Lesson 18. ENVIRONMENTAL FACTORS AFFECTING GROWTH OF MICROORGANISMS

Module 5. Microbial growth and nutrition

Lesson 18
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:
  • Temperature
  • Gas
  • pH
  • Osmotic pressure
  • Other factors
  • Microbial association
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).
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Fig. 18.1 Growth of microorganisms in different natural environmental conditions

Temperature affects living organisms in two opposing ways:
  • As temperature rises, chemical and enzymatic reactions proceed at a faster rate, and the growth rate increases. Above a certain temperature proteins are irreversibly damaged.
  • As temperature is increased within a certain range therefore, growth and metabolic activity increases up to a point where inactivation reactions set in. Above this temperature, cell functions fall sharply to zero.
Each microorganism thus has
  • A minimum temperature below which no growth occurs.
  • An optimum temperature at which growth is most rapid.
  • A maximum temperature, above which growth is not possible.
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.

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Fig. 18.2 Cardinal temperatures

18.1.1.2 Classification of microorganisms on the basis of growth temperature

Microorganisms can be broadly categorized into following groups on the basis of their growth temperatures as follows (Fig. 19.3):
  • Psychrophiles – (cold loving) 0 to 15°C
  • Psychrotrophs - (food spoilage) grow between 20 to 30°C
  • Mesophiles- (most human pathogens): 20 to 40°C
  • Thermophiles- (heat loving): 45 to 80°C
  • Hyperthermophiles (Archaea): 89 to 120°C
  • 183
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
  • Only a few key amino acids are different from mesophiles
  • Increase in salt bridges (ionic bonds) between amino acids
  • Densely packed hydrophobic interiors
  • Example of heat stable enzyme = Taq polymerase used in PCR, isolated from Thermus aquaticus
2. Membranes are more heat stable
  • Bacteria - saturated fatty acids (decrease fluidity) and stronger hydrophobic environment (greater interaction of fatty acid tails)
  • Archaea contain isoprene units - lipid monolayer and ether linkage
18.1.2 Gas requirements

Two gases that influence microbial growth

(1) Oxygen
  • Respiration - terminal electron acceptor
  • Oxidizing agent - toxic forms
(2) Carbon dioxide

18.1.2.1 Microorganisms classification based on oxygen requirements

Microorganisms vary in their need for or tolerance to oxygen during growth. There are thus four types of organisms with respect to oxygen needs and tolerances (Fig.18.4).
  • Aerobes – are capable of growing at full oxygen tension, and many can tolerate elevated levels of oxygen (greater than 21%).
  • Microaerophiles – are aerobes that can use oxygen only it is present at reduced levels in air.
  • Facultative organisms – are those that under appropriate nutrient and culture conditions can grow in either aerobic or anaerobic conditions.
  • Anaerobes –are those organisms that lack respiratory systems and thus cannot use oxygen as the final electron acceptor.
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.

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Fig. 18.4 Types of microorganisms on the basis of gaseous requirement

18.1.2.2 Other gas requirements
  • Microaerophiles - requires less than 10% of atmospheric O2 e.g. Campylobacter jejuni
  • Capnophiles - requires increased CO2 (5-15%) tension for initial growth e.g. S. pneumonia.
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.

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Fig. 18.5 Growth of microorganism at different pH

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):
  • Mild halophile – requires 1-6 % salt
  • Moderate Halophile – requires 6-15% salt
  • Extreme halophiles – requires 15-30% salt
  • Osmophiles – are those organisms capable of living in high sugar concentrations
  • Xerophiles – are those organisms capable of growing in very dry environments
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Fig. 18.6 Effect of sodium ion concentration on growth of microorganisms with different salt tolerances

18.1.4.2 Compatible solutes
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
This is achieved either by
  • Pumping inorganic ions into the cell from the environment or
  • Synthesizing or concentrating an organic solute
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
  • Radiation (solar, UV, gamma)
– Can all damage cells; bacteria have pigments to absorb energy and protect themselves.
– Endospores are radiation resistant.
– Deinococcus radiodurans: extremely radiation resistant
  • Extremely efficient DNA repair, protection against dessication damage to DNA.
  • Barophiles/barotolerant: microbes from deep sea; Baro means pressure. Actually require high pressure as found in their environment
Last modified: Monday, 5 November 2012, 7:15 AM