Colistin and β-lactam resistance in Escherichia coli isolates from bovines, swine, and humans.

Introduction: Colistin and β-lactams are widely investigated because of their effectiveness in the treatment of human diseases. This study investigated the phenotypic and genotypic profiles of colistin- and β-lactam-resistant Escherichia coli (n = 235) obtained from bovines, swine, and workers from a mixed slaughterhouse in Brazil.

Methodology: The disk diffusion method was used to test the resistance against β-lactams (amoxicillin, ampicillin, cefaclor, cefazolin, cefepime, cefotaxime, ceftazidime, ceftriaxone, imipenem, meropenem, and aztreonam). In order to test colistin resistance, the isolates were subjected to the minimum inhibitory concentration (MIC) technique using the broth microdilution method (BMD; 0.5 to 16 μg/mL) and polymerase chain reaction (PCR) assays targeting colistin- (mcr-1 to mcr-5) and β-lactam- (blaTEM, blaCTX-M, blaSHV, ampC) genes. The pmrAB mutation was further investigated.

Results: The isolates presented resistance, especially to ampicillin (cattle: 14/106, swine: 62/100, humans: 10/29) and amoxicillin (cattle: 7/106, swine: 61/100, humans: 8/29). One swine isolate was characterized as extended spectrum β-lactamase (ESBL) producer. The isolates obtained from swine presented higher frequencies of colistin resistance (13/100) when compared to isolates from bovines (5/106) and humans (0/29). Molecular assays concluded that the isolates presented blaTEM (swine: 67/100, humans: 7/29), ampC (swine: 1/100), and blaCTXM (swine: 1/100). The pmrAB complex presented mutations (T31S, P42A, I128N, G144S, H2R, N358Y, D283G, K15I).

Conclusions: This study highlights the presence of antimicrobial resistance and presents a method to verify these factors in the animal production chain.