Evaluation of nalidixic acid susceptibility testing for screening of clinical strains of Salmonella with decreased susceptibility to ciprofloxacin

Fluoroquinolones, in particular ciprofloxacin have recently become the drug of choice for antibiotic therapy of invasive Salmonella infections in many countries. however, several treatment failures due to Salmonella strains with decreased susceptibility to ciprofloxacin have been reported. These isolates are still considered to be susceptible to ciprofloxacin according to CLSI interpretive criteria (MIC≤1 μg/ml) while they can not be detected by disk diffusion test. CLSI has recommended nalidixic acid disk diffusion as an indicator test for screening of Salmonella isolates with decreased susceptibility to ciprofloxacin.In this study the value of nalidixic acid susceptibility testing for screening of clinical strains of Salmonella with decreased susceptibility to ciprofloxacin was evaluated. Salmonella spp. strains have been isolated from several provinces in Iran during 2007-2008. The strains were identified by standard biochemical tests and serology. The susceptibility of the isolates to nalidixic acid and ciprofloxacin was determined by disk diffusion method. MIC assay for ciprofloxacin was determined by E. test. 28 out of 53 Salmonella strains (52.8%) were resistant to nalidixic acid. Disk diffusion testing showed that all isolates were susceptible to ciprofloxacin however 8 (15%) resistant nalidixic acid isolates showed decreased susceptibility to ciprofloxacin (MIC= 0.125 μg/ml) when tested by E.test. The results showed that nalidixic acid disk diffusion test can be considered as a reliable and costeffective method for screening of Salmonella strains with low level resistance to ciprofloxacin.