INTRODUCTION

Raw cured shelf-stable meat sausages are very popular in Bulgaria. This group of products includes some low-acid dried sausages and high-acid fermented sausages with high microbial stability (9). The significance of Listeria monocytogenes as a foodborne pathogen is complex. The severity and case-fatality rate of the disease require appropriate preventive measures, but the characteristics of the bacteria make it unrealistic to expect that all food could be Listeria-free (16). According to Data (4), L.monocytogenes was isolated from various acidic foods, including sausages. The author noted that L.monocytogenes could be inactivated more easily by the stomach and intestinal pH and hence it would have higher infective dose, compared to some other food-borne pathogens. Recently it was noted that L.monocytogenes could possibly grow in sausage mix if there was fermentation failure (8). Also it was emphasized that during normal fermentation, drying and ripening there was a 10 to 100-fold decrease in L.monocytogenes counts. A low number of microorganisms could be found in the finished product which would not be able to multiply (11, 12). A possible benefit of the microbiological risk assessment (MRA) document for Listeria monocytogenes, developed jointly by the USFDA and the USDA for risk ranking of ready-to-eat foods in 20 food categories (one of them is raw cured dried meat products) was noted (7). There is opinion mentioned that the primary sources responsible for introduction of L. monocytogenes into the processing environment had not been clearly identified (10). In the case of processing of raw cured dried meat products, contamination of sausage ingredients played the main role.

Raw cured dried meat products can be included into the group of ready-to-eat (RTE) foods unable to support the growth of L. monocytogenes, set in Chapter 1. “ Food safety criteria” of the Commission Regulation (EC) No 2073/2005 in case of examination of pH and water activity (a_w) of the tested batch. EFSA opinion (6) concluded that the actual data did not provide a clear picture of the risk of listeriosis. Based on the dose-response relationship of L. monocytogenes, the risk of listeriosis is mostly related to products with high concentrations of the pathogen. Thus it is important to analyze the actual concentration of the pathogen in RTE products and especially the percentage of products with high concentrations.

The aim of this study was to collect actual data concerning the frequency of contamination with Listeria monocytogenes of some raw dried cured vacuum packed sausages very popular in Bulgaria, produced from Oct. 2004 till May 2008 in different meat producing plants.

MATERIALS AND METHODS

RESULTS

Data showing the detected Listeria spp. strains in raw cured dried meat products are presented in Table 1.

Table 1. Frequency of detection of Listeria spp. in raw cured dried meat products

Ten specimens were positive for presence of Listeria monocytogenes, equal to 6,75% of the 148 tested samples. In two other raw dried cured sausages L.welshimeri and L.innocua were found, but these species are not pathogenic for consumers. In the period before the official implementation of HACCP system (01.01.2006) in Bulgaria, 52 samples were examined and 5 Listeria monocytogenes isolates were found (~10%). 2,5 years after the HACCP implementation, 96 specimens from the same meat factories were tested and 5 Listeria monocytogenes isolates (5,2%) were detected. Samples taken from lots, produced in winter time were contaminated with Listeria monocytogenes more often (7 of all 10) than specimens taken during other seasons.

Results for morphological and biochemical characteristics of isolated Listeria spp. strains and their biochemical profile are shown in Table 2.

Table 2. Morphological and biochemical characteristics of Listeria spp. isolated from raw cured dried meat products

All L.monocytogenes isolates had similar morphological and biochemical characteristics and when tested with API Listeria, the biochemical profile was 6 510. L.innocua isolate showed lack of β-hemolytic ability, positive reaction to DIM, negative to XYL, RIB, G1P, and TAG, and biochemical profile 7 510. L.welshimeri isolate did not present β-hemolytic activity and manifested positive result to DIM, negative reaction to RIB and G1P and biochemical profile 7 711.

DISCUSSION

Results of the study showed presence of L.monocytogenes and some other representatives of Listeria spp. in ready-to-eat raw cured dried sausages, produced in Bulgarian meat plants. Frequency of reporting of L.monocytogenes was 6,75% of all tested samples.

On the other hand specimens examined before the HACCP implementation showed higher frequency of contamination (9,6%), compared to samples tested after application of this system (5,2%). All 10 isolates had similar characteristics and biochemical profile. One of the tested samples was positive for L.innocua and another showed presence of L.welshimeri. In Brazil there was evalution of raw cured meat products for occurrence of Listeria monocytogenes. The pathogen was detected in 13.3% of Italian type samples of salami, while L. innocua occurred in 6.5% of the Italian type and in 16.6% of the Milanese type (2). In Turkey Listeria spp. and L. monocytogenes from 63 (21%) and 35 (11.6%) of the tested Turkish style fermented sausages (sucuk) (3). In USA it was proven that the cumulative 3-y L.monocytogenes prevalence for dry and semidry fermented sausages was 3.25%. This kind of sausages is very popular also in Bulgaria and 4 of 10 of our L.monocytogenes isolates were isolated from this type of raw cured dried product (13). Opinion was noted that processing facilities were frequently contaminated with nonpersistent or persistent L. monocytogenes, which played an important role in contamination of the products. The contamination status of processing lines and machines was influenced by the compartmentalization of the processing line, with poor compartmentalization increasing the risk of L. monocytogenes presence (14). Data was presented that L. monocytogenes had the ability to adhere to stainless steel, but significant differences existed in the ability of various L. monocytogenes strains to attach to the surface.

Dust contaminated with L. monocytogenes was another source of contamination (5). Some results remarked that food safety incidents due to the growth of pathogenic bacteria such as Listeria monocytogenes in fermented sausage might provoke serious consequences for consumers (17).

To protect consumers from L.monocytogenes in ready-to-eat foods unable to support its growth, a ‘microbiological criterion’ set in the COMMISSION REGULATION (EC) No 2073/2005 for maximum level of 100 CFU/g products during their shelf-life was applied.

CONCLUSION

Frequency of contamination with L.monocytogenes of the tested raw cured dried sausages for the study period was 6,75%. The level of contamination was higher in the period before the implementation of the HACCP system in meat producing plants. Winter season was the time with higher prevalence of L.monocytogenes. Rarely some other Listeria spp. were found.