ESTIMATING THE PREVALENCE AND THE SENSITIVITY OF THREE MICROBIOLOGICAL SUBSTRATES IN A CULTURE-DEPENDENT DETECTION METHOD FOR LISTERIA MONOCYTOGENES USING A BAYESIAN MODELING APPROACH: APPLICATION TO A MINCED PORK MEAT SURVEY

摘要

The aim of this study was to estimate the prevalence and the population level (CFU/g) of Listeria monocytogenes in minced pork meat, as well as the sensitivity of three different culture media used for the detection of the foodborne pathogen by the application of a Bayesian modeling approach. Samples (n=100) purchased from local markets were tested for L. monocytogenes presence by plating in parallel onto three diagnostic media most commonly used in routine analysis: PALCAM, ALOA and RAPID'L.mono. Species confirmation was double-checked by performing biochemical tests and specific polymerase chain reaction. Twenty two (22) samples were identified as positive to L monocytogenes, so prevalence was 22%. Sensitivity was calculated at 72.7%, 68.2% and 77.3% for PALCAM, ALOA and RAPID'L.mono, respectively. Based on the results obtained by plating onto the culture media two Bayesian models were set-up for prevalence and sensitivity estimations using the WinBUGS vl.4 software, whereas another model for estimating pathogen level was set-up in the Excel and simulated with the @Risk v4.5 for Excel software to construct the posterior distribution. The first model for prevalence estimation utilized the data from the parallel use of all three substrates tested, while the second was developed by combining the data of the two chromogenic media ALOA and RAPID'L.mono. These models predicted satisfactorily the prevalence of the pathogen and the sensitivity of the culture methods (error<20%), therefore the second model was chosen due to its fewer parameters. An independent experiment was conducted to validate the developed model by analyzing extra samples (n=10). The Bayesian model estimated the prevalence of L. monocytogenes at 22.7% (95% confidence interval: 11.1%-36.5%) and the sensitivity of ALOA and RAPID'L.mono for the culture-dependent detection method at 65.8% (48.5%-82.2%) and 74.4% (58.1%-88.7%), respectively. The population of L monocytogenes in mince was estimated at 14 CFU/kg (8-23 CFU/kg) and 17 CFU/kg (11-26 CFU/kg) for ALOA and RAPID'L.mono, respectively. Validation results from the developed model showed good correlation between observed and predicted prevalence (error=-2.17%). The use of at least two culture media in parallel enhances the efficiency for L. monocytogenes detection.