## Modelling the probability of Listeria monocytogenes growing in cheese

**Characteristics of ***Listeria monocytogenes*

*Listeria monocytogenes*

http://www.dairyscience.info/newcalculators/listeria-d.asp

## **Concentration of ***Listeria monocytogenes *in raw milk

*Listeria monocytogenes*in raw milk

* ***Growth and survival of ***Listeria monocytogenes* in cheese

*Listeria monocytogenes*in cheese

**Minimum infective dose**

**Permitted concentration of ***Listeria monocytogenes* in food

*Listeria monocytogenes*in food

Current EC regulations (Regulation(EC) No 2073/2005) permit 100 CFU/g of *L. monocytogenes* in ready to eat food regardless of whether the pathogen can grow in the food. This is somewhat contentious in that some scientists argue that there is significant inherent risk in having this concentration of pathogen in any product that supports its growth.

**Overview of cheese borne listeriosis**

Cheese has been implicated in a number of major outbreaks of listeriosis in Europe and North America. An outbreak in California in 1985 was shown to be due to the consumption of Mexican-style fresh cheese (Linnan *et al*., 1988). There were some 86 cases of listeriosis; many involved mothers and their young children. At least twenty nine deaths occurred (33.7% mortality). Following investigation of the outbreak it has been suggested that the producers of the cheese may have mixed raw and pasteurised milk or that post pasteurisation contamination may have caused the problem (Silliker, 1986).

Fretz *et al* (2010) reported an outbreak of listeriosis in Austria and Germany due to the consumption of ‘Quargel’ cheese. The outbreak accounted for 14 cases in 2009, including four deaths.The cheese product was voluntarily withdrawn from the market on 23 January 2010. The outbreak was identified by molecular typing.

Following confirmation that a *L. monocytogenes* 1/2a isolate from the production plant was indistinguishable from the outbreak strain by genotyping Quargel cheese products were sampled at the production plant. The authors reported that investigations on January 13 yielded three different strains of* L. monocytogenes* 1/2a, including the outbreak strain, in numbers < 100 CFU/g per gram. However cheese sampled later (18 January 2010) yielded greater than 100 CFU/g

*L. monocytogenes*.

## Growth / no-growth models for predicting the growth of *Listeria monocytogenes* in Mexican-style fresh cheese after 42 days storage at at 10°C.

Bolton and Frank (1999) used binary and ordinal logistic regression to model the behaviour of *L. monocytogenes *in a cheese-based model system over a range of pH, salt and moisture content. Two models were developed, a binary regression and an ordinal logistic regression model. The binary regression model can be used to predict the probability of growth or no growth of *L. monocytogenes.* The ordinal logistic regression model can also be used to assess the probability of growth, but will also predict the probability of death or stasis (no growth) occurring within the range of data inputs used to formulate the model.

**Observations on the models**

**Literature cited**

Bajard, S., Rosso, L., Fardel, G. and Flandrois, J. P. (1996). The particular behaviour of *Listeria monocytogenes* under sub-optimal conditions. Int. J. Food Microbiol. 29:201–211.

Fretz R., Sagel, U., Ruppitsch W., Pietzka, A.T., Stöger, A., Huhulescu, S., Heuberger, S., Pichler, J., Much, P., Pfaff, G., Stark, K., Prager, R., Flieger, A., Feenstra O., Allerberger, F. (2010) Listeriosis outbreak caused by acid curd cheese ‘Quargel’, Austria and Germany 2009. Euro Surveill. 15(5):pii=19477. Available online: https://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19477 .

Goulet, V., Jacquet, C., Vaillant, V., Rebiere, I., Mouret, E., Lorente, C., Maillot, E., Stainer, F., Rocourt, J. (1995). Listeriosis from consumption of raw-milk cheese. Lancet 345, 1581-1582.