Physical and chemical control of growth and viability
The bacterial growth curve is known to have four vital phases. These phases are lag, exponential, stationery and death, and each has its own unique characteristics (Shastri, 2006). The environment under which the culturing takes place has a specific liquid nutrient. This environment does not allow external interference, and causes the nutrients available to diminish leading to the accumulation of metabolic products brought about by bacterial growth. The three phases, which are lag, log and stationery are such that they each have a distinct biochemical reaction that cause growth of cellular components that are quite important for the growth of cell, and division.
In the lag phase, the number of cells does not increase, but their sizes on the other hand increases. This phase makes a way for the exponential phase, and the activities that take place are the synthesis of enzymes and DNA by the bacteria. The next phase is the exponential, and here what happens is the increase in the number of bacterial cells (Taylor, 2001). The number of bacterial cells doubles up with every unit of time. During the stationery phase, there is nothing like decrease or increase of the bacterial cell population. The exponential growth of bacterial population is limited by the depleted nutrient supplies and the accumulation of waste products, meaning that in this phase death rate is equal to the growth rate (Shastri, 2006). In the transition process from exponential phase to stationery phase, the growth rate of the cellular components is not the same. This explains the difference in the biochemical composition of the cells between the two phases. The final phase is the death phase, where bacterial cells die and their replacement takes long. This situation is as a result of the accumulation of waste products which inhibit the growth of bacterial cells.
There are various methods used for measuring microbial growth. The parameters for measuring include Absorbance, density, viable count, and total cell count. However, the measures under our discussion are two, the total cell count, and viable count.
The total cell count involves counting the number of the microbes that are available in a given sample. In this case, all cells that are either dead or alive that are present in a liquid sample are counted with the use of a special microscopic glass slide known as “Petroff-Hausser Counting Chamber”. This gadget works by marking a grid on the glass slide using squares that have a known area. The grid of the chamber has around 25 squares, covering an area of 1 mm2 and a total volume of 0.02mm3. The process involves taking count of all the cells in large square, and then measuring the total number per ml sample (Shastri, 2006). Thus, if one square has 10 cells, then it means that the total count of the cells per ml sample is going to be: 10 cells x 25 square x 50/103 = 1.25/107. Direct counting can be taken in situation where there is dilute culture.
Viable count refers to a cell that has the potential to split and create more cell numbers. To ensure that the viable count is successful, the number of cells in a sample having the power to form colonies under a given environment is established. The assumption here is that each viable cell will end up forming one colony. Hence, the other name for viable count is colony count. There are two methods in which the plate count can be formed. The first one is the spread count method, which involves spreading of a volume culture (0.1 ml) on the agar surface using sterile glass spreader (Taylor, 2001). The plate is subjected to incubation to allow the growth of colonies, which are then counted. The second method is the pour plate method, where a given volume of the culture is put into the sterile Petri dishes. After mixing culture and melted agar medium, the plate under goes incubation and the resulting colonies are counted.
The two methods of controlling the growth of microorganisms are through killing and inhibiting. The killing method of controlling the growth of microorganisms involves the application of chemical agents with the sole purpose of killing microorganisms. Inhibiting method on the other hand, entails the application of the chemical agents with the aim of inhibiting the growth of microorganisms (Taylor, 2001). The chemical agents that terminate the life of the microbial cells are known as the cidal agents, while those that inhibit their growth are known as static agents. Thus, various chemical agents have been introduced to deal with the growth of specific microorganisms. These have given birth to various terminologies such as bactericidal and bacteriostatic. The former means termination of bacterial cells, and the later means inhibiting of bacterial cells growth. Thus, fungicide is used in killing fungi cells and bactericidal is used in killing bacterial cells. In the field of microbiology, the term sterilization means the total removal of the existing organisms in a substance. The process of sterilization involves the use of heat, radiation and chemicals in removing microbial cells.
Shastri, V. (2006). Microbes, Delhi: Gyan Publishing House.
Taylor, J. (2001).Microorganisms and Biotechnology, Cheltenham: Nelson Thornes