Microbial growth
Bacterial cell growth depends on a large number of cellular reactions
Synthesize small molecules (the blocks of macromolecules)
Provide various cofactors and coenzymes needed for enzymatic reactions
Reactions transform energy
The key reactions of cell synthesis are polymerizations that make macromolecules from monomers
The Concept of Exponential Growth
Most bacteria have shorter generation times than eukaryotic microbes
Generation time is dependent
growth medium
incubation conditions
Typical growth curve for population of cells grown in a closed system is characterized by four phases:
Lag phase: Interval between a culture is inoculated and growth begins
Log or Exponential phase: Cells in this phase are typically in the healthiest state
Stationary phase: Growth rate of population is zero, either an essential nutrient is used up or waste product of the organism accumulates in the medium
Death phase: If incubation continues after cells reach stationary phase, the cells will eventually die
Measuring Microbial Growth
Viable Counts
Turbidimetric Methods
Microscopic Counts
Limitations of microscopic counts
Cannot distinguish between live and dead cells without special stains
Small cells can be overlooked
Precision is difficult to achieve
Phase-contrast microscope required if a stain is not used
Cell suspensions of low density (<106 cells/ml) hard to count
measurement
living
reproducing population
Two main ways to perform plate counts:
Spread-plate method
Pour-plate method
Environmental Factors Affecting Growth
Acidity and Alkalinity
Osmotic Effects on Microbial Growth
Temperature
Oxygen and Microorganisms
Toxic Forms of Oxygen
Microorganisms can be classified into groups by their growth temperature optima
Mesophile: midrange temperature
Thermophile: high temperature
Psychrophile: low temperature
Hyperthermophile: very high temperature
Acidophiles: organisms that grow best at low pH (<6)
Alkaliphiles: organisms that grow best at high pH (>9)
Halophiles
Extreme halophiles
Nonhalophile
Halotolerant
Growth versus oxygen concentration.
(a) aerobic, (b)anaerobic, ( c) facultative, (d)microaerophilic, (e) aerotolerant anaerobe