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Antibiotic resistance genes in our food?

May 23, 2007
Courtesy American Society for Microbiology
and World Science staff

The food we eat could be promot­ing the dis­turb­ing spread of an­ti­bi­ot­ic-re­sist­ant in­fec­tions, sci­en­tists warn.

Harm­less bac­te­ria in our food car­ry genes that pro­vide re­sist­ance to an­ti­bi­otics, the re­search­ers ex­plained. Once in our bod­ies, they might trans­mit these genes to di­s­ease-caus­ing bac­te­ria, mak­ing them more dan­ger­ous.

Still Life With Food by Jan Da­v­idsz de Heem (1626-1683)
“The mag­ni­tude of an­ti­bi­ot­ic-re­sist­ant bac­te­ri­al con­tamina­t­ion in the food chain is tremen­dous,” said Hua Wang of Ohio State Un­ivers­ity in Co­lum­bus, Ohio, who pre­s­ents the find­ings May 23 at the Amer­i­can So­ci­e­ty for Mi­cro­bi­ol­o­gy an­nu­al meet­ing in To­ron­to. 

“Food could be an im­por­tant av­e­nue for an­ti­bi­ot­ic-re­sist­ant bac­te­ri­al ev­o­lu­tion and dis­semina­t­ion.”

An­ti­bi­ot­ic-re­sist­ant in­fec­tions arise when bac­te­ria evolve de­fenses against the an­ti­bi­otics with which hu­mans have been at­tack­ing them for years. Of­ten, the mi­crobes need­n’t evolve re­sist­ance on their own: they can get the re­sis­tance genes free from their neigh­bors.

The cul­prit, Wang said, is a pro­cess known as hor­i­zon­tal gene trans­fer, in which bac­te­ria near each oth­er can share ge­net­ic in­forma­t­ion. Gene trans­fer among di­s­ease-caus­ing bac­te­ria in hos­pi­tals is al­ready a rec­og­nized prob­lem. Re­search al­so shows that di­s­ease-caus­ing bac­te­ria can get genes from harm­less mi­crobes, in­clud­ing those in food, Wang said. Par­tic­u­larly wor­ri­some, she added, is that these or­gan­isms are plen­ti­ful and have a rich gene pools, and some are par­tic­u­larly ef­fi­cient at shar­ing genes.

Hua and col­leagues tested var­i­ous ready-to-eat foods in­clud­ing sea­food, meats, dairy, deli items and fresh pro­duce from gro­cery chain stores. Re­sist­ance gene-car­ry­ing bac­te­ria turned up through­out the sam­ples, ex­cept in pro­cessed cheese and yo­gurt, she said.

An­ti­bi­ot­ic-re­sist­ant in­fec­tions are a grow­ing pub­lic health prob­lem, said Mar­i­lyn Roberts of the Un­ivers­ity of Wash­ing­ton in Se­at­tle: de­pend­ing on the case, an an­ti­bi­ot­ic-re­sist­ant in­fec­tion could tri­ple a hos­pi­tal stay. A me­thi­cil­lin-re­sist­ant Staph­y­lo­coc­cus au­re­us in­fec­tion can cost thou­sands of dol­lars more to treat. In some cases, such as the new ex­ten­sively re­sist­ant tu­ber­cu­los­is, an­ti­bi­otics no long­er work, forc­ing doc­tors to take ex­treme meas­ures like re­mov­ing an in­fected lung.

The prob­lem is­n’t con­fined to the food sup­ply, Hua said: re­cent stud­ies have found re­sist­ance genes in bac­te­ria in ba­bies’ di­ges­tive tracts. Since these chil­dren had­n’t eat­en sol­id food yet, they must have ac­quired these genes some­where oth­er than the food sup­ply, sug­gest­ing re­sist­ance genes from the en­vi­ron­ment were in­volved, said Hua.

“An­ti­bi­otics and the con­tamina­t­ion of the en­vi­ron­ment is a med­i­cal prob­lem, an ag­ri­cul­tur­al prob­lem and a hu­man prob­lem. Eve­ry­body plays a role,” Roberts said. But there are things that can be done to min­i­mize con­tamina­t­ion in our food, Hua added. “Given the prop­er in­vest­ment of mon­ey, ef­fort and time we can iden­ti­fy the steps that need to be tak­en at the pro­cess­ing level.”

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The food we eat could be fueling the spread of antibiotic-resistant infections, scientists say. Harmless bacteria in our food may also carry genes that provide resistance to antibiotics, according to the researchers. Once in our bodies, they might transmit these genes to disease-causing bacteria, making them more dangerous. “The magnitude of antibiotic-resistant bacterial contamination in the food chain is tremendous,” said Hua Wang of the Ohio State University in Columbus, Ohio, who presents the findings May 23 at the American Society for Microbiology annual meeting in Toronto. “Food could be an important avenue for antibiotic-resistant bacterial evolution and dissemination.” The rise of antibiotic-resistant infections comes as bacteria evolve resistance to the antibiotics with which that humans have been attacking them for years or decades. Often, the microbes needn’t evolve resistance genes on their own: they can get them free from their neighbors. The culprit, Wang said, is a process known as horizontal gene transfer, in which bacteria nearby to each other can share genetic information. Gene transfer among disease-causing bacteria in hospitals is already a recognized problem. Research also shows that disease-causing bacteria can get genes from harmless microbes, including those in food, Wang said. Particularly worrisome, she added, is that these organisms are plentiful and have a rich gene pools, and some are particularly efficient at sharing genes. Hua and colleagues tested various ready-to-eat foods including seafood, meats, dairy, deli items and fresh produce from grocery chain stores. Antibiotic-resistance gene-carrying bacteria turned up throughout the samples, except in processed cheese and yogurt, she said. Antibiotic-resistant infections are a growing public health problem, said Marilyn Roberts of the University of Washington in Seattle: depending on the case, an antibiotic-resistant infection could triple a hospital stay. A methicillin-resistant Staphylococcus aureus infection in a hospital patient can cost thousands of dollars more to treat. In some cases, such as the new extensively resistant tuberculosis, antibiotics no longer work, forcing doctors to take extreme measures like removing an infected lung. The problem isn’t confined to the food supply, Hua said. Recent studies have shown antibiotic resistance genes in bacteria in babies’ digestive tracts. Since these children hadn’t eaten solid food yet, they must have acquired these genes somewhere other than the food supply, suggesting resistance genes from the environment were involved, said Hua. “Antibiotics and the contamination of the environment is a medical problem, an agricultural problem and a human problem. Everybody plays a role,” Roberts said. But there are things that can be done to minimize resistance genes in our food, Hua said. “Given the proper investment of money, effort and time we can identify the steps that need to be taken at the processing level to minimize” the contamination, said Hua.