Current mechanisms of resistance to antimicrobial agents in microorganisms causing infection in the patient at risk for infection.

The mechanisms of resistance encountered in bacteria causing infection in the patient at risk for infection are diverse. Most resistance currently seen is the result of plasmid transfer rather than mutational events. However, extensive use of antimicrobial agents in the hospital has caused the selection of organisms resistant to many agents by virtue of chromosomally mediated mechanisms. Staphylococcus aureus resistant to beta-lactams due to altered penicillin-binding proteins has become a problem in certain patients such as narcotic addicts and chronic care facility patients exposed to many beta-lactam antibiotics. S. epidermidis has also proved to be a problem in patients with indwelling foreign devices, and altered penicillin-binding proteins also make these organisms resistant to available penicillins and cephalosporins. Streptococcus fecalis has become increasingly resistant to aminoglycosides, erythromycin, and tetracyclines due to plasmid-mediated enzymes. Hemophilus influenzae resistant to both penicillins and chloramphenicol by virtue of beta-lactamases and chloramphenicol transacetylase has been encountered. Beta-lactamase-mediated resistance of Enterobacteriaceae, Escherichia coli, and Klebsiella pneumoniae to beta-lactam antibiotics has increased, and resistance of Serratia marcescens and Pseudomonas aeruginosa to aminoglycosides and penicillins is a widespread phenomenon. Mechanisms to reduce resistance will include not only careful attention to hygienic practices but also more appropriate use of antibiotics selecting the proper agent depending on the type of patient and environment in which the infection develops.