Abstract

The present study was designed to detect resistant site of nalidixic acid through transformation and plasmid curing of S. aureus strains isolated from buffalo milk with subclinical mastitis and workers’ hands. A total of 37 S. aureus isolates including 17 isolates recovered from buffalo milk infected with subclinical mastitis, in addition to 20 isolates recovered from workers’ hands. All 37 isolates were investigated by detection of the 23S rRNA gene and various other species specific genes including coa, nuc and clfA. The antibiotic resistance of S. aureus isolates was performed by the discs diffusion method using 19 antibiotics. Plasmid transformation method was carried out by transferring the plasmid isolated from S. aureus into competent Escherichia coli HB 101 in order to detection the resistant site of nalidixic acid. Plasmid curing was accomplished by preparing different concentrations of nalidixic acid (100, 150, 200, 250 and 300 µg/ml) and cultured transformed E.coli on LB agar supported with each of the aforementioned concentrations. The molecular results showed that six isolates (five isolates from milk samples and one from workers’ hands) were identified as S. aureus by coa, nuc, and clfA species specific primers. The six S. aureus isolates were found to be resistant to at least 5 antibiotics which included the nalidixic acid. The results of plasmid transformation revealed that E. coli was able to grow on LB agar supported with 100µg/ml, 150 µg/ml, 200 µg/ml and 250 µg/mlof nalidixic acid and failed to grow on 300 µg/ml concentration.