The growth of microbial cells is affected by different environmental conditions (both physical and chemical) such as pH, temperature, salinity, gas concentration (O2, CO2, and H2), water activity and osmosis. Knowing the different physical and chemical factors that affect the growth and survival of microbial cells in the environment will help to strengthen efforts geared towards their control, and thus reduce or eliminate their deleterious effects to mankind, animals and the environment. These features are vital in the characterization of microorganisms since microbes can be described by either their phenotypic or genotypic features. Phenotypic characteristics describe the physical appearance of microorganisms while genotypic characteristics describe microbes at the genetic level. Based on their phenotypes, microorganisms can be characterized with regards to their toleration of temperature, pH level, salinity, oxygen level, and nutritional requirements.
TEMPERATURE: Temperature affects the growth and survival of a microbial cell a great deal. Microorganisms only grow and thrive at optimum temperature fit for their survival as too hot or too cold a temperature will either inhibit or kill the microbial cell. Different microorganisms have different minimal or maximal temperature level they can tolerate in their environment. The simple form or unicellular nature of microorganisms makes them to be easily vulnerable to the varying temperature of their external environment. The metabolic activity of a microorganism increases with increase in temperature because enzymatic and other catabolic or anabolic reactions in the cell are doubled as the growth temperature increases. However, at very high temperatures (at which the organism cannot tolerate), microbial growth rate is slowed down.
High temperature is lethal to microbial cells because it denatures the organism’s proteins, enzymes and other metabolites vital to its survival. At very low temperature, microbial cells rarely function well because metabolic activities will progress at a very low pace. Maximum temperature, minimum temperature and optimum temperatures are often the three fundamental categories of temperature that influences the general growth and survival of every microbial cell. Above the maximum temperature, the growth of a microorganism is not possible. Below the minimum temperature, the growth of a microorganism is not possible. But at optimum temperature, the growth of a microbial cell is rapid and all enzymatic reactions progresses and functions properly. Notwithstanding the broad variety of microorganisms in the universe, microorganisms can also be categorized into five (5) different groups based on their temperature requirements (Figure 1).
MESOPHILES: Mesophiles are microorganisms that grow best at temperature ranges of 14 or 20 to 45oC. They are generally referred to as mesophilic microorganisms. Typical examples of mesophiles include Escherichia coli and Neisseria gonorrhoeae. Virtually all pathogenic microorganisms of man are mesophiles since they must thrive within the normal body temperature of the body which is 37oC. In addition, most microorganisms are mesophiles, and they are most widespread in nature and can be found in both cold and hot environments. The minimum temperature of mesophiles is between 14 and 20oC while their maximum temperature is 45oC. The optimum temperature of mesophiles is between 35oC and 39oC.
PSYCHROPHILES: Psychrophiles are microorganisms that can grow best at a very low temperature e.g. 0oC. They are usually found in environments that are constantly cold or frozen. Psychrophiles grow best at -12oC to 20oC. The minimum and maximum temperature of psychrophiles is 0oC and 20oC respectively while their optimum temperature is 15oC. A little heat or warmth can kill psychrophiles. Psychrophiles are majorly found in the Antarctic and Arctic regions of the earth and even in snow-prone regions. Examples of psychrophiles include Bacillus psychrophilus, Chlamydomonas nivalis, Polaromonas vacuolata, and other snow algae species. Some species of Pseudomonas, Vibrio, Alcaligenes, and Photobacterium are psychrophilic in nature.
PSYCHROTROPHS: Psychrotrophs are microorganisms that grow at OoC to 7oC but have a maximum temperature of around 35oC and an optimum temperature of between 20 and 30oC. They are mostly implicated in the spoilage of refrigerated foods. Psychrotrophs can also be called facultative psychrophiles due to their comparable temperature levels. Examples of psychrotrophs include Pseudomonas fluorescens and Listeria monocytogenes (a major food-pathogen found in diary and meat products).
