The publication of the complete genome sequence of L. monocytogenes EGD-e in 2001 [2] represents the transition from pregenomics to postgenomics in the field of L. monocytogenes research. As described briefly above, the availability of the L. monocytogenes genome sequence (and simultaneously the L. innocua sequence) has not only altered our view of the biology of this interesting pathogen, but also made it possible to use newly developed genome-based methods in the study of L. monocytogenes. The availability of the complete genomic sequences of at least one strain of each species of the genus Listeria in the near future will dramatically expand our genomic knowledge in this group of bacteria and will allow a fundamentally new view on the evolution of the genus Listeria.

Genomic information is, however, not only of major interest in basic science; this information is also vital for genome-based typing of bacteria, especially in such a heterogeneous group as the one represented by the different serovars found in L. monocytogenes. For many years, L. monocytogenes was usually characterized by serotyping and subtyped using pulse-field gel electrophoresis or ribotyp-ing. DNA microarrays based on sequence information of completely or partially sequenced genomes now provide an alternative means to resolve genetic differences among isolates and, unlike pulse-field gel electrophoresis and ribotyping, microarrays can additionally be used to identify specific genes associated with strains of interest. This approach was used successfully in several recent studies for selective discrimination of epidemic strains of L. monocytogenes [116-118]. Additionally, microarrays can also be used for direct detection ofpathogen-specific RNA or DNA in complex environmental samples (reviewed in Ref. [119]). Micro-array-based microbial detection systems will probably become routine - although the sensitivity of these systems currently limits their application for pathogen detection.

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