Pulsed-field gel electrophoresis (PFGE) analysis of Listeria monocytogenes isolates from different sources and geographical origins and representative of the twelve serovars.

Multiplex-PCR (MPCR) serogrouping and pulsed-field gel electrophoresis (PFGE) subtyping analysis are currently used by several public and private laboratories for the characterization of Listeria monocytogenes. In this study a set of 80 L. monocytogenes isolates belonging to the twelve serovars was used to investigate (i) the typeability of the rare serovars, (ii) the ability of PFGE analysis with ApaI and AscI to differentiate serovars within MPCR serogroups and (iii) the association of molecular types with the specific source or geographical origin of the isolates. With the exception of three isolates (rare serovars 4a and 4c) that were not amenable to restriction with ApaI, all the other analyzed isolates were subtyped by both enzymes. PFGE discriminated the 80 isolates into 62 combined ApaI and AscI PFGE patterns (pulsotypes), but could not differentiate serovars within MPCR serogroups, in which isolates from different serovars displaying the same pulsotype were found. Clustering analysis suggests that for some pulsotypes grouping according to Portuguese origin or source can be suggested. On the other hand, some L. monocytogenes clones are widely distributed. Two pulsotypes from Portuguese human isolates were identical to the ones displayed by human outbreak clones in the UK and in the USA and Switzerland, respectively, although they were not temporally matched. Computer-assisted data analysis of large and diverse PFGE type databases will improve the correct interpretation of subtyping data in epidemiological studies and in tracing routes and sources of contamination in the food industry.