Bacteriophage and some types of bacteria offer an alternate means for controlling foodborne pathogens at various stages of food production and processing, according to several researchers who spoke during several sessions of the 2013 ASM General Meeting, held in Denver last May.
Some of them described how native bacteria can outcompete pathogenic strains on plants during growth and harvest, while others outlined how bacteriophage can kill pathogens on plants before they are harvested or later during food processing.
“With 1 in 100 hospitalizations attributed to foodborne pathogens . . . novel, ecologically sound intervention strategies are essential,” says Jie Zheng, who leads a group investigating Salmonella outbreaks at the Food and Drug Agency (FDA) Center for Food Safety and Applied Nutrition. Sarah Allard presented the group’s work during the poster session “Foodborne Pathogens— Control.”
Zheng’s group sampled soils from Virginia, where Salmonella enterica is a frequent source of foodborne diseases, and identifıed 17 bacterial strains that inhibit Salmonella growth. One Bacillus strain, designated TS-15, is active in protecting tomatoes, a common vehicle for S. enterica. “TS-15 yielded a precipitous fıve-log drop in Salmonella cell inoculum, completely preventing pathogen recovery on fruit surfaces,” Allard says. Applying TS-15 to plants and soil reduces the pathogen, particularly during transplantation when seedlings can internalize Salmonella through injured roots. Several Bacillus species also are active against Escherichia coli, Shigella, Enterobacter, Listeria, and Staphylococcus aureus, and thus might prove useful in curbing these pathogens on leafy greens, peppers, meats, and cheeses, Zheng says.
Bacteriophages are abundant and target virtually all varieties of bacteria but are harmless to plants, animals, and humans. For example, phage that infect E. coli O157:H7 may prevent this pathogen from contaminating alfalfa sprouts and other leafy greens, according to Dandan Zhang of Purdue University in West Lafayette, Ind. In an agar-based model system, he found that T4 phage, applied as a seed coating prior to germination, can protect sprouting seedlings both inside and out.
Phage also might be used to control foodborne pathogens in milk and milk products, according to Lynn Haddad of the Université Laval in Quebec City, Quebec, Canada, who spoke during the symposium “Fermented Foods and Beverages: a Flavorful Blend of Culinary Tradition and Microbial Terroir.” She isolated a strain of phage from raw milk that targets S. aureus, but spares other bacteria that impart flavor and texture to cheeses. The phage, LH1- MUT, proved stable through cheese making and for at least one month after processing. Its stability means this phage can be added to raw milk as cheese making begins to prevent growth of staphylococcal bacteria or their production of enterotoxin, which can persist after the bacteria themselves are degraded.
Separately several years ago, FDA approved phage to target Listeria bacteria in cheeses and meats. However, high temperatures inactivate that phage, restricting its use to the later stages of food production—in the case of cheeses, to the ripening stage—and to food storage after processing.
Shannon Weiman is a freelance writer in San Francisco, Calif.