Microbial Nutrition, Growth, and Biofilms: Study Notes for MIC 205

Microbial Nutrition and Growth

Microbial Growth Patterns

Microbial growth refers to an increase in cell number, not cell size. Microbes can grow in various patterns depending on their environment and mode of reproduction.

• Discrete colony: Aggregation of cells visible on the surface of solid media.

• Dispersed cells: Single cells suspended in liquid media.

• Complex biofilm: A community of one or more species of bacteria attached to a surface.

Growth of Microbial Populations

Microbial populations grow rapidly, often following a logarithmic (exponential) pattern. This allows populations to reach millions of cells in a few hours.

• Arithmetic growth: Increase by a constant number.

• Logarithmic growth: Increase by a constant factor (doubling).

Clinical Sampling

Clinical specimens are human materials examined for the presence of microbes. Proper collection, technique, and timely delivery are essential for accurate diagnosis.

• Specimen types: Skin, blood, cerebrospinal fluid, stomach, urine, lungs, diseased tissue.

• Collection methods: Sterile swabs, needle aspiration, catheterization, surgical removal.

Type or Location of Specimen	Collection Method
Skin, accessible membrane	Sterile swab brushed across surface
Blood	Needle aspiration from vein
Cerebrospinal fluid	Needle aspiration from subarachnoid space
Stomach	Intubation, washing, or swabbing
Urine	Catheterization or clean catch
Lungs	Collection via coughing or catheter
Diseased tissue	Surgical removal (biopsy)

Culture Media

Culture media are used to grow microorganisms in the laboratory. Most microbes have never been grown in culture, but various media types exist for different purposes.

• Defined (synthetic) media: Exact chemical composition is known.

• Complex media: Contains nutrients from extracts; composition is not fully known.

• Selective media: Favors growth of specific microbes by inhibiting others.

• Differential media: Distinguishes between microbes based on metabolic reactions.

• Anaerobic media: Supports growth of anaerobes.

• Transport media: Maintains viability during transport.

Ingredient	Amount (g/L)
Glucose	1.0
Na2HPO4	16.4
KH2PO4	1.5
(NH4)2SO4	2.0
MgSO4·7H2O	0.2
CaCl2	0.01
FeSO4·7H2O	0.005
Water	To 1 L

Selective and Differential Media

Selective media inhibit unwanted microbes, while differential media reveal differences in microbial metabolism.

• Selective media example: Dextrose agar selects fungi by inhibiting bacteria.

• Differential media example: Blood agar distinguishes hemolytic activity of bacteria.

Phases of Microbial Growth

Microbial populations undergo distinct growth phases: lag, log (exponential), stationary, and death.

• Lag phase: Cells adjust to environment; no growth.

• Log phase: Rapid, exponential growth.

• Stationary phase: Growth rate slows; nutrients deplete.

• Death phase: Cells die due to lack of nutrients or toxic byproducts.

Microbial Nutrition

Nutritional and Physical Requirements

Microbes are classified by their nutritional and physical requirements, which determine their growth and survival.

• Energy sources: Phototrophs (light), Chemotrophs (chemical compounds).

• Carbon sources: Autotrophs (CO2), Heterotrophs (organic compounds).

• Physical factors: Temperature, pH, osmotic pressure, presence of other microbes.

Oxygen Requirements

Microbes vary in their need for oxygen and their ability to detoxify its toxic forms.

• Aerobes: Require oxygen; can detoxify toxic forms.

• Anaerobes: Cannot tolerate oxygen; lack detoxifying enzymes.

• Facultative anaerobes: Can grow with or without oxygen; prefer oxygen.

• Aerotolerant anaerobes: Can grow with or without oxygen; do not use oxygen.

• Microaerophiles: Require low levels of oxygen.

Toxic Forms of Oxygen

Toxic forms of oxygen are strong oxidizing agents that damage cellular components. These include singlet oxygen, superoxide radicals, peroxide anion, and hydroxyl radicals.

• Singlet oxygen (1O2): High-energy form generated during metabolism.

• Superoxide radicals (O2-): Formed during incomplete reduction of oxygen.

• Peroxide anion (O22-) and H2O2: Byproducts of lipid breakdown and SOD reactions.

• Hydroxyl radical (OH): Most reactive; formed by ionizing radiation.

Detoxifying Enzymes

Microbes use enzymes to neutralize toxic oxygen species.

• Superoxide dismutase (SOD): Converts superoxide to hydrogen peroxide.

