Characteristics of coliform bacteria

Written by cynthia ruscitto
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Characteristics of coliform bacteria
Coliform bacteria are found in the faeces of warm-blooded animals. (Dutch cows image by Robert Soen from

Though often associated with E. coli, coliform bacteria describes any of several microorganisms that, according to the Washington State Department of Health, are present in the environment as well as the faeces of warm-blooded animals. Genera making up the coliform group include Escherichia, Klebsiella, Enterobacter, Serratia and Citrobacter. Collectively, the group are called coliforms because of their physical and biochemical similarities. (See Reference 2)

Structural Characteristics

Coliform bacteria are prokaryotic cells. Unlike the cells of more complex organisms, prokaryotic cells lack a true nucleus and other membrane-bound organelles. (See Reference 5) Internally, coliforms consist of cytoplasm, DNA, RNA and ribosomes used to make proteins. A cell envelope surrounds the internal structures and consists of an inner cell membrane, a cell wall and an outer capsule. Coliforms have flagella that they use to move about. Some strains have surface bristles called fimbriae that allow them to adhere to other cells and surfaces. (See Reference 3)

Surface Antigen Characteristics

Coliforms have surface proteins called antigens that when identified, allow scientists to divide the coliforms into different types. Coliforms exhibiting certain combinations of surface antigens may be more likely to cause disease or show a preference for infecting specific parts of the body. The letters H, K and O are used to designate flagellar, capsular and somatic, or cell wall, surface antigens, respectively. (See Reference 3)

Staining Characteristics

Coliforms are Gram-negative bacilli, meaning they have a rod-like shape that appears pink rather than purple when stained with a crystal violet dye. Unlike the thick cell walls of Gram-positive organisms that retain the purple stain, Gram negative organisms have thin cell walls that fail to hold onto the dye. (See Reference 4)

Biochemical Characteristics

Coliforms ferment lactose and produce gas during the process. (See Reference 2) They are facultative anaerobic organisms, meaning they have the ability to thrive using either aerobic or anaerobic respiration. When their environment lacks the necessary oxygen for aerobic respiration, they obtain energy by switching over to anaerobic respiration. (See Reference 4) All coliforms produce toxic substances, called endotoxins, that are released when the organism disintegrates.(See Reference 2)

Disease-Causing Characteristics

According to the Washington State Department of Health, most strains of coliform bacteria are normal inhabitants of the human digestive tract and are harmless. They are opportunistic pathogens, meaning they only cause disease in people whose immune systems are compromised. Coliforms may cause intestinal, lung, wound, blood and urinary tract infections in susceptible individuals. According to Southern Illinois University-Carbondale, E. coli causes 40 per cent of all neonatal bacterial meningitis infections. (Reference 2 for all disease info)

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