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Physical Factors Affecting Bacterial Growth

    • In this physical factors affecting bacterial growth post we have briefly explained about physical factors in its environment affecting  bacterial growth including temperature, osmotic pressure, pH, and oxygen concentration.

    • Microbes’ growth is influenced by a variety of environmental factors in addition to dietary components. These variables play a crucial role in determining a microbe’s growth pattern.  
    • The rate of growth or death of a particular microbial species is influenced by a variety of physical factors in its environment including temperature, osmotic pressure, pH, and oxygen concentration.

    Physical Factors Affecting Bacterial Growth

    Temperature

    • Microorganisms as a group may develop in a wide range of temperatures, from below freezing to well above boiling. The minimum and maximum growth temperatures for any organism indicate the temperature range across which it can grow; this is normally around 25–30⁰C. At low temperatures, growth is slowed by the inefficiency of enzymes, as well as the hardening of lipids and the loss of membrane flexibility.
    • Temperature causes growth rates to increase until the optimum temperature is attained, after which they decrease. The temperature range of an organism’s enzyme systems, which are governed by their three-dimensional protein structures, determines its optimum and limiting temperatures.
    • The optimal temperature is often higher than the minimum temperature for growth. Once the ideal value is reached, the rate of development slows dramatically due to the loss of enzyme activity induced by denaturation.
    Factors affecting microbial growth

    Temperature

    • The majority of microorganisms thrive at temperatures between 20 and 45 degrees Celsius, which are referred to as mesophiles.
    • Thermophiles, on the other hand, have evolved to not just survive, but thrive at far higher temperatures. Typically, they may thrive at temperatures ranging from 40 to 80 degrees Celsius, with an ideal temperature of 50 to 65 degrees Celsius.
    • Extreme thermophiles have optimal values that are higher than this, and can withstand temperatures of above 100 degrees Celsius.
    • Psychrophiles are at the other end of the temperature spectrum; they can thrive at 0 degrees Celsius, with best growth happening at 15 degrees Celsius or lower. At temperatures exceeding 25 degrees Celsius, such creatures are unable to grow.
    • Psychrotrophs, on the other hand, have substantially higher temperature optima (20–30 degrees Celsius), despite the fact that they can thrive at 0 degrees Celsius. Because of their propensity to grow on chilled products, members of this category are typically commercially significant.
    Factors affecting microbial growth

    Temperature

    pH

    • The pH of their surroundings has a significant impact on microorganisms. We can designate minimal, optimal, and maximum values for growth of a given type, just as we do for temperature. Fungi have a larger pH range (between minimum and highest values) than bacteria. The majority of bacteria thrive in a neutral environment (pH 7).
    Factors affecting microbial growth

    pH

    • Many bacteria prefer somewhat alkaline environments, but only a few are acid tolerant, and even fewer are acidophilic. Fungi, on the other hand, like somewhat acidic environments and thus tend to outcompete bacteria when they exist.
    • During the production of growth media, the pH value is adjusted to the desired value by adding acid or alkali. Microorganisms’ metabolic activities typically cause them to modify the pH of their environment as they grow, thus it’s critical to set and maintain an appropriate pH in a laboratory growth medium.
    • This is accomplished through the employment of a suitable buffer mechanism. Phosphate buffers are extensively employed in microbiology laboratories because they allow medium to maintain a stable pH when acid or alkali is created.

    Oxygen

    • Oxygen is present as a major constituent (20 per cent) of our atmosphere, and most life forms are dependent upon it for survival and growth. Such organisms are termed aerobes. Not all organisms are aerobes however; some anaerobes are able to survive in the absence of oxygen, and for some this is actually a necessity.
    • Aerobic organisms require oxygen to act as a terminal electron acceptor in their respiratory chains. Such organisms, when grown in laboratory culture, must therefore be provided with enough oxygen to satisfy their requirements.
    • For a shallow layer of medium such as that in a petri dish, sufficient oxygen is available dissolved in surface moisture.
    • In a deeper culture such as a flask of broth however, aerobes will only grow in the surface layers unless additional oxygen is provided (oxygen is poorly soluble in water). This is usually done by shaking or mechanical stirring.
    • Obligate anaerobes cannot tolerate oxygen at all. They are cultured in special anaerobic chambers, and oxygen excluded from all liquid and solid media. 
    • Facultative anaerobes are able to act like aerobes in the presence of oxygen, but have the added facility of being able to survive when conditions become anaerobic. Aero tolerant anaerobes are organisms that are basically anaerobic; although they are not inhibited by atmospheric oxygen, they do not utilise it. 
    • Micro aerophiles require oxygen, but are only able to tolerate low concentrations of it (2–10 per cent), finding higher concentrations harmful. Organisms inoculated into a static culture medium will grow at positions that reflect their oxygen preferences.

    Oxygen

    Carbon dioxide

    • We discovered that autotrophic organisms can utilise carbon dioxide as a carbon source when grown in culture and given bicarbonate in their growth medium or kept in a CO2-rich environment.
    • Heterotrophic bacteria, on the other hand, necessitate a tiny quantity of carbon dioxide, which is absorbed into a variety of metabolic intermediates.
    • The failure of these organisms to grow when carbon dioxide is purposely removed from the environment demonstrates this dependence.

    Osmotic pressure

    • Osmosis is the process of water diffusing from a less concentrated solution to a more concentrated one over a semipermeable membrane, thereby balancing concentrations.
    • The osmotic pressure is the amount of pressure required to do this. Osmosis would cause a cell to lose water if it was placed in a hypertonic solution (one with a greater solute content) (plasmolysis).
    • This is why large concentrations of salt or other solutes are used to protect foods from microbial attack. Water from a dilute (hypotonic) solution would enter the cell in the opposite circumstance, causing it to enlarge and rupture.
    • Bacteria have solid cell walls that prevent them from bursting, which, along with their small size, makes them less susceptible to osmotic pressure fluctuations than other types of cells.
    • They can tolerate NaCl concentrations of between 0.5 and 3.0% in most cases. Haloduric (‘salt-tolerant’) bacteria can endure ten times higher salinities but prefer lower salinities, whereas halophilic (‘salt-loving’) bacteria are suited to thrive in high-salinity environments, such as those found in the Dead Sea in the Middle East.
    • In order to do this without plasmolysis occurring, they must build up a higher internal solute concentration, which they do by actively concentrating potassium ions inside the cell.

    Light

    • Light is required for photosynthesis in phototrophic organisms. In the laboratory, it’s important to make sure you’re using the right wavelength of light and that the source you’re using isn’t also a heat source.
    • Fluorescent light creates little heat, but it lacks the wavelengths needed by purple and green photosynthetic bacteria, which are above 750 nm.

    Further Readings

    Reference