HOST-MEDIATED MODIFICATION OF SAU3AI RESTRICTION IN LISTERIA-MONOCYTOGENES - PREVALENCE IN EPIDEMIC-ASSOCIATED STRAINS

Most major food-related outbreaks of listeriosis have been traced to a cluster of genetically related strains of serovar 4b (epidemic clone) . In spite of numerous searches, distinct bacteriologic or virulence-r elated features unique to these strains have eluded identification, al though a restriction fragment length polymorphism (RFLP) characteristi c of the epidemic clone has previously been described (W. Zheng and S. Kathariou, Appl. Environ. Microbiol. 61:4310-4314, 1995). We found th at DNAs from 75 strains which were derived from three separate outbrea ks and which had the epidemic clone-specific RFLP were also invariably resistant to digestion by Sau3AI and other restriction endonucleases sensitive to cytosine methylation at 5' GATC 3' sites. This modificati on of Sau3AI restriction was host mediated, as it did not persist when DNA was cloned and propagated in Escherichia coli, and was uncommon a mong other Listeria strains. Epidemic-associated strains with this mod ification were resistant to infection by phage propagated in a serotyp e 4b strain which was not known to be involved in an epidemic and whic h lacked the epidemic clone-specific RFLP. Screening for susceptibilit y to MboI digestion revealed that these epidemic strains lacked methyl ation of adenines at GATC sites. This type of modification was rare am ong Lister ia strains and was found in only three (of eight screened) strains of serovar 1/2b, possibly representing one clonal lineage.