Antibiotic resistance in Gram-positive cocci

Gram-positive cocci still predominate as a cause of nosocomial- and community-acquired infections. These organisms frequently reveal a high natural, intrinsic resistance to antimicrobials. Additionally, these bacteria are able to acquire resistance to frequently used drugs rapidly through selective pressure of the environment and via the genetic evolution of bacteria. The wide application of antimicrobials in medical and veterinary practice, usage of antibiotics in agriculture and common usage of antiseptics and disinfectants result in selective pressure. The use of antibiotics directly selects resistant variants to different antibiotics or disinfectants. The same genetic element (e.g. qac or smr) conferring resistance to some disinfectants are often present on the same plasmid conferring resistance to antibiotics. Selection of resistant variants occurs most frequently in the hospital environment. Staphylococcus aureus and enterococci are the most commonly isolated bacteria causing nosocomial infections. Among those giving therapeutic problems are methicillin-resistant staphylococci and vancomycin-resistant enterococci. Resistance to high levels of aminoglycosides or penicillins among hospital enterococcal strains can completely abolish synergism of the drugs. In these cases glycopeptides will be the drugs of choice in the treatment of serious infections. Recently S. aureus strains with decreased susceptibility to vancomycin has appeared. A mechanism for this elevated resistance, although intensively investigated, still remains unknown.