Microbial Nutrition and Growth: Study Notes

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Microbial Nutrition and Growth

Introduction to Microbial Growth

Microbial growth refers to the increase in the number of microorganisms in a population, primarily through the reproduction of individual cells. This process results in the formation of discrete colonies or complex communities known as biofilms.

Colony: An aggregation of cells arising from a single parent cell.
Biofilm: A collection of microbes living on a surface in a complex community.

Growth Requirements

Chemical and Energy Requirements

Microorganisms require various nutrients for energy and to build cellular structures. The most common elements needed are carbon, oxygen, nitrogen, and hydrogen. Microbes are classified based on their sources of carbon, energy, and electrons.

Autotrophs: Use carbon dioxide as a carbon source.
Heterotrophs: Use organic compounds as a carbon source.
Phototrophs: Obtain energy from light.
Chemotrophs: Obtain energy from chemical compounds.
Organotrophs: Obtain electrons from organic molecules.
Lithotrophs: Obtain electrons from inorganic molecules.

Table of four basic groups of organisms based on carbon and energy sources

Oxygen Requirements

Oxygen is essential for some organisms but toxic to others due to the formation of reactive oxygen species. Microbes are classified by their oxygen requirements:

Obligate aerobes: Require oxygen for growth.
Obligate anaerobes: Oxygen is toxic; they cannot survive in its presence.
Facultative anaerobes: Can grow with or without oxygen but grow better with it.
Aerotolerant anaerobes: Do not use oxygen but can tolerate its presence.
Microaerophiles: Require low levels of oxygen.

Thioglycolate medium showing oxygen requirements of organisms

Nitrogen, Phosphorus, Sulfur, and Growth Factors

Nitrogen is essential for the synthesis of amino acids and nucleotides. Some bacteria can fix atmospheric nitrogen. Other elements like phosphorus and sulfur are also required, along with trace elements and growth factors (organic molecules that some organisms cannot synthesize).

Growth Factor	Function
Amino acids	Components of proteins
Cholesterol	Used by mycoplasmas for cell membranes
Heme	Functional portion of cytochromes in electron transport
NADH	Electron carrier
Niacin (vitamin B3)	Precursor of NAD+ and NADP+
PABA	Precursor of folic acid, involved in nucleic acid synthesis

Physical Requirements for Growth

Temperature

Temperature affects protein structure and membrane fluidity. Microbes are classified by their preferred temperature ranges:

Psychrophiles: Grow best at low temperatures (below 20°C).
Mesophiles: Grow best at moderate temperatures (20–45°C).
Thermophiles: Grow best at high temperatures (above 45°C).
Hyperthermophiles: Grow best at extremely high temperatures (above 80°C).

Effects of temperature on microbial growth Categories of microbes based on temperature ranges Example of a psychrophile

pH

Microorganisms are sensitive to pH changes, which affect hydrogen bonding in proteins and nucleic acids.

Neutrophiles: Grow best at neutral pH (around 7).
Acidophiles: Grow best in acidic environments.
Alkalinophiles: Grow best in alkaline environments.

Water, Osmotic Pressure, and Hydrostatic Pressure

Water is essential for microbial metabolism. Osmotic pressure affects cell water balance, while hydrostatic pressure is important for barophiles (organisms living under high pressure).

Obligate halophiles: Require high salt concentrations.
Facultative halophiles: Can tolerate high salt but do not require it.
Barophiles: Thrive under high hydrostatic pressure.

Microbial Associations and Biofilms

Types of Relationships

Microorganisms can interact in various ways:

Antagonistic: One organism harms another.
Synergistic: Both benefit, but the relationship is not essential.
Symbiotic: Both benefit and the relationship is essential for survival.

Biofilms

Biofilms are complex communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix. They form through a process called quorum sensing, where microbes communicate and coordinate behavior based on cell density. Biofilms are common on medical devices and mucous membranes and are often more resistant to antibiotics.

Biofilm development Quorum sensing in bacteria

Culturing Microorganisms

Obtaining Pure Cultures

Pure cultures are obtained to study a single microbial species. The most common isolation techniques are the streak plate and pour plate methods. Aseptic technique is essential to prevent contamination.

Characteristics of bacterial colonies Streak-plate method of isolation Pour-plate method of isolation

Culture Media

Microbes are grown on various types of media, which can be liquid (broth) or solid (agar). Media types include:

Defined (synthetic) media: Exact chemical composition is known.
Complex media: Contains nutrients from extracts; composition is not precisely known.
Selective media: Favors or inhibits the growth of certain microbes.
Differential media: Distinguishes microbes based on metabolic reactions.
Anaerobic media: Supports growth of anaerobes.
Transport media: Used to transport clinical specimens safely.

Slant tubes containing solid media Selective medium example Blood agar as a differential medium Carbohydrate utilization tubes as differential media MacConkey agar as selective and differential medium Anaerobic culture system

Growth of Microbial Populations

Binary Fission and Generation Time

Most bacteria reproduce by binary fission, a process in which one cell divides into two identical daughter cells. The generation time is the time required for a cell to divide and is influenced by environmental conditions.

Binary fission

Population Growth Curves

Bacterial populations typically exhibit four phases of growth:

Lag phase: Cells adjust to their environment; little to no division.
Log (exponential) phase: Rapid cell division and population growth.
Stationary phase: Growth rate slows as resources become limited.
Death (decline) phase: Cells die faster than new cells are produced.

Arithmetic vs. logarithmic growth Growth curves of logarithmic growth Typical population growth curve Bacterial growth curve

Continuous Culture

A chemostat is used to maintain microbial populations in a specific phase of growth by continuously adding fresh medium and removing old medium. This is important in industrial microbiology.

Schematic of chemostat

Measuring Microbial Growth

Direct Methods

Microscopic counts: Counting cells under a microscope.
Electronic counters: Coulter counter and flow cytometry.
Serial dilution and viable plate counts: Estimating population size by counting colonies.
Membrane filtration: Used for small populations.
Most probable number (MPN): Statistical estimation based on dilution series.

Cell counter for estimating microbial numbers Serial dilution and viable plate count Membrane filtration to estimate microbial population size Most probable number method

Indirect Methods

Turbidity: Measuring cloudiness of a culture using a spectrophotometer.
Metabolic activity: Measuring products of metabolism.
Dry weight: Weighing biomass after drying.
Molecular methods: Detecting DNA sequences of unculturable microbes.

Turbidity and spectrophotometry

Case Study: Listeria monocytogenes

Listeria monocytogenes is a bacterium that can survive in diverse environmental conditions and cause central nervous system infections, especially in immunocompromised individuals. It can be identified using selective and differential media, and some antibiotics used to treat listeriosis inhibit bacterial enzymes.

Summary: Understanding microbial nutrition and growth is essential for culturing, identifying, and controlling microorganisms in clinical, industrial, and environmental settings.