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.