Resistance of Staphylococci to Antimicrobial Drugs

Hospital strains of S. aureus are usually resistant to a variety of different antibiotics. A few strains are resistant to all clinically useful antibiotics except vancomycin, and vancomycin-resistant strains are increasingly-reported. The term MRSA refers to Methicillin resistant Staphylococcus aureus. Methicillin resistance is widespread and most methicillin-resistant strains are also multiply resistant. A plasmid associated with vancomycin resistance has been detected in Enterococcus faecalis which can be transferred to S. aureus in the laboratory, and it is speculated that this transfer may occur naturally (e.g. in the gastrointestinal tract). In addition, S. aureus exhibits resistance to antiseptics and disinfectants, such as quaternary ammonium compounds, which may aid its survival in the hospital environment.

Staphylococcal disease has been a perennial problem in the hospital environment since the beginning of the antibiotic era. During the 1950's and early 1960's, staphylococcal infection was synonymous with nosocomial infection. Gram-negative bacilli (e.g. E. coli and Pseudomonas aeruginosa) have replaced the staphylococci as the most frequent causes of nosocomial infections, although the staphylococci have remained a problem, especially in surgical wounds.. S aureus responded to the introduction of antibiotics by the usual bacterial means to develop drug resistance: (1) mutation in chromosomal genes followed by selection of resistant strains and (2) acquisition of resistance genes as extrachromosomal plasmids, transducing particles, transposons, or other types of DNA inserts. S. aureus expresses its resistance to drugs and antibiotics through a variety of mechanisms.

Beginning with the use of the penicillin in the 1940's, drug resistance has developed in the staphylococci within a very short time after introduction of an antibiotic into clinical use. Some strains are now resistant to most conventional antibiotics, and there is concern that new antibiotics have not been forthcoming. New strategies in the pharmaceutical industry to find antimicrobial drugs involve identifying potential molecular targets in cells (such the active sites of enzymes involved in cell division), then developing inhibitors of the specific target molecule. Hopefully, this approach will turn up new antimicrobial agents for the battle against staphylococcal infections. In fact, in the past two years alternatives to vancomycin have been approved with the increase in VRSA (vancomycin resistant S. aureus) isolates.