Characterization of Antibiotic Resistance Pattern of Salmonella Spp. Isolated from Broiler Chickens, Farm Workers and Environment in Two Selected Districts of Bangladesh

Poultry farms act as an important source of transmission of antimicrobial-resistant (AMR) Salmonella to the environment as well as to humans. Indiscriminate and prophylactic use of antibiotics in poultry farms is one of the reasons behind it. Therefore, a cross-sectional study was conducted to determine the prevalence of Salmonella in broiler chickens, farm environments, as well as farmers to identify the risk factors for Salmonella colonization in broiler farms. This study further aimed at characterizing the AMR pattern of isolated Salmonella spp. and determining the prevalence of the carbapenem resistance gene (NDM-1).

Total 150 samples comprising cloacal swabs (n=50) from broiler chickens, farm sewage (n=50) and hand wash water (n=50) of farmers along with data on farm management were collected from 50 broiler farms of Mymensingh and Gazipur districts in Bangladesh. Cloacal swabs were collected from ten birds on each farm and pooled to make one sample. Farmers’ hand were washed directly in 100 ml of sterile distilled water which was then taken into a sterile tube and sealed. Voluntary and informed written consent was obtained before data collection. Salmonella was isolated using Xylose Lysine Deoxycholates (XLD) agar after pre-enrichment in buffered peptone water. Presumptive Salmonella colonies were observed by gram staining and assessed by catalase, sugar fermentation, indole, methyl red, and Voges Proskauer test. Biochemically confirmed Salmonella isolates were then confirmed by PCR with an Internal Transcribed Spacer (ITS) region of 16S-23S rRNA gene-specific primers as described by Chiu et al. (2005). Antimicrobial susceptibility testing was conducted to ten antibiotics (levofloxacin, ciprofloxacin, ceftazidime, ceftriaxone, cefotaxime, amoxicillin-clavulanic acid, colistin, doxycycline, imipenem, and meropenem) by disc diffusion technique (CLSI, 2015). Carbapenem resistance gene (NDM-1) was detected by PCR. Multivariable logistic regression was done to identify the risk factors associated with the Salmonella colonization in broiler chickens.

Salmonella was detected with an overall prevalence of 66% of the 150 samples. Among the three types of samples collected, the highest prevalence, 82%, was observed for cloacal swabs. Hand wash water collected from farmers exhibited the lowest Salmonella prevalence of 44%. Salmonella colonization in broiler chickens was significantly associated with ≤5 years experience of farmers (Odds ratio, OR:14.17, 95% Confidence interval, CI:1.75-114.7) and 11 to 20 day old (OR:18.55, 95% CI:1.18-292.2) as well as more than 20 day old (OR:15.71, 95% CI:1.11-222.67) birds. Overall, Salmonella was found resistant to colistin (88.9%), doxycycline (84.8%), ciprofloxacin (78.8%), ceftazidime (64.6%) and imipenem (36.4%). Lower resistance was observed for levofloxacin (15.2%) and meropenem (18.2%). Most importantly, 89.9% of the total isolates and almost all the cloacal swab isolates (97.6%) showed multidrug resistance (MDR). In genotypic resistance analysis, one broiler chicken isolate was found positive for the NDM-1 gene when the test was repeated three times with positive control in PCR.

High percentage of MDR Salmonella found in our study and the presence of carbapenem resistance gene suggest that a serious threat to public health may emerge. Further studies should be conducted with sufficient sample size to evaluate the situation.