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Academic
dissertation, 2004, Department of Food and Environmental Hygiene, Faculty of
Veterinary Medicine, University of Helsinki, Finland, 60 pp.
Persistent Listeria monocytogenes contamination in
food processing plants
Janne Lunden
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 2 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 plant contamination.
---------- 8< ----------
7 CONCLUSIONS
1. Persistent L. monocytogenes strains were observed to adhere
to stainless steel surfaces in higher cell numbers than non-persistent strains
after short contact times. Such enhanced adherence increases the possibility of
survival of the persistent strains due to increased resistance against
prevention methods and may have an effect on the initiation of persistent plant
contamination.
The adherence level of non-persistent strains was closer to the adherence level
of persistent strains after a long contact period. It appears therefore that
adherence over long contact times, in contrast to adherence over shorter periods
does not explain why some strains are persistent and some non-persistent.
2. The initial resistance of persistent and non-persistent L.
monocytogenes strains to disinfectants varied. Differences in resistance to
disinfectants may influence the survival of strains in food processing plants.
Both persistent and non-persistent strains adapted to all disinfectants
investigated, with the exception of potassium persulphate. The increase in
resistance was similar for persistent and non-persistent strains and did not
reach the concentrations of disinfectants used at food processing plants.
However, the adaptive response may have an effect on the survival of strains
when they encounter suboptimal disinfectant concentrations.
3. The persistent and non-persistent L. monocytogenes strains
cross-adapted to all disinfectants except potassium persulphate. Potassium
persulphate was the only agent that L. monocytogenes was observed not to adapt
or cross-adapt to, and thus it seems suitable for long-term use. However,
potassium persulphate did cause cross-adaptation of L. monocytogenes to the
other agents. The cross-adaptive responses appeared to be nonspecific as
cross-adaptation was observed between related and unrelated agents. Maintaining
high disinfectant efficiency of these agents by rotation may therefore not be
possible, even with agents with different mechanisms of action.
4. The PFGE types found in the raw materials had not established
themselves in the post heat-treatment lines. Persistent contamination therefore
appears not to be generally due to continuous recontamination by raw materials.
However, the contamination may have originated from raw materials. This
hypothesis is supported by several studies that show a high prevalence of L.
monocytogenes in raw materials. In addition, it is possible that had more raw
material samples been investigated in Study IV persistent strains would have
been found in raw materials.
Some of the strains were found only in one plant, suggesting that they may have
been plant-specific. However, it is difficult to determine whether a strain is
plant-specific or not. Increasing the number of samples might have resulted in
some of these strains being found in other plants as well. The persistent L.
monocytogenes strains were widely distributed in the food processing plants and
they more often contaminated cooked products than the non-persistent strains
illustrating their importance in food safety. The processing lines producing
fermented products were not persistently contaminated possibly due to the
presence of competing microbes on the processing surfaces.
5. Compartmentalization, and especially the separation of the
raw area from the post heat- treatment area, appeared to influence the
contamination status of the post heat-treatment
line. Poor separation of the raw area from the post heat-treatment line seemed
to increase contamination pressure. The processing machines were observed to be
contaminated with and to sustain L. monocytogenes contamination. Elimination of
contamination from complex processing machines was shown to be possible by
thorough disassembly of the machines and targeted sanitation. However, in
processing lines with inadequate compartmentalization, contamination appeared to
be reintroduced at some point due to poor hygiene barriers.
6. The L. monocytogenes contamination status of food products
reflects the contamination status of the corresponding food plant. Some of the
foods of one producer were found to be recurrently contaminated with one PFGE
type, indicating that the food processing plant or the processing line was
persistently contaminated. Some of the foods produced by several producers were
also recurrently contaminated with the same PFGE type, indicating that the PFGE
type may have been persistent in several plants.
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