Flow of resistance genes in the environment and from animals to man

Some antibiotic-resistant bacteria occur naturally in the environment but many arise by contamination with human and animal excreta in sewage, slurry and manure. These may be ingested as contaminants of water, animal feeds and cultivated foods or, in the case of animals, by licking their environment. Gene transfer has been demonstrated in voided calf faeces and the resultant transconjugants, subsequently ingested, colonized the calf gut. Identified strains of antibiotic-resistant Escherichia coli have been traced from the gut contents of pigs, calves and chickens to carcasses at slaughter and ultimately shown to colonize the gut of a human volunteer handling and eating the meat. Similarly, antibiotic-resistant strains of Salmonella typhimurium phage type DT204c, have been isolated from calves and humans indicating a possible zoonotic spread. The significance of these observations is discussed.