bacillus, any rod-shaped bacterium or, more particularly, a rod-shaped bacterium of the genus Bacillus. Some bacterium in the genus cause disease, for example B. anthracis is the cause of anthrax; others are useful in the production of antibiotics (e.g., gramicidin and bacitracin). Many organisms earlier classified as Bacillus species are now placed in different genera but continue to be referred to as baccili.

Any of the rod-shaped, gram-positive bacteria (see gram stain) that make up the genus Bacillus, widely found in soil and water. The term is sometimes applied to all rodlike bacteria. Bacilli frequently occur in chains and can form spores under unfavourable environmental conditions. Resistant to heat, chemicals, and sunlight, these spores may remain capable of growing and developing for long periods of time. One type sometimes causes spoilage in canned foods. Another, widespread bacillus contaminates laboratory cultures and is often found on human skin. Most strains do not cause disease in humans, infecting them only incidentally in their role as soil organisms; a notable exception is B. anthracis, which causes anthrax. Some bacilli produce antibiotics.

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