Assessing in-house monitoring efficiency by tracing contamination rates in cheese lots recalled during an outbreak of listeriosis in Austria.

A cluster of 34 cases of listeriosis was traced to consumption of quargel cheese, a sour milk specialty, in Austria, Germany and Czech Republic between 2009 and 2010. After recall from the retail market all soft cheese batches (n = 18) were sent for investigation and ISO 11290 based microbiological analysis revealed all red smear-ripened batches (16/18) to be positive for Listeria monocytogenes whereas mold ripened cheeses were negative. The 16 positive batches were grouped into three categories: those having exceeded shelf-life (G1), those around shelf-life (± 4 days, G2) and those within shelf-life (G3). Tracing the contamination levels as measured after recall (CLR) to the theoretical contamination level after processing (CL0) was considered to provide an estimate as to whether the in-house monitoring system would have been capable of unraveling the contamination scenario. Growth simulations starting from various hypothetical initial contamination levels of cheese at the plant and considering the potential variability in growth of L. monocytogenes due to model parameters and storage conditions suggested that a very low initial contamination level (e.g., <1 CFU/g or 5 CFU/100g) could justify the levels of L. monocytogenes enumerated in recalled samples of G1 and G2 lots. This in turn, may have resulted in low detection probability using ISO 11290:1996. In lots of G3 group, however, high initial contamination levels or temperature abuse at retail are inferred, based on simulated outputs.