THERMOPHILES: Thermophiles are microorganisms that grow at very high temperatures e.g. at 55oC or higher. Their optimum growth temperature is between 55 and 65oC while their minimum growth temperature is 45oC. Thermophiles are usually found in hot or thermal environments such as hot springs and boiling water containers or heaters. Compost sites, hot water discharges in industries, electric power plants and other natural and artificial hot environments are suitable habitats where thermophiles can thrive. Thermus aquaticus, the source of Taq polymerase enzyme used extensively in polymerase chain reaction (PCR) is a typical example of a thermophile. Other examples of thermophilic microorganisms include Bacillus stearothermophilus, Chaetomium thermophile, Cyanidium caldarium and certain other Archaea species. Thermophiles are heat-stable microorganisms and they are known to function properly at high temperatures.
HYPERTHERMOPHILES: Hyperthermophiles are thermophiles that grow at optimum temperatures of 80oC and above (Figure 1). Microorganisms that are hyperthermophiles are mainly found in boiling hot springs where the temperature ranges between 100 to 115oC and above. They are extreme thermophiles, and can rarely grow well below temperature of 55oC. Both thermophiles and hyperthermophiles are applied in various biotechnological and industrial processes due to their innate ability to withstand high temperature conditions. Most hyperthermophiles are Archaea since there is rarely any bacteria species that can grow and thrive at temperatures of 90oC and above. Typical examples of hyperthermophiles include Thermococcus celer, Pyrolobus fumarii, P. abyssi and Pyrodictium occultum. Microorganisms that grow optimally under one or more physical and chemical extreme conditions such as boiling hot spring, very high pH (12), very low pH (0), glaciers, and high salinity water bodies are generally called extremophiles. Extremophiles can grow and thrive successfully in various harsh conditions or environments where human beings and animals find too extreme for existence; and in which some other microbial cells cannot withstand.
Figure 1: Schematic illustration of the different groups of microorganisms based on their temperature requirements or tolerance.
pH (HYDROGEN ION CONCENTRATION): pH is the negative logarithm of the hydrogen ion (H+) concentration of a solution. It is a measure of the H+ activity of a solution, and it is expressed mathematically as: pH = – log [H+] = log (1/[H+]). The pH of a solution is measured or evaluated using a pH meter with scales that ranges from 0 to 14. The number 7 on the pH meter or scales (as shown in Figure 2) represents neutrality while figures below and above 7 represents acidic and basic (alkaline) levels respectively. The pH of a solution is used to indicate the acidity or alkalinity of a solution. Hydrogen ion concentration affects the growth and survival of microbial cells in the environment, and thus every microorganism has its own optimum pH range within which its growth and survival is possible. However, buffers are used in certain industrial productions to maintain the pH of the medium or solution at a constant level so that the hydrogen ion concentration of the solution does not fluctuate following the production of either acidic or basic metabolites by microorganisms.
A typical example of buffers is phosphate which is added to media in order to stop the inhibition of microbial growth by large pH changes. The production of acidic or basic metabolites by microorganisms into their surrounding environments also help to regulate the hydrogen ion concentration and thus help to maintain a suitable habitat for growth and survival. Due to the production of either basic or acidic metabolites by microbes, most culture media are buffered to stabilize the pH, and thus provide optimal environment for microbial growth. Acidity increases by decreasing the pH scale while alkalinity increases by increasing the pH scale. The internal environment of the cell (i.e. the cytoplasm) remains neutral or nearly close to pH 7 because the macromolecules found in the intercellular environment of a cell must be maintained at a neutral level in order to preserve them. Thus the optimal pH for the growth of a microbial cell is a measure of the hydrogen ion concentration of the external environment of the cell only. Microorganisms can be classified into three different types based on their hydrogen ion concentration levels as follows:
NEUTROPHILES: Neutrophiles are microorganisms that grow best at pH of between 5.5 and 8.0. Examples of neutrophiles include Pseudomonas aeruginosa, Staphylococcus aureus, Euglena, Paramecium and Escherichia coli among others. Various microorganisms can grow and survive at very low pH (acidic) or very high pH (alkaline) levels. Nevertheless, a good number of microorganisms (bacteria and protozoa inclusive) are neutrophiles because internal pH of the cell is near neutrality (pH 7).
