Microbial Growth and Nutrition: Study Notes

Microbial Growth and Nutrition

Learning Goals

• List and describe several factors that influence the growth and survival of bacteria.

• Describe the process of bacterial reproduction.

• Determine the generation (doubling time) of a population of bacteria.

• Describe the process of biofilm formation.

Microbial Growth: Nutrients

Essential Nutrients for Microbial Life

• Nutrients are substances required by organisms for energy, growth, and cellular structure.

• Common essential elements: carbon, oxygen, nitrogen, and hydrogen.

• Microbes obtain nutrients from a variety of sources, both organic and inorganic.

Sources of Carbon, Energy, and Electrons

• Organisms are classified by their carbon and energy sources:

  • Autotrophs: Use carbon dioxide (CO2) as their carbon source.

  • Heterotrophs: Use organic compounds as their carbon source.

  • Phototrophs: Use light as their energy source.

  • Chemotrophs: Use chemical compounds as their energy source.

Carbon Source	Energy Source	Example/Description
CO2 (auto-)	Light (photo-)	Photoautotrophs: Plants, algae, cyanobacteria (produce O2); green and purple sulfur bacteria (do not produce O2).
CO2 (auto-)	Chemical compounds (chemo-)	Chemoautotrophs: Hydrogen, sulfur, and nitrifying bacteria; some archaea.
Organic compounds (hetero-)	Light (photo-)	Photoheterotrophs: Green and purple nonsulfur bacteria, some archaea.
Organic compounds (hetero-)	Chemical compounds (chemo-)	Chemoheterotrophs: Most animals, fungi, protozoa, many bacteria.

Oxygen and Microbial Growth

Role of Oxygen

• Oxygen can support or hinder bacterial growth depending on the organism's requirements.

• Aerobic bacteria: Require oxygen for life processes (e.g., obligate aerobes, microaerophiles).

• Anaerobic bacteria: Oxygen is toxic (e.g., obligate anaerobes, aerotolerant anaerobes).

• Facultative anaerobes: Can grow with or without oxygen.

Toxic Forms of Oxygen

• Oxygen can form highly reactive species that damage cells:

  • Singlet oxygen (1O2): Highly reactive, strips electrons from molecules.

  • Superoxide (O2-): Combated by superoxide dismutase (SOD): $O_2^- + O_2^- + 2H^+ \rightarrow H_2O_2 + O_2$

  • Peroxide anion (O22-): Combated by catalase or peroxidase: $2H_2O_2 \rightarrow 2H_2O + O_2$

Oxygen Requirements Table

Type	Effect of Oxygen	Growth Pattern	Explanation
Obligate Aerobes	Only aerobic growth; oxygen required	Growth at top of tube	Presence of enzymes (SOD, catalase) allows toxic forms of oxygen to be neutralized
Facultative Anaerobes	Aerobic and anaerobic growth; greater growth in presence of oxygen	Growth throughout tube, more at top	Presence of enzymes (SOD, catalase); can use oxygen or not
Obligate Anaerobes	Only anaerobic growth; ceases in presence of oxygen	Growth at bottom of tube	Lacks enzymes to neutralize toxic oxygen
Aerotolerant Anaerobes	Only anaerobic growth; but continues in presence of oxygen	Growth evenly throughout tube	Presence of SOD allows partial tolerance to oxygen
Microaerophiles	Only aerobic growth; oxygen required in low concentration	Growth in middle of tube	Produce less SOD and catalase; sensitive to high oxygen

Examples of Oxygen Requirements in Human Body Sites

• Lungs: Mycobacterium tuberculosis (obligate aerobe)

• Blood/Lymph: Borrelia burgdorferi (microaerophile)

• Skin: Staphylococcus aureus (facultative anaerobe)

• Stomach: Helicobacter pylori (microaerophile)

• Large Intestine: Clostridium difficile (obligate anaerobe)

Nitrogen and Other Chemical Requirements

Nitrogen Requirements

• Anabolism ceases without sufficient nitrogen.

• Nitrogen is acquired from both organic and inorganic sources.

