 A strain is a subgroup within the species that has unique characteristics that distinguish it from other strains . These differences are often detectable only at the molecular level ; however , they may result in changes to the physiology or lifecycle of the bacterium . For example , a strain may gain pathogenic capacity , the ability to use a unique carbon source , the ability to take upon a particular ecological niche , or the ability to resist antimicrobial agents . Different strains of E. coli are often host @-@ specific , making it possible to determine the source of fecal contamination in environmental samples . For example , knowing which E. coli strains are present in a water sample allows researchers to make assumptions about whether the contamination originated from a human , another mammal , or a bird . 
