Microbiology Chapter 06: Microbial Nutrition and Growth

Microbial Nutrition and Essential Nutrients

Essential Nutrient: Refers to any substance that must be provided to an organism to sustain life.
Macronutrients: These are required by the cell in relatively large quantities. They play principal roles in cell structure and metabolic processes.
- Key examples include Carbon, Hydrogen, and Oxygen.
Micronutrients (Trace Elements): These are required in much smaller amounts compared to macronutrients.
- They are primarily involved in enzyme function and the maintenance of protein structure.
- Examples include Manganese ( $Mn$ ), Zinc ( $Zn$ ), and Nickel ( $Ni$ ).
Inorganic Nutrients: Molecules or atoms that contain a combination of atoms other than carbon and hydrogen.
- These are typically found in the Earth's crust, bodies of water, and the atmosphere.
Organic Nutrients: These contain both carbon and hydrogen atoms and are the products of living things.
- They range from simple molecules like methane ( $CH_4$ ) to large polymers such as carbohydrates, lipids, proteins, and nucleic acids.

Categorization by Carbon and Energy Sources

Carbon Sources:
- Heterotroph: An organism that must obtain its carbon in an organic form, usually by consuming other organisms.
- Autotroph: An organism that uses inorganic $CO_2$ as its carbon source.
  - Autotrophs have the capacity to convert $CO_2$ into organic compounds.
  - They are not nutritionally dependent on other living things.
Energy Sources:
- Phototroph: Microbes that use photosynthesis to gain energy from light.
- Chemotroph: Microbes that gain energy from chemical compounds.

Specific Nutritional Categories

Photoautotrophs: These are photosynthetic organisms. They produce organic molecules using $CO_2$ ; these molecules can be used by the photoautotrophs themselves and by heterotrophs.
Chemoautotrophs:
- Chemoorganic Autotrophs: Utilize organic compounds for energy and inorganic compounds as their carbon source.
- Lithoautotrophs: Rely entirely on inorganic minerals for energy and carbon; they require neither sunlight nor organic nutrients.
Chemoheterotrophs: Derive both their carbon and energy from organic compounds.
- These molecules are processed through metabolic pathways such as respiration or fermentation.
- Saprobes: Free-living organisms that feed on organic detritus from dead organisms. They act as decomposers of plant litter, animal matter, and dead microbes, effectively recycling organic nutrients.
- Parasites: Derive nutrients from the cells or tissues of a living host.
  - Pathogens: Parasites that cause tissue damage or death.
  - Ectoparasites: Live on the surface of the host body.
  - Endoparasites: Live within the organs and tissues of the host.
  - Intracellular Parasites: Live specifically within host cells (e.g., the leprosy bacillus and the syphilis spirochete).
  - Obligate Parasites: These are unable to grow or multiply outside of a living host.

Transport Mechanisms in Microbes

All necessary nutrients must be transported across the cell membrane, even in organisms possessing a cell wall.
Diffusion: The movement of atoms or molecules in a gradient from an area of higher density/concentration to an area of lower density/concentration. This is driven by inherent atomic and molecular movement.
Osmosis: The diffusion of water through a selectively (or differentially) permeable membrane.
- The membrane contains passageways allowing the free diffusion of water while blocking certain dissolved solutes.
- Water moves at a faster rate toward the side with less water (higher solute concentration) until the concentration is equalized on both sides.
Tonicity and its Effects:
- Isotonic: Water concentration is equal inside and outside the cell. Diffusion rates are equal in both directions.
- Hypotonic: The external environment has a lower solute concentration (more water) than the cell. Net diffusion of water is into the cell.
  - In cells with cell walls: The protoplast swells and pushes against the wall; the wall prevents bursting.
  - In cells without cell walls: The cell swells and may burst if water is not removed.
- Hypertonic: The external environment has a higher solute concentration (less water) than the cell. Water diffuses out of the cell.
  - In cells with cell walls: The cytoplasmic membrane shrinks away from the cell wall, a process known as plasmolysis.
  - In cells without cell walls: The cell shrinks and becomes distorted.
Endocytosis: A process where the cell encloses a substance in its membrane, forming a vacuole and engulfing it.
- Phagocytosis: Ingestion of whole cells or large solid matter (common in amoebas and white blood cells).
- Pinocytosis: Ingestion of liquids, such as oils or molecules in solution.

Environmental Factors: Temperature

Cardinal Temperatures: The specific range of temperatures permitting growth for a microbial species.
- Minimum Temperature: Lowest temperature for continued growth and metabolism; below this, activity is limited.
- Maximum Temperature: Highest temperature before growth stops and proteins denature.
- Optimum Temperature: The intermediate temperature that promotes the fastest rate of growth and metabolism.
Temperature Classification:
- Psychrophiles: Optimum temperature below $15^{\circ}C$ ; capable of growth at $0^{\circ}C$ . They cannot grow above $20^{\circ}C$ . Found in polar ice, deep oceans, and snowfields. Rarely pathogenic.
- Psychrotrophs: Optimum temperature between $15^{\circ}C$ and $30^{\circ}C$ . Can grow slowly in refrigerators. Examples include Staphylococcus aureus and Listeria monocytogenes, which cause food-borne illness.
- Mesophiles: Optimum temperature between $20^{\circ}C$ and $40^{\circ}C$ . Includes the majority of medically significant microbes. Human pathogens typically optimize between $30^{\circ}C$ and $40^{\circ}C$ .
- Thermoduric: Normally mesophiles that can survive short exposures to high temperatures. Examples include Giardia cysts and sporeformers like Bacillus and Clostridium.
- Thermophiles: Grow optimally above $45^{\circ}C$ . Range is typically $45^{\circ}C$ to $80^{\circ}C$ . Found in volcanic areas and compost piles.
- Extreme Thermophiles: Grow between $80^{\circ}C$ and $121^{\circ}C$ .

