Evaluation of Petrifilm EC method for enumeration of E. coli from soil.

AIMS: To evaluate the suitability of commercially available Petrifilm EC plates for enumeration of Escherichia coli from soil. METHODS AND RESULTS: A confirmed E. coli strain isolated from liquid swine manure was inoculated into sterilized sandy clay loam and loam soils at the concentrations of 10(2), 10(3), 10(5) CFU g(-1) of soil. The efficiency of recovery on Petrifilm EC plates for soils spiked with E. coli was compared with standard membrane filtration techniques on m-FC basal medium supplemented with 3-bromo-4-chloro-5-indoyl-beta-D-glucopyranoside (BCIG) and most probable numbers (MPN) techniques in E. coli medium with 4-methylumbelliferyl-beta-D-glucuronide (EC-MUG) broth. Petrifilm EC and m-FC (BCIG) methods were then assessed for the ability to recover E. coli from field soils applied with swine manure. No significant differences (P > 0.05) were observed between Petrifilm EC, m-FC (BCIG) and MPN methods for the recovery of E. coli from spiked samples, irrespective of soil type. However, recovery of E. coli from manure-applied field soil samples showed a significant difference (P < 0.05) between the Petrifilm EC method and the m-FC method in enumerating E. coli possibly as a result of false positives on m-FC. CONCLUSION: The Petrifilm EC method is suitable for the enumeration of E. coli from soil with a detection limit of 10 CFU g(-1) soil. SIGNIFICANCE AND IMPACT OF THE STUDY: The commercially available Petrifilm EC method is comparatively low cost, easy to use method for the enumeration of E. coli from soil without the need for further confirmation tests.