Fundamentals of Microbiology

Lesson 17. BACTERIAL NUTRITION

Module 5. Microbial growth and nutrition

Lesson 17

BACTERIAL NUTRITION

17.1 Introduction

The survival and sustenance bacteria in the laboratory, as well as in nature, depend on their ability to grow under certain chemical and physical conditions. An understanding of these conditions is a prerequisite for us to isolate, identify and characterize different types of bacteria. Such knowledge can also be applied to control the growth of microorganisms in practical situations.

17. 2 Common Nutrient Requirements and Nutritional Types of Microorganisms

Bacteria are composed of different elements and molecules, with water (70%) and proteins (15%) being the most abundant. Microorganisms are often categorized according to their energy source and their source of carbon.

17.2.1 Energy source

• Phototrophs use radiant energy (light) as their primary energy source.
• Chemotrophs use the oxidation and reduction of chemical compounds as their primary energy source.

17.2.2 Carbon source

Carbon is the structural backbone of the organic compounds that make up a living cell. Based on their source of carbon bacteria can be classified as autotrophs or heterotrophs.

17.2.2.1 Autotrophs

Require only carbon dioxide as a carbon source. An autotroph can synthesize organic molecules from inorganic nutrients.

17.2.2.2 Heterotrophs

Require organic forms of carbon. A heterotroph cannot synthesize organic molecules from inorganic nutrients.Combining their nutritional patterns, all organisms in nature can be placed into one of four separate groups which are

a) Photoautotrophs

Use light as an energy source and carbon dioxide as their main carbon source. They include photosynthetic bacteria (green sulfur bacteria, purple sulfur bacteria, and cyanobacteria), algae, and green plants. Photoautotrophs transform carbon dioxide and water into carbohydrates and oxygen gas through photosynthesis. Cyanobacteria, as well as algae and green plants, use hydrogen atoms from water to reduce carbon dioxide to form carbohydrates, and during this process oxygen gas is given off (an oxygenic process). Other photosynthetic bacteria (the green sulfur bacteria and purple sulfur bacteria) carry out an anoxygenic process, using sulfur, sulfur compounds or hydrogen gas to reduce carbon dioxide and form organic compounds.

b) Photoheterotrophs

Use light as an energy source but cannot convert carbon dioxide into energy. Instead they use organic compounds as a carbon source. They include the green nonsulfur bacteria and the purple nonsulfur bacteria.

c) Chemolithoautotrophs

Use inorganic compounds such as hydrogen sulfide, sulfur, ammonia, nitrites, hydrogen gas, or iron as an energy source and carbon dioxide as their main carbon source.

d) Chemoorganoheterotrophs

Use organic compounds as both an energy source and a carbon source. Saprophytes live on dead organic matter while parasites get their nutrients from a living host. Most bacteria, and all protozoans, fungi, and animals are chemoorganoheterotrophs.

17.2.3 Nitrogen source

Nitrogen is needed for the synthesis of such molecules as amino acids, DNA, RNA and ATP. Depending on the organism, nitrogen, nitrates, ammonia, or organic nitrogen compounds may be used as a nitrogen source.

17.2.4 Minerals

• Sulfur is needed to synthesisize sulfur-containing amino acids and certain vitamins. Depending on the organism, sulfates, hydrogen sulfide, or sulfur- containing amino acids may be used as a sulfur source.
• Phosphorus is needed to synthesize phospholipids, DNA, RNA, and ATP. Phosphate ions are the primary source of phosphorus.
• Potassium, magnesium, and calcium. These are required for certain enzymes to function as well as additional functions.
• Iron is a part of certain enzymes.
• Trace elements are elements required in very minute amounts, and like potassium, magnesium, calcium, and iron, they usually function as cofactors in enzyme reactions. They include sodium, zinc, copper, molybdenum, manganese, and cobalt ions. Cofactors usually function as electron donors or electron acceptors during enzyme reactions.

