Low nutrient medium and spiral-plate system for enumerating heterotrophic bacteria in soil

Identifying appropriate media to quickly and accurately enumerate bacteria in soil has been a continuing problem in microbiology research. Our objective was to determine the accuracy of the spiral plating technique to assess bacteria population in soil samples and its compatibility with low and high nutrient media. Colony numbers and the area covered by spreading organisms at 48, 72, and 144 hr were compared among four culture media (Standard Methods Agar [SMA] at 1%, 10%, and 100% strength, and Modified Egg Albumen Agar [MAA]) using two contrasting soils. The correlation between the spiral and the spread plating methods on MAA was tested using 17 soil samples. MAA gave the same or higher colony counts compared to full strength or diluted SMA. Colony size was larger on MAA than on 1% or 10% SMA. Confluent colonies that overgrew diluted and full strength SMA plates were largely absent from MAA plates. There were no differences between colony counts from spiral plating and spread plating methods using MAA. Colony numbers (log CFU g^-1 dry soil) from both methods on several soils were strongly related (R² = 0.97, n = 17, p < 0.01) and followed the relationship y = 1.008x - 0.125 where y represents the spread plate method and x the spiral plate method. Use of MAA media with the spiral plating technique provides automation of bacteria enumeration without sacrificing either numerical counts or the appearance of discrete colonies.