SS1 Biology Third Term: Micro-Organisms In Action

Introduction

Growth of micro-organisms, like all living things, do increase in size and multiply in number of cells using either the culture medium provided or any suitable surface such as moist bread as source of food. Micro-organisms can also increase in mass. Such an increase in size, mass or number of cells is regarded as growth in micro-organisms.

If environmental conditions of growth such as food, adequate temperature and humidity are favourable, such an increase in mass, size and number of cells of the colony is an index of growth of micro-organisms.

Ways of Measuring Growth in Micro-organisms

There are two major ways of measuring growth in micro-organisms. These are:

First method: A bacterial sample is inoculated into a nutrient agar (a clear liquid culture medium). As the bacterial population increases, the clear liquid medium becomes cloudy and turbid. Progressive increase in turbidity indicates a related increase in the number of bacterial cells. This property is used to measure bacterial growth.

In the laboratory, turbidity can be measured with a spectrophotometer. This instrument measures the amount of light that can pass through a liquid medium.

Second method: In this method, small samples of bacteria are taken from the nutrient agar at regular interval of time. Each sample is diluted several times. Each diluted sample is innoculated on to a nutrient agar medium in a petri-dish and incubated. The number of colonies formed in each petri-dish is counted. As each colony is formed by the multiplication of a single bacterium, the number of colonies indicate the number of living bacterial cells in the diluted sample. From this, the actual number of bacteria in the original sample can be calculated.

Beneficial Effects of Micro-Organisms

Some micro-organisms especially bacteria and fungi are beneficial to man in three major ways: in nature, medicine and in industries

In Nature

1. Compost formation: Micro-organisms especially bacteria aid compost formation through the decay of dead organisms and humus
2. Nitrogen fixation: Certain bacteria (Rhizobium leguminosarium) aid nitrogen fixation into plants through the root nodules of leguminous plants.
3. Maintenance of soil fertility: Most saprophytic bacteria, due to their decomposition activities, release nutrients into the soil which aid its fertility through nitrogen cycle
4. Digestion of cellulose: Some bacteria living in the rumen of ruminant animals like cattle, sheep and goat help such animals to digest cellulose in the rumen
5. Decomposition: Micro-organisms, especially saprophytic bacteria also aid the decomposition of dead of dead plants and animals thereby releasing nutrients to the soil
6. Silage making: Some bacteria are also useful in silage making which involves the preservation of pasture crops for future use.
7. Sewage treatment: Bacteria are also involved in the decomposition of sewage into harmless substance

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SS1 Biology Third Term: Micro-Organisms In Action