Soil particles inhibit colony formation of Escherichia coli on the surface of agar media

When enumerating Escherichia coli in serial dilution of soil using selective media, soils was found to exert an inhibitory effect on colony formation on the surface of the agar plates. The inhibitory effect did not appear to be due to either soil bacteria or soluble chemicals but to the soil particles themselves. No inhibition was observed when the cells were enumerated by the most probable number (MPN) method. Some alternative plating methods, such as pour-plating or overlaying, were effective in removing the inhibitory effect. Removal of soil particles by simple sedimentation was also effective in enhancing the enumeration efficiency. These observations would be practically important for enumerating bacteria in the soil environment.