Gram-negative, nonsporing, facultative rod-shaped bacteria that ferment lactose with gas formation within 48 hours at 35 degrees C. Examples of coliform bacteria are members in the genera Escherichia ( e.g. E. coli), Klebsiella (e.g. K. pneumoniae), Enterobacter (e.g. E. cloacai), and Citrobacter (e.g. C. freundii).