Microbial Nutrition and Growth: Study Guide

Microbial Nutrition

Essential Nutrients

Microbes require specific nutrients to survive, grow, and reproduce. These nutrients are classified based on their quantity and function:

• Macronutrients: Needed in large amounts; essential for cell structure and metabolism. Examples: carbon, hydrogen, oxygen.

• Micronutrients (Trace Elements): Needed in small amounts; important for enzyme function and protein structure. Examples: manganese, zinc, nickel.

Categorizing Nutrients by Carbon Content

• Inorganic nutrients: Do not contain both carbon and hydrogen. Examples: water, salts, gases.

• Organic nutrients: Contain both carbon and hydrogen; often products of living things. Examples: carbohydrates, proteins, lipids, nucleic acids.

Chemical Composition of Microbial Cytoplasm

• Water constitutes about 70% of cell components.

• Organic compounds make up 97% of dry cell weight.

• Elements CHONPS (carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur) account for 96% of dry cell weight.

Nutritional Categories of Microbes

• Heterotroph: Obtains carbon from organic sources.

• Autotroph: Uses inorganic CO2 as carbon source; can convert CO2 into organic compounds.

• Phototroph: Uses light as energy source.

• Chemotroph: Uses chemical compounds for energy.

Autotrophs and Heterotrophs

• Photoautotrophs: Photosynthetic; produce organic molecules using CO2.

• Chemoautotrophs: Use inorganic minerals for energy and carbon.

• Chemoheterotrophs: Derive both carbon and energy from organic compounds; includes saprobes (decomposers) and parasites (pathogens).

Other Important Nutrients

• Potassium (K): Protein synthesis and membrane function.

• Sodium (Na): Cell transport.

• Calcium (Ca): Stabilizes cell wall and endospores.

• Magnesium (Mg): Chlorophyll component, stabilizes membranes and ribosomes.

• Iron (Fe): Cytochrome proteins in respiration.

• Zinc (Zn): Regulatory element for eukaryotic genetics.

Microbial Transport Mechanisms

Diffusion and Osmosis

Microbes transport nutrients across membranes using passive and active mechanisms:

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water through a selectively permeable membrane.

Effects of Osmotic Conditions:

• Isotonic: Equal water concentration inside and outside; no net movement.

• Hypotonic: Water enters cell; cell may swell.

• Hypertonic: Water leaves cell; cell shrinks (plasmolysis).

Passive and Active Transport

• Passive Transport: Includes simple diffusion and facilitated diffusion (uses protein channels).

• Active Transport: Requires energy (ATP) and membrane proteins; moves substances against concentration gradient.

Endocytosis

• Phagocytosis: Ingestion of whole cells or large particles.

• Pinocytosis: Ingestion of liquids or molecules in solution.

Microbial Growth and Environmental Factors

Temperature Requirements

Microbes are classified by their optimal growth temperatures:

• Psychrophiles: Optimum below 15°C; grow at 0°C.

• Psychrotrophs: Optimum 15–30°C; can grow at refrigerator temperatures.

• Mesophiles: Optimum 20–40°C; most human pathogens.

• Thermoduric: Survive high temperatures briefly; usually mesophiles.

• Thermophiles: Optimum above 45°C; extreme thermophiles up to 121°C.

Oxygen Requirements

Microbes differ in their need and tolerance for oxygen:

• Aerobes: Require oxygen; possess enzymes to detoxify oxygen by-products.

• Obligate Aerobes: Cannot grow without oxygen.

• Microaerophiles: Require small amounts of oxygen; harmed by atmospheric levels.

• Facultative Anaerobes: Use oxygen if present, but can grow without it.

• Anaerobes: Cannot use or detoxify oxygen; die in its presence.

• Aerotolerant Anaerobes: Do not use oxygen but can survive in its presence.

Other Environmental Factors

• pH: Most microbes grow between pH 6 and 8; acidophiles and alkalinophiles thrive at extremes.

• Osmotic Pressure: Halophiles require high salt; osmophiles tolerate high solute concentrations.

• Radiation: Phototrophs use light; some microbes produce pigments to protect against light damage.

• Pressure: Barophiles live under high pressure; rupture at normal atmospheric pressure.

Microbial Associations and Biofilms

Types of Microbial Associations

• Mutualism: Both organisms benefit.

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits, the other is harmed.

• Antagonism: Competition; production of inhibitory compounds.

• Synergism: Cooperative interaction; not obligatory.

Biofilms

Biofilms are complex communities of microbes attached to surfaces. They form through a series of steps:

• Pioneer bacteria colonize a surface.

• Secrete extracellular material for attachment.

• Other species join and contribute to the matrix.

• Biofilms release bacteria to become free-living.

• Quorum sensing: Communication between cells to monitor population size.

Bacterial Growth and Population Dynamics

Binary Fission

Bacteria reproduce by binary fission, a process involving:

• Cell enlargement

• Chromosome duplication

• Formation of a septum

• Division into two daughter cells

Population Growth

Bacterial populations grow exponentially under favorable conditions. The generation time (doubling time) is the time required for one cell to divide into two.

• Average generation time: 30–60 minutes

• Equation for population size: $N_t = N_0 \times 2^n$ where $N_t$ is the total number of cells at time t, $N_0$ is the starting number, and $n$ is the number of generations.

Bacterial Growth Curve

Growth in a closed system follows four phases:

• Lag phase: Adjustment, no rapid growth.

• Exponential (log) phase: Rapid, geometric increase.

• Stationary phase: Birth and death rates equal; nutrients depleted.

• Death phase: Cells die at an exponential rate; some remain viable but nonculturable.

Measuring Bacterial Growth

• Viable Plate Count: Dilute and plate samples; count colonies to estimate population size.

• Turbidity: Cloudiness of a culture indicates population size; measured with a spectrophotometer.

• Direct Microscopic Count: Count cells on a grid under a microscope.

• Coulter Counter: Electronic device counts cells as they pass through a detector.

• Flow Cytometer: Measures cell size and differentiates live/dead cells.

• Genetic Probing: Uses PCR to quantify bacteria in samples.

Summary Table: Essential Nutrients

Element	Source	Function
Carbon	CO2, organic compounds	Cell structure, metabolism
Hydrogen	H2O, organic compounds	Maintains pH, forms hydrogen bonds
Oxygen	O2, H2O	Metabolism, cell structure
Nitrogen	N2, NO3-, NH3	Amino acids, nucleic acids
Phosphorus	PO43-, H3PO4	Nucleic acids, ATP
Sulfur	SO42-, H2S	Amino acids, vitamins
Potassium	K+	Protein synthesis, membrane function
Sodium	Na+	Cell transport
Calcium	Ca2+	Cell wall, endospore stability
Magnesium	Mg2+	Chlorophyll, membrane stability
Iron	Fe2+, Fe3+	Respiration
Zinc	Zn2+	Enzyme regulation

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