microbial physiology
Chapter 7: Microbial Growth
Measurement of Microbial Growth
Focuses on the methods for measuring microbial growth and the phases of growth, especially unicellular bacteria.
Definition of Growth
Growth in biological systems is defined as the orderly increase of chemical components.
Increase in mass does not necessarily reflect biological growth.
Balanced Growth: Occurs in suitable media, where an increase in biomass coincides with increases in protein, RNA, DNA, and intracellular water, maintaining constant chemical composition.
Simplifies measuring bacterial culture growth rate, as any single component's increase reflects the overall growth rate.
Mathematical Representation of Growth
Balanced growth is modeled as a first-order autocatalytic reaction where the increase is proportional to the mass of bacteria present.
Doubling Time (g): Average time for cell doubling; for example, if g = 18 minutes, a growth rate constant (k) is derived from growth equations:
[ \dfrac{dN}{dt} = kN ]
Growth rate constant (k) indicates growth speed, changing with bacterial concentration.
Doubling time is calculated from growth rate as:
[ g = \dfrac{0.693}{k} ]
Growth data plotted using logarithmic scales results in straight lines with slope = k/2.303.
Growth Phases
Exponential/Growth Phase:
Rapid doubling of cells occurs; size and mass increase.
Growth rate makes initial measurements simpler.
Stationary Phase:
Growth rate stabilizes as nutrient depletion occurs; cell size may decrease.
Population exhibits resistance to environmental stress.
Death Phase:
Results from depletion of nutrients or accumulation of toxic products.
Death follows an exponential pattern where die-off is reflected in logarithmic plots.
Lag Phase
The transition from stationary to growth phases featuring chemical composition changes before growth resumes; duration linked to previous phase length.
Arithmetic Growth
Under specific situations, growth can become arithmetic rather than exponential.
Often results when essential nutrients are depleted or presence of growth inhibitors.
Example: Adding phenylalanine analogue preventing proper protein function leads to growth stagnation.
Measurement Approaches
Cell Mass Measurement
Direct mass measurement involves filtration and weighing of dried cells, commonly less sensitive and accurate for bacterial counts.
Indirect methods involve optical density measurements using a spectrophotometer; it relates absorbance to cell concentration.
Cell Number Measurement
Direct counting using specialized slide counting chambers or electronic counters like the Coulter counter provides efficient and sensitive enumeration.
Viable count differentiates between live and dead cells using plating techniques and colony counting.
Growth Yields
Defined as the net cell mass change, influenced by nutrient concentrations in culture.
Certifies active uptake of nutrients to cells; values indicate efficiency of substrate conversion into biomass.
Continuous Culture Systems
Include systems like chemostats and turbidostats.
They maintain a steady-state culture with constant growth rates by regulating nutrient supply proportional to cell removal rates through siphoning.
Offers a model to study growth regulation mechanisms efficiently.
Chapter 8: Effect of the Environment on Microbial Growth
Interaction Between Microbial Cells and Environment
The composition of intracellular solutes in microbes is often higher than in the external environment.
Cell Membrane Functions
The cell membrane is selectively permeable, regulating nutrient transport through embedded proteins.
Transport Mechanisms
Passive Diffusion:
Movement occurs along concentration gradients without energy expenditure.
Facilitated Diffusion:
Membrane proteins assist specific solutes' transport without energy use.
Active Transport:
Solutes move against concentration gradients using ATP or proton motive force.
Secondary Active Transport:
Utilizes established gradients for the co-transport or counter-transport of molecules across membranes.
Group Translocation
Involves chemically modifying a substrate during transport (e.g., phosphorylation of sugars).
Characterized by the phosphotransferase system (PTS) that conserves energy while transporting nutrients into cells.
Specific Transport Examples
Different solutes are transported using various methods; for example:
Glycerol enters through facilitated diffusion.
Maltose via an active transport system utilizing energy adjustments through the concentration of the substrate.
This comprehensive overview synthesizes microbial growth measurement methods, growth-phase characteristics, and the transport mechanisms of nutrients across cell membranes.