ABSTRACT : Reasons for persistent Listeria monocytogenes food plant contamination were investigated. Properties important in the survival of persistent and non-persistent L. monocytogenes strains in food processing plants were examined, and factors in food processing lines that predispose to persistent contamination were identified.
Persistent L. monocytogenes strains showed higher adherence levels to stainless steel surfaces than non-persistent strains after short contact times. Because, adherence to stainless steel increases resistance against sanitation procedures, efficient adherence over a short period may have an effect on the initiation of persistent plant contamination. Differences in initial minimum inhibitory concentrations (MICs) of disinfectants observed between L. monocytogenes strains may also have an effect on the survival of these strains in food processing environments. Persistent and non-persistent L. monocytogenes strains were observed to adapt to quaternary ammonium compounds (QACs), tertiary alkylamine and sodium hypochlorite at 10ºC and 37ºC. Persistent and non-persistent strains adapted to similar levels. The adaptive response was observed after a 2-h sublethal exposure, indicating rapid response of the cells. The highest increase in resistance was over 15-fold. Although the increased resistance did not exceed the concentrations of disinfectants used at food processing plants, it may influence the survival of cells when the concentration of the disinfectant is sublethal due to inadequate sanitation procedures.
Since, all disinfectants caused cross-adaptation of L. monocytogenes, maintaining a high disinfectant efficiency by rotation is difficult. The only disinfectant that L. monocytogenes was not observed cross-adapt to was potassium persulphate. However, potassium persulphate caused cross-adaptation of L. monocytogenes to the other disinfectants, which reduces the effectiveness of these agents. Cross-adaptation was not seen only to disinfectants with similar mechanisms of action but also to disinfectants with different mechanisms of action, indicating non-specific responses. Persistent and non-persistent L. monocytogenes strains were observed in all meat and poultry processing plants. The persistent strains were often widely spread in the processing plant, contaminating two or more processing lines. Important factors sustaining contamination were complex processing machines and poor compartmentalization of processing lines. The elimination of L. monocytogenes from processing machines was difficult but shown to be possible with regular and thorough disassembly and for example alkali-acid-alkali washes. Compartmentalization, especially the separation of the raw area from the post heat-treatment area, seemed to affect the contamination status of processing lines, with poor compartmentalization increasing contamination. L. monocytogenes contamination of final food products reflects the contamination status of the manufacturing food plant. Some L. monocytogenes pulsed-field gel electrophoresis (PFGE) types were found repeatedly from the product of one producer, indicating a persistent contamination in the food plant. Some PFGE types were also found repeatedly from the products of different producers, indicating persistence of these types in several plants. In conclusion, persistent L. monocytogenes plant contamination appears to be the result of the interaction of several different factors. Properties influencing survival, including enhanced adherence to food contact surfaces and adaptation to disinfectants, in addition to such predisposing factors in the processing line as complex processing machines and poor compartmentalization may lead to persistent L. monocytogenes plants contamination.
Persistent L. monocytogenes strains showed higher adherence levels to stainless steel surfaces than non-persistent strains after short contact times. Because, adherence to stainless steel increases resistance against sanitation procedures, efficient adherence over a short period may have an effect on the initiation of persistent plant contamination. Differences in initial minimum inhibitory concentrations (MICs) of disinfectants observed between L. monocytogenes strains may also have an effect on the survival of these strains in food processing environments. Persistent and non-persistent L. monocytogenes strains were observed to adapt to quaternary ammonium compounds (QACs), tertiary alkylamine and sodium hypochlorite at 10ºC and 37ºC. Persistent and non-persistent strains adapted to similar levels. The adaptive response was observed after a 2-h sublethal exposure, indicating rapid response of the cells. The highest increase in resistance was over 15-fold. Although the increased resistance did not exceed the concentrations of disinfectants used at food processing plants, it may influence the survival of cells when the concentration of the disinfectant is sublethal due to inadequate sanitation procedures.
Since, all disinfectants caused cross-adaptation of L. monocytogenes, maintaining a high disinfectant efficiency by rotation is difficult. The only disinfectant that L. monocytogenes was not observed cross-adapt to was potassium persulphate. However, potassium persulphate caused cross-adaptation of L. monocytogenes to the other disinfectants, which reduces the effectiveness of these agents. Cross-adaptation was not seen only to disinfectants with similar mechanisms of action but also to disinfectants with different mechanisms of action, indicating non-specific responses. Persistent and non-persistent L. monocytogenes strains were observed in all meat and poultry processing plants. The persistent strains were often widely spread in the processing plant, contaminating two or more processing lines. Important factors sustaining contamination were complex processing machines and poor compartmentalization of processing lines. The elimination of L. monocytogenes from processing machines was difficult but shown to be possible with regular and thorough disassembly and for example alkali-acid-alkali washes. Compartmentalization, especially the separation of the raw area from the post heat-treatment area, seemed to affect the contamination status of processing lines, with poor compartmentalization increasing contamination. L. monocytogenes contamination of final food products reflects the contamination status of the manufacturing food plant. Some L. monocytogenes pulsed-field gel electrophoresis (PFGE) types were found repeatedly from the product of one producer, indicating a persistent contamination in the food plant. Some PFGE types were also found repeatedly from the products of different producers, indicating persistence of these types in several plants. In conclusion, persistent L. monocytogenes plant contamination appears to be the result of the interaction of several different factors. Properties influencing survival, including enhanced adherence to food contact surfaces and adaptation to disinfectants, in addition to such predisposing factors in the processing line as complex processing machines and poor compartmentalization may lead to persistent L. monocytogenes plants contamination.
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