The possible effect of a sanitization program on intraspecies differentiation of Listeria monocytogenes strains isolated from a fish processing plant

Forty-seven Listeria monocytogenes strains isolated during a year in a selected Polish fish-processing plant as well as 7 L. monocytogenes strains of different origins (including a reference strain) were analyzed in our studies. Strains were isolated from raw fish fillets (flounder), frozen coated flounder fillets, coating ingredients, and the processing environment. Isolation of strains covered the period of a sanitization program introduced in the plant. L. monocytogenes was identified using conventional microbiological methods and the PCR technique. RAPD (random amplified polymorphic DNA) technique for fingerprinting was applied to analyze the intraspecies diversity. Six RAPD types (A – F) and seven unique strains were revealed as a result of fingerprinting with one persistent type isolated from July 1999 to February 2000. It was detected for the first time after one month of sanitization. Its occurrence could have been promoted by clone selection either due to ineffective disinfection or to resistance against the disinfectant. As L. monocytogenes mostly occurred on frozen products, this indicates that contamination could start during product freezing, cold storage, or handling. The results revealed that there is a crucial need for preparing sanitization schemes precisely targeted at L. monocytogenes to avoid its recurrence as persistent ‘in – house' strains. The possibility of incorrect interpretations of classical microbiological test results as well as the necessity to introduce assays based on nucleic acid analysis into epidemiological investigations were emphasized.