Correlation of Escherichia coli O157-associated Virulence Genes in Beef Trim with Precipitation Levels and Temperature

Introduction: The frequency of contamination of meat products by pathogenic microorganisms fluctuates because of many variables such as month of testing and differences in temperature, rainfall, cattle source and plant processing practices.

Purpose: The association of various virulence genes associated with the enterohemorrhagic pathogen, Escherichia coli O157, was examined in beef trim samples to determine if there were a regular relationship between their frequency in these products and the environmental factors of precipitation and temperature.  Such information could be useful for risk assessments and for designing sampling and testing plans to maximize the frequency of detection of pathogens in trim.

Methods: Samples (375 g) were collected throughout calendar year 2012, enriched for at least 8 hours and screened by PCR for the virulence-associated genes rfb, eae, stx₁, and stx₂ and by lateral flow (LF) to detect the O157 antigen.  Also, two Salmonella-specific genes were detected.  Two groups of four meat processing establishments each (upper Midwest [UM] and central Midwest [CM]; 99,067 and 122,869 samples, respectively) were compared using the frequency of virulence-associated genes and rainfall amount and temperature using data from the U.S. Climatological Divisions.

Results: For UM plants, annual average temperature and rainfall ranged from 49-52°F and 1.2-3.6”/month, respectively.  CM plant temperature and rainfall was 63-68°F and 1.8-2.5”/month, respectively.  The frequency of screen-positive samples for enterohemorrhagic E. coli (EHEC; stx⁺ and eae⁺) was 6.6% and 17.9% for UM and CM plants, respectively.  The average number of signals per sample was 0.97 and 0.55 for UM and CM plants, respectively.  Generally, plants in warmer weather tended to have a higher frequency of EHEC screen-positive samples and a higher average number of signals per sample.  However, there was considerable variation between plants.

Significance: Beef trim samples in plants during warmer weather (seasonally and annually) tend to have higher percentages of samples screen-positive for E. coli O157 virulence-associated genes.  More sophisticated statistical analyses may be able to more fully assess the relative contributions of temperature and rainfall to these findings.  Such plants could consider these factors in designing their product sampling plans; future microbial hazard risk assessments might be informed by these observations.