Effect of Growth Media and Incubation Time on the Culturability of Soil Bacteria

Abstract

The effect of growth medium and incubation time were investigated by determining the number of colonies that were visible on triplicate plates of three different media on daily intervals. The viable counts were significantly different between the three different media even after the first day and this continued for seven days. Colony counts on the three media reached their maximum on the third day of incubation. The use of Nutrient agar (NA) and Plate count agar (PCA) resulted in increasing counts with increasing incubation time respectively. Counts obtained from the Nutrient agar were higher than that of Plate count agar followed by the soil extract agar (SEA) which had the lowest count and a longer bacteria lag phase due to the low nutrient content in the soil extract agar. The Plate count agar formed more distinct colonies than the Nutrient agar and Soil extract agar. During the incubation period, colonies appeared on the Nutrient agar even on the seventh day. These characteristics of the Nutrient agar to support a higher number of colonies for a long period of time could be attributed to its nutrient composition. The colony counts upon incubation for seven days ranged from a mean of 3.0x10 5 cfu/g on Soil extract agar, 6.1x10 5 cfu/g on Plate count agar and 7.6x10 5 cfu/g on Nutrient agar for example A which was a cultivated loamy soil and 1.3x10 5 cfu/g also on Soil extract agar, 3.9x10 5 cfu/g on Plate agar and 5.6x10 5 cfu/g Nutrient agar for soil sample B which was a non-cultivated sandy soil. Longer incubation time increased colony count on the different growth media because it was so obvious on the Soil extract agar. No visible colonies were observed from the first to the third day on the Soil extract agar, visible colonies appeared on the fourth day of incubation. Also in the other growth media, there was an increase in colony count on the third day upon incubation.