Antibiotic and disinfectant resistance of Salmonella isolated from egg production chains

To investigate the contamination of Salmonella and its drug resistance in egg production chains, 111 Salmonella strains of different serotypes isolated from egg production chains were used in the study. The minimum inhibitory concentrations (MICs) of antibiotics and disinfectants against Salmonella isolates were determined, meanwhile, antibiotic and disinfectant resistance genes were amplified. The results showed that the resistance frequency of trimethoprim (TMP, N=100, P=90.09%) was highest among Salmonella isolates and all isolates were sensitive to amoxicillin and clavulanate (AMC), ceftiofur sodium (CFS) and gentamicin (CN), respectively. There were six different antibiotic resistance profiles, and TMP profile was the most prevalent type (N=36, P=32.43%). 52.25% of Salmonella isolates appeared multi-drug resistance. The MICs of benzalkonium chloride (BC) and cetylpyridinium chloride (CPC) against Salmonella strains ranged from 8 to 128 μg/mL and 8 to 256 μg/mL, respectively. Compared to quality control strain Escherichia coli ATCC10536, 101 Salmonella isolates (P=90.99%) had dual resistances to BC and CPC. 109 Salmonella (P=98.20%) were co-resistant to antibiotic and disinfectant. Detection of drug resistance genes showed that blaTEM gene was dominant (N=49, P=44.14%). The qnrA, qnrB and qepA genes were not detected. Only qacEΔ1 gene (N=63, P=56.76%) was detected among the disinfectant resistance genes. There was a significant correlation between sul1 gene and qacEΔ1 gene (P < 0.01). S. Derby showed multi-resistances to TMP, oxytetracycline (OTC), amoxicillin (AML) and ciprofloxacin (CIP). Eleven antibiotic resistance genes were found in S. Derby, in which the prevalence of qacEΔ1 gene was 81.25% (N=52). Besides, the drug resistance frequency and the prevalence of drug resistance genes in internal farm environment were higher than those in external environment. High frequency of drug resistances and high prevalence of drug resistance genes were detected in all links of the egg production chains, including package, storage and sale. Our results showed that severe antibiotic and disinfectant resistances existed in egg production chains. Therefore, further hygiene supervision should be implemented to prevent and control Salmonella, and standardize the use of antibiotics and disinfectants.