The minimum, maximum and optimum temperature for an organism is known as its cardinal temperatures. The cardinal temperatures for any organism are dependent upon several factors, including the age of the culture and the supply of nutrients. An organism, while not growing at unfavourable temperatures, may still endure them.
For example, many organisms can survive in freezing environments; these forms may be called psychroduric or cryoduric. Organisms that survive at high temperatures are known as thermoduric types. Bacillus speared good examples of the latter, due to their formation of heat-resistant spores.
2. Acidity or Alkalinity (pH):
Microorganisms also have certain pH (hydrogen ion concentration) needs, as reflected by their growth responses in various media. An operational definition of pH is simply the negative logarithm of the hydrogen ion (H+) concentration.
The scale of values representing this property of solutions extends from 0 to 14. Aqueous solutions also contain hydroxyl ions, (OH–), which are important in the control of the alkalinity of a solution.
A neutral state, in which the concentrations of hydrogen and hydroxyl ions are equal, is represented by pH value of 7. In general, pure water should have this pH. Solutions with pH values from 0 to 6.9 are acidic, while those having values from 7.1 to 14 are basic or alkaline.
Because of the logarithmic nature of the pH units, a change in one such unit corresponds to difference of ten times in the hydrogen ion concentration. Consider, for example, a preparation with a pH of 0.
The (H+) here is 10° or 1 normal (N). By comparison, a solution having a pH of 1 has a hydrogen concentration represented by 10°or 1/10 normal, i.e., N/10, of the first preparation.
Various indicators and electronic pH meters are commonly used to deterimne the hydrogen concentrations of prepartions. Certain pH indicators have particular importance in a bacteriological laboratory, as they are used to demonstrate the recomposition of various carbohydrates and related substances.
Common examples of indicators incorporated into bacteriological media include: (1) bromoresol purple; (2) litmus and (3) phenol red. Necessary adjustments to obtain the desired pH usually can be made by the careful addition of standard acids, such as hydrochloric acid (HC1), or bases such as potassium or sodium hydroxide (KOH and NaOH).
Some organisms can be found growing in sulphur springs containing sulphuric acid with a pH of less than 2, others in ammoniated solutions at a pH greater than 8. Fungi, as a rule, grow well at an acid pH range of 5.5 to 6.
This property is used in the preparation of selective media for these organisms, since many contaminating bacteria cannot grow under these conditions. Similarly, the causative agent of Asiatic cholera, Vibrio comma, can tolerate a pH of 8.
This fact is used in the preparation of isolation media or the organisms, since it must be separated from other typical organisms comprising the enteric flora found in feces. As a rule, microorganisms appear to prefer a more neutral pH, between 7 and 7.5. Therefore, acidic and alkaline solutions can exert disinfecting effects on various organisms.
3. Growth Media:
The combining of various substances into nutritive concoctions has long been an integral part of microbiology. Such media are used for the isolation of important organisms from various materials such as dairy product, foods, soil, water, and clinical specimens.
Most organisms found in these situations are heterotrophic in nature, thus simplifying the task of combing the essential nutrient components into a suitable medium. Some of the important ingredients of media are briefly described below.
4. Nitrogen Sources:
Nitrogen is a component of cellular proteins, nucleic acids, and vitamins. Microorganisms must therefore be supplied with this element in some form. Many can use ammonium salts, e.g., ammonium chloride (NH4C1), while others require the break-down products of proteins, such as peptones (partially hydrolyzed proteins), peptides, and amino acids.
Some bacteria (e.g., Bacillus spp.) produce extra-cellular protein- digesting enzymes (proteases) which breakdown gelatin and other proteins into smaller components, like peptides and amino acids.
These can then be brought into the cells for further metabolic action. Organisms that are able to grow when supplied with ammonium salts usually will grow in the presence of organic nitrogen. A good example is the bacterium Escherichin coli.
5. Carbon and Energy Sources:
Carbon is the most basic structural element of all living forms. It is obtained by organisms from carbohydrates, proteins (peptones, etc. and lipids, in essence from all organic nutrients and carbon dioxide.
The catabolism (metabolic decomposition) of organic compounds results in the production of amino acids, sugar, fatty acids, and other related compounds. Such materials may function in anabolism (constructive metabolism), thus producing the enzymatic and structural proteins, nucleic acids, carbohydrates, and other biochemical compounds peculiar to the organism.
The same compounds may be involved in energy metabolism, producing the impetus for growth and reproduction, or as storage products rich in energy-yielding chemical bonds.
Carbohydrate sources of energy and carbon include starch, glycogen, various pentose (5-carbon monosaccharides) such as arabinose, hexose (6-carbon) monosaccharides such as dextrose, and disaccharides such as lactose, sucrose, and maltose.
In utilizing the polysaccharides (starch and glycogen) the organism must produce extracellular enzymes to bring about the degradation of the complex compound into smaller molecules which can enter the cell.
The enzyme amylase, for example, degrades starch into maltose units, which can then be transported into the cell. Subsequently, the enzyme maltase splits the molecule into
two dextrose units for use in further metabolic activities.
This catabolism not only yields the various building blocks for proteins, polysaccharides, lipid, and nucleic acid biosynthesis, but results in the production of energy with which cells can perform anabolic reactions.
There usually are exceptions to any rule. Some will be discussed in other chapters in relation to forms of energy metabolism which function solely with amino acids, as with the Stick land reaction found in Clostridium spp.