Question

1. Explain how the plate count method can be an understatement for bacterial abundance and an overestimate for fungal abundance in the soil.

2. From an ecological standpoint, what can be shown from the buried slide technique that can not be shown by the plate count technique?

Answer 1

1) The plate count method is a viable cell counting method that allows one to identify the number of actively growing/dividing cells in a sample. Bacterial growth follows three phases; the lag phase, the log phase, and the stationary phase. In the standard plate count method, a sample is diluted with sterile saline or phosphate buffer diluent until the bacteria are dilute enough to count accurately. The final plate in the series should have between 30 and 300 colonies. Bacterial abundance in the soil is determined to a great extent by nutrient uptake and turbidity, which is an important index for bacterial growth and cell biomass. As the plate count method doesn't incorporate spectrophotometric (turbidimetric analysis), it misses out on one of the most essential factors behind bacterial growth in the soil, proving to be an inadequate method for determining bacterial abundance in the soil. Due to the morphological and physiological differences observed in fungi populations, the plate count method might produce exaggerated readings when counting fungal populations. Therefore, the plate count method can be an understatement for bacterial abundance and an overestimate for fungal abundance.

2) The buried slide technique places the side at an inconspicuous site(below the soil) for a period of two weeks under incubation, so it is not inadvertently or knowingly disturbed. The collected microbes are stained with Rose Bengal Dye and observed via microscopy using oil immersion on the 100X objective. Since the collected sample has little external interference, it is a useful tool for determining colonial growth in bacterial populations. Although there is relatively less disturbance, the glass slide does alter the dynamics of the soil ecosystem. In spite of this, different nutrient amendments can be added to the soil for the voluntary control of microbial growth and interactions. This allows for easier, more systematic observation of soil microbiology and helps to identify a multitude of organisms that are present in the environment. This feat cannot be accomplished by the plate count technique, because such microbiological parameters in the soil cannot be assessed with the same ease and vividness.