Microbial Nutrition and Growth: Chapter 6 Notes

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Microbial Nutrition and Growth

Growth Requirements

Microbial growth refers to the increase in the population of microbes, primarily due to the reproduction of individual cells. This growth can result 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.

Streak plate with isolated colonies and a biofilm

Nutritional 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 and energy:

Autotrophs: Use carbon dioxide as a carbon source (make their own food).
Heterotrophs: Use organic compounds as a carbon source (consume food).
Phototrophs: Obtain energy from light.
Chemotrophs: Obtain energy from chemical compounds.

Carbon Source	Energy Source	Examples
CO2 (auto-)	Light (photo-)	Photoautotrophs: Plants, algae, cyanobacteria
CO2 (auto-)	Chemical compounds (chemo-)	Chemoautotrophs: Hydrogen, sulfur, nitrifying bacteria
Organic compounds (hetero-)	Light (photo-)	Photoheterotrophs: Green and purple nonsulfur bacteria
Organic compounds (hetero-)	Chemical compounds (chemo-)	Chemoheterotrophs: Most animals, fungi, protozoa, many bacteria

Table of nutritional types 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 (ROS):

Obligate aerobes: Require oxygen for growth.
Obligate anaerobes: Oxygen is toxic; they cannot survive in its presence.
Facultative anaerobes: Grow better with oxygen but can survive without it (e.g., E. coli).
Aerotolerant anaerobes: Do not use oxygen but can tolerate it.
Microaerophiles: Require low levels of oxygen (2–10%).

Toxic forms of oxygen Thioglycolate broth tubes showing oxygen requirements

Nitrogen, Phosphorus, Sulfur, and Growth Factors

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

Nitrogen cycle and nitrogen-fixing bacteria

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

Table of growth factors and their functions

Physical Requirements for Growth

Temperature

Temperature affects the structure of proteins and the fluidity of membranes. Microbes are classified based on their preferred temperature ranges:

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

Effect of temperature on microbial growth rate and colony growth at different temperatures Temperature ranges for microbial growth Psychrophile habitat and cells

pH

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

Neutrophiles: Grow best at neutral pH (6.5–7.5); most pathogens are neutrophiles.
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 and hydrostatic pressure influence microbial survival:

Halophiles: Require or tolerate high salt concentrations.
Barophiles: Live under high hydrostatic pressure, such as in deep ocean environments.

Culturing Microorganisms

Culture and Inoculation

Microorganisms are cultivated in nutrient-rich media. Inocula can be obtained from environmental, clinical, or stored specimens. Pure cultures are obtained using aseptic techniques and isolation methods such as streak plates and pour plates.

Streak plate method of isolation Pour plate method of isolation

Colony Characteristics

Bacterial colonies can be described by their shape, margin, elevation, size, texture, appearance, pigmentation, and optical properties.

Colony morphology chart Bacterial colony on agar plate

Biofilms and Microbial Associations

Microbes often live in complex associations, including antagonistic, synergistic, and symbiotic relationships. Biofilms are structured communities formed through quorum sensing, making microbes more resistant to environmental stresses and antimicrobial agents.

Biofilm development stages Quorum sensing and biofilm formation

Culture Media Types

Various types of media are used to culture microorganisms:

Defined (synthetic) media: Exact chemical composition is known.
Complex media: Contains nutrients from partially digested organic sources; composition is not precisely known.
Selective media: Favors or inhibits the growth of certain microbes.
Differential media: Distinguishes microbes based on their biological characteristics.
Anaerobic media: Supports the growth of anaerobes.
Transport media: Used to preserve and transport clinical specimens.

Solid, liquid, and semi-solid culture media Slant tubes with solid media Defined medium ingredients

Examples of Selective and Differential Media

Blood agar: Differentiates bacteria based on hemolysis patterns (alpha, beta, gamma).
MacConkey agar: Selects for Gram-negative bacteria and differentiates lactose fermenters.

Blood agar hemolysis patterns Hemolysis of Streptococci Carbohydrate utilization tubes as differential media MacConkey agar results MacConkey and Blood Agar media ingredients

Anaerobic Culture Systems

Anaerobic jars and chambers are used to cultivate anaerobic bacteria by removing oxygen from the environment.

Anaerobic culture system

Preserving Cultures

Refrigeration: Short-term storage.
Deep-freezing: Long-term storage in glycerol solution.
Lyophilization: Freeze-drying for decades-long preservation.

Growth of Microbial Populations

Binary Fission

Most bacteria reproduce by binary fission, a process in which one cell divides into two identical daughter cells. This process involves DNA replication, septum formation, and cell division.

Binary fission steps

Generation Time and Growth Curves

Generation time is the period required for a bacterial cell to divide. Microbial populations grow exponentially (logarithmically) under optimal conditions.

Arithmetic growth: Linear increase in cell number.
Logarithmic growth: Exponential increase in cell number.

Arithmetic vs. logarithmic growth curves

Continuous Culture

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

Measuring Microbial Reproduction

Estimating microbial numbers is essential for clinical, environmental, and industrial applications. Methods include:

Direct methods: Microscopic counts, electronic counters (Coulter counter, flow cytometry), serial dilution and viable plate counts, membrane filtration, most probable number (MPN).
Indirect methods: Turbidity measurement using spectrophotometry.

Cell counter for estimating microbial numbers Serial dilution and viable plate count Membrane filtration method Turbidity and spectrophotometry

Case Study: Listeria monocytogenes

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