Characteristics of gentamicin resistance in nosocomial infections

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.

Evidence for a chromosomal site specifying amikacin resistance in multiresistant Serratia marcescens

We studied 21 strains of amikacin-resistant Serratia marcescens from three different U.S. cities, Twenty of the 21 strains contained conjugative R plasmids mediating gentamicin and tobramycin resistance. Amikacin-resistant S. marcescens from two cities predominated in protracted outbreaks. Conversely, the amikacin-resistant Charleston strain (serotype 02/03:nonmotile) was isolated from only four patients during an outbreak of gentamicin- and tobramycin resistant, amikacin-susceptible S. marcescens (serotype O19:O17). Five different representative amikacin-resistant S. marcescens, each containing a single conjugative plasmid, elaborated a nontransferable aminoglycoside (6')-N-acetyltransferase [AAC(6')] with similar substrate profiles in addition to other transferable aminoglycoside-modifying enzymes. One amikacin-resistant S. marcescens cured of its plasmid and another naturally occurring plasmid-free amikacin-resistant S. marcescens elaborated only AAC(6')-1. These data support the concept of a chromosomal locus in S. marcescens for AAC(6')-1 which commonly coexists with plasmid-mediated genes for aminoglycoside-modifying enzymes.

Antibiotic resistance in Providencia stuartii isolated in hospitals

A total of 238 isolates of Providencia stuartii obtained from infected patients in six Dublin hospitals were grouped by using serological and bacteriocin typing methods and tested for sensitivity to a number of antimicrobial agents. Most isolates were resistant to several of these agents. Resistance to tetracycline, resistance to penicillin, resistance to polymyxin, and probably resistance to nitrofurantoin was intrinsic. Plasmid screening coupled with resistance transfer studies showed that both chromosome-encoded and plasmid-coded resistance mechanisms were clinically important. Ampicillin resistance was both chromosomally and plasmid encoded, whereas resistance to kanamycin and resistance to carbenicillin were exclusively plasmid encoded. Gentamicin resistance was more common than kanamycin resistance, and although gentamicin-resistant strains contained aminoglycoside acetyltransferase activity, no association could be demonstrated with plasmid deoxyribonucleic acid in the strains tested. Unlike minimal inhibitory concentrations for kanamycin, minimal inhibitory concentrations for gentamicin varied over a wide range. P. stuartii isolated obtained from several different countries were tested for comparison. As a group, these strains were less resistant, but they did exhibit similar resistance properties.