A total of 30 Escherichia coli isolates from 50 random samples of diarrheic calves were screened against resistance to sulfonamides. Minimum inhibitory concentration (MIC) by microdilution method indicated that 25/30 (83.3%) were found resistant to sulfadimidine, 27/30 (90%) were resistant to sulfadiazine, while 22/30 (73.3%) were found resistant to sulfamethaxole. These phenotypically sulfonamide resistant isolates were then probed for the presence of Sul genes (Sul-1-3) by polymerase chain reaction (PCR) using specific primers. PCR results revealed that 23/25, 25/27 and 20/22 isolates carried Sul-1 gene, respectively. Interestingly, all these isolates were found negative for the presence of Sul-2 and Sul-3 genes. Of note, no Sul genes could be verified in 02 phenotypically resistant isolates. The abundance of Sul1 gene and absence of Sul-2 and Sul-3 genes indicating a clonal expansion of sulfonamide resistant E. coli that might be linked to excessive abuse of sulfonamides in animals.

Key words: Diarrhea, E. coli, Minimum inhibitory concentration, Sul genes, Sulfadimidine resistance