strongly Gram positive aerobe and facultative anaerobe
temperature range for growth of 12-45 degrees
grow on all ordinary media as typical colonies with a wavy margin and small projections
the spores are resistant to chemical disinfectants and heat;the spores of many stains will resist dry heat at 140 degrees for 1-3 h and boiling or steam at 100 degrees for 5-10 min. However,autoclaving at 121 degrees destroys them in 15 min
b) Pathogenesis
i) Clinical Infection
anthrax arises by inoculation through the skin of material from infected animals or their products
the resulting lesion of cutaneous anthrax is often described as a malignant putsule
coagulation necrosis of the centre of the putsule results in the formation of a dark-coloured eschar which is later surrounded by a ring of vesicles containing serous fluids and an area of oedema and induration which may become extensive
inhalation of spores in dust or wool fibres may result in respiratory anthrax : wool-sorter's disease
this condition carries a high mortality due to the intense inflammation,haemorrhage and septicaemia which result from the multiplication of organisms in bronchi and spread to lungs,lymphatics and bloodstream
ii) Virulence determinants
two determinants had been discovered :- capsule,which is unusual in being a polypeptide of D-glutamic acid,which inhibit opsonophagocytosis
the other determinants is the plasmid-encoded toxin complex comprising three proteins,all of which are required for pathogenicity
the protective antigen binds the complex to receptors on the macrophage surface;after proteolysis,oedema factor and lethal factor are released and,after endocytosis,work inside the cell by blocking the adenyl cyclase pathway
the main effect of this toxin is to increase vascular permeability,which leads to shock
c) Laboratory diagnosis
the fully developed malignant putsule may be difficult to swab and the central necrotic area gives a poor yield. Fluid aspirated from the surrounding vesicles,when present,is more likely to yield anthrax bacilli
animal inoculation was also used to confirm the organism. Guinea pigs may be inoculated subcutaneously with a suspension of bacilli. Death usually ensues within 2-3 days and post mortem reveals numerous bacteria on cardiac puncture or impression of splenic cells. These specimen when stained with polychrome methylene blue show large,blue bacilli surrounded by a red granular-stained capsule which this appearance is given only by B.Anthracis
confirmation in vitro is also made by simple biochemical and physiological reactions. Demonstration of non-motility,gelatin liquefaction,growth in straight chains and enhanced growth aerobically,will generally identify B.Anthracis completely
it is occasionally necessary to isolate B.Anthracis from potentially contaminated material such as animal haor,hides or soil. This task is particularly difficult because of the large number of non-pathogenic Bacillus spp. found in the environment
d) Treatment
susceptible in vitro to a wide range of antimicrobial agents and many have been used successfully in the treatment of anthrax in man
penicillin remains the drug of choice whereas most stains tested have also been found to be sensitive to macrolides,aminoglycosides,tetracyclines and chloramphenicol
Bacillus Cereus
a) Description
large Gram positive bacilli
motile
lacks glutamic acid capsule
saprophyte and frequents soil,water and vegetation
b) Pathogenesis
spores of B.cereus are particularly heat-resistant and most strains produce toxins
widespread in the environment and found in most raw foods especially cereals such as rice
most episodes of food poisoning are due to preformed toxin and vomiting is the main symptom,occurring within 6 h of ingestion
B.cereus strains may also produce an enterotoxin,which is heat labile and formed in the intestine,causing a diarrhoeal form of food poisoning
c) Laboratory diagnosis
by testing the food,a high numbers of B.cereus,often 100000000 or more per gram,in the absence of other food poisoning bacteria are sufficient to make the diagnosis
large facultatively anaerobic Gram-positive bacilli which produce anthracoid colonies on blood agar after overnight incubation at 37 degrees are almost certain to be B.cereus
d) Management and Control
no specific treatment and the disease may be easily prevented by proper cooling and storage of food
all dishes should be freshly prepared and eaten
rice,in particular,should not be stored for long periods above 10 degrees
