Towards the definition of pathogenic microbe

Identification and typing of spoilage and pathogenic microorganisms have become major objectives over the past decade in microbiology. In food, strain typing is necessary to ensure food safety and for linking cases of foodborne infections to suspected items. Recent advances in molecular biology have resulted in the development of numerous DNA-based methods for discrimination among bacterial strains. Here, we present the use of Simple Sequence Repeats (SSR, or Microsatellites) for bacterial typing. SSRs are a class of short DNA sequence motifs that are tandemly repeated at a specific locus. Computer-based screen of the complete genomic DNA sequences of various prokaryotes showed the existence of tens of thousands well distributed SSR tracts. Mono Nucleotides Repeats (MNRs) are the majority of SSR tracts in bacteria, therefore selected MNR loci were analyzed for variation among strains belonging to three bacterial species: Escherichia coli, Listeria monocytogenes and Vibrio cholerae. High levels of polymorphism in the number of repeats was observed. The finding that most of the MNR tracts are variable in bacterial genomes, but stable at the strain level, allows the use of MNRs for bacterial strains identification. The variation in MNR tracts enables the separation between virulent and non-virulent strain groups and further discriminates among bacterial isolates, in the three tested bacterial species. The uncovered MNR polymorphism is important as a genome-wide source of variation, both in practical applications (e.g. rapid strain identification) and in evolutionary studies. This multi-locus MNR strategy could be applied for high throughput bacterial typing by assigning an “identity number” for each strain based on MNR variations. The developed typing technology should include the fingerprint database for large bacterial strain collections and a high throughput scanner. This accurate and rapid tool can have a major role in decreasing the incidences of food-related outbreaks and will contribute to limit epidemics.