Occurrence and characteristics of Listeria in foods produced in Northern Ireland.

The incidence of L. monocytogenes and other Listeria species was determined for 513 food samples produced in Northern Ireland. Selected L. monocytogenes isolates were typed using MEE and RFLP analysis. The overall incidence of Listeria in the foods examined was 35% (Listeria monocytogenes 18.3%). The incidence of Listeria and L. monocytogenes in four different food categories (graded I-IV according to decreasing extent of processing) being: I, 11.1% Listeria and 4.7% L. monocytogenes; II, 27.1% Listeria and 12.2% L. monocytogenes; III, 89.1% Listeria and 50% L. monocytogenes and IV, 100% Listeria and 100% L. monocytogenes. Within food categories I and II, the incidence of Listeria on occasions varied markedly between similar products produced by different processors. The most frequently isolated species was L. innocua, followed by L. monocytogenes, L. seeligeri and L. welshimeri. The L. monocytogenes isolates were predominantly serogroup 1. A modified USDA method was the most productive of four enrichment procedures investigated. Over the one-year duration of the survey, a distinctive Listeria microflora could be discerned in products from certain processors and was confirmed in some cases by MEE and RFLP typing of L. monocytogenes isolates.