Transmission of resistance genes can and does occur in a variety of surroundings. The question that remains is how often successful transfers cause clinical problems. Resistance genes can persist in the environment both in their original bacterial hosts and in environmental organisms. There was resistance to antibiotics long before antibiotics were used in medicine e.g. 2,000 year old antibiotic resistant bacteria have recently been isolated from glaciers in the Canadian Arctic. In the presence of antibiotics, resistant bacteria will have a competitive advantage.
Resistance becomes a problem when bacteria causing a disease withstand antibiotic therapy.
Increased resistance to antibiotics may therefore cause clinical problems and may shorten the useful life span of some antibiotics. Cases of human salmonellosis caused by multiple resistant Salmonella typhimurium definitive phage type 104 are occurring in the UK and the majority of these arise from contact with animals or contaminated food. Antibiotics are only used to treat Salmonella in a small minority of cases in compromised patients but their use in those cases can be a case of life and death. Campylobacters, Clostridia and Yersinias may also develop resistance. It is not clear whether any of the vancomycin resistant enterococci found in man and causing disease belong to species or strains derived from animals but at present the suggestion is that they are not.