Endemic aminoglycoside resistance in gram-negative bacilli: epidemiology and mechanisms

Isolates of gentamicin-resistant gram-negative bacilli from clinical specimens peaked at nine to 10 per month in 1973-1974. Instituting barrier-type precautions during 1974-1977 was associated with a sustained 87% reduction in resistant Enterobacteriaceae. The number of resistant Pseudomonadaceae fell temporarily by 28%, paralleling gentamicin usage. During an endemic 15-month period in 1976-1977 nonenzymatically mediated resistant Pseudomonas aeruginosa often emerged after aminoglycoside therapy in patients who had prior carriage of sensitive strains of the same serotype (P = 0.002); this resistance was associated with wound or sputum isolates (P = 0.003). Resistant Enterobacteriaceae more often demonstrated the converse, that is, spread of urinary tract isolates with enzymatically mediated resistance from patients not on aminoglycoside therapy. These findings suggest that control measures to minimize occurrence of resistant bacilli include barrier-type precautions for patients with resistant Enterobacteriaceae, evaluation of transfers and readmissions as a source of resistant organisms, and reduction of aminoglycoside use to decrease the selection of nonenzymatic resistance.