Microbial Growth Physiology PDF

# Microbial Growth - The word growth is used in prokaryotes in general and in some eukaryotic microbes to express the increase in the number of microbial cells over the original number with which the culture began its growth. - That is, growth in bacteria expresses reproduction and comes as a natur...

# Microbial Growth - The word growth is used in prokaryotes in general and in some eukaryotic microbes to express the increase in the number of microbial cells over the original number with which the culture began its growth. - That is, growth in bacteria expresses reproduction and comes as a natural result. - Their number increases without an increase in the size of their individuals. ## The Growth Curve - The growth of microorganisms is represented graphically by drawing the curve that shows the relationship between: - the incubation time (t) - the logarithm of the number of viable cells (log n). - This curve is characterized by the presence of four phases, which are: ### Lag phase - Adding microbes to a new medium does not mean doubling their number immediately, due to the new conditions. - This also does not mean that they will enter a dormant phase - Studies have shown that a single cell in this phase increases in size, is physiologically active, and synthesizes new protoplasm. - The microbial cell in these new conditions may suffer from a deficiency in its enzymes, and it needs time to adapt to this medium. Therefore, we say that this cell is dormant only in terms of its division. - The duration of this phase varies according to: - Type of microbe - Age of the cultured microbe: If the microbes are young, this phase will be shorter. - Nature of the new medium: The duration of this phase is limited if the medium is suitable and does not contain growth inhibitors or antibiotics. - Quantity of cultured microbes: The larger the quantity, the shorter the duration of this phase as well. ### Logarithmic phase (Exponential Phase) - Microbial growth accelerates. - Growth rate is constant - The growth curve takes the form of a straight line (when the logarithm of the number of cells is placed against time). - The increase in the number of cells reaches its maximum due to the short time of division. - Microbial cells are similar in their chemical composition, metabolic activity, and the rest of their physiological characteristics. - The time period of this phase is related to the beginning of the phase that follows it. ### Stationary phase - The logarithmic phase of growth ends after several hours. - The rate of reproduction begins to decrease until it balances with the rate of cell death. - The number of living cells remains constant, and thus the stationary phase begins. - The occurrence of this phase is attributed to several reasons: - the lack of nutrients and - the accumulation of toxic growth products - The length of this phase depends primarily on: - the microbial type and - its sensitivity to the surrounding conditions. The greater sensitivity leads to the shorter phase period. ### Decline phase - The phase is also called the death phase. - The rate of death of microbial cells is high, due to the depletion of nutrients and the accumulation of growth-inhibiting substances. - The duration of this phase varies depending on the microbial type. - Some Gram-negative cocci die within 72 hours. - Other types remain live for months or several years. ## Microbial growth measurement Microbial growth (increase in the number of cells formed as a result of division) can be estimated in several ways: - **Measurement of the total number of microbial cells:** - directly using a microscope, or - indirectly by colony counting. - **Measurement cell mass:** - directly by estimating the increase in dry or wet weight, or through one of the elements of the cellular contents(C,N), or - indirectly depending on the degree of medium turbidity. - **Measurement of cellular activity:** - This is an indirect method based on comparing enzymatic activity. The output is the amount of growth to be measured. You can follow one of the following methods: ### A- Estimation of the total number of cells - This count is done directly under the microscope in colored preparations. - A known volume of the microbial culture or suspension (0.01 ml) is spread evenly over a known area of a glass slide. - The resulting membrane is fixed and colored, then the individual microbes are counted by examining it under the microscope field. - The number of cells in (1 ml) of the suspension is calculated. - Special plates (as Petroff hausser, hemocytometer) are used, which have a counting chamber that helps to carry out the count accurately and quickly without coloring. - The disadvantages of this method are that it gives the total number of living and dead cells, and that it is not sufficient in the case of very dilute suspensions. ### B- Counting colonies in plates - This method depends on cultivating microbes in suitable solid food media. - 1 (ml) of the culture to be examined is taken and inoculated into the solid culture medium. - The dishes are incubated at a suitable temperature. - The colonies growing in them are counted after a certain period of time. - For example: (24 hours) for E. coli bacteria, three days for Azotobacter bacteria. - The studied sample is gradually diluted to a degree that allows the appearance of a number of colonies in the dishes ranging between (30 - 300) colonies. - By knowing the dilution, we can know the number of microbes present in (1 ml). - This method gives good results when cultivating bacteria in milk, water, food, etc., because it is an easy and accurate method. ### C- Turbidimetric method - Microbes grow in liquid media, and this growth is accompanied by an increase in the turbidity of the medium due to the increase in the number of cells in it. - If we pass a light beam through the medium, the bacterial cells absorb and scatter some of these light rays. - The amount of light absorbed and scattered is proportional to the mass of cells in the medium, as the increase in the number of cells leads to an increase in the scattering and absorption of light. - To measure the extent of this scattering, either a spectrophotometer or a turbidimeter is used. - This method is widely used in studying bacterial growth due to its speed and accuracy. - It cannot be used in media that are: - heavily colored - contain suspended materials other than bacterial cells, - or when the growth of the culture is weak. - It should be noted that this method takes into account both dead and living cells. ### D. Estimating the increase in dry weight of the cell: - This method is used in the case of microbial cultures with dense growth. - The cells must be washed well from impurities and suspended materials, and - It is also considered one of the insensitive methods.