Clinical pathogens, especially Gram-negative bacteria developing resistance to third-generation cephalosporins, are making clinical outcomes more complicated and serious. This study was undertaken to evaluate the distribution of CTX-M-type extended-spectrum β-lactamases (ESBLs) in Tamil Nadu, India. For this study, clinical samples were collected from five different hospitals located in Tamil Nadu and the ESBL-producing Gram-negative isolates were characterized. MIC was performed using cefotaxime and ceftazidime. The bla
ESBL-producing genes were screened using multiplex PCR for the genes, CTX-M group-1, -2, -8, -9, -26. The conjugation studies were performed using
Escherichia coli
AB1157 as a recipient for the isolates harbouring plasmid-borne resistance following broth-mating experiment. In total, 1500 samples were collected and 599 Gram-negative bacteria were isolated that included
E. coli
(n=233),
Klebsiella pneumoniae
(n=182),
Pseudomonas aeruginosa
(n=79),
Citrobacter
spp. (n=30),
Proteus mirabilis
(n=28),
Salmonella
spp. (n=21),
Acinetobacter baumannii
(n=12), Serratia spp. (n=6),
Shigella
spp. (n=4),
Morganella morganii
(n=3) and
Providencia
spp. (n=1). MIC results showed that 358 isolates were resistant to cefotaxime and ceftazidime. Further, ESBL gene-amplification results showed that 19 isolates had CTX-M group-1 gene including
E. coli
(n=16), K. pneumoniae (n=2) and
P. aeruginosa
(n=1) whereas one
M. morganii
isolate had CTX-M group-9, which was plasmid-borne. Through conjugation studies, 12/20 isolates were found to be involved in the transformation of its plasmid-borne resistance gene. Our study highlighted the importance of horizontal gene transfer in the dissemination of plasmid-borne bla
CTX-M-type resistance genes among the clinical isolates.