Gram-positive bacilli
A. Bacillus sp.
1. Characteristics
a. Aerobic, spore-forming, gram-positive rod
2. Epidemiology
a. 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 nature
i. 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. Pathogenesis
a. B. anthracis
i. capsule: antiphagocytic
ii. toxins: edema toxin and lethal toxin that mediate tissue
destruction
iii. cutaneous, pulmonary (“wool sorters”), and GI
b. B. cereus
i. enterotoxins and pyogenic toxins
ii. diarrheal and emetic food poisoning
iii. post-traumatic eye infections, endocarditis, bacteremia.
B. Listeria monocytogenes
1. Characteristics
a. Small, thin, catalase-positive GPR
2. Epidemiology
a. Can be normal GI flora, found in the environment, as
well as animals
b. transmission through ingestion of contaminated food
(meat and dairy); also mother to fetus in colonized moms
3. Pathogenesis
a. Listeriolysin: hemolysin and cytotoxin permitting
survival within macrophages
b. bacteremia and meningitis, usually in neonates or
immunosuppressed
C. Corynebacterium sp.
1. Characteristics
a. pleomorphic, catalase + GPR
2. Epidemiology
a. normal URT and skin flora; also NF of animals
b. person to person or endogenous
3. Pathogenesis
a. C. diphtheriae
i. diphtheria toxin: exotoxin that destroys cells
Respiratory dipth: pseudomembrane exudates; respir
compromise; toxin affects organs
Cutaneous: non-healing ulcers
b. C. jeikeium
i. resistance to multiple antibiotics
ii. septicemia, wound infections
D. Laboratory diagnosis
1. Specimen collection, transport, processing
a. In general, no special requirements
b. Bacillus cereus from suspected food poisoning
i. Heat or alcohol shock specimen; only spores will survive
c. Listeria from placenta
i. difficult to isolate from this source; cold enrichment at 4C
for several weeks-months
2. Direct detection
a. 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 identification
a. Bacillus
i. grow well on BAP; CAP
ii. various colony morphologies and colors: large, flat,
spready, wrinkly; white, gray, green
iii. some are beta-hemolytic
iv. Gram stain and colony morphology are usually
sufficient; full ID not warranted
v. commonly considered a contaminant
Bacillus sp. on Gram stain
http://textbookofbacteriology.net/Anthrax.html
Left: B. cereus
Right: B. anthracis
http://textbookofbacteriology.net/Anthrax.html
3. Culture and identification
b. Listeria
i. grow well on BAP and CAP
ii. small white or translucent colonies; beta hemolytic
iii. CAMP positive
iv. Motile; umbrella motility
Listeria sp. on Gram stain
http://www.geocities.com/CapeCanaveral/3504/gallery.htm
3. Culture and identification
c. Corynebacterium
i. most grow well on BAP and CAP
ii. C. diphtheriae requires selective and differential media
(freshly prepared)
Cystine-tellurite: black or gray colonies
Tinsdale’s: black with dk brown halos
iii. others are infrequently ID’d, common contaminants
iv. Listeria and Coryne ID’d by API strip if required
Corynebacterium sp. on Gram stain
http://www.life.umd.edu/classroom/bsci424/PathogenDescriptions/Corynebacterium.htm
C. diphtheriae on Tellurite agar
4. Susceptibility testing and therapy
a. MIC guidelines exist, but only test in severe
disease
b. Bacillus: Vanco, cipro
c. Listeria: Pen, Amp
d. 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 rods
Some are partially acid-fast
Difficult to recognize clinically and difficult to isolate
Nocardia, Streptomyces, Rhodococcus, Tsukamurella
A. Nocardia
1. 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 Pathogenesis
a. World-wide inhabitants of soil and water, responsible
for decomposition of plant material
b. Can be colonizers or cause infection following
traumatic inoculation or inhalation
c. Intracellular pathogens, prevent destruction in
phagocytes, tropism for neural tissue
3. Spectrum of disease
a. Immune competent: skin infections
i. Mycetoma
ii. Lymphocutaneous
iii. Skin abscesses or cellulitis
b. Immune compromised: pulmonary and disseminated
i. pulmonary disease is non-specific, so risk factors
add it to differential
ii. 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 diagnosis
a. 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 tissues
b. 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
Gram stain of Nocardia
http://home.primus.com.au/royellis/nocast.html
a. Culture
i. can grow on BAP, CAP, and fungal media
ii. may take 48-72 h to grow, so can be overgrown by normal
flora
Selective media: BCYE contains polymixin, anisomycin
and vanco
iii. Colony morphology is variable: wrinkled, dry, crumbly;
chalky-white to orange-tan
b. Identification
i. Growth rate, colony morphology, gram stain
ii. Urease +: cleaves urea, increases pH  pink
iii. Lysozyme resistant: can grow in its presence
iv. Species id: use of casein, xanthine an tyrosine and starch
hydrolysis: Quad plates, hold for 1 month
Colonial morphologies of selected Nocardia sp.
http://www.toyama-mpu.ac.jp/hp/clla/saikin/senshokuzou/gram_right.html
Quad plate for ID of Nocardia sp.
http://path.upmc.edu/cases/case315.html
5. Susceptibility testing and antimicrobial therapy
a. Standardized method are available; ? in vitro-in vivo
correlation; routinely requested for every patient
b. Trimethoprim/sulfamethoxazole is primary drug of
choice