ACIDOPHILES: Acidophiles are microorganisms that grow best at low pH (e.g. 3). They grow optimally between pH 0 to 5.5 (acidic). Most prokaryotic and eukaryotic cells are acidophiles, but fungi are more tolerant to acidic conditions than bacteria. The integrity of the cytoplasmic membrane of acidophiles is crucial to maintaining their growth and survival in acidic conditions as increase in pH up to neutrality is capable of destroying their cytoplasmic membrane, and thus lyse the organism. Examples of acidophiles include Ferroplasma acidarmanus, Thiobacillus thiooxidans, sulfolobus acidocaldarius, and Lactobacillus acidophilus amongst others.
ALKALIPHILES: Alkaliphiles or alkalophiles are microorganisms that grow at very high pH levels (e.g. 9 and above). They are usually found in environments that are highly alkaline in nature (e.g. high-carbonated soils). Typical examples of alkaliphiles are Bacillus species (e.g. B. firmus) which thrives in extreme alkaline conditions.
Figure 2: The pH scale.
SALINITY AND WATER ACTIVITY: The salinity (sodium chloride concentration) of a solution or the environment of microorganisms can adversely affect the optimal growth and survival of microbial cells as different microbes have different salt tolerance levels or requirements. Water activity (aw) is the actual amount of water available to microorganisms for growth and survival. Water activity values are usually between 0 and 1, and it varies across different microorganisms. The process by which water diffuses from a region of high concentration to a region of low concentration is known as osmosis; and this phenomenon is exemplified by microbial cell owing to the fact that they are constantly surrounded by moist environments. Water tends to diffuse into the cell of a microorganism through osmosis because the internal environment of the cell (i.e. the cytoplasm) has a higher solute concentration than the external environment. However, when microbes enter environments where the solute concentration of the environment is more concentrated than the contents of the cytoplasm, then the reverse will be the case as fluid will flow from the cell to the environment. This may lead to plasmolysis or dehydration, and the destruction or lyses of the cell if not contained. Such a condition is known as hypertonic because high sugar, water and salt rush out of the cell.
In hypotonic condition, water rushes into the cell, and this leads to the bursting of the microbial cell. While some microorganisms cannot grow at very low aw, others known as osmotolerant microorganisms (e.g., Saccharomyces species and Staphylococcus aureus) can grow and survive even at low aw levels. Osmotolerant microorganisms maintain high internal solute concentration in order to retain water which is vital for growth since they are found in environments with low aw levels. Microorganisms that require high sodium chloride concentration for growth or can thrive in environments with high salinity are generally known as halophiles. Some examples of halophiles include Dunaliella and Halobacterium. Halotolerant microorganisms are those organisms that can tolerate or survive in environments with very low aw levels. Microbial cells can either be non-halotolerant, Halotolerant, halophilic or extremely halophilic based on their ability to withstand different concentrations of solute and water potentials. Microorganisms that can grow in environments with high sugar or salt concentrations are known as osmophiles while those that can grow and survive in dry environments are called xerophiles.
Oxygen concentration: According to their oxygen requirements, microorganisms (both pathogenic and non-pathogenic organisms) are grouped into different categories based on how they obtain or tolerate oxygen (O2) concentration in their environment. Microorganisms can be classified as obligate aerobes, facultative anaerobes, aerotolerant anaerobes, microaerophilic organisms and strict (obligate) anaerobes.
Oxygen requirements, pH (hydrogen ion concentration), temperature requirements and osmotic pressure and/or water activity all form the physical requirement that is essential for the unperturbed growth and development of microbial cells. The chemical requirements for microbial growth and development are usually dependent on the nutritional requirements of the individual organisms, and these are usually provided for in their culture/growth medium. Examples of the chemical requirements for microbial growth include nitrogen, phosphorus, carbon and sulphur among others.
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