ORIGINAL ARTICLE Quinolone resistance among Salmonella enterica and Escherichia coli of animal origin Mehrnaz Rad & Mojhgan Kooshan & Hooman Mesgarani Received: 21 April 2010 / Accepted: 25 August 2010 / Published online: 4 September 2010 # Springer-Verlag London Limited 2010 Abstract Escherichia coli and Salmonella enterica are major pathogens of worldwide importance in the animal industry. Antimicrobial therapy is an important tool in reducing the enormous losses caused by these infectious agents, however, resistance to existing antimicrobials espe- cially quinolones and fluoroquinolones is widespread and of concern to veterinarians. Iranian isolates of E. coli and S. enterica from different species of animals were examined for resistance to quinolones and fluoroquinolones. Thirty-five S. enterica isolates were recovered from different animal origins. Twenty-five E. coli strains were also isolated from poultry with colibacillosis. Eleven strains of E. coli were isolated from cloacal swabs from healthy chicken. All of the E. coli strains were identified by biochemical tests. Gene invA was detected in all of the Salmonella isolates. Serogroups of Salmonella were determined by colored rapid latex test. There was no relationship among serogroups of Salmonella and resistance to quinolones. In vitro antibiotic activities of four antibiotic substances against the isolates were determined by disk diffusion test. Forty percent of S. enterica and 96% of E. coli were found to be resistant to nalidixic acid. Fifty-six percent, 72%, and 36% of E. coli strains were resistant to ciprofloxacin, enrofloxacin, and norfloxacin, respectively. However, 11.42%, 22.85%, and 5.71% of Salmonella strains were resistant to ciprofloxacin, enrofloxacin, and norfloxacin, respectively. Keywords Escherichia coli . Salmonella enterica . Quinolone resistance Introduction Antimicrobials are valuable tools to treat infectious disease and to maintain healthy and productive animals. However, the treatment of whole herds and flocks with antimicrobials for disease prevention and growth promotion has become a controversial practice. Sick animals are sometimes treated individually, but often whole flocks or herds of animals are treated at once, including animals that are not ill. In addition, antimicrobials are used in the absence of disease to prevent diseases during times when animals may be susceptible to infections. This practice is very common on poultry farms in countries where infections caused by enteric pathogens are severe. Such misuse and/or inappro- priate usage affect a larger number of animals, because it usually involves treating a whole herd, which increases the likelihood of selecting for organisms that are resistant to the antibiotic. Of particular concern is the emergence of resistance to the quinolones and fluoroqinolones, which, because of their low toxicity and relatively broad-spectrum coverage, are extremely valuable for treating infections (Yang et al. 2004). Mechanisms of resistance to fluoroqui- nolones include alterations in DNA gyrase and topoisomerase IV, and decreased intracellular accumulation of the antimicro- bial agent due to modifications of membrane proteins (Stevenson et al. 2007; Yang et al. 2004). However, several studies demonstrated that quinolone-resistant Escherichia coli strains have fewer virulence factors than quinolone- susceptible strains (Martinez-Martinez et al. 1999; Seputiene et al. 2006; Soto et al. 2006; Vila et al. 2002). Quinolone (nalidixic acid) and fluoroquinolones (norflox- acin, ciprofloxacin, and enrofloxacin) are classes of synthetic antimicrobial agents with an excellent activity against E. coli and other gram-negative bacteria used in human and veterinary medicine (Seputiene et al. 2006). However, the M. Rad (*) : M. Kooshan : H. Mesgarani Department of Pathobiology, School of Veterinary Medicine, Ferdowsi University of Mashhad, P.O. Box 91775-1793, Mashhad, Iran e-mail: mehrnazrad@yahoo.com Comp Clin Pathol (2012) 21:161–165 DOI 10.1007/s00580-010-1078-2