Environmental Factors Affecting Growth Of Bacterial Population

Beside nutrition there are certain environmental factors required for the proper growth of bacterial colony. These factors are generally considered under the optimum conditions to grow. Like any other living organism, bacterial cells are also susceptible to its surrounding environment and presence or absence of certain features of surrounding effect them as much, those can be seen on the growth of entire bacterial population directly or indirectly. Some common environmental factors are discussed here.

Oxygen Requirement

Depending on the utilization of oxygen for growth bacteria are generally divided into two categories. Aerobic are those bacteria those grow well in the presence of oxygen and utilize oxygen for their growth. Aerobic bacterias can be further divided in to two categories, obligate aerobes and facultative anaerobic. Obligate aerobes can grow only in the presence of oxygen and do not show growth curves in the absence of oxygen. While facultative anaerobes are those bacteria those generally grow in the presence of oxygen but can also show growth curves in the absence of oxygen. Most bacteria of medical importance are facultative anaerobes. Anaerobic are those bacteria those can grow in the absence of oxygen e.g. clostridia. Obligate anaerobic bacteria are those, those grow only in the absence of oxygen and will die on the exposure to oxygen. Microserphilic bacteria are those bacteria, those grow best in the presence of low oxygen tension.

Exact reason for the intolerance of oxygen by obligate anaerobic bacteria is not known. It is studied that in the presence of oxygen hydrogen peroxide and other toxic substances are accumulated in the bacterial cell. In aerobic bacteria there is an enzyme catalase which is useful in splitting these toxic substances is absent in anaerobes. Another suggested reason is that, essential enzymes of anaerobic bacteria are active only in the absence of oxygen i.e. in the reduced state. In the presence of oxygen anaerobic bacterias cannot utilize its enzyme for biochemical reactions.

As we know by studding metabolic pathways of bacteria. Aerobic bacteria utilizes oxygen as the receptor of electron in ATP synthesis. This operation is done by some hydrogen receptors in anaerobic bacterias. Facultative anaerobes can act in both type of pathways.

Carbon Dioxide

Almost all bacteria require carbon dioxide for their growth. This requirement is easily full filled by the atmospheric carbon dioxide, or produced endogenously by cellular metabolism. Some bacteria like Brucella require much higher amount of carbon dioxide (5 – 10 %) for growth, specially on fresh isolation.

Temperature

Every bacterial species have its own temperature requirement. If bacteria are cultured out of its normal temperature range, it would not show proper growth. so, we can say, bacterial growth does not occur above or below its normal temperature range. The temperature at which bacteria shows best growth pattern is called its optimum temperature. In case of most pathogenic bacteria optimum temperature is 37 degree celsius.

Bacteria which grow best at temperature of 25 – 40 C are called mesophilic. Psychrophilic bacteria are those that grow best at temperature below 20 C, some of them even growing at temperature as low as – 7 C. they are soil and water saprophytes. Another group of non-pathogenic bacteria, called thermophiles, row best at high temperature, 55 – 80 C. they may cause spoilage of under processed canned food. Extremely thermophilic bacteria have been identified which can grow at temperature as high as 250 C.

Some thermophilic bacteria like Bacillus strarothermophilus forms spores that are exceptionally thermoresistant.

Bacterial cells show different pattern towards the effect of heat over them. Heat is an important method for the destruction of microorganisms (sterilization), moist heat causes coagulation and desaturation o proteins and dry heat causing oxidation and charring. Moist heat is more lethal than dry heat. The lowest temperature that kill bacteria under standard conditions in a given time is known as thermal death point. Most vegetative mesophilic bactera, under moist conditions, have a thermal death point between 50 and 65 C and most spores between 100 and 120 C.

Moisture And Drying

As any other living organism water is also essential for the bacterial cell. Water in bacterial protoplasm help and provide environment to do various metabolic activities. Hence, drying may be lethal to cells. However, effect of drying varies in different bacterial species.

Some bacteria like Treponema pallidum are highly sensitive towards drying, while other like staphylococci can withstand drying for months. Spores are particularly resistant to desiccation and may survive in the dry state for several decades. Drying in vacuum in the cold (freeze drying or lyophilisation) is a method for the preservation of bacteria, viruses and many labile biological materials.

H-Ion Concentration

Bacteria are sensitive to change in pH of the growth medium. Each species of bacteria have certain pH range. Bacteria do not growth if the pH range of medium is above or below the pH range required by the bacterial growth. pH at which bacteria shows its best growth is called its optimum pH. Most of the pathogenic bacteria grow well on neutral or slightly alkaline pH (pH 7.2 – 7.6).

Some acidophilic bacteria such as lactobacilli grow under acidic conditions. Other such as cholera vibrio, are very sensitive to acid but can tolerate high degree of alkalinity. Strong solution of acid or alkali (5 % hydrochloric acid or sodium hydrochloride) readily kill most bacteria, though mycobacteria are exceptionally resistant to them as well.

Light

Bacteria (except the phototrophic species) grow well in the dark. Bacterial cells are sensitive to ultraviolet light and other radiation. Culture of bacteria die if exposed to sunlight. Exposure to light may influence pigment production. Photochromogenic mycobacteria form a pigment only on exposure to light and not when incubated in the dark.

Osmotic Effect

Because of the presence of cell wall, bacterial cells are more tolerant to the osmotic variation than any other cell. Even then sudden exposure to hypertonic solution may cause osmotic withdrawal of water and shrinking of protoplasm – plasmolysis. This occur more readily in Gram negative cells than in Gram positive bacterial cell. Sudden transfer from a concentration solution to a distilled water may cause plasmoptysis.

Mechanical And Sonic Stress

Even after having cell wall, bacterial cells are sensitive to mechanical and sonic stress. Bacterial cells can be ruptured by mechanical stress such as grinding or vigorous shaking with glass beads. They may also be disintegrated by exposure to ultrasonic vibration.

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine

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