Pigs and other farm animals are harbouring major reservoirs of antibiotic-resistant bacteria

Sep 8 2004

Pigs and other farm animals are harbouring major reservoirs of antibiotic-resistant bacteria, according to research presented today at the Society for General Microbiology’s 155th Meeting in Trinity College Dublin, by researchers from the University of Leeds.

The scientists were concerned about the effects that decades of use of antibiotics to treat infections, prevent diseases, or promote growth, have had on the spread of antibiotic resistance genes in common farm bacteria.

“The European Commission banned some growth promoting antibiotics in 1999, and all growth promoters will be banned by 2006,” says Melanie Thompson of the School of Biochemistry and Microbiology at Leeds University. “But the use of these antibiotics in animal husbandry for many years to treat illness, prevent infections and increase the growth rate and food efficiency of the animals has exerted a Darwin-style selective pressure on the different types of bacteria which survive in farm animals. For years we have been actively selecting for bacteria which possess genes capable of antibiotic resistance.”

In other countries the effect of a ban on growth-promoting antibiotics has been an increase in their use as veterinary medicines, meaning that their overall use hardly alters. The antibiotics used for animals are either structurally very similar or identical to ones used in human medicine. If resistant bacteria from farm animals pass through the food chain to infect people, some commonly used antibiotics for human medicine could become ineffective, and people could suffer infections which would not respond to conventional therapies.

Bacteria originally from food animals can reach people through poor hygiene, improper food handling and inadequate cooking. The scientists looked at the types of bacteria and the antibiotic resistance they already possess at a commercial pig farm to assess the probability of resistant bacteria being present in food intended for human consumption.

“We found that some of the bacteria taken from pig faeces could easily transfer resistance genes to laboratory bacteria,” says Melanie Thompson. “This was not surprising, but it is worrying, since if bacteria can transfer genes in a laboratory, they are likely to be able to do it in other enclosed places, such as inside the human gut.”

The research team is now looking at different samples taken from different groups of pigs ranging in age from their farrowing to their finishing to see how the bacteria which contaminate them change. The groups will also be exposed to different farming practices to find out if this alters the types of bacteria they are carrying.

The scientists have so far looked at 40 different samples of bacteria from pigs and discovered resistance genes to tetracyclines, ciprofloxacin, and ampicillin, although so far no resistance has been found to extended-spectrum cephalosporins.