High Prevalence of Multidrug-Resistant Haemolytic Escherichia coli in Colombian Pig Farms

<b>Introduction:</b> Haemolytic <i>Escherichia coli</i> (<i>E. coli</i>) is commonly associated with enteric disease in pigs and is frequently used as a phenotypic marker for enterotoxigenic <i>E. coli</i> (ETEC). This study aimed to characterise the resistance and virulence profiles of haemolytic <i>E. coli</i> isolated from Colombian pig farms. <b>Methods</b>: A total of 367 faecal samples from sows and pigs across all production stages were collected and analysed for the presence of haemolytic <i>E. coli</i>. Resistance and virulence genes associated with ETEC was detected by multiplex PCR, and antimicrobial susceptibility profiles were determined using broth microdilution and disc diffusion. <b>Results</b>: Haemolytic <i>E. coli</i> were identified in 40.3% of samples (<i>n</i> = 148 non-duplicate isolates), with the highest prevalence observed in growing piglets (47.1%). ETEC occurred in 5.4% of isolates. All isolates exhibited resistance to at least three antimicrobial classes (MDR), with high levels of resistance to tetracycline (98.0%), neomycin (97.3%), chloramphenicol (95.9%), sulfamethoxazole (93.9%), trimethoprim (91.9%), ampicillin (91.9%), nalidixic acid (82.4%), and ciprofloxacin (79.7%). Colistin resistance was observed in 5.4% of isolates, mediated by <i>mcr</i>1 or <i>mcr</i>3, while cefotaxime resistance (8.8%) was extensively associated with blaCTX-M. <b>Conclusions</b>: These findings reveal a concerning burden of MDR <i>E. coli</i> in Colombia's pig-producing regions and indicate that haemolysis alone is a poor indicator of ETEC. Integrating farm-level antimicrobial use data with genomic analyses will be essential to identify drivers of AMR and guide effective stewardship in the Colombian pig industry.