MICROBIOLOGY Study Guide - 2012

CHAPTER 9. MICROBIAL PHYSIOLOGY

9.2. MICROBIAL NUTRITION

All physiological processes, such as motility, GROWTH AND REPRODUCTION, spore and capsule formation, and toxin production, can only take place with a continuous supply of energy.

Through Nutrition, an Organism acquires the substances required for the synthesis of Cellular Structures as well as the energy source for all vital metabolic processes. KEY FEATURES OF Microbial Nutrition include the uptake of nutrients across the entire Cell surface and a remarkably High Metabolic Rate.

The Cell wall and cytoplasmic membrane of microorganisms are impermeable to many high-molecular-weight compounds (such as Polysaccharides, Lipids, and Proteins); therefore, these substances are first broken down by extracellular Enzymes—released by the Cells into the external environment—into simpler compounds (mono- and Disaccharides, Amino Acids, organic acids, glycerol, etc.). This process, unique to microorganisms, is known as extracellular Digestion.

Culture media used for cultivating microorganisms in laboratory and industrial settings must meet the following minimum requirements:

✵ they must contain all the elements required for cell building;

✵ these elements must be in a form that microorganisms can readily assimilate;

✵ they must have an optimal pH level;

✵ the media must be sterile.

Culture media vary in consistency, composition, and purpose.

Based on their consistency, media are classified as liquid, solid, or semi-solid. Solid and semi-solid media are prepared by adding Agar-agar or gelatin to liquid media. Typically, 1.5–2.0% agar-agar is added to prepare solid media, and 0.2–0.5% for semi-solid media.

The composition of culture media is determined by the Nutritional Requirements of microorganisms. Depending on The Nature of their initial components, media are categorized as natural, synthetic, or semi-synthetic.

Natural media consist of natural substrates (such as meat, milk, vegetables, etc.). Examples include meat-peptone broth, hydrolyzed milk, brewers' wort, Yeast extract, hay infusion, and potato medium. Determining the precise Chemical composition of such media is quite challenging. Lactic acid Bacteria are particularly fastidious regarding their nutritional requirements; therefore, they are cultivated in milk, hydrolyzed milk, whey, brewers' wort, or specially formulated media.

Synthetic media are formulated from known chemical compounds in precise quantities. Escherichia coli is nutritionally undemanding and is thus able to grow on synthetic media of relatively simple composition.

Semi-synthetic media contain both defined components and undefined substrates. For instance, yeast autolysate or meat-peptone broth may be added to a synthetic base.

Depending on their application, media are divided into selective and differential-diagnostic media.

Selective media are used to isolate specific groups of microorganisms from their natural habitats.

Differential-diagnostic media are used for the rapid identification of microorganisms based on their characteristic traits.

Nutritional requirements of microorganisms. Based on The chemical composition of microorganisms, for the Biosynthesis of Essential macromolecules The Cell must acquire substances containing Macronutrients C, O, H, N, S, P, Ca, Fe, Mg, and micronutrients Mn, Co, Mo, Cu, Zn, among others. Macronutrients are required in relatively large quantities, ranging from 0.2 to 0.5 g/L, whereas micronutrients are needed in very low concentrations, from 0.1 to 0.001 mg/L. Mineral ions participate in the Regulation of cellular osmotic pressure, pH, and E_h of the medium. The primary function of micronutrients is the activation of various enzymes.

Among all the aforementioned elements, carbon plays the most crucial role in microbial nutrition. Based on their carbon source, microorganisms are divided into: autotrophs (from the Greek autos meaning self, and trophe meaning nourishment), which utilize CO₂ for biosynthetic purposes, and heterotrophs (from the Greek heteros meaning other), which obtain carbon from Organic compounds.

The highest degree of heterotrophy is exhibited by microorganisms that are obligate or facultative parasites (from the Greek parasitos meaning one who eats at the table of another). Facultative heterotrophic parasites include pathogenic bacteria responsible for infectious diseases in humans, animals, and plants; obligate parasites, which can only survive inside a host cell, include rickettsiae, chlamydiae, Viruses, and certain Protozoa.

Another major group of heterotrophs consists of saprotrophs (from the Greek sapros meaning rotten, and phyton meaning plant), which derive their nutrition from decaying plant or animal Tissues. The majority of bacteria and micromycetes belong to saprotrophs.

For many heterotrophs, CARBOHYDRATES serve as the optimal and most accessible organic carbon source. Monosaccharides—specifically hexoses and pentoses—are utilized particularly widely. Certain groups of microorganisms are also capable of using organic acids, primary alcohols, cyclic compounds, and other substances as carbon sources.

Nitrogen and sulfur are incorporated into cellular Organic compounds as amino groups and sulfhydryl groups of amino acids. Some bacteria take up these two elements in an oxidized state—as nitrates and sulfates. Therefore, they are first reduced and then utilized in biosynthetic processes. Most bacteria utilize nitrogen in a reduced form, such as amino acids or urea. Sulfides or Sulfur-Containing Amino Acids (e.g., Cysteine) can serve as sources of sulfur.

Growth factors are substances that are not synthesized by many bacteria but are essential for them to build organic cellular components. Consequently, they must be present in the culture medium used to grow microorganisms. Growth factors include:

✵ amino acids, which are required for Protein Synthesis;

✵ Purines and Pyrimidines, used for the synthesis of Nucleic Acids;

✵ Vitamins, which act as prosthetic groups or active sites of certain enzymes.

Microorganisms that require growth factors are called auxotrophs. Microorganisms that synthesize the necessary growth factors on their own are called prototrophs.

9.2.1. Nutritional Types of Microorganisms

Dividing microorganisms into two main categories—autotrophs and heterotrophs—proved clearly insufficient to reflect the full diversity of their nutritional and Energy Requirements. Therefore, the Classification of microorganisms by nutritional type includes such core criteria as the carbon source, energy source, and electron donor. Based on these criteria, all microorganisms can be divided into four groups (Table 6).

Class="center">Table 6. Classification of microorganisms by Nutritional types

Anabolic processes in microbial cells require Energy Expenditure. Microorganisms capable of utilizing solar radiation as an energy source are called phototrophs (photosynthetic). Microorganisms that derive energy from redox reactions are called chemotrophs.

Depending on the oxidized substrate, known as the electron donor, microorganisms are divided into lithotrophs (from Greek lithos meaning stone), which use inorganic compounds as electron Donors, and organotrophs, which utilize organic compounds for this purpose.