Possible link between bacterial resistance and use of antibiotics and biocides.

Low-level plasmid-mediated resistance to cationic biocides such as chlorhexidine (CHX), quaternary ammonium compounds (QACs), amidines, and acridines has been observed in antibiotic-resistant strains of Staphylococcus aureus and Staphylococcus epidermidis (1, 4, 5), and it has been postulated that strains in which qac genes are present might have enhanced survival in the clinical environment. Extensive use of cationic biocides could lead to the selection of staphylococcal strains showing resistance to both antibiotics and biocides (9, 11), but the clinical relevance of this possibility remains contentious (8). Although plasmid-mediated resistance to biocides has also been found in gram-negative bacteria, it has been proposed (8) that intrinsic resistance in these organisms is of greater significance. Resistance to both antibiotics and biocides in gramnegative organisms is more likely where less specific mechanisms are involved, e.g., the outer membrane may act as a nonspecific exclusion blanket thereby preventing the uptake of chemically unrelated molecules (10, 11). There have, however, been some instances where biocides have been claimed to select for resistant gram-negative bacteria. Stickler et al. (12) observed resistance to CHX, QACs and at least five antibiotics for gram-negative bacteria isolated from urinary tract infections and proposed that the widespread use of CHX was responsible for selecting antibiotic-resistant strains. There was no evidence of plasmid-linked resistance association (although the possibility was raised but not proven that CHX-resistant strains were acting as more efficient recipients of plasmids conferring antibiotic resistance). Dance et al. (2) isolated a strain of Proteus mirabilis, responsible for a hospital outbreak, that was resistant to CHX and antibiotics. However, there was no evidence of a genetic linkage between these resistances, which were considered to be intrinsic rather than plasmid mediated. Selection of antibiotic-resistant bacteria by chlorination of drinking water and treated sewage has also been described, and the basis of linked biocide-antibiotic resistance in organisms isolated from aquatic sources is of potential public health importance (9).