Micro08105 MICROBIAL GROWTH AND REQUIREMENTS Dr Shahzad Ali

Micro-08105 MICROBIAL GROWTH AND REQUIREMENTS Dr. Shahzad Ali Assistant Professor Department of Wildlife and Ecology UVAS, Ravi Campus, Pattoki 3(2 -1)

MICROBIAL GROWTH The Growth of Bacterial Cultures [page 171(204)] 1. Bacterial Division 2. Generation Time 3. Phases of Growth

Bacterial Division Bacterial growth refers to an increase in bacterial numbers, not an increase in the size of the individual cells. Bacteria normally reproduce by binary fission (Figure 6. 12 ). [page 171(204)] A few bacterial species reproduce by budding; they form a small initial outgrowth (a bud) that enlarges until its size approaches that of the parent cell, and then it separates. Some filamentous bacteria (certain actinomycetes) reproduce by producing chains of conidiospores carried externally at the tips of the filaments. A few filamentous species simply fragment, and the fragments initiate the growth of new cells.

Generation Time The time required for a cell to divide (and its population to double) is called the generation time. It varies considerably among organisms and with environmental conditions, such as temperature. Most bacteria have a generation time of I to 3 hours; others require more than 24 hours per generation. If binary fission continues unchecked, an enormous number of cells will be produced. If a doubling occurred every 20 minutes- which is the case for E. coli under favorable conditions- after 20 generations a single initial cell would increase to many cells.

Phases of Growth When a few bacteria are inoculated into a liquid growth medium and the population is counted at intervals, It is possible to plot a bacterial growth curve that shows the growth of cells over time ( Figure 6. 15), There are four basic phases of growth: (173/206) 1. Lag PHASE 2. Log PHASE 3. Stationary PHASE 4. Death phase

Phases of Growth Lag PHASE For a while, the number of cells changes very little because the cells do not immediately reproduce in a new medium. This period of little or no cell division is called the lag phase, and it can last for 1 hour or several days. During this time, however, the cells are not dormant. The microbial population is undergoing a period of intense metabolic activity involving, in particular, synthesis of enzymes and various molecules. (The situation is analogous to a factory being equipped to produce automobiles; there is considerable tooling-up activity but no immediate increase in the automobile population. )

Phases of Growth Log PHASE Eventually, the cells begin to divide and enter a period of growth, called the log phase, Cellular reproduction is most active during this period, and generation time reaches a constant minimum. Because the generation time is constant, a logarithmic plot of growth during the log phase is a straight line. The log phase is the time when cells are most active metabolically and is preferred for industrial purposes where, for example, a produce needs to be produced efficiently.

Phases of Growth Stationary PHASE If log growth continues unchecked, large numbers of cells could arise. For example, a single bacterium (at a weight of 9. 5 X 10 - 13 g per cell ) dividing every 20 minutes for only 25. 5 hours can theoretically produce a population equivalent in weight to that of an 80, 000 -ton aircraft carrier. In reality, this does not happen. Eventually, the growth rate slows, the number of microbial deaths balances the number of new cells, and the population stabilizes. This period of equilibrium is called the stationary phase. What causes exponential growth to stop is not always clear. The exhaustion of nutrients, accumulation of waste products, and harmful changes in p. H may all play a role.

Phases of Growth Death phase The number of deaths eventually exceeds the number of new cells formed, and the population enters the death phase, or logarithmic decline phase. This phase continues until the population is diminished to a tiny fraction of the number of cells in the previous phase or until the population dies out entirely. Some species pass through the entire series of phases in only a few days; others retain some surviving cells almost indefinitely

Direct Measurement of Microbial Growth The growth of microbial populations can be measured in a number of ways. 1. Some methods measure cell numbers; 2. Other methods measure the population's total mass, which is often directly proportional to cell numbers. Population numbers are usually recorded as the number of cells in a milliliter of liquid or in a gram of solid material.

Direct Measurement of Microbial Growth Plate Counts The most frequently used method of measuring bacterial populations is the plate count. An important advantage of this method is that it measures the number of viable cells. One disadvantage may be that it takes some time, usually 24 hours or more, for visible colonies to form. This can be a serious problem in some applications, such as quality control of milk, when it is not possible to hold a particular lot for this length of time. Plate counts assume that each live bacterium grows and divides to produce a single colony.

Direct Measurement of Microbial Growth Plate Counts This is not always true, because bacteria frequently grow linked in chains or as dumps Therefore, a colony often results, not from a single bacterium, but from short segments of a chain or from a bacterial clump. To reflect this reality, plate counts are often reported as colony- forming units (CFU). When a plate count is performed, it is important that only a limited number of colonies develop in the plate. When too many colonies are present, some cells are overcrowded and do not develop; these conditions cause inaccuracies in the count.

Direct Measurement of Microbial Growth Plate Counts The U. S. Food and Drug Administration convention is to count only plates with 25 to 250 colonies, But many microbiologists prefer plates with 30 to 300 colonies. To ensure that some colony counts will be within this range, range the original inoculum is diluted several times in a process called serial dilution (Figure 6. 16).

Serial Dilutions 1. Let's say, for example, that a milk sample has 10, 000 bacteria per milliliter. If 1 ml of this sample were plated out, there would theoretically be 10, 000 colonies formed in the Petri plate of medium. Obviously, this would not produce a countable plate. 2. If 1 ml of this sample were transferred to a tube containing 9 ml of sterile water, each milliliter of fluid in this tube would now contain 1000 bacteria. If 1 ml of this sample were inoculated into a Petri plate, there would still be too many potential colonies to count on a plate. 3. Therefore, another serial dilution could be made. One milliliter containing 1000 bacteria would be transferred to a second tube of 9 ml of water. Each milliliter of this tube would now contain only 100 bacteria, and if 1 ml of the contents of this tube were plated out, potentially 100 colonies would be formed- an easily countable number. (Figure 6. 16 -page 175 (208))

Pour Plates and Spread Plates A plate count is done by either 1. Pour plate method 2. Spread plate method POUR PLATE METHOD The pour plate method follows the procedure shown in Figure 6. 17 a. (page 176/209) Either 1. 0 ml or 0. 1 ml of dilutions of the bacterial suspension is introduced into a Petri dish. The nutrient medium, in which the agar is kept liquid by holding it in a water bath at about 50°C, is poured over the sample, which is then mixed into the medium by gentle agitation of the plate.

POUR PLATE METHOD When the agar solidifies, the plate is incubated. With the pour plate technique, colonies will grow within the nutrient agar (from cells suspended in the nutrient medium as the agar solidifies) as well as on the surface of the agar plate. DRAWBACKS This technique has some drawbacks because some relatively heatsensitive microorganisms may be damaged by the melted agar and will therefore be unable to form colonies. Also, when certain differential media are used, the distinctive appearance of the colony on the surface is essential for diagnostic purposes. Colonies that form beneath the surface of a pour plate are not satisfactory for such tests.

Spread plate method To avoid these problems, the spread plate method is frequently used instead (Figure 6. 17 b ). A 0. 1 ml inoculum is added to the surface of a pre-poured, solidified agar medium. The inoculum is then spread uniformly over the surface of the medium with a specially shaped, sterilized glass o r metal rod. This method positions all the colonies on the surface and avoids contact between the cells and melted agar.