Friday, February 27, 2004

I get listerious;
or, lazy internet research

Over at the cheese diaries, Anne is debating the microbiology of her refrigerator. Her fear of listeria lead me to wonder about some things. First of all, as Steingarten has shown for American cheese (and Ed Behr for french époisses), pasteurized milk is actually the culprit for many listeriosis outbreaks. I was not able to track down an easy database to check this out more thoroughly, but at least one randomly chosen outbreak from 2000 confirms my suspicions. From MMWR:
Various members of the Hispanic immigrant community made the Mexican-style fresh soft cheese from raw milk in their homes. Inspectors found unlabeled homemade cheese in all three of the small local Latino grocery stores they visited in Winston-Salem. In addition, many persons regularly sold the cheese in parking lots and by going door-to-door. Owners of two local dairies reported selling raw milk. Milk samples were obtained from these two Forsyth County dairies and from three dairies in neighboring counties. L. monocytogenes isolates were obtained from nine patients, three cheese samples from two stores, one cheese sample from the home of a patient, and one raw milk sample from a manufacturing grade dairy. All 14 isolates had indistinguishable PFGE patterns, indicating a common link.

NCDA&CS conducted an investigation at a manufacturing grade dairy farm to determine the potential source of L. monocytogenes contamination. NCDA&CS collected milk samples from all 49 cows in the herd and samples from the bulk milk storage tanks. Milk from each cow was tested for somatic cell count to identify mastitic cows. Milk from each cow also was tested for presence of L. monocytogenes. Repeated testing did not identify any cow with milk confirmed positive for L. monocytogenes, suggesting that the cows were not infected and that L. monocytogenes may have originated from environmental contamination.

In other words, they couldn't find it in the cows or the dairies, but they did find it in one milk sample -- but they won't say where it's from. They also fail to demonstrate that all the cheeses were made with the raw milk. In short, as they are forced to admit/obfuscate, the contamination was most likely environmental: i.e., dirty processing of clean milk -- which means it's irrelevant whether it was pasteurized or not.

My second question was to what extent listeria can develop after a long time, like a couple months in the fridge. I was unable to find a good source for that either, but I learned some interesting things from Ariel Maoz, et al., "Temporal Stability and Biodiversity of Two Complex Antilisterial Cheese-Ripening Microbial Consortia," Applied and Environmental Microbiology 69, no. 7 (July 2003), p. 4012-4018.

When the traditional "old-young smearing" procedure is used for the production of red-smear cheeses, pathogenic microorganisms such as Listeria monocytogenes may also be transferred from the mature to the fresh cheeses. Due to its ubiquitous nature, its ability to grow at refrigeration temperatures, and its tolerance to low pHs (below pH 5.0) and high (up to 10%) sodium chloride levels, it is very difficult to control L. monocytogenes in a cheese environment.
The "ubiquity" suggests that we all eat low levels of listeria all the time, and, as with many bacteria, the problem arises when you hit a much more serious than usual infestation. Therefore, long storage times could create a problem where there was none, if the conditions change to encourage a modest amount of listeria to blossom into an enthusiastic colony. Anyway, the authors investigate the antilisterial effects of two different "microfloral consortia" (i.e., the other bacteria) from red-smear cheeses, which are not defined but appear to include limburger and vacherin mont d'or (they found over 400 different strains!):
The antilisterial activity of consortium R as determined in situ on soft cheese is displayed in Fig. 3. In both experiments A and B, L. monocytogenes WSLC 1364 grew to high numbers on the control cheeses (M) from initial contamination levels of 40 and 60 CFU/ml of brine, respectively. At the same initial contamination levels, the pathogen was inhibited completely by consortium R. Except for one positive enrichment at day 41 (experiment A), it was also inhibited completely at initial contamination levels of 340 (experiment A) and 270 (experiment B) CFU/ml of brine, respectively. These results indicate a stable antilisterial activity of consortium R over 6 months. In experiment B, the antilisterial activity of consortium R was tested additionally at an initial contamination level of 1,600 CFU/ml of brine. Even then it showed a total inhibition of L. monocytogenes. The high incidence of Listeria on red-smear cheeses indicates that many of the industrial surface consortia exhibit no significant antilisterial activity or, alternatively, are contaminated by resistant Listeria strains....

Data on the in situ antilisterial activity of consortium K, derived at the beginning (sampling point A) and at the end (sampling point B) of the 6-month period, are depicted in Fig. 3. In both cases consortium K inhibited L. monocytogenes at initial contamination levels of 75 (sampling point A) and 70 (sampling point B) CFU/ml of brine. At contamination levels of 240 (sampling point A) and 280 (sampling point B) CFU/ml of brine, however, listerial growth could not be inhibited to the same extent. While comparison with the control consortium M still demonstrates a certain inhibitory effect, Listeria analysis yielded positive results after about 3 and 2 weeks. Our data demonstrate significant differences between consortia R and K, revealing a lower antilisterial potential of the latter, which decreased additionally within 6 months. Interestingly, this change in antilisterial activity was accompanied by a significant change of the species within each consortium.

What this means is that we don't know why some cheeses are able to fend off listeria and others can't. The authors dowplay the effect of biodiversity on antilisterial activity, because consortium K lost out to the listeria at high contamination levels. However, the uselesness of the (much less diverse) control consortium (M) by comparison suggests that biodiversity is a significant factor -- something best achieved with raw milk. Of course, it would be relevant to know what "normal" listeria concentrations are like, becasue if they are like the higher numbers that overwhelmed K, then the debate is academic. The fda's dose-response model predicts a median death rate of 1 in 769,231 servings at 1 x 1010 cfu/serving (the whole paper is here), but you'd have to know a lot more math than I do to correlate that with the levels Moaz et al. give for their brine. The FDA found levels that high in less than .01 percent of every dairy category, including soft fresh cheese and unpasteurized milk.

In sum, nothing conclusive, but this all does tend to reinforce the suspicion that if anything, raw milk may be safer than pasteurized, if handled properly. Also, it is possible to turn something harmless into something dangerous in your fridge. Please read the scary things at the CDC and the FDA for actual medical advice.


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