Escherichia coli is a major mastitis pathogen and a well-known cause of clinical mastitis in dairy cows. Ubiquitously present in organic material like manure and bedding, E. coli can reach high concentrations in the cow’s environment. Although being described as generally self-limiting and transient, the infection causes substantial economic losses due to a dramatic reduction in milk yield, or in some cases loss of the animal. The severity of the inflammatory response is strongly determined by host factors which adds another layer of complexity to management of the disease on herd level. A high environmental pressure with strains adapted to the mammary gland, paired with a high number of immune compromised cows can create an outbreak like situation. This article focuses on recent insights in characteristics of the microorganism, host-pathogen interaction, epidemiology, economic impact and strategies for prevention and management of E. coli mastitis.
Belonging to the family of the Enterobacteriaceae, E. coli are a group of gram negative, rod-shaped bacteria which are typical commensals in the gastrointestinal tract of warm blooded mammals. They belong to the group of coliform bacteria and have the ability to ferment lactose and sustain anaerobic conditions (Kaper et al., 2004). Escherichia coli are a very versatile group of bacteria and can thrive in many different environmental scenarios outside the gut.
Virulence and fitness factors
There has been an ongoing debate whether an infection of the udder results from contamination of the mammary gland with just any E. coli strain or if there are strains that are specifically mastitis associated. Several studies were conducted with the goal to identify specific virulence factors in E. coli strains isolated from mastitis cases. They found great genetic variation of virulence genes and no association with clinical signs or persistence of infection (Bean et al., 2004, Wenz et al., 2006, Suojala et al., 2011). This lead to the conclusion that mastitis causing E. coli are typical commensals and that the outcome of an infection is mainly determined by cow factors (Burvenich et al., 2003).
With the advent of genome sequencing and more refined molecular techniques evidence accumulates that not all E. coli strains are equally likely to cause mastitis. The majority of strains isolated from mastitis cases, so called mammary pathogenic E. coli (MPEC), belong to phylogroup A, a phylogenetic group which is generally considered to be more commensal but also includes significant pathogens (Crossman et al., 2010). An examination of different strains from this group on genomic level revealed that MPEC are more closely related than expected by chance or
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