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Bacillus anthracis
Etiologic agent of anthrax. Considered 1st 'microorganism' to cause human disease [2]. Used as 1st model in development of Koch's postulates.
Epidemiology:
1) found in soil
2) endemic in Iran, Turkey, Pakistan, Sudan
-> natural resistance of humans is > than that of livestock Clinical characteristics:
1) extracellular pathogen
2) resistant to phagocytosis
3) rapidly proliferating
4) rapidly lethal
Pathology:
Capsule:
1) consists of poly-D-glutamic acid
2) confers resistance to phagocytosis
Endospores:
1) oxygen is required for sporulation, but not for germination
2) sporulation does NOT take place in vivo
3) spores can survive in dry earth for decades
4) spores are destroyed by
a) boiling to 10 minutes or autoclaving
b) treatment with oxidizing agents (H2O2, permanganate)
c) dilute or vaporized formaldehyde
5) spores are resistant to:
a) drying
b) heat
c) ultraviolet light
d) gamma irradiation
e) many disinfectants
6) polymerase chain-reaction (PCR) can detect spores in contaminated agricultural products
Virulence factors: derived from 2 plasmids, pXO1 & pXO2
1) pXO1 (184.5 kB)
a) 3 exotoxins are transcribed from genes expressed on pXO1
1] protective antigen (PA)
2] edema factor (EF)
3] lethal factor (LF)
b) the 3 exotoxins form 2 binary complexes:
1] edema toxin: edema factor & protective antigen
2] lethal toxin: lethal factor & protective antigen
2) pXO2 (95.3 kB)
a) encodes 3 genes (capA, capB & capC) involved in the synthesis of the polyglutamyl capsule of B anthracis
b) the capsule inhibits phagocytosis of the vegetative form of B anthracis
c) transcription of the capsule genes is regulated by a B anthracis derived transcription factor AcpA
3) expression of virulence is regulated by host-specific factors
a) elevated temperature (> 37 C)
b) carbon dioxide concentration > 5%
c) serum components
d) expression of toxin & capsule genes is mediated by transcriptional activator AtxA
4) both plasmids are required for virulence*
Laboratory:
- Bacillus anthracis in specimen
- microscopic characteristics:
- gram-positive rod with 'box-car' appearance
- size: 1.0-1.5 by 4-10 uM (large)
- non-motile
- encapsulated
- chain-forming
- Bacillus anthracis serology
- Bacillus anthracis Ag in tissue
- Bacillus anthracis DNA
- aerobic bacterial culture on blood agar
a) non hemolytic or weakly hemolytic, grayish-white, rough colonies
b) if medium contains bicarbonate in the presence of excess CO2, colonies are smooth & mucoid
Management:
1) see anthrax
2) ciprofloxacin, levofloxacin, or doxycycline generally with a 2nd agent
3) alternative agents
a) other flouroquinoline
b) beta-lactam (if susceptible)
c) rifampin
d) clindamycin
e) chloramphenicol
* Anthrax vaccines have been made by rendering virulent strains free of one or both plasmids (pX01 & pXO2).
1) Pasteur: an avirulent pXO2-carrying strain that is encapsulated but does not express exotoxin
2) Sterne: an attenuated pXO1-carrying strain that can synthesize extoxoin, but does not have a capsule
Related
anthrax (woolsorter's disease)
B. anthracis edema toxin
B. anthracis lethal factor (LF)
B. anthracis lethal toxin
B. anthracis protective antigen
General
Bacillus
Properties
KINGDOM: monera
DIVISION: SCHIZOMYCETES
References
- Harrison's Principles of Internal Medicine, 14th ed.
Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 897
- Dixon TC et al,
Anthrax.
NEJM 341:815, 1999
PMID: 10477781
- Medical Knowledge Self Assessment Program (MKSAP) 15,
American College of Physicians, Philadelphia 2009