Module 4: Microbial Nutrition, Growth, and Ecology

Principles of Microbial Nutrition

• Nutrition is defined as the process by which organisms obtain nutrients.

• Nutrients are chemical substances required by an organism for growth.

• Nutrients for microorganisms are categorized into different groups based on requirement levels and origin:

  • Macronutrients: Nutrients required in large amounts by the cell.

  • Micronutrients: Nutrients required in small amounts, often referred to as trace elements.

  • Essential Nutrients: Substances that an organism cannot produce on its own and must be supplied in its diet. These can be organic or inorganic.

  • Growth Factors: Specific organic substances that an organism requires because they cannot synthesize them through their metabolism. These typically include amino acids or vitamins.

Microbial Growth and Reproduction

• Generation Time: Also known as doubling time, this is the amount of time it takes for a whole population to double in size. This metric is used because individual cell division is difficult to observe directly.

• Binary Fission: The primary method of bacterial reproduction.

  • It is a form of asexual reproduction.

  • The resulting daughter cells are genetically identical to the mother cell.

  • Bacteria do not possess the machinery for mitosis.

  • Process of Binary Fission:

    1. The cell undergoes elongation.

    2. The chromosome replicates to create a copy.

    3. The cell undergoes a process of pinching, dividing into two identical daughter cells.

The Bacterial Growth Curve in a Closed System

• A closed system is an environment where no additional nutrients are added and no waste products are removed (e.g., a test tube inoculated with a loopful of bacteria).

• The growth curve plots the bacterial population over time and consists of four distinct phases:

  1. Lag Phase: A period following initial inoculation where there is no increase in population. Bacteria are adjusting to the new environment. The more the new environment differs from the previous one, the longer the lag time. During this phase, bacteria turn on appropriate genes for enzymes.

  2. Logarithmic (Log) Phase: A period of rapid growth and division occurring under optimum conditions. There are high levels of available nutrients and minimal waste products.

  3. Stationary Phase: The population levels off. At this stage, the death rate of the population is equal to the division rate. This occurs as nutrients begin to deplete and waste products build up, making the environment toxic.

  4. Death (Decline) Phase: Nutrients are at a minimum or entirely used up, while waste products are at a maximum. High acid content or other toxic metabolic byproducts lead to the death of the organisms.

Biofilms and Microbial Communities

• Definition: A biofilm is a complex ecosystem and highly structured microbial community that forms as a slime layer on the surface of objects.

• Formation Process:

  1. Planktonic Cells: Free-swimming cells in a solution.

  2. Attachment: Planktonic cells attach to a surface.

  3. Sessile State: Once attached, the cells become sessile and begin to divide and colonize the surface.

  4. EPS Release: Cells begin to release extracellular polymeric substance (EPS), an extracellular material that builds the matrix of the biofilm.

  5. Maturation: Other organisms may attach to the existing structure, creating a thick matrix with embedded cells.

• Characteristics and Impacts:

  • Biofilms adhere very strongly and are difficult to remove physically.

  • Dental Plaque: Formed by Streptococcus bacteria releasing mutants that create a slime layer on teeth.

  • Pet Water Dishes: A slime layer often forms on these dishes, requiring vigorous scrubbing to remove.

  • Medical Complications: Biofilms can form inside long-term catheters, potentially leading to persistent infections.

Physical Factors Affecting Microbial Growth

Temperature Classifications

• Psychrophiles: Cold-loving organisms that live in very cold temperatures. Their optimum temperature range is between $0^{\circ}C$ and $20^{\circ}C$, with $15^{\circ}C$ being the norm. They are found in environments like the Antarctic and cold oceans.

• Psychrotrophs: Organisms that prefer temperatures between $4^{\circ}C$ and $25^{\circ}C$. They are responsible for food spoilage in refrigerators.

• Mesophiles: Middle-loving organisms with an optimum range between $20^{\circ}C$ and $45^{\circ}C$. This includes room temperature and human body temperature. Most human pathogens fall into this category.

• Thermophiles: Heat-loving organisms that prefer temperatures from $50^{\circ}C$ to $70^{\circ}C$. They are found in hot springs and steam vents, such as those in Iceland. These are often Archaea, which are more primitive.

• Extreme Thermophiles: Organisms that grow at even hotter temperatures beyond standard thermophiles.

