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 objecti ve 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 144hr were compared among four culture media (Standard Meth ods 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 ga ve the same or higher colony counts compared to full strength or diluted SM A. 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 pl ating and spread plating methods using MAA. Colony numbers (log CFU g(-1) d ry soil) from both methods on several soils were strongly related (R-2 = 0. 97, n = 17, p < 0.01) and followed the relationship y = 1.008x - 0.125 wher e y represents the spread plate method and x the spiral plate method. Use o f MAA media with the spiral plating technique provides automation of bacter ia enumeration without sacrificing either numerical counts or the appearanc e of discrete colonies.