Gentamicin-resistant Pseudomonas aeruginosa: Mayo Clinic Experience, 1970-1976.

During the period 1970 through 1976, there were 144 patients from whom gentamicin-resistant Pseudomonas aeruginosa (minimum inhibitory concentration [MIC], more than 5 microgram/ml) was isolated. In 20(21 percent) of the 95 patients who acquired such organisms within our institutions, the occurrence was considered clinically significant. Factors that favored the appearance of gentamicin-resistant P. aeruginosa included prolonged hospitalization, previous antibiotic treatment, increased gentamicin usage, underlying disease, and instrumentation (70 percent). Virulence of gentamicin-resistant isolates appeared less than that of susceptible organisms, with bacteremia due to these isolates occurring in only three cases. Resistant isolates with MICs for gentamicin of 8 to 16 microgram/ml were more susceptible to tobramycin than to amikacin, whereas isolates with MICs for gentamicin of 64 microgram/ml or greater were more susceptible to amikacin than to tobramycin. Eighty percent of all strains were susceptible to 128 microgram/ml or less of carvenicillin. Favorable results occurred in 12 or 13 cases treated with gentamicin plus carbenicillin, whereas treatment with either of these agents alone resulted in failure or relapse in 7 of 14 cases.