Antimicrobial Resistance and Presence of Integrons in Salmonella Isolated from Animals and Humans in the United States of America and Uganda

Abstract

Salmonella has been cited as one of the leading causes of food borne illness world wide and in the United States (US), as well as an indicator organism for studying antimicrobial resistance (AMR) trends. The objective of this study was to characterise AMR patterns of Salmonella isolates from animals and humans in North Dakota, US, and Kampala, Uganda, and determine the association between the observed AMR and presence of class 1 integrons. Salmonella isolates were collected from the Veterinary Diagnostic Laboratory (VDL) at North Dakota State University and the North Dakota Department of Health respectively from 2003-2008. Samples were also retrieved from archives present at the Microbiology Department, Faculty of Veterinary Medicine at Makerere University in Kampala, Uganda. AMR profiles were determined using a panel of 15 antimicrobials as per the manufacturer's instructions (Sensitire, Trek Diagnostics System, Westlake, Ohio). Screening for the class 1 integrons was done using PCR with primers specific for the int 1. Out of 359 Salmonella isolates tested, 24. 79% were resistant to ~5 antimicrobials while 36.2% were resistant to at least 2. Pan susceptible isolates were mostly (65.05%) from human isolates. The most common multidrug resistant (MDR) phenotype among the tested isolates was the classic ACSSuT penta-resistance at 29.06% (50/172). The highest resistance frequency was seen against Tetracycline (39.6%) and Streptomycin (34.7 %), while 5.2% (17) of the isolates were resistant to Nalidixic acid and 56 (15.7%) to Ceftiofur. A total of20.7% (57/276) of the ND samples tested positive for presence of class 1 integrons. Class 1 integron was significantly associated (p< 0.05) with AMR to Ampicillin, Kanamycin, Tetracycline, Streptomycin and Sulfisoxazole. Of all Ugai.dan Salmonella isolates tested, 94.4% (68/72) were resistant to 2':2 antimicrobials. The highest resistance was observed against Sulfisoxazole and Trimethoprim-Sulphamethoxazole, and 45.8% of human and 46.2% of cattle isolates tested positive for presence of class 1 integrons. Presence of class 1 integron was significantly associated (p< 0.05) with AMR to Tetracycline and Amoxicillin. DNA sequencing of the class 1 integron variable regions identified several resistance genes including aadAJ, dfrA 7, and dfrA5 gene. The data indicated high AMR among antimicrobials widely used in veterinary and human medicine. Also, AMR was observed against drugs whose veterinary use is restricted, implying possible horizontal transmission. A good proportion (47.9% in Uganda and 29.85% in ND) of the Salmonella isolates from clinical cases of salmonellosis were MDR (resistant to 2':2) isolates bearing class 1 integron.