Characterization of Antimicrobial Resistance in Escherichia coli Isolated from Diarrheic and Healthy Weaned Pigs in Catalonia

Simple Summary Postweaning diarrhea (PWD) is a significant concern in the swine industry, causing substantial economic losses due to decreased growth rates, increased mortality, and the extensive use of antimicrobials. Certain Escherichia coli (E. coli) pathovars are frequently implicated in PWD cases among piglets. Regular surveillance and testing of E. coli susceptibility to different antimicrobials is essential. This helps veterinarians and farmers select appropriate treatments, avoid unnecessary antibiotic use, and prevent the further spread of antimicrobial resistance (AMR). Herein, we tested the susceptibility to fourteen antimicrobials of 251 E. coli strains isolated from fecal samples of diarrheic (n = 148) and apparently healthy piglets (n = 103) in farms in Catalonia. The results of this study showed that 41.4% of the E. coli were multidrug-resistant (MDR) strains, presenting high resistance to conventional veterinary antimicrobials such as erythromycin, amoxicillin, streptomycin, tetracycline, lincospectin, sulfamethoxazole/trimethoprim, and florfenicol. A special concern was also observed for human last-resort antimicrobials, like quinolones and colistin. Moreover, pigs suffering from diarrhea had a higher frequency of infection with MDR strains than the healthy ones. To reduce the incidence and impact of PWD in pig herds, optimization of antimicrobial therapies had to be implemented with other strategies, such as good hygiene practices, optimizing nutrition, managing stress levels, and employing proper vaccination protocols. Abstract Postweaning diarrhea (PWD) is a multifactorial concern in the swine industry that leads to high antibiotic consumption, usually without testing susceptibility, increasing the risk of the selection of Escherichia coli-resistant strains. In this study, 251 E. coli strains isolated from fecal samples of diarrheic (n = 148) and apparently healthy piglets (n = 103) in farms in Catalonia were tested against their susceptibility to fourteen different antimicrobials. The phenotypic antimicrobial resistance (AMR) testing revealed high levels of AMR, with 41.4% of the isolates presenting a multidrug-resistant (MDR) profile. More specifically, resistance to class D (prudence) antimicrobials such as erythromycin (99.6%), amoxicillin (95.2%), streptomycin (91.6%), tetracycline (88.8%), lincospectin (64.5%), and sulfamethoxazole/trimethoprim (60%) was very high, as well as to class C (caution) antimicrobials such as florfenicol (45%). A special concern was observed for antimicrobial category B (restrict), like quinolones and colistin, that both presented a high rate of resistance. Colistin use was substantially reduced in Spain, but resistance is still present in weaned pigs, presenting a MIC90 of 4 μg/mL. This suggests that reducing antibiotic use is not enough to eliminate this AMR. Finally, it was found that piglets suffering diarrhea were more commonly carriers of MDR strains than the healthy ones (49.3% vs. 35%, p = 0.031). Therefore, given the high rates of resistance to the most commonly used antimicrobials, especially in diseased pigs, a new non-antibiotic-based approach should be implemented for the management of PWD.