Mechanisms of antimicrobial resistance: their clinical relevance in the new millennium.

Antimicrobials show selective toxicity. Suitable targets for antimicrobials to act at include the bacterial cell wall, bacterial protein and folic acid synthesis, nucleic acid metabolism in bacteria and the bacterial cell membrane. Acquired antimicrobial resistance generally can be ascribed to one of five mechanisms. These are production of drug-inactivating enzymes, modification of an existing target, acquisition of a target by-pass system, reduced cell permeability and drug removal from the cell. Introduction of a new antimicrobial into clinical practice is usually followed by the rapid emergence of resistant strains of bacteria in some species that were initially susceptible. This has reduced the long-term therapeutic value of many antimicrobials. It used to be thought that antibacterial resistance was mainly a hospital problem but now it is also a major problem in the community. Organisms in which resistance is a particular problem in the community include members of the Enterobacteriaceae, including Salmonella spp. and Shigella spp., Mycobacterium tuberculosis, Streptococcus pneumoniae, Haemophilus influenzae and Neisseria gonorrhoeae. Multi-resistant Gram-negative rods, methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci are major causes of concern in the hospital setting. Prevalence of antibacterial resistance depends both on acquisition and spread. Decreasing inappropriate usage of antimicrobials should lessen the rate of acquisition, and spread can be minimised by sensible infection control measures.