracis found in soil; causes anthrax

i. transmission through inhalation of spores, breaks in skin/mucous membranes, or ingestion

b. B. cereus found in naturei. transmission through traumatic introduction, contaminated

medical equipment, or ingestion of contaminated food

c. Many other sp. found in the environment; generally considered non-pathogens

3. Pathogenesisa. B. anthracis

i. capsule: antiphagocyticii. toxins: edema toxin and lethal toxin that mediate tissue

destructioniii. cutaneous, pulmonary (“wool sorters”), and GI

b. B. cereusi. enterotoxins and pyogenic toxinsii. diarrheal and emetic food poisoningiii. post-traumatic eye infections, endocarditis, bacteremia.

B. Listeria monocytogenes1. Characteristics

a. Small, thin, catalase-positive GPR

2. Epidemiologya. Can be normal GI flora, found in the environment, as well as animalsb. transmission through ingestion of contaminated food (meat and dairy); also mother to fetus in colonized moms

3. Pathogenesisa. Listeriolysin: hemolysin and cytotoxin permitting survival within macrophagesb. bacteremia and meningitis, usually in neonates or immunosuppressed

C. Corynebacterium sp.1. Characteristics

a. pleomorphic, catalase + GPR

2. Epidemiologya. normal URT and skin flora; also NF of animalsb. person to person or endogenous

3. Pathogenesisa. C. diphtheriae

i. diphtheria toxin: exotoxin that destroys cells Respiratory dipth: pseudomembrane exudates; respir

compromise; toxin affects organs Cutaneous: non-healing ulcers

b. C. jeikeiumi. resistance to multiple antibioticsii. septicemia, wound infections

D. Laboratory diagnosis1. Specimen collection, transport, processing

a. In general, no special requirementsb. Bacillus cereus from suspected food poisoning

i. Heat or alcohol shock specimen; only spores will survive

c. Listeria from placentai. difficult to isolate from this source; cold enrichment at 4C

for several weeks-months

2. Direct detectiona. Bacillus: gram positive rods; size varies with species

i. Spore stain: Malachite green stains spores, safranin stains vegetative cells

ii. location of spores helps in ID if required

b. Listeria: small GPR; may form short chains and be confused with streptococci

c. Corynebacterium: palisading GPR; Chinese letters

3. Culture and identificationa. Bacillus

i. grow well on BAP; CAPii. various colony morphologies and colors: large, flat,

spready, wrinkly; white, gray, greeniii. some are beta-hemolyticiv. Gram stain and colony morphology are usually

sufficient; full ID not warrantedv. commonly considered a contaminant

http://textbookofbacteriology.net/Anthrax.html

Bacillus sp. on Gram stain

http://textbookofbacteriology.net/Anthrax.html

Left: B. cereus Right: B. anthracis

3. Culture and identification

b. Listeriai. grow well on BAP and CAPii. small white or translucent colonies; beta hemolyticiii. CAMP positiveiv. Motile; umbrella motility

http://www.geocities.com/CapeCanaveral/3504/gallery.htm

Listeria sp. on Gram stain

3. Culture and identification

c. Corynebacteriumi. most grow well on BAP and CAPii. C. diphtheriae requires selective and differential media

(freshly prepared) Cystine-tellurite: black or gray colonies Tinsdale’s: black with dk brown halosiii. others are infrequently ID’d, common contaminantsiv. Listeria and Coryne ID’d by API strip if required

http://www.life.umd.edu/classroom/bsci424/PathogenDescriptions/Corynebacterium.htm

Corynebacterium sp. on Gram stain

C. diphtheriae on Tellurite agar

4. Susceptibility testing and therapy

a. MIC guidelines exist, but only test in severe disease b. Bacillus: Vanco, ciproc. Listeria: Pen, Ampd. C. diptheriae: anti-toxin plus pen or erythro

i. vaccine DPT to toxoid

e. Other corynes: all suscep to vanco

Aerobic actinomycetes

Branching, filamentous, gram-positive rodsSome are partially acid-fastDifficult to recognize clinically and difficult to isolateNocardia, Streptomyces, Rhodococcus, Tsukamurella

A. Nocardia1. Characteristics

a. beaded, gram-positive, variably acid-fast (depends on media), catalase positive, strict aerobe

b. Form substrate hyphae (filaments along agar surface) and aerial hyphae (in air)

c. 11 species, N. asteroids, N. brasiliensis are most common

2. Epidemiology and Pathogenesisa. World-wide inhabitants of soil and water, responsible for decomposition of plant materialb. Can be colonizers or cause infection following traumatic inoculation or inhalationc. Intracellular pathogens, prevent destruction in phagocytes, tropism for neural tissue

3. Spectrum of diseasea. Immune competent: skin infections

i. Mycetomaii. Lymphocutaneousiii. Skin abscesses or cellulitis

b. Immune compromised: pulmonary and disseminatedi. pulmonary disease is non-specific, so

risk factors add it to differentialii. organism can spread hematogenously

from primary site, resulting in brain and/or skin lesions (dissemination = poor prognosis)

Nocardia mycetoma

http://www.thelancet.com/journals/laninf/article/PIIS1473-3099(09)70347-0/abstract

4. Laboratory diagnosisa. Specimen collection

i. No special requirements; multiple specimens may be required to increase sensitivity

ii. single isolate is of questionable significance

iii. form micro-colonies or granules in tissuesb. Direct detection

i. characteristic gram-stain, but also may be difficult to see

ii. modified acid-fast positive 1% sulfuric acid rather than 3%

HCl as decolorizer

http://home.primus.com.au/royellis/nocast.html

Gram stain of Nocardia

a. Culturei. can grow on BAP, CAP, and fungal mediaii. may take 48-72 h to grow, so can be overgrown by normal

flora Selective media: BCYE contains polymixin, anisomycin

and vancoiii. Colony morphology is variable: wrinkled, dry, crumbly;

chalky-white to orange-tan

b. Identificationi. Growth rate, colony morphology, gram stainii. Urease +: cleaves urea, increases pH pinkiii. Lysozyme resistant: can grow in its presenceiv. Species id: use of casein, xanthine an tyrosine and starch

hydrolysis: Quad plates, hold for 1 month

http://www.toyama-mpu.ac.jp/hp/clla/saikin/senshokuzou/gram_right.html

Colonial morphologies of selected Nocardia sp.

http://path.upmc.edu/cases/case315.html

Quad plate for ID of Nocardia sp.

5. Susceptibility testing and antimicrobial therapya. Standardized method are available; ? in vitro-in vivo correlation; routinely requested for every patientb. Trimethoprim/sulfamethoxazole is primary drug of choice