Measures to ensure microbial food safety, when applied at a "mild" sublethal level, single or in combination, are commonly termed "hurdles" for bacterial growth by food technologists. These "hurdles" may differ in their nature, intensity and/or time - intensity profile. From a microbiological point of view, all types of "hurdles" represent environmental stress factors. Microorganisms have to cope with these situations. This adaptation is achieved by utilizing genomic information already present in the bacterial cell or by changing the genomic information (e.g. mutations) and thereby opening new metabolic pathways. Generally, mutations are stochastic, but environmental stress may induce mutations at specific genomic sites, called "adaptive mutations". This paper deals with two aspects: firstly, to briefly review the "hurdle" concept as a "microbial stress" phenomenon; secondly to demonstrate the considerable biological variations in microbial growth. Consequently, an experiment was conducted where a batch of biochemically similar food isolates of Yersinia enterocolitica was grown under different technological conditions and subsequently compared to laboratory reference strains originating from clinical cases. Food isolates were found to react heterogeneously, whereas reference strains behaved in a uniform way.