Aminoglycoside resistance among Pseudomonas aeruginosa isolates with an unusual disk diffusion antibiogram

Abstract

In recent years, a number of clinical microbiology laboratories have isolated Pseudomonas aeruginosa with the unusual aminoglycoside disk diffusion result of resistance to both amikacin and gentamicin but susceptibility to tobramycin (AFGrTS). A total of 39 isolates of P. aeruginosa reported to have this resistance pattern were retested by the standard National Committee for Clinical Laboratory Standards disk diffusion procedure; 30 strains (77%) were confirmed to be ArGFTS. These 30 isolates were further examined for susceptibility to those aminoglycosides by agar dilution and broth micro-and macrodilution methods. Only 27, 27, and 23% of the isolates appeared to be ArGrTS by agar, broth microdilution, and broth macrodilution testing, respectively. Most of the remaining isolates were resistant to all three aminoglycosides when tested by broth dilution and resistant only to gentamicin when tested by agar dilution. The percentages of strains resistant to any particular aminoglycoside by agar dilution, broth microdilution, and broth macrodilution, respectively, were 43, 80, and 70 for amikacin, 97, 93, and 100 for gentamicin, 100, 100, and 100 for netilmicin, 30, 87, and 93 for sisomicin, and 13, 57, and 50 for tobramycin. These results indicate that strains showing the unusual aminoglycoside antibiogram are less susceptible to aminoglycosides in general and should probably be considered borderline resistant to all aminoglycosides. The efficacy of aminoglycosides in the treatment of infections produced by these strains is unknown.

Postoperative Surgical Site Bacterial Infections and Drug Susceptibility Patterns at Gondar University Teaching Hospital, Northwest Ethiopia

“…According to the current study, methicillin resistant CoNS were 77.3% which is in line with the study that reported 72.5% resistant strains [37]. The present study also showed a single isolate of CoNS resistant to vancomycin which is less from the isolates found India [37]. The emergence of vancomycin resistance in CoNS in our teaching hospital may pose therapeutic problems, and therefore the empirical antibiotic treatment of suspected infections caused by CoNS should be prescribed according to antimicrobial susceptibility testing.…”

“…Methicillin resistant CoNS have become the predominant pathogen and increasing dramatically in hospitalized patients [35,36]. According to the current study, methicillin resistant CoNS were 77.3% which is in line with the study that reported 72.5% resistant strains [37]. The present study also showed a single isolate of CoNS resistant to vancomycin which is less from the isolates found India [37].…”

Synergistic activity of aminoglycoside-beta-lactam combinations against Pseudomonas aeruginosa with an unusual aminoglycoside antibiogram

“…These isolates were recovered from different patients at The Mount Sinai Hospital in New York, NY (four isolates), from the Saint Joseph Hospital in Omaha, Nebraska (five isolates), and from the Veterans Administration Medical Center in Omaha (one isolate). All strains were identified as P. aeruginosa by previously described methods [5][6][7] and showed the ArGrT s antibiogram when tested by the NCCLS disk diffusion procedure [25]. Multiple passaging was avoided to prevent possible loss of antibiotic resistance.…”

“…Agar dilution was performed by the NCCLS standard method [26] as described earlier [5]. Test strains were spot inoculated onto Mueller-Hinton agar (MHA) (BBL, Cockeysville, Md) with a multipoint inoculator.…”

“…These isolates, by disk diffusion testing, are resistant to amikacin and gentamicin, but show susceptibility to tobramycin (ArGrTS). Recent data from our laboratory indicate that ArGrT S strains should probably be considered borderline resistant to all aminoglycosides [5]. The ArGrT s are apparently resistant by membrane impermeability since representative isolates were shown not to produce aminoglycoside modifying enzymes.…”