Microbial Nutrition, Growth, and Environmental Influences: Study Notes

Nutrition and Growth of Bacteria

Introduction: The Soil Bacterium Streptomyces

Bacteria exhibit diverse growth strategies and play crucial roles in natural environments. Streptomyces is a notable soil bacterium known for producing secondary metabolites, including antibiotics and compounds responsible for the characteristic smell of soil.

• Vegetative growth: Filamentous cells grow by tip elongation without cell division.

• Aerial hyphae and sporulation: Under nutrient limitation, sporulation occurs via multiple fission, forming many spores from a single filamentous cell.

• Secondary metabolites: Examples include geosmin (soil odor) and streptomycin (antibiotic).

• Binary fission: The primary mode of bacterial cell division, where one cell splits into two.

Microbial Growth and Its Control

Learning Objectives

• Understand how microbes are cultured and how their growth is measured.

• Gain an overview of the dynamic nature of microbial growth.

• Appreciate the influence of the environment on microbial growth.

• Identify major techniques for the control and prevention of microbial growth.

Culturing Microbes and Measuring Their Growth

Feeding the Microbe: Cell Nutrition

Microbial cells require a variety of nutrients for growth, which can be classified as macronutrients and micronutrients. The chemical composition of a typical bacterial cell, such as Escherichia coli, is dominated by a handful of elements and macromolecules.

• Macronutrients: Required in large amounts (C, O, N, H, P, S, K, Na, Ca, Mg, Cl, Fe).

• Micronutrients: Required in minute amounts (trace metals and growth factors).

• Major macromolecules: Proteins, lipids, polysaccharides, nucleic acids.

Carbon and Nitrogen Sources

• Heterotrophs: Obtain carbon from organic compounds.

• Autotrophs: Synthesize organic compounds from CO2.

• Nitrogen: Sourced from ammonia (NH3), nitrate (NO3-), nitrogen gas (N2), or organic compounds.

Other Macronutrients

• Phosphorus (P): Essential for nucleic acids and phospholipids.

• Sulfur (S): Found in certain amino acids and vitamins.

• Potassium (K), Magnesium (Mg), Calcium (Ca), Sodium (Na): Required for enzyme function and cellular stability.

Micronutrients: Trace Metals and Growth Factors

Many enzymes require metal ions or small organic molecules as cofactors for catalysis. Iron is especially important for cellular respiration and redox reactions.

Element	Function
Boron (B)	Quorum sensing, antibiotics
Cobalt (Co)	Vitamin B12, transcarboxylase
Copper (Cu)	Respiration, photosynthesis
Iron (Fe)	Cytochromes, catalases, peroxidases
Manganese (Mn)	Enzyme activator, photosystem II
Molybdenum (Mo)	Nitrogenases, reductases
Nickel (Ni)	Hydrogenases, urease
Selenium (Se)	Formate dehydrogenase, selenocysteine
Tungsten (W)	Formate dehydrogenases
Vanadium (V)	Nitrogenase
Zinc (Zn)	Polymerases, DNA-binding proteins

Growth Factors

• Organic micronutrients, often vitamins, required for enzyme function as coenzymes.

• Some microbes synthesize all their growth factors; others require supplementation from the environment.

Growth Media and Laboratory Culture

Types of Culture Media

Culture media are nutrient solutions used to grow microbes in the laboratory. They can be solid or liquid, and their composition can be defined or complex.

• Defined media: Exact chemical composition is known.

• Complex media: Contains digests of organic materials; composition is not precisely known.

• Selective media: Inhibits growth of some microbes while allowing others to grow.

• Differential media: Contains indicators to distinguish between different metabolic reactions.

Example: EMB Agar

• Contains eosin and methylene blue dyes, toxic to Gram-positive bacteria.

• Differentiates lactose fermenters (dark purple/green sheen colonies) from non-fermenters (colorless/light colonies).

Colony Morphology

Colony morphology refers to the visible characteristics of microbial colonies on solid media, which can aid in identification.

