Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment
Incidence of Listeria monocytogenes in Fresh and Smoked Fish in Iran
A. Akhondzadeh Basti, T. Zahrae Salehi
The psychrotrophic pathogen Listeria monocytogenes is a widespread microorganism that has been isolated from many foods, including fish and shellfish (Farber 1991; Fuchs and Surendran 1991; Dillon et al. 1992; Dillon and Patel 1992; Jemmi and Keusch 1992; Ben Embarek 1994; Heinitz and Johnson 1997; Weagant et al. 1988). The several outbreaks that have occurred since 1979 have implicated vegetables and dairy products (Dillon et al. 1992) as well as undercooked fish and shellfish (Heinitz and Johnson 1997) as vehicles of infection. An epidemic of prenatal listeriosis in New Zealand suggested a possible link to the consumption of raw fish. That outbreak resulted in five deaths from 22 cases (Dillon and Patel 1992). A few cases have identified undercooked fish and shellfish (Heinitz and Johnson 1997). However, many sources of Listeria outbreak are never established.
Listeria monocytogenes is ubiquitous in nature and able to grow at low temperatures and at salt concentrations up to 10%; enhanced virulence of L. monocytogenes has been associated with growth at 4°C (Heinitz and Johnson 1997). Freezing has no detrimental effect on its viability; it is also fairly thermostable (Dillon et al. 1992).
Smoked fish is a ready-to-eat food commodity that is kept frozen or refrigerated and is often consumed without further cooking or heating. Older individuals who consume Listeria-contaminated smoked fish are at a higher risk of Listeria infection than other populations (Dillon et al. 1992).
Because of increasing demand for cultivated fish and the tradition of consuming raw, undercooked, and ready-to-eat smoked fish in Iran, the incidence of L. monocytogenes in three common types of cultivated fish from fish markets and fish farms in two provinces of Iran was investigated for the first time in this study.
Three hundred whole ungutted cultivated fresh coldwater (rainbow trout Oncorynchus mykiss) and warmwater (silver carp Hypophtalmichthys molitrix and common carp Cyprinus carpio) fishes as well as 40 ready-to-eat traditionally hot-smoked (≥80°C) and lightly salted (3%) silver carp were obtained aseptically from two fish markets (20 retail shops) and three fish farms in Tehran and Gilan Province, Iran. The fish farms were located near the city and a polluted river and were fertilized with large-animal manure.
Each fish was transported in a sterile plastic bag on ice to the laboratory for analysis. To isolate Listeria, the modified Canadian version of the U.S. Food and Drug Administration Listeria isolation protocol (Dillon et al. 1992) was used, with some modifications. In brief, a 25-g portion of various fish parts (with skin) was added to 225 mL of Listeria enrichment broth base (LEB; Merck) with potassium thiocyanate (37.5 g/L; Merck) and nalidixic acid (15 µg/mL; Sigma). The LEB was incubated at 25°C for 48 h. After incubation, 0.1 mL of LEB was streaked onto a Listeria-selective agar plate with nalidixic acid (50 µg/mL). The plate was incubated for 24–48 h at 35°C. Suspected Listeria colonies had a transparent yellow-green appearance and were streaked onto brain heart infusion broth (BHI; Merck) plates for purity and incubated at 35°C overnight.