Fluctuation in contamination dynamics of L. monocytogenes in quargel (acid curd cheese) lots recalled during the multinational listeriosis outbreak 2009/2010

For the first time it has been possible to determine the contamination level of Listeria monocytogenes in the very cheese lots of acid curd cheese that caused a multinational outbreak between 2009/2010. The listeriosis outbreak accounted for 34 clinical cases and eight deaths. The cheese, which was distributed in Austria, Germany, the Czech Republic, Poland and Slovakia, was recalled on the 23rd January 2010. All recalled lots were immediately investigated after call back from the retail market. The company manufactured two different cheese types, (i) red smear ripened--and (ii) mold coated/white veined--acid curd cheese. Depending on the lot production dates, cheese samples (n=1045) were analyzed at three different time points: (i) beginning to mid shelf-life (lot nos. 15-18; production period 5.1.2010-13.1.2010); (ii) end of shelf-life (lot nos. 9-18; production period 21.12.2009-13.1.2010) and, (iii) ≤46days after the expiry date (lot nos. 1-18; production period 1.12.2009-13.1.2010). Qualitative and quantitative examinations of cheese samples were performed according to ISO 11290-1&2. Examination of the samples, according to ISO 11290-1, resulted in 16 L. monocytogenes positive (red smear type) and two negative lots (mold coated type). These results were confirmed by a combined enrichment/real-time PCR method. The contamination values obtained by quantitative ISO 11290-2 varied from ≤log 2 cell forming units (CFU)/g to log 8.1CFU/g. Three out of sixteen L. monocytogenes positive lots revealed a contamination level of ≤log 2CFU/g at the beginning of their shelf-life when stored at 4°C. Nevertheless, by increasing the storage life and/or the storage temperature (15, 22°C) the contamination level could be raised to between log 5 and log 6CFU/g. Our data indicate that 81.3% (13/16) of the recalled red smear quargel cheese lots were highly contaminated with L. monocytogenes. All this implies that the main contamination of the quargel cheese took place during the red smear process and that quargel cheese can easily support growth of L. monocytogenes.