Chapter 7-Microbial Nutrition, Ecology, and Growth: Study Notes

Microbial Nutrition, Ecology, and Growth

Microbial Nutrition

Microbial nutrition refers to the process by which microorganisms acquire chemical compounds (nutrients) from their environment to sustain life. Understanding the types and sources of nutrients is essential for studying microbial growth and metabolism.

• Bioelements: Basic requirements for life, including carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, iron, sodium, chlorine, and magnesium.

• Essential nutrients: Substances (elements or compounds) that an organism must obtain from outside its cells.

• Macronutrients: Required in large quantities; play principal roles in cell structure and metabolism (e.g., proteins, carbohydrates).

• Micronutrients (Trace elements): Required in small amounts; involved in enzyme function and maintenance of protein structure (e.g., manganese, zinc, nickel).

Types of Nutrients

• Organic nutrients: Contain carbon and hydrogen atoms and are usually products of living things. Examples include methane (CH4), carbohydrates, lipids, proteins, and nucleic acids.

• Inorganic nutrients: Atoms or molecules that contain a combination of atoms other than carbon and hydrogen. Examples include metals and their salts (e.g., magnesium sulfate, ferric nitrate, sodium phosphate), gases (oxygen, carbon dioxide), and water.

Chemical Composition of Cells

Cells are composed primarily of water and organic compounds, with proteins being the most prevalent. Six elements make up the majority of cell mass:

• Carbon

• Hydrogen

• Oxygen

• Phosphorus

• Sulfur

• Nitrogen

Essential Biological Nutrients

Microorganisms require various essential nutrients for growth and metabolism. The following table summarizes the sources, forms, and significance of key elements:

Element	Forms Found in Nature	Sources/Reservoirs	Significance to Cells
Carbon	CO2 (gas), CO32- (carbonate), organic compounds	Air, sediments/soils, living things	Essential for structure and function; CO2 used in photosynthesis, found in cell walls/skeletons
Nitrogen	N2 (gas), NO3- (nitrate), NO2- (nitrite), NH4+ (ammonium), organic nitrogen	Air, soil, water, organisms	Used for amino acid and nucleic acid synthesis; some microbes fix N2
Oxygen	O2 (gas), oxides, H2O	Air, soil	Required for aerobic metabolism; component of organic/inorganic compounds
Hydrogen	H2 (gas), H2S, CH4, organic compounds	Water, swamps, mud, volcanoes, organisms	Maintains pH, transfers energy, solvent for reactions
Phosphorus	PO43- (phosphate)	Rocks, soil	Component of DNA, RNA, ATP, phospholipids
Sulfur	SO42- (sulfate), sulfhydryl groups	Mineral deposits, soil	Found in amino acids, vitamins, stabilizes proteins
Potassium	K+	Mineral deposits, ocean water, soil	Protein synthesis, membrane transport
Sodium	Na+	Same as potassium	Membrane actions, osmotic pressure
Calcium	Ca2+	Oceanic sediments, rocks, soil	Cell wall stabilization, endospore resistance
Magnesium	Mg2+	Geologic sediments, rocks, soil	Chlorophyll central atom, enzyme function
Chloride	Cl-	Ocean water, salt lakes	Membrane transport, osmotic regulation
Zinc	Zn2+	Rocks, soil	Enzyme cofactor, genetic regulation
Iron	Fe2+, Fe3+	Rocks, soil	Respiratory proteins (cytochromes)
Micronutrients	Copper, cobalt, nickel, molybdenum, manganese, iodine	Geologic sediments, soil	Cofactors for specialized enzymes

Classification of Nutritional Types

Microorganisms are classified based on their carbon and energy sources:

• Heterotrophs: Obtain carbon from organic sources (e.g., proteins, carbohydrates, lipids, nucleic acids).

• Autotrophs: Use CO2 (inorganic gas) as their carbon source; not nutritionally dependent on other living things.

• Growth factors: Essential organic compounds that cannot be synthesized by the organism (e.g., essential amino acids, vitamins).

Energy Source Classification

• Chemotrophs: Gain energy from chemical compounds.

• Phototrophs: Gain energy through photosynthesis.

