Many of the pathogens relevant to the dairy industry are gram negative and normally considered heat sensitive. Recently, however, there is growing evidence that some strains possess a markedly higher heat resistance compared to the typical type strains. This enhanced heat resistance may even, in some cases, be transferable and it is plausible that certain scenarios could select for these isolates. While the initial focus was on hospital infections due to recurring bacteriological problems on heat-treated medical equipment, it has become increasingly clear that several of these strains are found in the food industry including the dairies.

The specific genes and variants causing enhanced heat resistance have now been found in isolates from many different gram negative bacteria such as Salmonella, pathogenic E. coli and Chronobacter sakazakii, and the genetic background for the increased resistance has been partly elucidated. Examples will be given of the tolerance of these isolates to different stresses, and the potential importance for dairy processes will be discussed.