Microbial Nutrition and Growth: Study Guide

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Microbial Nutrition and Growth

Overview of Microbial Growth

Microbial growth refers to the increase in population size and number of microorganisms. Growth is the primary purpose of microbial metabolism, and it can be observed as discrete colonies on solid media, each arising from a single parent cell.

Microbial growth = increase in population size and number.
On solid media, colonies are aggregations of cells from a single parent.
Generation time: the time required for a bacterial cell to grow and divide; varies with environmental conditions.

Bacterial Growth Curve

The bacterial growth curve describes the population changes over time, typically plotted as number of organisms (y-axis, logarithmic scale) versus time (x-axis). There are four distinct phases:

Lag phase: Cells adjust to new environment, synthesize enzymes, slow doubling.
Logarithmic (log) phase: Rapid population increase, binary fission dominates, cells are most susceptible to antibiotics, best phase for Gram staining.
Stationary phase: Nutrients deplete, waste accumulates, cell division rate equals cell death rate.
Death phase: Cell death rate exceeds division rate, population declines, some cells survive as endospores.

Example: Bacillus anthracis endospores can remain viable for decades.

Equation for Bacterial Population Growth

To calculate the size of a bacterial population:

Where:

= total number of cells
= number of cells at start
= number of generations

Measuring Microbial Reproduction

Serial dilution and standard plate count: Used to estimate population size; viable count is 30-300 colonies per plate.
Turbidity (spectrophotometric method): Measures cloudiness to estimate population size indirectly.

Growth Requirements

Chemical and Energy Requirements

Microbes require various elements and compounds for growth, including carbon, hydrogen, oxygen, nitrogen, trace elements, and growth factors.

Carbon: Essential for all organic molecules.
Hydrogen & Oxygen: Components of water and organic molecules.
Nitrogen: Needed for amino acids, proteins, nucleic acids; acquired from organic and inorganic sources.
Phosphorus: Required for phospholipids, DNA, RNA, ATP.
Sulfur: Component of certain amino acids, vitamins, and disulfide bonds.
Trace elements: Required in small amounts (e.g., vitamins).
Growth factors: Organic chemicals that some organisms cannot synthesize (e.g., vitamins, amino acids, purines, pyrimidines, cholesterol, NADH, heme).

Classification by Carbon and Energy Source

Autotrophs: Use inorganic carbon (CO2).
Heterotrophs: Use organic carbon (proteins, carbohydrates).
Chemotrophs: Obtain energy from chemicals.
Phototrophs: Obtain energy from light.
Humans: Chemoheterotrophs.
Plants: Photoautotrophs.

Comparison Table: Photoautotrophs vs. Chemoheterotrophs

Type	Carbon Source	Energy Source	Example
Photoautotroph	CO2 (inorganic)	Light	Plants, Cyanobacteria
Chemoheterotroph	Organic compounds	Chemicals	Humans, Most bacteria

Oxygen Requirements

Aerobes: Require oxygen for aerobic respiration.
Anaerobes: Oxygen is toxic; use anaerobic respiration or fermentation.
Facultative anaerobes: Can use aerobic respiration, fermentation, or anaerobic respiration.
Obligate: Must have or must avoid oxygen.
Aerotolerant: Can tolerate oxygen but do not use it.

Table: Oxygen Requirements

Type	Oxygen Use	Example
Obligate Aerobe	Requires O2	Mycobacterium tuberculosis
Obligate Anaerobe	Cannot tolerate O2	Clostridium botulinum
Facultative Anaerobe	Uses O2 or not	Escherichia coli
Aerotolerant Anaerobe	Tolerates O2	Lactobacillus

Nitrogen Fixation

Nitrogen fixation is the reduction of nitrogen gas (N2) to ammonia (NH4+) by certain bacteria, making nitrogen available for biological use. This process is essential for life on earth.

Example: Rhizobium species fix nitrogen in plant roots.

Other Chemical Requirements

Phosphorus: Needed for membranes, nucleic acids, ATP.
Sulfur: Needed for amino acids, vitamins, protein structure.
Trace elements: Required in small amounts (e.g., iron, zinc).
Growth factors: Organic molecules some microbes cannot synthesize.

Physical Requirements for Growth

Temperature

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

Psychrophiles: Grow best at low temperatures (0-20°C).
Mesophiles: Grow best at moderate temperatures (20-40°C); includes human pathogens.
Thermophiles: Grow best at high temperatures (40-70°C).
Hyperthermophiles: Grow best at very high temperatures (70-110°C).

Table: Microbial Temperature Preferences

Type	Optimal Temperature Range	Example
Psychrophile	0-20°C	Psychrobacter
Mesophile	20-40°C	Escherichia coli
Thermophile	40-70°C	Thermus aquaticus
Hyperthermophile	70-110°C	Pyrococcus furiosus

pH

Microbes are sensitive to pH, which affects protein structure and DNA stability.

Neutrophiles: Grow best at neutral pH (6-7).
Acidophiles: Grow best in acidic environments (pH 1-6); e.g., Helicobacter pylori.
Alkaliphiles: Grow best in alkaline environments (pH 8-14).

Water and Osmotic Pressure

Water is essential for dissolving nutrients and metabolic reactions. Osmotic pressure affects cell survival:

Hypotonic solutions: Cells swell; cell wall limits swelling in bacteria.
Hypertonic solutions: Cells shrink (plasmolysis); growth inhibited or cell death.
Obligate halophiles: Require high salt concentrations.
Endospores and cysts: Can survive dry environments for years.

Microbial Associations and Biofilms

Microbial Relationships

Antagonistic: One organism harms another.
Synergistic: Both benefit, but not required for survival (e.g., Corynebacterium in the eye).
Symbiotic: Both benefit and are dependent (e.g., E. coli making vitamins in the gut).

Biofilms

Biofilms are complex aggregations of microorganisms on surfaces, formed via quorum sensing. They consist of multiple species and are embedded in an extracellular polymeric matrix (glycocalyx).

Form on most surfaces with moisture (e.g., medical devices, mucous membranes, environmental surfaces).
Biofilm formation involves secretion of extracellular matrix, adherence, nutrient sequestration, and protection.
Cells in biofilms exhibit altered gene expression and can share resistance factors.
Biofilm-associated infections are difficult to treat due to protection by matrix and continuous release of free-living cells.

Table: Biofilm Locations and Importance

Location	Importance
Medical devices	Persistent infections, difficult to treat
Mucous membranes	Can cause chronic infections
Environmental surfaces	Wastewater treatment, nutrient cycling

Quorum sensing inhibitors and inhibitory chemicals are being researched to prevent biofilm formation.

Key Definitions

Chemoheterotroph: Organism that uses organic compounds for both energy and carbon.
Photoautotroph: Organism that uses light for energy and CO2 for carbon.
Mesophile: Microbe that grows best at moderate temperatures.
Psychrophile: Microbe that grows best at low temperatures.
Thermophile: Microbe that grows best at high temperatures.
Hyperthermophile: Microbe that grows best at very high temperatures.
Growth curve: Graph showing population changes over time.
Lag phase: Adjustment period before rapid growth.
Logarithmic phase: Period of rapid population increase.
Stationary phase: Population size remains constant.
Biofilm: Aggregation of microorganisms on a surface, embedded in a matrix.
Anaerobe: Organism that cannot tolerate oxygen.
Aerobe: Organism that requires oxygen.
Facultative anaerobe: Organism that can grow with or without oxygen.
Catalase: Enzyme that breaks down hydrogen peroxide into water and oxygen.