Environmental Factors: Gases

Oxygen ( $O_2$ ): Has the greatest impact on microbial growth. It is a powerful oxidizing agent.
Toxic Oxygen Products: Oxygen can be transformed into reactive products such as:
- Singlet oxygen ( $O$ ): Destroys cells by oxidizing membrane lipids.
- Superoxide ion ( $O_2^-$ ): Highly reactive.
- Hydrogen peroxide ( $H_2O_2$ ): Toxic; used as a disinfectant.
- Hydroxyl radicals ( $OH^-$ ): Highly reactive.
Detoxification Enzymes:
1. Superoxide dismutase: Converts superoxide ion into hydrogen peroxide.
2. Catalase: Converts hydrogen peroxide into water and oxygen.
Oxygen Usage Categories:
- Obligate Aerobes: Cannot grow without oxygen (e.g., Bacillus species, Mycobacterium tuberculosis).
- Microaerophiles: Require a small amount of oxygen but cannot grow at normal atmospheric levels (e.g., Helicobacter pylori).
- Facultative Anaerobes: Use oxygen if present but can grow without it via anaerobic metabolism (e.g., E. coli, Staphylococci).
- Obligate Anaerobes: Lack enzymes to process toxic oxygen and die in its presence (e.g., many oral and intestinal bacteria).
- Aerotolerant Anaerobes: Do not utilize oxygen but can survive/grow in its presence because they have alternate detox mechanisms (e.g., Lactobacilli, Streptococci).
Capnophiles: Organisms that grow best at higher $CO_2$ tensions than normally present. Important for isolating Neisseria (gonorrhea), Brucella, and Streptococcus pneumoniae.

Other Environmental Factors

pH: Most organisms grow between pH $6$ and $8$ .
- Acidophiles: Thrive in acidic environments. Euglena mutabilis (pH $0-1$ ), Thermoplasma (pH $1-2$ ), Picrophilus (can live at pH $0$ ).
- Alkalinophiles: Thrive in alkaline conditions. Natromonas (pH $12$ ), Proteus (neutralizes urine to infect the urinary system).
Osmotic Pressure:
- Osmophiles: Live in high solute concentrations.
- Halophiles: Prefer high salt.
  - Obligate Halophiles: Require at least $9\%$ $NaCl$ ; optimal at $25\%$ (e.g., Halobacterium).
  - Facultative Halophiles: Resistant to salt but don't require it (e.g., S. aureus grows on $0.1\%$ to $20\%$ $NaCl$ ).
Radiation: Phototrophs use light. Light can produce toxic oxygen; some microbes produce yellow carotenoid pigments to neutralize this. UV and ionizing radiation are used for microbial control.
Pressure: Barophiles exist under pressures over $1,000$ times atmospheric pressure and will rupture at normal pressure.

Microbial Associations and Symbioses

Symbiosis: Close partnership between two organisms.
- Mutualism: Obligatory and mutually beneficial.
- Commensalism: One partner (commensal) benefits; the other is unaffected.
- Parasitism: Host provides nutrients/habitat; parasite harms the host.
Non-Symbiotic Associations:
- Antagonism: Competition between free-living species. Antibiosis occurs when one microbe releases inhibitory chemicals (antibiotics) to destroy competitors.
- Synergism: Relationship that benefits both but is not necessary for survival (e.g., gum disease, dental caries).
Biofilms: Mixed communities of microbes attached to a surface and each other.
- Formation: Pioneer colonizer attaches; others follow and secrete extracellular material.
- Quorum Sensing: Bacteria release chemicals to interact with nearby cells.
- Characteristics: Extremely resistant to eradication; different physical/biological properties (pH, oxygen) throughout the structure compared to planktonic (free-living) bacteria.

Bacterial Growth and Binary Fission

Binary Fission Process:
1. Parent cell enlarges.
2. Chromosome is duplicated.
3. Cell envelope pulls to the center.
4. Cell wall forms a central septum.
5. Cell divides into two daughter cells.
Generation (Doubling) Time: The time required for a complete fission cycle.
- Average: $30-60$ minutes.
- Shortest: $10-12$ minutes.
- Longest: Mycobacterium leprae ( $10-30$ days).
Math of Growth:
- $N_t = (N_i)2^n$
- $N_t$ = total number of cells at time $t$ .
- $N_i$ = starting number of cells.
- $n$ = generation number.

The Bacterial Growth Curve

Lag Phase: Newly inoculated cells adjust and synthesize components but are not yet multiplying at the maximum rate.
Exponential (Log) Phase: Growth increases geometrically. Cells are most vulnerable to heat and antimicrobials during this phase.
Stationary Phase: Growth rate equals death rate. Caused by depleted nutrients/oxygen and waste accumulation.
Death Phase: Cells die at an exponential rate. Some may enter a Viable Nonculturable State (VNC) where they remain dormant and cannot be grown on medium.

Analyzing Population Size

Turbidometry: Measuring cloudiness (turbidity) in nutrient solution; higher turbidity equals larger population.
Direct Cell Count: Counting cells microscopically using a hemocytometer.
Coulter Counter: Electronic scanner of fluid passing through a pipette.
Flow Cytometer: Similar to Coulter counter but differentiates between live and dead cells and measures size.
Genetic Probing: Uses real-time PCR to quantify organisms in samples.

Questions & Discussion

Question: Which term describes an organism deriving energy and carbon from organic molecules?
Answer: Chemoheterotroph.
Question: Which association involves one organism benefitting and one being harmed?
Answer: Parasitism.
Question: What is the correct order of the bacterial growth curve?
Answer: Lag phase, Exponential phase, Stationary phase, Death phase.