Phenotypic Characterization of Antimicrobial Resistance in Salmonella enterica Isolates Associated with Cattle at Harvest in Mexico

Introduction: In recent years, the emergence of antimicrobial resistance in Salmonella has become a major public health concern worldwide. Salmonella is a common food contaminant that can develop resistance to antimicrobials used to treat human or animal illnesses. Consequently, there is an increasing concern regarding the use of antimicrobials in food animals such as beef cattle, since this could be a potential factor in the dissemination of antimicrobial-resistant Salmonella. 

Purpose: The objective was to determine the antimicrobial susceptibility profiles among Salmonella enterica isolates from cattle presented at harvest in Mexico. 

Methods: A total of 351 Salmonella isolates from fecal samples (n=31), beef hides (n=105), and beef carcasses (n=215), were included in the study to test for antimicrobial susceptibility. Isolates were isolated from samples collected in three abattoirs in Mexico. Susceptibility profiles were determined using the Sensititre automated antimicrobial susceptibility system with the Gram Negative NARMS plate format CMV3AGNF. The MIC breakpoints for each of the 14 antimicrobials tested were interpreted using the CLSI standards for microdilution broth methods, when available, and with the NARMS breakpoints. 

Results: Overall, a total of 205 (58.4%) Salmonella enterica isolates tested exhibited resistance to at least one or more antimicrobial. Furthermore, all 351 isolates exhibited 20 different resistance phenotypes. Resistance to tetracycline (40.2% of the isolates were resistant), and resistance to nalidixic acid (21.1%) were observed most often. Additionally, the most common multidrug-resistant phenotypes present in the isolates shared resistance to chloramphenicol, streptomycin, tetracycline, and trimethopin/sulfamethoxazole (11.3%), resistance to ampicillin, tetracycline, and trimethopin/sulfamethoxazole (3.4%), and resistance to ampicillin, streptomycin, and tetracycline (2.5%).  

Significance: Understanding the diversity of antibiotic-resistant Salmonella isolated from food animals can promote control measures in livestock production practices globally. This can subsequently aid in the prevention of the spread of antimicrobial resistance pathogens such as Salmonella to human environments.