Each sample was thoroughly mixed with 4.5mL of 0.1% peptone buffer (Lab M, Lancashire, UK) and then vortexed for 2×30 s to obtain soil suspensions. The soil suspension dilutions per sample were plated on the SMAC (sorbitol MacConkey)-BCIG(5-bromo-4- chloro-3-indoxyl--d-glucuronide) agar (Lab M, Lancashire, UK) containing 100gmL−1 of rifampicin and 25gmL−1 of nalidixic acid. The inoculated plates were incubated at 37 ◦C for 16 h and then the colonies counted.