Comparing the Behavior of Multidrug-Resistant and Pansusceptible Salmonella during the Production and Aging of a Gouda Cheese Manufactured from Raw Milk

Abstract

Outbreaks of salmonellosis have been linked to the consumption of cheese, and emerging multidrug-resistant (MDR) strains of Salmonella may be more virulent and more tolerant than less resistant strains to stresses encountered in food production, which may enhance the survival of these resistant strains in cheese. This study was conducted to compare the behavior of MDR and pansusceptible Salmonella strains during the manufacture and aging of Gouda cheese and compare pathogen recovery via several rapid and traditional methods. Cheeses were manufactured from raw milk inoculated with a six-strain cocktail of either MDR or susceptible Salmonella Newport and Salmonella Typhimurium at initial levels of ~20 CFU/ml. Samples of milk, whey, curd, and finished cheese were analyzed using eight enrichment and detection protocols. Overall, changes in pathogen levels observed throughout manufacture and aging did not differ significantly between MDR and susceptible Salmonella strains. Salmonella counts increased significantly during manufacture to a mean of 734 CFU/g on day 1 followed by a significant decrease over 60 days of aging to <1 CFU/g. Although levels fell and stayed below the direct plating detection limit of ≥5 CFU/g after 54 days on average, viable cells remained detectable after enrichment for an average of 210 ± 40 days. The International Organization for Standardization methods with and without PCR detection provided the most accurate results, and the remaining methods, notably those with selective primary incubation, produced results that disagreed significantly with the true result. Overall, our findings suggest that MDR Salmonella strains may not pose a greater threat to cheese safety than do non-MDR Salmonella strains.