Microbial Growth and Nutrients

Microbial Growth

• Macrobes (like humans) grow by increasing in size.
• Microbes grow by increasing in cell number through binary fission (bacteria, protists, algae) or budding (yeasts).
• Bacteria thrive in diverse habitats due to their small size, minimal nutrient needs, diverse metabolisms and tolerance of varied physical conditions.

Nutrients

• All organisms require carbon, energy, and electrons/hydrogen.
• Organisms are grouped into four categories based on their carbon and energy sources:
  • Photoautotrophs obtain energy from light and carbon from the environment.
  • Chemoautotrophs obtain energy and carbon from chemicals in the environment.
  • Photoheterotrophs obtain energy from light and carbon from organic compounds.
  • Chemoheterotrophs obtain energy and carbon from organic compounds.
• Essential elements include CHONPS, trace elements (Zn, Mn, Mg, etc.), vitamins, and growth factors.
• Carbon is an energy source and building block found in carbohydrates.
• Nitrogen is used in amino acids, purines, and pyrimidines.
• Cyanobacteria and Rhizobium can perform nitrogen fixation, converting N2 to NH3, which is an important process for fertilizing soil.
• Oxygen serves as the final electron acceptor in the electron transport chain during aerobic respiration and is found in carbohydrates.
• Toxic forms of oxygen that form during aerobic respiration include:
  • Superoxide radical (O2-), which superoxide dismutases detoxify and convert.
  • Peroxide anion (O22-), is detoxified by catalase.
• Sulfur (S) is found in cysteine, methionine, and co-enzymes; Phosphorus (P) is found in nucleic acids, ATP, and phospholipids.
• Trace elements such as Cu, Fe, Zn, Co, Na, Cl, and K act like enzyme cofactors, while Co is used for B12 and Fe is used for heme and ETC cytochromes.
• Growth factors include organic substances required in small amounts that organisms cannot synthesize themselves like vitamins and essential amino acids.

Physical Factors

• Temperature influences the 3-D configuration of biological molecules like enzymes and phospholipids in membranes.
• Minimum, maximum, and optimum growth temperatures exist for microbes.
• Cold temperatures slow enzymes, while heat irreversibly denatures them.
  • Psychrophiles thrive below 15°C.
  • Mesophiles thrive between 20-40°C, and most pathogens have an optimal temperature = 37°C.
  • Thermophiles thrive above 45°C, with some toleratimg temperatures as high as 110°C.
• pH levels interfere with hydrogen bonding in proteins and nucleic acids, destroying their structure.
  • Acidophiles thrive in acidic environments (pH 0.1-5.4)
  • Neutrophiles thrive in neutral environments (pH 5.4-8.0) and include most human pathogens.
  • Alkaliphiles thrive in basic environments (pH 7-11.5).
• Water is essential to dissolve enzymes and nutrients, as well as for metabolism.
  • Osmotic pressure is exerted on membranes by dissolved substances in solution.
  • Hyperosmotic environments cause cells to shrink due to water loss causing plasmalysis.
  • Hypoosmotic environments cause cells to take up water, resulting in walled cells becoming turgid and wall-less cells bursting.
  • Halophiles need high salt concentrations (10-35%) to grow and survive.
• Hydrostatic pressure increases by 1 atm for every 10 m of depth.
  • Bacteria can survive to a depth of 7,000 m; barophiles require pressure.

Ecological Associations

• Microbes form relationships.
  • Antagonistic relationships occur when one organism harms or kills another.
  • Synergistic relationships occur when members cooperate for benefits exceeding those achieved alone, but can live separately.
  • Symbiotic relationships occur when organisms live in close nutritional or physical contact and are interdependent, which means members usually don't live apart from each other.
• Biofilms are the result of complex relationships between organisms, and group behavior differs from individual behavior.
• Quorum sensing involves bacteria responding to signals from fellow bacteria and responding with new behaviors.

Microbial Culture

• Koch's second postulate requires a suspected pathogen to be isolated and cultivated in pure culture, requiring an inoculum and medium.
• The majority of microbes never are grown in pure culture.
• Special culture techniques include:
  • Anaerobic culture uses reducing media containing oxygen-binding compounds.
  • Low oxygen culture for microaerophiles and capnophiles, using CO2 incubators and candle jars.
  • Special media and containers are needed for transporting clinical specimens.
  • Preserving cultures includes refrigeration, deep freezing, and lyophilization (freeze drying).

Growth of Microbial Populations

• Microbial growth occurs through binary fission, where the cell duplicates its components and splits into two daughter cells, leading to logarithmic growth.
• Generation time is the time it takes for cells to double in number (20 minutes to 20 hours)

Growth Curve

• The phases of the growth curve are:
  • Lag phase
  • Log phase, in which growth occurs at an exponential rate.
    • X-axis shows time (linear); Y-axis shows growth (logarithmic) on a semi-log graph.
    • All cells do not divide at the same time, meaning division is non-synchronous.
    • Log phase has a finite duration because increases in bacteria number leads to nutrients being depleted and wastes accumulating.
    • The limiting factor for the log phase is the rate of energy production from ATP.
  • Stationary phase, in which nutrients are limited, wastes may be at toxic levels, O2 is inadequate and pH is suboptimal, leading to new cells being produced at the rate of old cell death.
  • Decline/death phase, in which wastes have built up and nutrients are depleted.
  • Colonial growth, which is exponential growth on solid surfaces that leads to colony formation.
  • All cells in a colony are descendents of original founding cell and a colony is considered a clone.
  • All phases of growth curve occur simultaneously in a colony and growth is non-synchronous.

Measuring Microbial Growth

• Direct methods:
  • Viable cell measurement
  • Membrane filtration
  • Petroff-Hauser counting chamber
  • Electronic counters (Coulter counter, flow cytometers)
• Indirect methods:
  • Turbidity measurement with a spectrophotometer
  • Metabolic product measurement
  • Dry weight measurement