Aseptic Technique

Aseptic technique is essential for transferring microbes without contamination. The streak plate method is commonly used to obtain pure cultures.

Measuring Microbial Growth

Microscopic Counts

Direct microscopic counts involve counting cells using a counting chamber. This method is quick but cannot distinguish live from dead cells without special stains.

Fluorescent Staining

Fluorescent stains can differentiate live and dead cells, and can be used to identify phylogenetic groups in environmental samples.

Viable Counting (Plate Counts)

Viable counts measure the number of living cells capable of forming colonies. The spread-plate and pour-plate methods are commonly used, with results expressed as colony-forming units (CFU).

Turbidimetric Measures

Turbidity (cloudiness) of a cell suspension is measured using a spectrophotometer. Optical density (OD) is proportional to cell number within certain limits.

Dynamics of Microbial Growth

Binary Fission and the Microbial Growth Cycle

Bacteria typically reproduce by binary fission, resulting in exponential population growth. The microbial growth curve in batch culture includes lag, exponential, stationary, and death phases.

Quantitative Aspects of Microbial Growth

Exponential growth can be described mathematically. The generation time (g) is the time required for the population to double.

• Number of cells after n generations: $N = N_0 \times 2^n$

• Generation time: $g = t / n$

• Specific growth rate: $k = 0.693 / g$

Continuous Culture

Continuous culture systems, such as the chemostat, allow for the constant growth of microbial populations under steady-state conditions. Both growth rate and cell density can be independently controlled.

Biofilm Growth

Biofilms are structured communities of microbes attached to surfaces and embedded in a self-produced matrix. Biofilms are important in natural, medical, and industrial contexts.

Alternatives to Binary Fission

Some bacteria reproduce by budding or hyphal growth, forming mycelia and arthrospores. These alternative modes of division are common in certain groups such as actinomycetes.

Environmental Effects on Microbial Growth

Temperature

Microorganisms are classified by their temperature optima:

• Psychrophiles: Cold-loving, optimum ≤ 15°C

• Mesophiles: Moderate temperatures, optimum 20–45°C

• Thermophiles: Heat-loving, optimum 45–80°C

• Hyperthermophiles: Extreme heat, optimum >80°C

pH

Microbes are also classified by their pH optima:

• Neutrophiles: pH 5.5–7.9

• Acidophiles: pH < 5.5

• Alkaliphiles: pH ≥ 8

Osmolarity

Water availability (aw) is critical for microbial growth. Halophiles require high salt concentrations, while osmophiles and xerophiles tolerate high sugar or dry environments, respectively.

Oxygen

Microbes are classified by their oxygen requirements:

• Obligate aerobes: Require oxygen.

• Facultative anaerobes: Can grow with or without oxygen.

• Microaerophiles: Require reduced oxygen levels.

• Aerotolerant anaerobes: Tolerate oxygen but do not use it.

• Obligate anaerobes: Killed by oxygen.

Summary Table: Oxygen Relationships of Microorganisms

Group	O2 Relationship	Type of Metabolism	Example	Habitat
Obligate aerobe	Required	Aerobic respiration	Micrococcus luteus	Skin, dust
Facultative anaerobe	Not required, better with O2	Aerobic/anaerobic respiration, fermentation	Escherichia coli	Large intestine
Microaerophile	Required at low levels	Aerobic respiration	Spirillum volutans	Lake water
Aerotolerant anaerobe	Not required, unaffected by O2	Fermentation	Streptococcus mutans	Oral cavity
Obligate anaerobe	Harmful/lethal	Fermentation/anaerobic respiration	Methanobacterium formicicum	Sewage sludge

Control of Microbial Growth

Microbial growth can be controlled by physical methods (heat, radiation, filtration) and chemical agents. Details of these methods are covered in subsequent topics.

Further Reading

For more details, refer to the recommended textbook: Brock Biology of Microorganisms.