17.2.5 Water

17.2.6 Growth factors

Growth factors are organic compounds such as amino acids, purines , pyrimidines, and vitamins that a cell must have for growth but cannot synthesize itself. Organisms having complex nutritional requirements and needing many growth factors are said to be fastidious (Table 17.1).

Table 17.1 Nutritional types of microorganisms

17.3 Types and Composition of Microbiological Media

Culture media are needed to grow microorganisms in the laboratory and to carry out specialized procedures like microbial identification, water and food analysis, and the isolation of particular microorganisms. A wide variety of media is available for these and other purposes. Media can be classified on the basis of physical state, chemical composition and functional type (Table 17.2).

Table 17.2 Types of microbiological media

17.3.1 Classification by physical state

17.3.1.1 Liquid Media

In these media Growth seen as cloudiness or particulates and examples are Broths, milks, or infusions (Fig.18.2)

17.3.1.2 Semisolid media

They have clot like consistency at room temperature and are used to determine motility and to localize reactions at a specific site. Example: SIM for hydrogen sulfide production and indole reaction

17.3.1.3 Solid media

A firm surface on which cells can form discrete colonies. These can be liquefiable and non-liquefiable useful for isolating and culturing bacteria and fungi example: gelatin and agar (liquefiable); rice grains, cooked meat media, potato slices (non-liquefiable)

Fig. 17.2 Classification of media on the basis of physical state

17.3.2 Classification by chemical content

17.3.2.1 Synthetic media

Their compositions are precisely chemically defined, contain pure organic and inorganic compounds ex. glucose salts medium contains glucose plus inorganic salts in known amounts plus

17.3.2.2 Complex (non-synthetic) media

Contains at least one ingredient that is not chemically definable (extracts from plants and animals), no exact formula / tend to be general and grow a wide variety of organisms. For example nutrient broth which contains beef extract, yeast extract, peptone, NaCl. The exact composition of these is not known.

17.3.3 Classification by function

17.3.3.1 General purpose media

Such media are used to grow as broad a spectrum of microbes as possible and are usually non-synthetic; contain a mixture of nutrients to support a variety of microbes. Examples: nutrient agar and broth, brain-heart infusion, trypticase soy agar (Fig. 17.3).

17.3.3.2 Enriched media

Contain complex organic substances (for example blood, serum, growth factors) to support the growth of fastidious bacteria e.g. blood agar and Thayer-Martin medium (chocolate agar) which contains heated blood which turns a chocolate brown color.

17.3.3.3 Selective and differential media

These contain one or more agents that inhibit the growth of certain microbes but not others e.g. Mannitol salt agar (MSA), MacConkey agar, Hektoen enteric (HE) agar.

17.3.3.4 Differential media

Allow multiple types of microorganisms to grow but display visible differences among those microorganisms. MacConkey agar can also be used as a differential medium.

17.3.3.5 Miscellaneous media

• Reducing media: absorbs oxygen or slows its penetration in the medium, used for growing anaerobes or for determining oxygen requirements
• Carbohydrate fermentation media: contain sugars that can be fermented and a pH indicator; useful for identification of microorganisms
• Transport media: used to maintain and preserve specimens that need to be held for a period of time.

Fig. 17.3 Enrichment and selective media

17.4 Use of Culture Media

Different media are used for cultivation of bacteria under laboratory conditions and bacteria from their natural niche or mixed population can be isolated and identified with appropriate selection of media (Fig. 17.4).

Fig. 17.4 Different types of media used for isolation of bacteria from mixed population

17.5 Composition of Media

In the following sections composition of some commonly used media are given (Table 17.3-17.8).

Table 17.3 Nutrient agar (General purpose medium)

Table 17.4 A chemical defined medium for E. coli

Table 17.5 Plate count agar (standard methods)

Table 17.6 Potato dextrose agar (yeasts and moulds)

Table 17.7 Brilliant green lactose bile broth (selective broth for coliforms)

Table 17.8 MacConkey agar (selective agar for coliforms)