• All cells recycle nitrogen for amino acids and nucleotides.

• Nitrogen fixation by certain bacteria is essential for life on Earth.

Other Chemical Requirements

• Phosphorus and sulfur are required for nucleic acids, ATP, and proteins.

• Trace elements: Required in small amounts (e.g., iron, copper, zinc).

• Growth factors: Organic chemicals that cannot be synthesized by certain organisms (e.g., vitamins, amino acids).

Growth Factor	Function
Amino acids	Components of proteins
Heme	Functional portion of cytochromes in electron transport
NADH	Electron carrier
Vitamins	Coenzymes for metabolism

Physical Factors Affecting Microbial Growth

Temperature

• Temperature affects protein structure and membrane fluidity.

• Each bacterium has an optimum temperature for growth, but can survive within a range.

• If too low, membranes become rigid; if too high, membranes become too fluid.

• Psychrophiles: Grow best at cold temperatures (0–20°C).

• Psychrotrophs: Grow at 0–30°C, can spoil refrigerated food.

• Mesophiles: Grow best at moderate temperatures (20–45°C); most human pathogens.

• Thermophiles: Grow best at high temperatures (45–80°C).

• Hyperthermophiles: Grow at very high temperatures (80–120°C).

Example: Listeria monocytogenes

• Can grow at refrigeration temperatures; found in animal meat and milk.

• Refrigeration does not control its growth, making it a food safety concern.

pH and Salt Concentration

• Microbes have optimal pH ranges for growth; extremes can inhibit growth.

• Halophiles: Grow optimally at high salt concentrations.

• Salt (halo)tolerant: Grow best without salt, but can tolerate low concentrations.

• High salt (hypertonic) environments cause water to leave the cell, leading to plasmolysis.

• Extreme halophiles resist plasmolysis by maintaining high internal solute concentrations.

Microbial Growth: Reproduction and Population Growth

Bacterial Reproduction: Binary Fission

• Bacteria reproduce asexually by binary fission.

• Process:

  1. Cell replicates its DNA.

  2. Cytoplasmic membrane elongates, separating DNA molecules.

  3. Cross wall forms; membrane invaginates.

  4. Cross wall forms completely.

  5. Daughter cells may separate.

• Growth results in approximately doubling of cell size and number.

Population Growth and Generation Time

• Generation time: The time required for a bacterial population to double in number.

• Bacterial growth is typically exponential (logarithmic): $N_t = N_0 \times 2^n$ Where $N_t$ = number of cells at time t, $N_0$ = initial number of cells, $n$ = number of generations.

• Example: Starting with 100 bacteria, replicating every 30 minutes, after 8 hours: $n = \frac{8 \text{ hours} \times 60}{30} = 16$ generations $N_t = 100 \times 2^{16} = 6,553,600$

Phases of Bacterial Population Growth

• Lag phase: Adaptation, little to no cell division.

• Log (exponential) phase: Rapid cell division and population growth.

• Stationary phase: Growth rate slows, nutrients deplete, waste accumulates.

• Decline (death) phase: Cells die faster than they divide.

Biofilms and Dormancy

Biofilm Formation

• Biofilms are complex communities of microorganisms attached to surfaces and embedded in a self-produced matrix.

• Stages of biofilm development:

  1. Free-swimming microbes attach to a surface.

  2. Cells produce extracellular matrix and become sessile.

  3. Quorum sensing signals coordinate behavior and shape.

  4. Water channels form, allowing nutrient and waste exchange.

  5. Some cells disperse to colonize new sites.

• Biofilms are medically significant due to their resistance to antibiotics and immune responses.

Persister Cells and Endospore Formation

• Persister cells: Metabolically inactive cells that survive stress (e.g., antibiotics) and can revive when conditions improve.

• Endospores: Highly resistant, dormant structures formed by Bacillus and Clostridium species in response to nutrient limitation.

• Endospores can survive extreme heat, desiccation, and chemicals; germinate when conditions become favorable.

• Medically important diseases caused by endospores: Anthrax, tetanus, botulism, antibiotic-resistant colitis.