Enumeration and growth of naturally occurring Listeria spp. in unpackaged ham

A total of 301 unpackaged retail ham samples were tested for the presence and number of Listeria spp. after 7 days at 5 °C to simulate domestic storage. Thirteen samples (4.3%) contained Listeria monocytogenes, with the highest count being 1.6×103 cfu g−1. Thirteen samples contained other Listeria spp. Genotyping showed that only one L. monocytogenes isolate from the 14 tested was of a type previously identified in New Zealand human cases.Listeria-contaminated batches were incubated at 5 °C over approximately 3 weeks to assess the growth rate of natural contaminants. None contained L. monocytogenes, but growth occurred in one sample containing Listeria welshimeri and four containing Listeria innocua. Growth was usually slow at 0.002–0.004 log h−1. In one sample, L. innocua grew at 0.02 log h−1 although the maximum number reached was only 4.0–5.0×103 cfu g−1. In five other samples little growth, if any, occurred. Growth of naturally occurring Listeria spp. at 5 °C was therefore generally slower than predicted by the Pathogen Modelling Programme (PMP) or did not occur.