Module 5. Microbial growth and nutrition Show
ENVIRONMENTAL FACTORS AFFECTING GROWTH OF MICROORGANISMS 18.1 Introduction The activities of microorganisms are greatly affected by the chemical and physical conditions of their environments. Different organisms react to their environment in different ways. An environment that is harmful to one microorganism may be beneficial to another. Sometimes an organism can tolerate an adverse condition in which it is unable to grow. There are a number of such factors which affect microbial growth as follows:
18.1.1 Temperature Environmental temperature is one of the most important factors affecting the growth rate of microbes. There is a minimum temperature, below which growth does not
occur. As we rise above the minimum, rate of growth increases in accordance with the laws governing the effect of temperature on the chemical reactions that make up growth. These reactions are mostly enzyme catalyzed. The minimum and maximum temperatures for microbial growth vary widely among microorganisms and are usually a reflection of the temperature range and average temperature of their habitat (Fig. 18.1).
Fig. 18.1 Growth of microorganisms in different natural environmental conditions Temperature affects living organisms in two opposing ways:
Each microorganism thus has
The optimum temperature is always closer to the maximum rather than the minimum. These three temperatures called the cardinal temperatures are usually characteristic of each type of organism (Fig. 18.2). They are not completely fixed however, because they can be modified by other environmental factors; especially the chemical composition of the medium. The maximum growth temperature usually reflects the inactivation of one or more key proteins in the cell. The factors affecting minimum temperatures are less clear. It may result from the ‘freezing’ of the cyto-plasmicmembrane, impairing its ability to transport nutrients or form proton gradients. Experiments have shown that adjustment in membrane lipid composition can cause changes in minimum temperature. 18.1.1.1 Variation in cardinal temperature Cardinal temperatures vary greatly throughout the microbial world. Optimal temperatures vary from 40°C to higher than 100°C. Growth of different bacteria can range from below freezing to above boiling, though no one organism can grow over this whole range. Most bacteria have a temperature range of about 30°C, although some have broader ranges than others.
18.1.1.2 Classification of microorganisms on the basis of growth temperatureMicroorganisms can be broadly categorized into following groups on the basis of their growth temperatures as follows (Fig. 19.3):
Fig. 18.3 Classes of microorganisms on the basis of growth temperature How can thermophiles and hyperthermophiles thrive at high temperatures?1. Enzymes are more heat stable
2.
Membranes are more heat stable
18.1.2 Gas requirementsTwo gases that influence microbial growth(1) Oxygen
(2) Carbon dioxide 18.1.2.1 Microorganisms classification based on oxygen requirements
Although oxygen is found as a cellular component, most organisms need molecular oxygen for respiration. In these organisms, oxygen serves as the terminal electron acceptor and
such organisms are referred to as ‘obligate aerobes’, e.g. Nitrobacter. As opposed to this, there are organisms which do not use molecular oxygen as terminal electron acceptor although oxygen is a component of their cellular material. In fact, molecular oxygen is toxic to these organisms and these are, called as ‘obligate anaerobes’, e.g. clostridia. In these organisms nitrate, sulphate or organic compounds serve as electron acceptors. Some microorganisms can also grow either in the presence or
absence of molecular oxygen and these are termed as facultative anaerobes, e.g. E. coli. Some, of these have a fermentative energy yielding metabolism but are not sensitive to the presence of molecular oxygen, while others can shift from a respiratory to a, fermentative metabolism depending upon the presence or absence of oxygen. Obligate anaerobes are unable to detoxify some of the byproducts of oxygen metabolism. Aerobes have enzymes that decompose toxic oxygen products. Anaerobes lack these
enzymes. In addition to these major classes there are organisms which grow best at reduced oxygen pressure but are obligate aerobes and these are called ‘microaerophilic’, e.g. most lactobacilli.
