| Listeria monocytogenes is one of the most important food-borne intracellular pathogen which can cause human serious listeriosis such as meningitis, sepsis, encephalitis, fetal death, and premature births. People especially immuno-compromised people who consumed the food contaminated with L. monocytogenes may suffer from such serious infection. More and more studies showed that the prevalence of L. monocytogenes in food products especially in RTE food products is high and we now have fundamental insight in the contamination pattern of L. monocytogenes in food chain and lineage classification of I. monocytogenes. The objective of this study is to (i) establish a rapid detection method for L. monocytogenes based on PCR technique; (ii) study on the prevalence of L. monocytogenes in different kinds of food products and sewages in local Chinese markets; (iii) investigation of L. monocytogenes contamination pattern in local Chinese food market and the tracing of two clinical isolates by RAPD analysis; (iv) lineage classification of the Chinese L. monocytogenes isolates based on the partial sequence of the actA gene and study on the invasion and multiplying ability of strains within different lineages.1. Rapid detection of Listeria monocytogenes based on PCRA polymearase chain reaction (PCR) assay targeting the hly gene was developed for detecting L. monocytogenes in pure cell cultures and on artificially contaminatedpork, water and milk. 850-bp PCR product was detected in all 43 L. monocytogenes strains. In contrast, the PCR product was not detected both in all other Listeria spp., including Listeria innocua, Listeria ivanovill, Listeria seelingeri, Listeria welshimeri, or Listeria grayi and in non-Listeria bacteria, indicating that the primer set we use was highly specific for L. monocytogenes. The detection limit of the PCR assay for pure cell cultures was 105 cfu per ml pure cell culture. However, the assay could detect as few as 101 cfu of L. monocytogenes in 25 g of pork and 25 ml milk and water following 16 h ofenrichment in Listeria Enrichment broth (LEB) at 37 C. Further studies showed that thedetection limit of DNA template is 6.5pg and the total assay time including enrichment was approximately two days. These results suggest that the PCR assay based on amplifying hly gene can be used to detect L. monocytogenes on pork, milk, water and possibly other types of food products rapidly, specifically and sensitively.2. Prevalence of L. monocytogenes in different kinds of food products and sewages in local Chinese marketsIn this study we attempted to investigate the prevalence of L. monocytogenes in different kinds of food products and market sewages in local Chinese markets. 1352 of fresh food product samples, 854 of ready-to-eat (RTE) food products, 645 of sewage samples from four markets (market A, B, C and D), 120 of packaged RTE food product samples from two supermarkets and 420 of meat product samples of freshly slaughtered hogs were collected for the isolation and identification of L. monocytogenes. Our results showed that the overall prevalence of L. monocytogenes for fresh food products and sewage samples in the four markets was 2.01%, with prevalence ranging from 0.00% which was found in vegetables in market A, B, bean products in market A, C and fresh poultries in market C to 5.56% which was found in fresh meat products in market A.Statistic analysis showed that the average prevalence of fresh meat (4.83%) was significantly higher than that of other food products (p<0.05) suggesting that fresh meat may be suitable for the growth and spread of L. monocytogenes. No significant differences were found between the prevalence of L monocytogenes in each market. Overall prevalence for RTE food products in those four markets was 2.47% with prevalence ranging from 0.00% to 6.67%. No significant differences were found between the prevalence of L. monocytogenes in each market and also no significant differences were found between the prevalence of L. monocytogenes in each kind of RTE food products. T... |
PDF Full Text Request