Antibiotic and Disinfectant Resistant Bacteria in Rivers of the United States

We examined natural water sources in the U.S. for the presence of bacteria resistant to natural and human-produced antimicrobial agents. The impact of these aquatic organisms on diseases in man and animals is unknown but resident bacteria in water represent a potentially important reservoir of resistance genes. Further, the presence of resistant organisms is usually an indication of some selective pressure in the past which has enabled these bacteria to arise. Usage of antibiotics and chemotherapeutic agents in veterinary medicine and agriculture can result in contamination of natural water sources. Clinical use in humans can also result in the introduction of such compounds into water. Since some studies have suggested a link between antibiotic resistance and resistance to disinfectants, we determined the nature of bacteria in water with respect to these two resistance traits. Our previous sampling of U.S. rivers demonstrated the widespread resistance of resident bacteria to a variety of synthetic and semisynthetic antimicrobial agents. We have investigated further the resistance to sulfa drugs which represent a class of compounds that are not found in nature except as a result of use in animals or humans. Bacteria resistant to sulfadiazine and sulfamethoxazole-trimethoprim were found in all rivers examined. In addition, these resistant organisms were not killed by other antibiotics and many contained plasmids which are common vehicles for transfer of genes among bacteria. Over forty percent of the organisms also contained integron sequences, DNA elements responsible for the transfer of blocks of genetic information. The sensitivity of sulfa-resistant bacteria to disinfectants was also tested. It was found that, in general, organisms possessing integrons were able to survive higher concentrations of benzalkonium chloride. The fact that resistance to these agents has been maintained in natural populations suggests that these traits may confer some additional advantage to bacteria who have received and maintained the respective genetic information.