Bacillus is a genus of rod-shaped aerobic bacteria forming heat resistant sporesa. If applied by Gram stain they can easily colorize but tend to be decolorized easily as well, so as to appear Gram variable.
They are generally motile with peritrichous flagella, the anthrax bacillus being a notable exception.
Members of this group exhibit great diversity in their properties. The genus includes psychrophilic, mesophilic and thermophilic species. The maximum temperatures for vegetative growth ranging from about 25 C to above 75 C and the minimum from about 5 C to 45 C. their salt tolerance varies from less than 2 % to 25 % NaCl.
Their spores are ubiquitous, being found in soil, dust, water and air and constitute the commonest contaminants in bacteriological culture media.