Carbapenems are one of the most important classes of antibiotics used in humans and are an important agent against multi-drug resistant bacteria.
Now, for the first time, bacteria that carry a transmissible carbapenem resistance gene have been found in agricultural animals in the U.S. The research was published Dec. 5 in Antimicrobial Agents & Chemotherapy, a journal of the American Society for Microbiology.
Carbapenem-resistant bacteria are not uncommon in hospitals. However, in the U.S., carbapenems are not used in agriculture because of their importance to human health.
"It's a surprise that they would show up in livestock," said corresponding author Dr. Thomas Wittum, professor and chair of veterinary preventive medicine at The Ohio State University.
Like many antibiotic resistance genes, the carbapenem resistance gene in this report, called bla IMP-27, is carried by a plasmid. Plasmids are small pieces of independent DNA that can move easily from one bacterium to another, including across species.
Additionally, the particular plasmid on which bla IMP-27 was found has one of the widest host ranges of any plasmid, said first author Dixie Mollenkopf, a graduate student in Wittum's lab.
This combination of attributes, and the fact that carbapenem resistance was recently designated an urgent threat to public health by the Centers for Disease Control & Prevention, led the researchers to investigate whether bacteria with carbapenem resistance genes such as bla IMP-27 might be present in agricultural animals or in agricultural settings, Wittum said.
The investigators used gauze swabs to obtain samples from floors and walls of pens as well as swiffers to collect environmental and fecal samples from a 1,500-sow, farrow-to-finish pig farm during four visits over five months. Despite all of the work they had put into the study, they were still surprised to find carbapenem-resistant bacteria growing in the agar plates, Mollenkopf said. Nonetheless, the numbers of isolates bearing bla IMP-27 were few.
Furthermore, the resistance gene was present primarily in environmental samples from the farrowing operation, and the investigators failed to find it in pigs being finished for slaughter.
"There is no evidence the pigs carried the gene into the (human) food supply," Wittum said.
Still, finding the gene at all on this particular farm was somewhat mysterious because no new livestock were introduced on it during the past 50 years, Wittum said. The farm had bred all of its animals during this time.
Carbapenems are a subset of beta-lactam antibiotics. Βeta-lactam antibiotics that are not carbapenems are legal for use on farms in the U.S. This study's results support the investigators' hypothesis that the use of ceftiofur, a beta-lactam antibiotic, might be selecting for resistance to carbapenems on farms.
The investigators suggest that in light of their results, it will be important to monitor farms to ensure that they do not become a source of bacteria with carbapenem resistance genes such as of bla IMP-27 within the human population.