Anaerobic conditions

The greatest population change from summer to winter was a reduction of 75 percent in the actinomycetes group. Aerobic bacteria declined 67 percent and anaerobic bacteria decreased 30 percent in both soils. The fungi remained unchanged throughout the year in both soils. It is also noticed that assumptions are linked to the technique used (dilution plate count technique). The technique is based on the principle that complete detachment and dispersion of cells from the soil will give rise to discrete colonies when incubated on a petri plate containing nutrient media. It is assumes that a complete dispersion of sample will occur, media will be suitable for the growth of organisms, and once on the media, organisms will not interact (Internet 1).

There are many potential inaccuracies when using the dilution plate technique, which result in an underestimate of the total viable population of cells. The factors responsible for the underestimation are: 1- clumps of cells remain aggregated or attached to soil particles; 2- cells are killed in the dilution medium; 3- spores fail to germinate; 4- adsorption of cells on pipette walls; 5- high selectivity of the plating medium and incubation conditions (Skinner et al., 1952). All these factors might affect the numbers of microbes forming on the agar plates and it is generally accepted that plate counts account for < 10% of the total population (Internet 3).

Although, two counts, one for fungi and one for bacteria, have been taken into account for the calculations, when they are under the minimum colonies number that should have been counted. Indeed, plates with between 25 and 250 colonies are suitable for counting. A plate with fewer than 25 colonies is inaccurate because a single contaminant could influence the results and, a plate with greater than 250 colonies is extremely difficult to count. Those two plates have however been taken into account to further enhance conclusions, as they do not contradict them.

Experience 2 Ammonification in soil is the conversion of organic forms of nitrogen (for example, nitrogen in proteins in dead plants and soil animals) to ammonium. The ammonification process is carried out by a wide range of soil organisms. Many different types of bacteria and fungi are involved. Ammonification is also known as nitrogen mineralization and occurs when soil dwelling saprophytic bacteria decompose dead organic matter, which are composed of complex nitrogen containing compounds such as proteins and amino acids, and nucleic acids these bacteria use the nitrogen they obtain to create their own amino acids and proteins and release the excess nitrogen as ammonium which can then be used by plants (Porteous, 2000).

Results show that the lowest rate of ammonification occurred in tubes1, 9 and 10. Tubes 9 and 10 are controls, thus no ammonification was expected from those tubes, as no microorganisms were inoculated into them. A colour change indicates production of ammonia, and the intensity of that colour change is proportional to the quantity of ammonia produced. Microorganisms inoculated tubes were all coloured, exception of tube 1 (inoculated with Bacillus cereus). Tube 1 contained Bacillus cereus, the faint yellow colouration of the tube, indicated that Bacillus cereus did not perform a high level of ammonification during incubation.

Although tube 4 (also inoculated with Bacillus cereus) developed a slight orange colouration. Thus, it can be concluded that Bacillus cereus do participate to ammonification processes to a certain extent. Following this reasoning, it appears that all three microorganisms have an ammonifying power. Although, according to the colouration intensity, Proteus vulgaris has more power than Bacillus cereus and Pseudomonas fluorescens has more power than Proteus vulgaris. Indeed one notable feature of Proteus vulgaris is the ability to degrade urea to ammonia, by production of the enzyme urease. And, Pseudomonas fluorescens is known to be an ammonifier (Conn and Bright, 1919).

The speed with which this ammonia is formed within a soil varies with the physical and chemical composition of the soil together with the number and physiological efficiency of the various organisms taking part in the process (Greaves, 1922). The formation of ammonia in the soil is the result exclusively of the conjoint activity of numerous lower organisms of very widely different, characters.

All three microbes inoculated are bacteria, it is therefore not surprising that both soil tubes (9 and 10) displayed a more intense colouration as bacteria and fungi have ammonifying power. Soils contain a mixture of bacteria and fungi, thus ammonifying power of every type of microbes accumulates. Moreover, it is quite likely that the organisms are even more efficient in the soil in the mixed cultures than they are in the pure cultures, because the transforming of protein nitrogen to ammonia is a complex process which must proceed by steps and some organisms must be more efficient than are others in specific phases of the reaction (Greaves, 1922).

Experience 3 Denitrification is the microbial reduction of nitrate to di-nitrogen gas. It is a step in the nitrogen cycle, which involves the reduction of nitrates into nitrite, nitrous oxide, ammonia, or elemental nitrogen. It is carried out by certain forms of denitrifying bacteria in the soil and serves as an important part of the breakdown of dead organism. It is responsible for the loss of much of the soil’s natural and synthetic fertilizers. This process is favoured most in warm, anaerobic conditions.

If denitrification has occurred in the incubated tubes, gas formation should be observable in the Durham tubes, or a colour change should developed when reagents are added. Pseudomonas aeruginosa is well known amongst microbiologist for its denitrification abilities and physiological versatility. Pseudomonas aeruginosa is a denitrifying bacterium, a microorganism whose action results in the conversion of nitrates in soil to free atmospheric nitrogen. It is commonly found in terrestrial soil, and can grow in a variety of low-nutrient conditions (Doudoroff and Palleroni, 1974).

The production of fermented milk products such as cheese, yogurt, and fermented milks have developed over the centuries as a means of preserving the nutritive value of milk and providing a safe, attractive food (Roberts and Skinner, 1983). It is …

Microbial community plays an important role in soils. These organisms are diverse and dynamic, performing beneficial activities such as decomposing organic matter, making nitrogen available and, sometimes, suppressing certain undesirable soil pathogens (Graham & Mitchell, 1997). The active part of …

Metabolic differences are used for the taxonomy of bacteria as bacteria divide asexually, as each bacterial species exhibit metabolic differences on a variety of substrates. A bacteria’s ability to metabolise a substrate depends on the microorganism having the necessary enzymes …

|Experiment 2. The gas produced in respiration | | | |[pic] | |(a) Label two boiling tubes A and B and place a known weight or number of living organisms in A. | | |Place an equal quantity of dead …

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