Nutritional Categories Table

Category	Carbon Source	Energy Source	Examples
Photoautotroph	CO2	Sunlight	Plants, algae, cyanobacteria
Chemoautotroph	CO2	Simple inorganic chemicals	Methanogens, some bacteria
Photoheterotroph	Organic compounds	Sunlight	Purple and green non-sulfur bacteria
Chemoheterotroph	Organic compounds	Organic compounds	Protozoa, fungi, many bacteria

Transport Mechanisms Across Cell Membranes

Microorganisms use various mechanisms to transport substances across their cell membranes:

• Passive transport: Does not require energy; substances move down their concentration gradient.

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

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

  • Facilitated diffusion: Solutes require a carrier protein for transport.

• Active transport: Requires energy and carrier proteins; can move substances against their concentration gradient.

  • Carrier-mediated active transport: Uses permeases and pumps; ATP is used to move solutes.

  • Group translocation: Transported molecule is chemically altered during passage.

  • Bulk transport: Includes endocytosis (phagocytosis, pinocytosis) and exocytosis.

Osmotic Solutions and Effects on Cells

• Isotonic solution: Water concentration is equal inside and outside the cell; no net movement.

• Hypotonic solution: Net diffusion of water into the cell; may cause swelling or bursting.

• Hypertonic solution: Water diffuses out; cell shrinks (plasmolysis).

Environmental Factors Influencing Microbial Growth

Microbial growth and metabolism are affected by several environmental factors:

• Temperature: Microbes have minimum, maximum, and optimum growth temperatures.

  • Psychrophiles: Optimum below 15°C.

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

  • Thermophiles: Optimum above 45°C.

• Oxygen requirements: Oxygen can be toxic; microbes have adaptations:

  • Obligate aerobe: Requires oxygen.

  • Facultative anaerobe: Can use oxygen or grow without it.

  • Microaerophile: Requires small amounts of oxygen.

  • Obligate anaerobe: Cannot tolerate oxygen.

  • Aerotolerant anaerobe: Does not use oxygen but tolerates its presence.

• pH: Most microbes are neutrophiles (pH 6–8); acidophiles and alkalinophiles grow at extreme pH.

• Osmotic pressure: Halophiles require high salt; osmotolerant microbes resist salt.

• Barometric pressure: Barophiles survive under high pressure.

Microbial Associations

Microorganisms interact in various ways, forming symbiotic and nonsymbiotic relationships:

• Symbiotic: Organisms live in close nutritional relationships.

  • Mutualism: Both members benefit.

  • Commensalism: One benefits, the other is unaffected.

  • Parasitism: Parasite benefits, host is harmed.

• Nonsymbiotic: Free-living relationships.

  • Syntrophy: Members cooperate and share nutrients.

  • Amensalism: One member is inhibited or destroyed by another (antagonism, competition).

Biofilms

Biofilms are complex communities of microorganisms attached to surfaces and embedded in an extracellular matrix. They communicate via quorum sensing and dominate natural environments.

• Cells synthesize an adhesive matrix and coordinate gene expression via inducer molecules.

• Biofilms enhance survival and resistance to environmental stress.

Microbial Growth and Population Dynamics

Microbial growth occurs at the cellular level (increase in size) and population level (increase in number). Most bacteria divide by binary fission.

• Binary fission: Parent cell enlarges, duplicates its chromosome, and divides into two daughter cells.

• Generation time: Time required for a complete fission cycle (doubling time).

• Exponential growth: Each cycle doubles the population.

Population size calculation:

• $N_f = (N_i)2^n$

• Where $N_f$ = final number of cells, $N_i$ = initial number of cells, $n$ = number of generations

Population Growth Curve

Microbial populations display a predictable growth curve in laboratory settings:

• Lag phase: Adjustment, little growth.

• Exponential (log) phase: Maximum growth rate.

• Stationary phase: Growth rate equals death rate; nutrients depleted.

• Death phase: Cells die exponentially.

Methods of Analyzing Population Growth

• Turbidometry: Measures cloudiness (turbidity) of culture media.

• Viable colony count: Counts living cells capable of forming colonies.

• Direct cell count: Manual or automated counting of cells (e.g., Coulter counter, flow cytometer).

Example: Escherichia coli with a doubling time of 20 minutes, starting with 5 cells, will have $N_f = 5 imes 2^9 = 2560$ cells after 3 hours (9 generations). Additional info: Some context and details were inferred to clarify fragmented points and ensure completeness.