• Catalase and Peroxidase: Convert hydrogen peroxide to water and oxygen.

Oxygen Classifications

Microbes are classified based on their oxygen requirements and ability to detoxify toxic forms.

• Aerobes: Use oxygen as final electron acceptor; detoxify toxic forms.

• Anaerobes: Use fermentation or other electron acceptors; cannot detoxify.

• Facultative anaerobes: Can use aerobic or anaerobic metabolism; detoxify.

• Aerotolerant anaerobes: Use fermentation; possess some detoxifying enzymes.

• Microaerophiles: Require low oxygen; limited detoxification.

Nitrogen Requirements

Nitrogen is essential for protein and nucleotide synthesis. Some bacteria fix atmospheric nitrogen into ammonia, making it available for other organisms.

• Nitrogen fixation: Conversion of N2 gas to NH3 by bacteria such as Azospirillum and Rhizobium.

• Sources: Organic and inorganic compounds, recycling of amino acids and nucleotides.

Other Chemical Requirements

Microbes require phosphorus, sulfur, trace elements, and growth factors for cellular functions.

• Phosphorus: Needed for membranes, DNA, RNA, ATP.

• Sulfur: Component of amino acids and vitamins.

• Trace elements: Minerals and ions found in water.

• Growth factors: Organic chemicals not synthesized by some organisms (e.g., vitamins, amino acids).

Effects of Temperature on Growth

Microbes grow best at specific temperatures, which affect protein and membrane function. They are classified by their temperature preferences.

• Psychrophiles: 5–20°C

• Mesophiles: 15–45°C

• Thermophiles: 40–80°C

• Extreme thermophiles: 80–121°C

Hydrogen Ion Concentration (pH)

Most microbes are sensitive to pH changes, which affect protein and nucleic acid structure.

• Neutrophiles: Grow best at pH 6.5–7.5.

• Acidophiles: Prefer acidic environments.

• Alkalinophiles: Prefer alkaline environments up to pH 11.5.

Physical Effects of Water

Water is essential for microbial metabolism. Some microbes tolerate extreme environments.

• Obligate halophiles: Grow in up to 30% salt.

• Facultative halophiles: Tolerate high salt concentrations.

• Barophiles: Live under extreme pressure.

Biofilms: Structure and Significance

Characteristics of Biofilms

Biofilms are complex microbial communities attached to surfaces. They exhibit social structure, resistance to antimicrobials, and reduced metabolism.

• Attachment: On abiotic surfaces, host tissues, or liquid-air interfaces.

• Protection: Biofilm structure protects microbes and allows growth.

• Resistance: Biofilm-associated bacteria are up to 1000x more resistant to antibiotics.

• Molecular filter: Reduces drug activity.

Common Biofilms

Biofilms are found in diverse environments, including natural and artificial surfaces.

• Examples: Dental plaque, river stones, flower vases, legume root nodules, sewage bioreactors, water pipes, dental units, contact lens cases.

Biofilms and Infection

Biofilms are implicated in many infections, especially those associated with medical devices.

• Examples: Pacemakers (Staphylococcus aureus), cystic fibrosis pneumonia (Pseudomonas aeruginosa), urinary catheters (Escherichia coli), orthopedic devices (Staphylococcus epidermidis), contact lenses (various bacteria).

• Prevalence: 80% of bacterial infections involve biofilms.

Biofilm Heterogeneity

Biofilms contain microhabitats with gradients of oxygen, pH, and growth factors, leading to varied metabolic activity.

• Microhabitats: Different regions support different metabolic processes.

• Gradients: Oxygen and pH vary within the biofilm.

Model of Biofilm Development

Biofilm formation involves several steps: initial interaction, stable adhesion, microcolony formation, exopolysaccharide synthesis, and maturation.

• Gene regulation: Attached cells change gene expression, increase capsule production, and alter metabolism.

• Quorum sensing: Cell-to-cell signaling regulates biofilm development.

Quorum Sensing

Quorum sensing is a mechanism by which bacteria monitor population density and regulate gene expression using signaling molecules called autoinducers.

• Autoinducers: Unique to each species; regulate specific genes.

• Example: Switching between flagella and capsule genes for biofilm formation.

Biofilm Prevention

Biofilm formation can be inhibited by blocking quorum sensing. Furanones produced by red algae block homoserine lactone signaling molecules.

• Furanones: Natural compounds that prevent biofilm development.