Antibiotic Residues and Zinc Concentrations in the Livers and Kidneys of Portuguese Piglets-Relationship to Antibiotic and Zinc Resistance in Intestinal Escherichia coli.

Metal ions such as zinc and copper have been used as alternatives to antibiotics, to improve animal health and growth rates in pig farming. This study aims to determine antibiotic residues and Zn concentration in piglets' livers (n = 56) and kidneys (n = 60); and to examine the correlation between the use of Zn and antibiotics, and resistance to Zn and antibiotics of Escherichia coli isolated from piglets' faeces (n = 60). Samples were collected from randomly selected healthy piglets (n = 60); antibiotic residues were quantified by ultra-high-performance-liquid-chromatography time-of-flight mass spectrometry (UHPLC-ToF-MS); Zn was quantified using flame atomic absorption spectrometry (FAAS); microbiological methods were used for E. coli isolation, antibiotic susceptibility, and Zn minimal inhibitory concentration; and Real-Time PCR was used for gene detection. The presence of antibiotic residues and Zn concentrations in the liver was found to be negatively correlated, whilst no significant difference was observed in the kidney. In E. coli isolated from piglet faeces considered to be susceptible or multi-drug-resistant, no significant difference was found between Zn concentrations in the liver and in the kidney, which appears to indicate that Zn accumulated in the liver and in the kidney does not promote resistance to antibiotics in E. coli. The isolates showed tolerance to Zn which would suggest that antibiotic resistance and phenotypic tolerance to Zn in these isolates are not related. The genes zitB and zntA associated to Zn tolerance, were predominantly found in the more resistant Zn isolates. The findings provide insights on how Zn use in pig production maintains antibiotic resistance and metal tolerance in bacteria, with implications for One Health.