Listeria monocytogenes: knowledge gained through DNA sequence-based subtyping, implications, and future considerations

The purpose of subtyping is to differentiate bacterial isolates beyond the classification of species or subspecies. Subtyping methods can be grouped into two broad categories based on the cellular components targeted: (1) phenotypic subtyping methods that differentiate isolates by the enzymes, proteins, or other metabolites expressed by the cell, and (2) molecular subtyping methods that discriminate isolates based on interrogation of nucleic acid sequences. The two major types of molecular subtyping methods include band-based methods based on fragment pattern data or DNA fingerprints, and methods that generate DNA sequence data. Molecular subtyping methods have shown that Listeria monocytogenes isolates can be classified into four genetic lineages or divisions. Although band-based molecular subtyping methods continue to serve as the gold standard for routine molecular subtyping of most clinically important foodborne pathogens, including L. monocytogenes, the explosion of recently completed and ongoing DNA sequencing projects, and thus available DNA sequence data, have stimulated efforts to develop highly discriminatory and high-throughput DNA sequence-based subtyping methods for L. monocytogenes. L. monocytogenes represents one of the most highly sequenced human pathogens; more than 20 genome sequences are currently available for this organism. This review provides an overview of the concepts behind subtyping and discusses the application of molecular subtyping methods, with an emphasis on DNA sequence-based subtyping methods to characterize L. monocytogenes.