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Physical Parameters for Bacterial Growth and Growth Curve

Bacteria are divided into aerobes and anaerobes based on their dependence on oxygen for growth. Air is necessary for aerobic bacteria to grow.

Physical Parameters for Growth

Gaseous Environment

Bacteria are divided into aerobes and anaerobes based on their dependence on oxygen for growth and survival. Air is necessary for aerobic bacteria to grow. Depending on their environment, they could be facultative or obligate aerobes. The growth of oxygen-dependent aerobes, such as Cholera bacillus, is dependent on oxygen. Faculty anaerobes that require oxygen grow more rapidly in the presence of oxygen than when not provided with oxygen. The vast majority of bacteria present in the human gut are facultative anaerobes.

Clostridia are an anaerobic bacterium that grows without oxygen. Exposure to oxygen can even kill anaerobes. Low oxygen tension is the best environment for microaerophilic bacteria to grow. Oxygen from the atmosphere is the final electron acceptor in the respiration process (aerobic respiration) of aerobes. When carbon and energy are oxidized, carbon dioxide and water are produced. As ATP is converted from adenosine diphosphate (ADP) into adenosine triphosphate (ATP), phosphate bonds are formed that store energy. Oxidative phosphorylation is the process involved. In anaerobic respiration (anaerobic bacterial respiration), anaerobic bacteria use compounds such as nitrates or sulphates instead of oxygen as electron acceptors. These bacteria use fermentation in anaerobic metabolism more commonly than any other process.

An enzyme breaks down complex organic compounds like glucose into simpler compounds without oxygen, using enzymes. Several organic products are formed from this process, including organic acids, organic alcohols, and gaseous products (carbon dioxide and hydrogen). Escherichia coli, for instance, ferments glucose to produce gas and acid, It can also ferment lactose. Organic phosphate is introduced into intermediate metabolites during fermentation to produce energy-rich phosphate bonds. Phosphorylation at the substrate level describes this process. ADP is converted into ATP by these energy-rich phosphate groups.

Carbon dioxide is a requirement for all bacteria to grow. Bacterial cells obtain carbon dioxide from the atmosphere or from their cellular metabolism. For example, Brucella abortus needs about 5 to 10% of carbon dioxide in order to grow. These bacteria are called capnophiles.

Temperature Requirements

The temperature at which bacteria can grow varies among bacteria. The temperature range for each species restricts growth to the maximum and minimum of this range. An optimal temperature is the temperature at which growth occurs most efficiently. A temperature of 37°C is ideal for the majority of pathogenic bacteria. Mesophilic bacteria, such as Escherichia coli, prefer temperatures between 25 and 40°C.

The most psychrophilic bacteria grow at temperatures below 20°C. Those saprophytes inhabit soil and water and can lead to food spoilage in refrigerators. Temperatures of 55-80°C are ideal for thermophilic bacteria to grow. Under-processed canned food can be spoiled by them. In thermophiles, spores such as those produced by Geobacillus stearothermophilus, have a high degree of thermo-resistance.

Other Physiological Requirements

Moisture, drying, hydrogen ion concentrations, light, osmotic effects, mechanical stress, and ultrasonic stress also affect bacterial growth and multiplication.

Bacterial Growth Curve

A bacterium will grow in a defined way when seeded into suitable liquid media and then incubated. A growth curve can be generated by making bacterial counts after inoculation and plotting them against time. Phases of the curve are as follows:

Lag phase - The number of cells does not increase significantly following seeding in a culture medium, but the size of the cells may increase. It takes some time for the cells to adapt to the new environment. It is essential to build up adequate quantities of enzymes and metabolic intermediates for the multiplication process to take place. By the end of lag phase, maximum cell size has been reached. During the lag phase, the incubation period will be either long or short according to the species, the size of the inoculum, the kind of medium, and the environment's temperature.

Log (logarithmic) or exponential phase - Cells divide exponentially with time after the lag phase, or geometrically as time goes on. An example of a straight line can be seen if one plots the logarithm of viable count against time. Smaller cells are present at this stage and stain similarly.

Stationary phase - When nutrients and toxic products have been depleted, cell division stops after a varying period of exponential growth. During each cell division, just enough new cells are formed to replace dying cells. In other words, the newly formed cells equal the number of dying cells. Consequently, the viable counts remain unchanged. The cells often show irregular staining during this phase, indicating gram-variable character. In this phase, sporulation occurs.

Phase of decline - Cell death is the reason for the decrease in population during this phase. In addition to nutritional exhaustion and toxic accumulation, autolytic enzymes can also contribute to cell death.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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