Wise use of antibiotics combats
bacterial resistance

Mary Aegerter, Special reporter for the Food Safety, Farm to Table Conference

Food contamination may occur during any step of bringing food commodities from the farm to the table, university researchers from around the nation told participants of the 6th annual Food Safety, Farm to Table Conference held May 27 and 28 in Moscow, Idaho.

According to the researchers, food contamination with such bacterial pathogens as E. coli 0157:H7, Campylobacter jejuni, and Salmonella may come from contact with water, soil, wildlife, or from preparation by the consumer. The researchers listed hazard analysis critical control points (HACCP), identification of all possible sources of contamination throughout every step of food handling and processing, as one of the most beneficial activities one can take in improving food safety. Other recommendations included irradiation and vacuum steam pasteurization. Overuse of antibiotics, another popular way to prevent food contamination, can lead to bacteria quickly developing antibacterial resistance, the researchers agreed.

Antibiotics, introduced 50 years ago, were hailed as miracle drugs that would rid the world of infectious disease. That they haven't is testimony to bacterial versatility and adaptability. Today, more microorganisms are resistant to more antibiotics, David White of North Dakota State University told conference participants.

Bacterial resistance is driven largely by antibiotic use, says White. His work on calf scours has shown that antibiotic resistance definitely correlates with antibiotic use, even mirroring year-to-year fluctuations in that usage. The highest resistance is often to drugs that are the easiest to obtain those sold over the counter.

Many bacteria that cause food-borne illness can infect many animal species. We can get E. coli 0157:H7 or Salmonella typhimurium directly from beef or indirectly through contamination of other food by birds or other animals. Chickens carry most Campylobacter jejuni infections, and eggs can carry Salmonella enteritidis. If these bacteria are resistant to antibiotics in other animals, they'll be resistant to antibiotics when they infect us.

Antibiotics usually kill bacteria by interfering with an essential aspect of bacterial life. Penicillin inhibits the synthesis of the bacteria cell wall a structure lacking in animals. The fluoroquinolones interfere with bacterial reproduction. Sulfa drugs interfere with essential bacterial enzymes, and erythromycin inhibits bacterial protein synthesis.

Bacteria resistant to these antibiotics use one of several mechanisms. One is to simply inactivate the antibiotic. Another is to alter the antibiotic's target in the bacteria so that the antibiotic can't work on it. Finally, some bacteria simply prevent the antibiotic from getting inside them, while others actively pump to the outside any antibiotic that enters.

If each bacterial strain had to make its own resistance to every antibiotic, there might be a better chance of staying even. But this isn't the case, for bacteria may acquire genes for resistance from other bacteria. Interestingly, the other bacteria need not be of the same species: Salmonella typhimurium can give resistance genes to other Salmonella typhimurium, to E. coli , or to any other bacteria living nearby.

In addition, these resistance genes often come in sets of up to seven different resistances. These sets might be part of the bacteria's own DNA. The ACSSuT resistances carried by Salmonella typhimurium DT104, R-type ACSSuT are most probably contained in two sets. (They carry resistance genes for ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline.) Or the set may be contained in a separate piece of DNA called a plasmid. These plasmids can be passed from one bacterial strain to another when two bacteria come together.

Although plasmids can be passed between bacteria, they usually aren't passed on to following generations, unless they have become part of the bacterial DNA. Plasmids can actually " jump" into and out of the bacterial DNA. When the bacteria reproduce, the plasmids can jump into the bacterial DNA. When bacteria contact one another, plasmids can jump out of the bacterial DNA and be passed from one cell to the other. Talk about versatility!

Bacteria normally lose resistance to antibiotics to which they aren't exposed. But because the set of multiple resistances travels as a unit, bacteria can maintain resistance to antibiotics they aren't exposed to anymore or acquire resistance to antibiotics they've never seen. A resistance gene only needs to be part of a set. When bacteria are exposed to one of the antibiotics, the whole set will be maintained. Margaret Davis of WSU found that although chloramphenicol hasn't been used in animals for many years, Salmonella typhimurium in cattle is still resistant to the antibiotic at a high level. This may be because the resistance gene for chloramphenicol is part of the ACSSuT set.

Antibacterials are all the rage today and occur in or on soaps, cleaners, lotions, antibacterial toys, cutting boards, pens, and cheese graters. One car company plans to put antibacterials on its steering wheels. No prescriptions needed; just take a trip to the appropriate store. And because consumers are scared (Remember the Consumer Reports expose on contaminated chicken in grocery stores?) these products sell.

Triclosan is one of these antibacterials. White said that while Triclosan appears to reduce plaque and gingivitis when it's part of oral health products, there's no evidence that it works in soaps or detergents or on the surfaces of toys and other items.

When various methods of disinfecting hands were tested, Triclosan was no more effective than plain soap. The best products were those that have been used for years in hospitals: isopropanol, chlorhexidine, and povidone iodine. And the best advice for disinfecting your hands is the same as always: wash your hands in soap and water.

Unfortunately, even though these products don't work well, they are not innocuous. A bacterial gene for resistance to Triclosan has been found. If it's part of a set, it can carry other resistances along with it wherever it goes and into whatever species of bacteria it encounters. This has happened with the antibacterial product Pine Sol. E. coli exposed to Pine Sol were two to eight times more likely to be resistant to antibiotics than E. coli that hadn't been and visa versa. The resistances travel together.

What can we do? Use antibiotics wisely. Doctors, veterinarians, and potential users need education about resistance. Companies should be discouraged from enticing us through discounts and promotions to buy antibiotics. We should use antibiotics only when necessary and give them only in the right dosage for the right time period. Antibiotics should be rotated, and we should be alert to new resistance patterns.

Of course, there will be new antibiotics. Researchers are working now to develop drugs that will inhibit the pumps bacteria use to dump out antibiotics. But the timeline to market is 10 years, and development will cost millions of dollars.

Return to the Table of Contents for the July 1998 issue

Return to the Agrichemical & Environmental News Index