Antimicrobials resistance of animal Salmonella spp. isolates

Salmonella is an important pathogen of economic significance in both humans and animals. The majority of Salmonella infections in humans are self-limiting. In some patients the infection may be more severe and antimicrobial therapy essential, but infections by antimicrobial-resistant Salmonella strains may cause treatment failure. In the last years the emergence and spread of multi-drugs resistant Salmonella enterica strains represent a severe concern worldwide (1). The aim of this work was to evaluate the antimicrobial resistance profile of 176 Salmonella spp. isolates obtained from different animal samples, in particular from swine, poultry and reptiles. Tigecycline was included in the trial to verify the susceptibility of Salmonella to this antimicrobial. Disk diffusion method was carried out with the following antibiotics: Nalidixic Acid, Ciprofloxacin, Enrofloxacin, Ampicillin, Amoxicillin/Clavulanic Acid, Cefotaxime, Cefalotin, Ceftazidime, Gentamycin, Kanamycin, Streptomycin, Amikacin, Tobramycin, Tetracycline, Tigecycline, Sulfonamide, Trimethoprim, Trimethoprim/Sulfamethoxazole, Colistin, Nitrofurantoin, Chloramphenicol, Florfenicol. MIC was evaluated with microdilution method for antibiotics that resulted ineffective against the majority of the tested isolates and for Tigecycline. Disk diffusion method and MIC were performed as suggested by CLSI; the antimicrobial breakpoints for MIC evaluation were the following: Streptomyicin ≥ 64 µg/ml, Kanamycin ≥ 64 µg/ml, Tetracycline ≥ 16 µg/ml and Tigecycline ≥ 2 µg/ml (2). Most of the antibiotics tested resulted effective against more than 80% of isolates. In particular 88.64% and 98.30% of isolates resulted susceptible to Enrofloxacin and Amoxicillin/Clavulanic Acid, respectively, two of the most used antibiotic in veterinary medicine. For Nitrofurantoin and Chloramphenicol, the percentages of susceptible isolates were 80.68% and 93.18%, respectively. More than 90% of isolates were susceptible to cephalosporins. Low susceptibility to Kanamycin, Streptomycin, Tetracycline, Tigecycline and Sulfonamide was observed with 48.30%, 82.95%, 68.18%, 81.25% and 75.00% non-susceptible isolates, respectively. One hundred and seventy-one isolates (97.14%) resulted non-susceptible to at least one antibiotic, and a great variability in resistance patterns was observed. All but 1 of the 120 Tetracycline non-susceptible isolates were confirmed as resistant; in particular 42 isolates showed a MIC ≥ 1024 µg/ml. For Streptomyicin, 48 of the 146 non-susceptible isolates resulted resistant with MIC test. Fourteen of the 85 non-susceptible isolates for Kanamycin were confirmed as resistant; 11 isolates showed a MIC ≥ 2048 µg/ml. As concerns Tigecycline, a new antibiotic, 44 of the 143 non-susceptible isolates showed a MIC value higher than the breakpoint, in particular 24 isolates showed a MIC of ≥ 2 µg/ml, 17 isolates of ≥ 4 µg/ml and 3 isolates of ≥ 8 µg/ml. All isolates resistant to Tigecycline were also resistant to Tetracycline, 32/44 isolates showed a MIC for Tetracycline ≥ 1024 µg/ml. Our results confirm that different animal species may be reservoirs of antimicrobial-resistant Salmonella, that could represent a threat for animal and human therapy.