There has been speculation that multidrug-resistant (MDR) strains are generated by subtherapeutic antibiotic use in food animals and that such strains result in increased resistance to lethality by food processes such as heat and irradiation. The objective of this study was to evaluate the heat resistance of 20 strains, namely an MDR and a non-multidrug-resistant (NMDR) strain of each of 10 Salmonella serotypes isolated from cattle or cattle environments. MDR and NMDR Salmonella serotypes studied included Montevideo, Typhimurium, Anatum, Muenster, Newport, Mbandaka, Dublin, Reading, Agona, and Give. For phase I, stationary-phase cultures of the strains were aliquoted into sterile capillary tubes and immersed in a temperature-controlled water bath at 55, 60, 65, and 70 degrees C for appropriate times. Survivor curves were plotted for each temperature, and a best-fit linear regression was derived for each temperature. D-values (decimal reduction times) and z-values (changes in temperature required to change the D-values) were calculated for each strain. Although there was no overall significant difference in the heat resistance of MDR and NMDR serotypes, NMDR serotypes generally appeared to have slightly higher heat resistance than NMDR serotypes, especially at 55 and 60 degrees C. The highest relative heat resistance (highest z-values) was exhibited by Salmonella Anatum. Notably, the relative heat resistance of NMDR Salmonella Agona was similar to that of NMDR Salmonella Anatum and had the highest D-values at all four temperatures. For phase II, three serotypes (regardless of resistance profile) with the highest relative heat resistance and their drug-resistant counterparts were selected for thermal inactivation in ground beef patties cooked to endpoint temperatures. Salmonella Agona was able to survive in ground beef cooked to an internal temperature of 71 degrees C. Results of these studies suggest drug resistance does not affect the heat resistance of Salmonella and that serotype or strain is an important consideration in risk assessment of the pathogen with regard to survival at cooking temperatures.

Pathogenic Escherichia coli strains on raw or insufficiently cooked foods are of public health concern as serious disease may result from their ingestion. Therefore, many commercial producers of beef products screen for E. coli O157:H7 before shipment. While Salmonella is not considered an adulterant on raw beef products, it is used as an indication of process control. To detect these microorganisms, rapid screening methods are often used to provide results within 8-24 hours after sampling. During 2005-2008, about 971,389 samples from several commercial beef production plants were tested using a rapid screening method based on the polymerase chain reaction to determine if they were presumptively positive for bacterial cells carrying Salmonella or Shiga toxin-producing E. coli-specific genes. Of the product lots sampled (trim, ground beef, and variety meats), 15% were positive for the stx(1) and/or stx(2) (Shiga toxin genes), 9.1% for the eae gene (the attaching and effacing gene [eae] encoding intimin), 3.0% for an rfb gene region (encoding the O157-specific O side chain polysaccharide), and 1.67% for Salmonella by the polymerase chain reaction assay. In general, lots of ground beef showed the lowest frequency of contamination, and variety meats (by-products of carcass evisceration), the highest. Overall, 4.6%, 4.6%, and 0.8% samples were screen-positive for enteropathogenic E. coli, enterohemorrhagic E. coli, and E. coli O157, respectively. Of the E. coli O157-positive samples, 14% were also Salmonella positive. The frequency of screen-positive samples increases during the summer months, probably because of the prevalence of climatic conditions more conducive to microbial growth. The presence of fecal organisms in beef products suggests a failure of sanitary controls during processing and the more prevalent relatives of E. coli O157, Shiga toxin-producing Escherichia coli, enteropathogenic E. coli, and enterohemorrhagic E. coli, serve as more sensitive indicators of contamination than O157 strains alone.

No abstract

No abstract

No abstract