Coliform bacteria are facultative anaerobic, Gram-negative bacterium with no ability of forming spores. They are capable of fermenting lactose in the incubation at 35 ºC – 37 ºC. They are commonly present in the feces of warm-blooded animals, including humans. They are also present in aquatic environment. Coliform are usually be tested for as an indicator of food and water quality. The presence of these bacteria in food and water indicates fecal contamination. The common genera of this type of bacteria include Citrobacter, Escherichia, Enterobacter, Hafnia, and Klebsiella.

Citrobacter: Aside from human intestines, this genus of bacteria can also be found in soil and wastewater. Although the ability to ferment lactose is the common characteristics of coliform, C. koseri is non-lactose fermenter.

Escherichia: E. coli is one of the most studied organisms by researchers. It can be found in intestines of warm-blooded animals. It is also known as thermotolerant coliform due to its ability to grow at 44 ºC – 45 ºC. Other members of this genus such as E. vulneris and E. fergusonii are found in wounds and blood.

Enterobacter: Species like E. cloacae, E. aerogenes, E. gergoviae, and E. agglomerans are opportunistic pathogens and can cause many diseases such as meningitis, bacteremia, pneumonia, and urinary tract infections. It is also the leading cause of intensive care units (ICU) infections in United States.

Hafnia: This genus consists of only one species, H. alvei. It causes systemic infections like septicemia and pneumonia.

Klebsiella: This genus can be found everywhere. K. pneumoniae is the cause of pneumonia, usually difficult to be treated. The mortality rate is 50%, even with microbial therapy.