18.1.3 pH Each organism has a pH range within which growth is possible, and most have well defined pH optima. Most natural environments have pH values between 5 and 9 and most organisms have pH optima in this range. Very few species can grow at pH values below 2 or above 10. Organisms capable of living at low pH are called acidophiles (Fig.18.5). Those capable of living at very high pH are called alkaliphiles. As a group, fungus tend to be more acid tolerant than bacteria. Many grow optimally at pH 5 or below, and a few grow well at pH values as low as 2. Some bacteria are also acidophilic and some cannot grow at neutral pH (obligate acidophiles). These include several species of Thiobacillus, and several genera of the archaea including Sulfolobus.
18.1.3.1 Extracellular versus intracellular pH Despite the pH requirements of particular organisms for growth, the optimal growth pH represents the pH of the extracellular environment only. The intracellular pH must remain near neutrality to prevent destruction of acid-or alkali-labile macromolecules in the cell. In extreme acidophiles and extreme alkalinophiles, the intracellular pH may vary by several units from neutrality. Most cells grow best between pH 6-8. The internal pH of an extreme acidophile has been measured at 4.6 units while the internal pH of an extreme alkalinophile has been measured at 9.5 units. 18.1.4 Osmotic pressure Water activity (aw) is a measure of the availability of water in an environment. aw is the ratio of the vapor pressure of the air in equilibrium with a substance or solution to the vapor pressure of pure water. The water activity can vary from 0 to 1. The more dissolved solutes in a mixture the lower the water activity. Osmosis is the process by which water diffuses from a region of low solute concentration (more water) to a region of high solute concentration (less water). Usually cellular cytoplasm has a higher solute concentration than the cells environment. The cell thus usually has what is called a positive water balance and the tendency is for water to diffuse into the cell. If however the cells environment has a low aw there is tendency for water to flow out of the cell. 18.1.4.1 Halophiles In nature osmotic effects are of interest mainly with high concentrations of salts. Microorganisms found in the sea usually have specific
requirement for the sodium ion and also grow optimally at aw of sea water. These are called halophiles (salt loving). Halophiles are divided into mild halophiles, moderate halophiles and extreme halophiles depending on their salt requirements. Halotolerant microbes are those which can tolerate a reduction in aw but grow best in the absence of the added solute. Thus on the basis of compatible solute requirement, we can categorize microorganism into
following groups (Fig. 18.6):
When an organism grows in a medium with low water activity, it can obtain water from its environment only by increasing its own internal solute concentration
The solute used inside the cell for adjustment of cytoplasmic water activity must be non-inhibitory to the biochemical processes taking place within the cell. These compounds are thus called compatible solutes. Compatible solutes are all highly water soluble sugars or sugar alcohols, other alcohols, amino acids or their derivatives or potassium. Potassium is used only in the case of extreme
halophiles, whether bacteria or archaea. Compatible solutes may be synthesized directly by the microorganism or accumulated from the environment e. g. K+ or glycinebetaine. The concentration of compatible solutes in a cell is a function of the level of external solutes. However, maximal amount of compatible solute made or that can be accumulated is a genetically determined characteristic. Different organisms thus tolerate different water activities. Non-halotolerant, halotolerant,
halophilic, and extremely halophilic microorganisms are essentially defined by their genetic capacity to produce or accumulate compatible solutes. 18.1.5 Other factors
– Can all damage cells; bacteria have pigments to absorb energy and protect
themselves.
What are the environmental factors that affect microbial growth?Warmth, moisture, pH levels and oxygen levels are the four big physical and chemical factors affecting microbial growth. In most buildings, warmth and moisture are the biggest overall issues present.
What are the 5 factors that affect microbial growth?Environmental Influences on Microbial Growth. Temperature.. Osmotic Pressure.. Oxygen Concentration.. What factors affect microbial growth quizlet?List the six factors which directly affect the growth of microorganisms.. pH, acidity or alkalinity of the food.. moisture or available water.. Temperature.. microbial inhibitors.. Nutrients available.. Gaseous atmosphere.. What are the 5 environmental factors?Air, water, climate, soil, natural vegetation and landforms are all environmental factors. By definition, the environmental factors affect everyday living, and play a key role in bringing health differences across the geographic areas.
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