Jul 20, 2010 (CIDRAP News) A recent study of a modest sample of US retail beef products found little difference between the levels of bacteria in grass-fed and conventionally raised beef, despite marketing claims that grass-fed beef is safer.
Reporting in Foodborne Pathogens and Disease, researchers said they found no significant differences in total coliform bacteria, Escherichia coli, or Enterococcus species. They also looked at antimicrobial resistance and found mixed evidence, with some signs of increased resistance in bacteria isolated from conventional beef as compared with grass-fed beef.
"Taken together, these data indicate that there are no clear food safety advantages to grass-fed beef products over conventional beef products," says the report by investigators from Purdue University in Indiana and Zhejiang University in Hangzhou, China.
They write that grass-fed beef products are often marketed as safer than conventional grain-finished beef because of the potential effects of the grass diet on gut microbes. But they suggest that other factors, including where and how beef is processed and whether cattle are fed preventive antimicrobials, may have larger effects on contamination in finished products. Grass-fed cattle are more likely to be processed in small facilities and are less likely to be given preventive antibiotics, the authors say.
In view of these factors, they hypothesized that grass-fed beef would have higher overall contamination rates but that conventional products would carry bacteria with higher levels of antimicrobial resistance.
To test this, they collected and analyzed samples of conventional beef and beef labeled as grass-fed from retail stores in Illinois and Indiana between July 2008 and March 2009. Fifty conventional samples were collected from four outlets, and 50 grass-fed samples were gathered from 10 sources, including retail stores, farm stores, and farmers' markets. About two thirds of the samples in each set were solid cuts of beef such as steaks, and the rest were ground beef.
The samples were washed according to a standard protocol, and the wash solution was then tested for bacteria. The bacteria also were tested for resistance to various antimicrobials.
Testing showed no difference in overall coliform contamination of the sample sets (conventional beef, 2.6 log-10 colony-forming units [CFU] per milliliter of rinsate; grass-fed beef, 2.7 CFU/mL of rinsate). Likewise, solid cuts and ground beef did not differ in coliform levels.
Enterooccus species were isolated from 62% of the conventional samples and 44% of the grass-fed samples, a difference that did not reach significance (P=.07) However, the difference was greater for ground beef: 75% for conventional versus 41% for grass-fed (P<.05).
The two sample sets had equal overall levels of E coli contamination, at 44%. For solid cuts of meat, the conventional products had a higher level of E coli than the grass-fed ones, but this was reversed for ground beef. Neither difference was significant. No E coli O157:H7 or Salmonella was found in any of the samples.
The investigators found no differences in the percentages of resistant E coli from conventional versus grass-fed beef. However, Enterococcus species from conventional beef were more frequently resistant to daptomycin and linezolid than were the same species from grass-fed beef (P<.05), they report. They also found that both E coli and Enterococcus from conventional beef showed higher resistance to several antibiotics than the same species from grass-fed beef, in terms of the amount of drug needed to inhibit the organism.
Despite finding few significant differences between the two kinds of products, the authors conclude there was an "overall trend" for more Enterococcus species in conventional beef generally and for more E coli in grass-fed versus conventional ground beef. They also write that the findings on resistance show "a possible trend for bacteria isolated from conventional products to be more antimicrobial-resistant."
The researchers note that the grass-fed beef products were not labeled as organic and thus were not subject to the ban on antimicrobial use that exists in organic beef production. They suggest that the resistance found in bacteria from the grass-fed products may have been related to therapeutic use of antibiotics in the grass-fed cattle, as preventive use in grass-fed cattle is not practical.
Their report also says grass-fed and conventionally raised cattle can be processed in the same facilities, which suggests that cross-contamination could account for the resistant Enterococcus species found on grass-fed products. They say further research is needed to better identify the sources of contamination on farms, during transportation, and at processors.
Scott J. Wells, DVM, PhD, director of education at the University of Minnesota Center for Animal Health and Food Safety in St. Paul, called the study interesting but said the small sample size limits the conclusions.
"The low sample size limits inference generally as well as limits authors from further evaluating differences between specific subtypes of retail beef products," he commented by e-mail. He added that the low prevalence of E coli O157 and Salmonella in the beef supply means that a much larger sample would be needed to compare levels in grass-fed versus conventional beef.
Wells commented further, "Contamination of retail beef samples by certain bacterial pathogens is a complex process, with multiple points of potential contamination . . . and several risk factors that could potentially mitigate the outcome. It's not likely to be as simple as grass-fed vs conventional fed."
He observed that other factors such as farm sizes and farm management systems can contribute to the level of bacteria in beef. "In short, the article is interesting but I'm left with more questions," he said.
Zhang J, Wall SK, Xu L, et al. Contamination rates and antimicrobial resistance in bacteria isolated from "grass-fed" labeled beef products. Foodborne Pathogens Dis 2010 (published early online Jul 10) [Abstract]