| Summary: | We investigated the prevalence of extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli strains in dairy farm wastewater in Chiang Mai, Thailand. We analyzed wastewater samples collected from 150 dairy farms and found that 88.7% of the farms (n = 133) were positive for ESBL-producing E. coli. Multiplex polymerase chain reaction (PCR) amplification was performed to characterize the presence of bla CTX-M, bla TEM, and blaSHV in ESBL-producing isolates. blaCTX-M was found in all isolates (n = 133), followed by blaTEM (80/133, 60.2%), whereas blaSHV was not detected in any isolate. blaCTX-M and blaTEM were present in 60.2% (80/133) of the isolates, and 39.8% (53/133) isolates carried bla CTX-M alone. Subgroup analysis showed that CTX-M-1 was the most prevalent subgroup among the isolates (129/133, 97.0%), followed by CTX-M-8 (2/133, 1.5%) and CTX-M-9 (2/133, 1.5%). The distribution of the phylogenetic groups was as follows: group A (100/133, 75.2%), followed by B1 (14/133, 10.5%), D (6/133, 4.5%), F (6/133, 4.5%), B2 (4/133, 3.0%), and E (3/133, 2.3%). Based on enterobacterial repetitive intergenic consensus PCR (ERIC-PCR) and dendrogram analysis, 24 isolates were classified into clades I (n = 21), II (n =1), and III (n =2). Minor genetic differences were found in all clade I isolates. Our data suggest that the circulating of ESBL-producing E. coli carried at least one bla gene strain distributed in dairy farm wastewater in Chiang Mai.
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