Oxygen Requirements

• Aerobic Metabolism Toxicity: Aerobic growth can produce toxic free radicals and chemicals. Organisms must have specific enzymes to neutralize them:

  • Superoxide Dismutase: Breaks down the superoxide ion ($O_2^-$ anion) into hydrogen peroxide ($H_2O_2$).

  • Catalase: Breaks down toxic hydrogen peroxide into oxygen and water ($H_2O$).

• Oxygen Requirement Classifications:

  • Obligate (Strict) Aerobe: Must have oxygen to survive.

  • Obligate (Strict) Anaerobe: Cannot live with oxygen; it is lethal to them.

  • Facultative Anaerobe: Can live without oxygen but grows better when oxygen is present.

  • Aerotolerant: Does not use oxygen for growth but is not harmed by it; grows equally well with or without it.

  • Microaerophile: Requires small amounts of oxygen for growth.

  • Capnophile: Requires a low oxygen environment with a high carbon dioxide ($CO_2$) content.

• Thioglycolate Broth Lab Test: Used to determine oxygen requirements by stabbing a needle into the medium and observing growth after 48 hours.

  • Pink area: Oxygen is available.

  • Yellow area: No oxygen present.

  • Growth Patterns:

    • Growth only at the top: Obligate aerobe.

    • Growth only at the bottom: Obligate anaerobe.

    • Growth throughout, but heavier at the top: Facultative anaerobe.

    • Growth in a thin band just below the pink area: Microaerophile.

    • Uniform growth throughout: Aerotolerant.

pH Requirements

• Acidophiles: Prefer a pH below $5.5$. Example: Algae thriving in the acidic Lemonade Spring hot springs in Yellowstone.

• Alkalophiles: Prefer a high pH between $8$ and $10.5$. Example: Red Archaea in alkaline salt lakes (pH $10$). Certain alkalophiles can grow in hand soaps, which is a safety concern for wound care.

• Neutrophiles: Prefer a pH within one point of neutral ($6$ to $8$). Most human pathogens are neutrophiles.

Osmotic and Barometric Pressure

• Osmosis: Diffusion of water from high water concentration to low water concentration. Bacterial cells usually have higher internal solute concentrations than their environment, creating pressure against the cell wall.

• Osmophiles: Thrive in high-solute environments.

  • Halophiles: Specifically love high salt environments, such as the Great Salt Lake in Utah or the ocean.

• Light: Photosynthetic organisms require specific light intensities to capture sunlight energy to build sugar molecule bonds.

• Barometric Pressure:

  • Barotolerant: Can survive increased atmospheric pressure.

  • Barophiles: Require high pressure to grow, such as organisms living at the bottom of the ocean floor near steam vents. They are difficult to culture in a lab.

Microbial Ecology and Symbiosis

• Definitions:

  • Population: A group of organisms of the same species.

  • Community: Multiple populations of different species living together and interacting.

  • Symbiosis: The interaction between different species in an environment.

• Types of Symbiotic Relationships:

  • Commensalism: One species benefits while the other is unaffected (not harmed, not helped).

  • Amensalism: One species harms another while remaining unaffected itself (e.g., bacteria producing a bactericide that kills neighbors without providing a direct benefit to the producer).

  • Mutualism: Both species benefit from the interaction. Example: Lichen, a combination of fungus and algae (or bacteria). The fungus provides shelter, and the algae provides sugar via photosynthesis.

  • Neutralism: Species coexist without any positive or negative effect on each other.

  • Parasitism: One species benefits at the expense of another (the host is harmed).

Classification by Carbon and Energy Source

• Carbon Source:

  • Autotrophs: Obtain carbon from inorganic carbon dioxide ($CO_2$).

  • Heterotrophs: Obtain carbon from organic compounds (e.g., sugar).

• Combined Classifications:

  • Photoautotrophs: Derive energy from light and carbon from $CO_2$; these are photosynthetic.

  • Chemoautotrophs: Derive energy from inorganic compounds (e.g., sulfur dioxide, nitrates) and carbon from $CO_2$.

  • Chemoheterotrophs: Derive both energy and carbon from organic compounds. This includes humans and many microorganisms.

    • Saprobes: A type of chemoheterotroph that feeds on dead or decaying matter (e.g., fungi breaking down leaf litter).

    • Parasites: A type of chemoheterotroph that obtains organic compounds from a living host.

  • Photoheterotrophs: Derive energy from sunlight but obtain carbon from organic compounds.