The latest from mBio raises some important questions about the safety of my favorite pizza topping, but it also highlights the fact that antibiotic use in livestock can impact food safety in unexpected ways.
Antibiotics used as growth promoters or to treat disease in livestock can eventually end up in meat, and regulators in the US and EU have set limits on the concentrations of antibiotics in meat for consumption by humans. Researchers at the University of Copenhagen, in Denmark, and University College Cork, in Ireland, set out to determine whether these antibiotics might interfere with the process of fermentation in products like pepperoni, salami, or chorizo - sausages that are fermented using lactic acid producing bacteria in a curing process many cultures have employed for hundreds of years. Today, sausage manufacturers inoculate sausage meat with lactic acid producing bacteria in an effort to control the fermentation process and ensure the final product is acidic enough to kill pathogens that might have existed in the raw meat.
Hanne Ingmer of the University of Copenhagen is the lead author of the study. She says fermented sausages occasionally cause serious bacterial infections, but it's never been understood why that might be.
In small-scale experiments in the lab (Meat in syringes! Best microcosm ever.), Ingmer and her colleagues added the antibiotics oxytetracycline or erythromycin to meat inoculated with lactic acid producing bacteria and pathogens Escherichia coli O157:H7 and Salmonella enterica. They followed the progress of the fermentation and tracked the survival of the pathogens. Ingmer says several different starter cultures of lactic acid producing bacteria were sensitive to these antibiotics and hence did not acidify the sausage meat effectively - results that could explain why people sometimes get sick from eating fermented sausage.
"At low concentrations and at regulatory levels set by authorities, we could see that the lactic acid bacteria are more susceptible to the antibiotics than the pathogens are," says Ingmer. "So basically, we can have a situation where residual antibiotics in the meat can prevent or reduce fermentation by the lactic acid bacteria but these concentrations do not effect survival or even multiplication of pathogens."
Hence, says Ingmer, the results show antibiotics can potentially have a paradoxical effect that would increase the risk of foodborne illness: antibiotic residues reduce the effectiveness of bacteria that should make the sausages safe but don't affect the bacteria that can make you sick.
Although the results raise an alarm for the manufacture of processed meats, Ingmer stresses that it is important to conduct similar tests in manufacturing facilities. "The majority of sausages are manufactured at a commercial scale. It has to be addressed whether this is a problem in a real life facility," says Ingmer.
Manufacturers with good quality control systems can catch problems in fermentation, Ingmer says, preventing large-scale outbreaks of foodborne illness by testing the final product before it's distributed, but random batch testing could well miss a batch that didn't ferment properly, putting the public at risk.
How can we be sure antibiotic residues don't interfere with the safety of these products? Ingmer sees two possible solutions. If antibiotics are present in meat, boosting the survival and activity of the lactic acid producing bacteria is important. In the future, Ingmer hopes to work with manufacturers to develop cultures of lactic acid producing bacteria that tolerate low levels of antibiotics.
But the ultimate solution to the problem of antibiotics in meat may be harder to achieve. "The obvious solution is to eliminate the use of antibiotics as growth promoters and closely monitor the use of antibiotics in treating farm animal diseases," Ingmer says. The European Union and other countries have banned the use of antibiotics in livestock as growth promoters, Ingmer points out, a move the US is unlikely to follow very soon.