Listeria monocytogenes is a gram-positive, foodborne pathogen. It is widely distributed in the environment and occurs naturally in many raw foods. Listeria monocytogenes is psychrotrophic and halotolerant and can, under otherwise optimal conditions, grow in the range of 1 to 45 °C and between 0 and 10% NaCl. As a consequence it may grow in many food products with extended shelf lives. Products that do not receive a heat treatment by the consumer, including ready-to-eat (RTE) products such as cheeses, meat, and fish delicatessen products, may contain high levels of L. monocytogenes when eaten, and many of these types of foods have been associated with listeriosis. In general, populations in foods are low (0-103 cfu / g with 90-99% being below 102 cfu / g and less than 1% being between 103 and 104 cfu / g; however, higher concentrations (105 - 107 cfu/g) have been reported.
So, we are eating it all the time if we like eating RTE cheeses, processed meats and CSS, so how many people are getting sick from listeriosis? and what does listeriosis do to us?
Listeria monocytogenes may be divided into 13 different serotypes, all of which may cause listeriosis. The vast majority of cases, however, is caused by serotypes 1/2a, 1/2b and 4b. The ingestion of high numbers of L. monocytogenes is a significant threat to health for people in risk groups such as immunocompromised, elderly, foetuses, and newborn babies. In these groups, the mortality from listeriosis is high, typically 20-30%. Because L. monocytogenes can readily be isolated from smoked fish and because inoculation trials have demonstrated significant growth in such products, the risk of getting listeriosis from consuming these types of products is taken very seriously.
There are no documented (published) cases of listeriosis from cold-smoked salmon. So how potent is Listeria monocytogenes?
The virulence of L. monocytogenes varies with the strain.
In a recent study it was found that the majority (63.2%) of L. monocytogenes isolated from cold-smoked salmon belonged to lineage II, whereas clinical isolates and 30% of fish isolates belonged to lineage I. These findings suggest that the pathogenicity potential of isolates from cold-smoked salmon may be lower than predicted from its prevalence.
mm.. that sounds like it is worthy of further investigation..
Why do the Standards for RTE products vary?
The levels of L. monocytogenes found in cold-smoked salmon are not different from those in other RTE food products so the Standards should be the same.
What is a safe level of L. monocytogenes in cold smoked salmon?
A WHO and FAO team in 2001 concluded as part of an expert consultation that 99% of all listeriosis cases would be eliminated if levels of L. monocytogenes were kept below 1000 cfu L.m./g at point of consumption assuming a consumption pattern equivalent to that of the USA.
For the purpose of setting standards it should be accepted that it is not possible to produce certain seafood products consistently free of L. monocytogenes.
So, what is the level typically used?
In many cases it is an almost zero tolerance level?! Where did that one come from?
FSANZ
For processed ready-to-eat finfish, such as smoked salmon, a level of up to 100 colony forming units (cfu) is permitted in one of the five samples, with the other four samples being "not detected" in 25g. This decision arose due to the absence of a listericidal step during processing of smoked fish. Therefore, there is the possibility of occasional low level contamination occurring, as
L.monocytogenes is frequently present in the processing environment. However, scientific opinion indicates that low numbers of L. monocytogenes are unlikely to cause illness in consumers other than possibly the most severely immunocompromised. However, as L.monocytogenes can grow in the fish during refrigerated storage, the application of the microbiological limits until the end of the products shelf life would mean that processors may need to impose on their processes a "not detected" process outcome. This would ensure that any growth of bacteria from levels below the detection during the shelf life of the product would not exceed 100/g.
But the WHO/FAO Team in 2001 said that no cases of listeriosis would occur at 1000/g?
Are their listericidal steps in the production of cold-smoked salmon?
HACCP
A hazard analysis of L. monocytogenes in cold-smoked salmon production, distribution and use reveals that with current processing and storage practices, no listericidal step (CCP) exists. Thus the organism survives processing, recontamination will occur and the typical storage conditions [vacuum-packed, chill-stored (5°C),
NaCl at 3–6% (water phase salt) and pH of approx. 6.2] may not prevent growth to hazardous levels. Sporadically, high levels of the organism are detected. However, it must be emphasised that control measures that prevent or limit growth after production can be introduced, e.g. by frozen storage or by limiting shelf-life.
Several studies have documented that L. monocytogenes grows well in cold-smoked salmon. Such studies are typically using inoculated packs to evaluate growth potential. Growth in naturally contaminated products is mostly not nearly as pronounced. This may be explained by the inhibition of growth by naturally occurring lactic acid bacteria. Although cold-smoked salmon is regarded as “one” product, it actually covers a diverse range of products where NaCl levels range from as low as 1.5% NaCl to almost 7%. Also, smoking procedures vary and some types of smoking may be inhibitory to growth of the organism.
Several measures that can prevent growth (hence constituting a CCP) could be implemented:
• The use of freezing for storage/distribution (quality issues);
• >10% NaCl [(WPS) salty flavour issue though] or the addition of other inhibitory factors;
• Lactates with or without a CO2-atmosphere;
• Lactic acid bacteria and/or their bacteriocins and bacteriophages;
• Selected salt-smoke combinations, paying special attention to the phenol content of the smoke (research needed);
• Eliminating L.m. after packaging using processes such as ultra-high pressure, irradiation, etc (research needed).
So, again, what level should we use?
Following the International Commission on Microbiological Specifications of Foods in 2002, a performance criterion (PC) was the required outcome of a step, or a combination of steps that contribute to assuring a food safety objective (FSO) is met.
It has been demonstrated that in naturally-contaminated cold-smoked salmon stored at 5°C, a 1 log increase occurs during a 3-week storage period. Therefore, if a shelf life limit of less than three weeks (at 5°C) is used, the PC of 10 cfu L.m./g at the end of the processing line will allow the FSO to be met.
Most processors will set a PC of <10 cfu L.m./g to build in safety margins. However, at present, there is no consensus on what this safety margin should be.
The take home message for processors is that noone wants to admit to having Listeria monocytogenes in their processing plants and so the dialogue to getting a "safe" level is not likely to proceed very fast, as history has already shown.
The take-home message for consumers is that processors generally have excellent hygiene processes such that the levels clearly do not reach danger points at the end of shelf life as demonstrated by a "zero" figure on listeriosis in CSS. It is highly likely that L. monocytogenes growth is restricted by lactic acid bacteria, smoke compounds and salt levels such that final levels are consistently very low.