Razi Journal of Medical Sciences

    Background & Aim: Nalidixic acid is a quinolone antibiotic with excellent in vitro and in vivo activity against salmonella. It is often the first choice for treating drug resistance salmonella infections. Nalidixic acid resistance can often lead to resistance to fluoroquinolones such as ciprofloxacin. In this survey the extent of salmonella infections, the salmonella strains involved, antibiotic susceptibility patterns, and the MIC values towards nalidixic acid were investigated. Patients and Methods: During one year period(2005-2006), a total of 1333 diarrheal stool samples were collected from hospitalized patients. Stool cultures were performed on differential and selective media for salmonella isolation. A total of 45 salmonella spp.(species) were isolated(3.4%). Species identification were achieved by agglutination with species-specific antisera. Antibiotic susceptibilities were determined by disk-diffusion method(Kirby-Baure). The minimal inhibitory concentration(MIC) of drug-resistance salmonella isolates was performed by E.test. The study was a descriptive work. Data was analyzed by SPSS sftware. Based on difference ratio hypothesis there were no significant differences between the two methods. Results: A total of 45 salmonella spp.(3.4%) were isolated from 1333 stool samples. Agglutination tests with specific antisera indicated that 9 of them belonged to S. enteritids(20.0%), 6 S. typhimurium(13.3%), 4 S. montevideo(8.9%), 3 S. paratyphi C(6.7%), 2 S. paratyphi B(4.4%), 1 S. muenchen(2.2%), 1 S. derby(2.2%), 1 S. schwarzengrund(2.2%), 1 S. arizonea(2.2%) and 13(28.9%) untypable strains. All of the isolates were agglutinated with only anti O-antisera and non showed any reactions with anti-H antisera. Of the 13 untypable strains, 10(22.2%) belonged to the salmonella serogroup C and the remaining 3(6.7%) were serogroup B. Antibiogram tests indicated that 11(24.4%) of the salmonella isolates were resistant to Nalidixic acid in the disk diffusion agar method. However, determination of MIC values with E.test indicated that only 9(20.0%) of these strains showed MIC values within resistant range. Conclusion: The highest rate of nalidixic acid resistance was seen within the non-typhoidal salmonella strains. These strains are widely distributed within our environment and are the major etiological agents of human salmonellosis. Eleven strains were nalidixic acid resistant in the disk-diffusion method whereas, only 9 showed resistant trait with E.test. The MIC of the resistant isolates to nalidixic acid was ≥ 32µg//ml. Despite its high cost, it is therefore concluded that E.test gives a better and more accurate identification of drug-resistance trait as compared to disk diffusion agar method.