Endemic resistance to amikacin among hospital isolates of gram-negative bacilli: implications for therapy

We reviewed the records of the microbiology laboratory of the Veterans Administration Medical Center, Bronx, New York in order to determine the prevalence, epidemiology and complete antibiotic susceptibility profile of amikacin-resistant aerobic and facultative gram-negative bacilli isolated from clinical specimens submitted for culture between January 1, 1980 and May 1, 1981. Of more than 5000 gram-negative rods isolated during this 16-month period, 2.8% were determined to be resistant to amikacin by the disc diffusion method. Eighty-eight of the amikacin-resistant organisms were unique isolates derived from cultures on 74 patients located throughout the hospital. Urine (51%) and sputum (27%) were the predominant sources of specimens yielding resistant strains. These organisms represented seven different genera of Enterobacteriaceae (58%) or Pseudomonas aeruginosa (31%) and other glucose non-fermenting species (11%). Resistance to amikacin was usually associated with resistance to gentamicin, tobramycin and most of the other antimicrobials tested. Twenty percent of isolates were susceptible to only a single antimicrobial, and another 5% were resistant to every agent routinely tested. Although geographic clustering of a small number of amikacin-resistant organisms occurred twice (a strain of Proteus mirabilis on the spinal cord injury service and a strain of P. aeruginosa on one medical ward), the vast majority of isolations were consistent with a pattern of endemic resistance.