Antibiotic Use in Agriculture under Pressure

Following the recent announcement that the European Commission is moving to cut the use of antibiotics in agriculture, the German Ministry of Food, Agriculture and Consumer Protection has announced a package of measures to gain better control of the use of antibiotics in animal husbandry. In the UK, antibiotic use is reported to be on the rise again after a prolonged period of decline but the report stresses that the use of antimicrobials per tonne of feed has risen only slightly.

Researchers in the US have reported that antibiotics in feed encourage gene exchange between gut bacteria, which may lead to the development of antimicrobial resistance (AMR).

A study published this week in the online journal, mBio® shows that adding antibiotics to pig feed causes microorganisms in the guts of these animals to start sharing genes that could spread antibiotic resistance.

The study by Heather Allen and her colleagues at the USDA National Animal Disease Center (NaDC) in Ames, Iowa, adds to our knowledge about what happens to the microorganisms that populate animal digestive tracts when they are exposed to low but long-term levels of antibiotics. The researchers studied how two in-feed antibiotic formulations affect prophages, segments of DNA found in bacteria that can encode antibiotic resistance genes and other genes that bacteria may use.

Prophages can cut themselves out of the larger chromosome of bacterial DNA in a process called induction, then replicate and package themselves as viruses. These viruses explode the cell from the inside then move on to infect other organisms and deliver their genes.

Dr Allen, who is lead author on the study, says when pigs were fed antibiotics, the numbers of antibiotic resistance genes carried by the phages remained steady but the microorganisms still reacted to the presence of antibiotics. Prophages underwent a significant increase in induction when exposed to antibiotics, indicating that medicating the animals led to increased movement of prophage genes among gut bacteria.

She said: "Induction of the prophages is showing us that antibiotics are stimulating gene transfer. This is significant because phages have previously been shown to carry bacterial fitness genes such as antibiotic resistance genes."

Studies that explore the impacts of in-feed antibiotics most often focus on the bacterial residents of the gut, according to Dr Allen but phages and other viruses move a significant amount of genetic information around the community. This makes changes in prophage induction an important collateral effect of antibiotic treatment, she says. Resistance genes are the unit of currency among microbes experiencing the duress of an antibiotic, so following the movement of genes is arguably more important than following certain changes in bacterial communities. And if bacteria in humans acquire resistance genes from animals, there can be serious health consequences.

Finally, we return to the coming ban on conventional battery cages in the EU. Free-range egg producers in the UK are clearly feeling the pressure of margins squeezed by the need to invest in new housing, high feed prices and the threat of their product becoming viewed as a commodity.

Add to that concerns of eggs from battery cages coming into the country – in shell or as egg products – and it is no surprise that the mood at their recent association meeting was somewhat downbeat. They should gain some comfort from the latest survey of the country's consumers, 80 per cent of whom said they want action to stop eggs laid by hens in illegal barren battery cages coming into the UK.