Evaluation of antibiotic resistance in bacterial strains isolated from sewage of slaughterhouses

BACKGROUND: The frequent and incorrect use of antibiotics in animal husbandry has led to the spread of antimicrobial resistance. The issue should be addressed in a One Health vision, since human and animal health, as well as the environment are interconnected. Wastewater from slaughterhouses can be contaminated with multidrug-resistant bacteria, representing a possible cross-contamination route. This study evaluated the presence of antibiotic-resistant bacteria in wastewater from slaughterhouses to highlight their role in the spread of antibiotic resistance. METHODS: In total, 18 out 35 slaughterhouses in Sicily (Italy) were considered. For each wastewater system the samples were collected before any treatment, and cultured within 24 h of sampling. The isolates were identified by API20NE, API20E profiles (bioMerieux, France). Using the Kirby Bauer test, the isolated strains were classified as resistant (R), intermediately resistant (I), or sensitive (S) to the tested antibiotics. RESULTS: Proteus mirabilis (85.7%) and Peudomonas aeruginosa (27.3%) were the most frequently isolated microorganisms, followed by Enterobacter cloacae, Aeromonas hydrophila, and Citrobacter freundii. The most resistant strains belonged to Enterobacter spp., which showed the highest resistance rate for macrolides, penicillins and cephalosporins. Aeromonas spp. was resistant to carbapenems while E. coli showed the highest resistance to colistine. Finally, Citrobacter spp. was completely resistant to macrolides, rifampicin, and tetracycline. CONCLUSIONS: High resistance levels were detected for some antibiotics widely used in human medicine such as macrolides, penicillins, and cephalosporins. Resistance to carbapenems and colistine, which are now considered key drugs in human medicine for the treatment of severe multidrug-resistant infections, was also reported. Thus, continuous monitoring on the correct use of antibiotics in animal husbandry is necessary. KEY MESSAGES: • Wastewater from slaughterhouses may be a vehicle of antibiotic-resistant bacteria. • The One Health approach should be used for the management of health security, food safety and antimicrobial resistance.