Dissemination of Extended-Spectrum β-Lactamase-Producing Enterobacterales in Organic Fertilizers: A One Health Perspective from Southwestern Colombia

Extended-spectrum β-lactamase (ESBL)-producing bacteria are a growing public health concern within the One Health framework. This study aimed to characterize ESBL-producing Enterobacterales in industrial and artisanal organic fertilizers marketed in southwestern Colombia. Five commercial fertilizer brands were analyzed using a selective culture on ceftriaxone supplemented media (4 µg/mL), antimicrobial susceptibility testing by broth microdilution to determine minimum inhibitory concentrations (MICs), phenotypic synergy testing for ESBL confirmation, and polymerase chain reaction (PCR) to detect blaTEM, blaSHV, and blaCTX-M genes. Overall, 18.6% of the samples showed growth of ceftriaxone-resistant Enterobacterales, predominantly Escherichia coli and Klebsiella pneumoniae. ESBL producers accounted for 84% of the isolates, all of which carried at least one bla gene, predominantly blaCTX-M. Statistically significant differences in bacterial growth frequency were observed among fertilizer types, with higher positivity rates observed in manure-based artisanal formulations (p < 0.05). Whole-genome sequencing of selected isolates identified Klebsiella pneumoniae ST37 and Escherichia coli ST224, both harboring blaCTX-M-55 and additional resistance and virulence determinants. These findings demonstrate that organic fertilizers, particularly manure-derived products, may act as reservoirs and potential dissemination routes for clinically relevant antimicrobial-resistant bacteria. This is the first study in Colombia documenting the presence of ESBL-producing bacteria in organic fertilizers. These results underscore the need to incorporate surveillance of these products into national policies under a One Health perspective.