Bacillus, (genus Bacillus), any of a genus of rod-shaped, gram-positive, aerobic or (under some conditions) anaerobic bacteria widely found in soil and water. The term bacillus has been applied in a general sense to all cylindrical or rodlike bacteria.
The largest known Bacillus species, B. megaterium, is about 1.5 μm (micrometres; 1 μm = 10−6 m) across by 4 μm long. Bacillus frequently occur in chains.
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