Restriction Enzyme Analysis in the Epidemiology of Listeria Monocytogenes

Listeria monocytogenes is a foodborne bacterial pathogen which is transmitted to humans via consumption of dairy products and vegetables. The purpose of this study was to examine the genetic diversity of L. monocytogenes by restriction enzyme analysis and to localize the hemolysin gene by hybridization with a synthetic oligomer specific for the β-hemolysin (β- listeriolysin) toxin gene. Reference strains of serotypes 1A (1/2A), IB (1/2B), 1C (1/2C), 3A, 3B, 3C, 4A, 4B, 4AB, 4C, 4D, and 4E and 39 isolates obtained from food and clinical specimens were serotyped and genomic DNA analyzed for restriction enzyme patterns. Three groups of isolates were examined: two groups were derived from environmental samples and food products, and unrelated sporadic clinical cases. The third set consisted of isolates recovered during the recent listeriosis outbreak in Los Angeles County, California. Isolates were identified as serogroup 1A (factor 1) or as serotype 4B (factor 6). Reference strains exhibited a distinct electro- phoretic pattern after Hha 1 digestion. Isolates assigned to the same serotype exhibited different restriction patterns after Hha 1 digestion. However, when restriction fragments were blotted using the Southern method and probed with a synthetic oligonucleotide specific for the β-hemolysin gene, hybridization occurred with a limited number of restriction fragments. Ten strains recovered from Mexican-style soft cheese incriminated in the California listeriosis outbreak were antigenically identified as serogroup 4B (factor 6). All 10 strains exhibited identical restriction patterns after cleavage with Hha I. Only a single restriction fragment (0.9 kb) hybridized with the β-hemolysin gene probe. This suggests a common contaminating source of Listeria for the cheese samples examined.