Research on quantitative and contamination level of Listeria monocytogenes from pre-packaged refrigerated ready-to-eat meat in Chengdu City

Abstract

ObjectiveIn order to find out the contamination situation and potential risk of Listeria monocytogenes in refrigerated ready-to-eat prepackaged foods. This study carried out a quantitative contamination risk survey of L. monocytogenes for ready-to-eat cooked meat products with short shelf life.MethodsWe collected 64 samples of ready-to-eat cooked meat from Chengdu， and carried out qualitative and quantitative detection of L. monocytogenes according to traditional culture method and RT-PCR molecular detection method. Additionally， the second generation of the whole genome sequencing was performed after DNA extraction of the isolated strains， and relevant epidemiological characteristics were analyzed.ResultsThe results showed that L. monocytogenes were isolated from 6 samples of cooked meat （6/64）. All 6 L. monocytogenes positive samples were produced by the same manufactory， of which 5 were three platters of marinated meat. The results of quantitative detection showed that the contamination level of L. monocytogenes in three platters of marinated meat was 30-200 CFU/g， and the contamination level of cooked pork samples with positive L. monocytogenes was lower as <10 CFU/g. Six isolates were identified as L. monocytogenes harboring LIPI-3 virulence island related genes by whole genome sequencing analysis. In silico MLST analysis found that the main ST types were ST3 （n=2） and ST121 （n=2）， followed by ST8 and ST9. Single nucleotide polymorphisms （SNP） results showed that 23 SNP difference was detected between L. monocytogenes strains LM22071911 and LM22080802， and 13 SNP differences were detected between L. monocytogenes strains LM22062706 and LM22080806. All four strains above were isolated from the same factory， indicating that there was a risk of direct transmission of two clonal groups of L. monocytogenes in the processing of the factory.ConclusionSome refrigerated ready-to-eat prepackaged foods sold in Chengdu are facing the contamination by L. monocytogenes， and it is necessary to strengthen the control in processing.

Keywords

• refrigerated ready-to-eat food
• prepacked food
• listeria monocytogenes
• quantitative detection
• foodborne pathogens