Development of a polymerase chain reaction assay to detect enteric bacteria in ground beef

Pathogenic strains of Escherichia coli, Salmonella, Shigella and Citrobacter can potentially cause foodborne diseases if consumed in contaminated beef products. The purpose of this study was to establish a polymerase chain reaction (PCR) assay to screen ground beef for these enteric bacteria. With the use of homologous phoP gene sequences as the target, a PCR yielded a 302-bp product from E. coli, Salmonella, Shigella and Citrobacter. No amplification was observed with genomic DNA from 48 other food-related bacterial strains. The detection limits for crude cell lysates of E. coli O157:H7, Salmonella Typhimurium, Shigella flexneri and Citrobacter freundii were 9.2×102, 1×101, 9.5×102, and 1.4×102 total cfu, respectively. Ground beef samples inoculated with as few as 1 cfu/g of these bacteria yielded positive results with the PCR following 10 h of enrichment. In addition to cultural methods, this PCR assay provides a presumptive alternative to detect E. coli, Salmonella, Shigella or Citrobacter in ground beef. However, further steps are needed to differentiate the organisms in the presumptive samples.