1. Microbial Growth Microbiology

2. Microbial Growth • In microbiology growth is defined as an increase in the number of cells. • Knowledge of how microbial populations expand is useful when designing methods to control microbial growth.

3. Factors that Affect Microbial Growth • Physical Factors • Temperature • pH • Osmotic pressure • Chemical Factors

4. Physical Requirements • A. Temperature • Optimal growth temperature • Permissible range • human pathogens optimal = 37°C

5. Physical Requirements • Temperature • Psychrophile: cold loving • Range: 0C-20C • Mesophile: moderate temp. loving • Range: 20C-40C • Thermophile: heat loving • Range: 40C-100C

6. Physical Requirements • B. pH • Acidophiles- “acid loving” • Acidity inhibits most microbial growth and is used frequently for food preservation (Ex: pickling) • Certain bacteria, such as those in sauerkraut and yogurt, prefer acidic environments of 6.0 or below. • Fungi tend to live in slightly acidic environments pH 5-6. • Molds and yeast grow in wider pH range, but prefer pH between 5 and 6. • Neutrophiles- • most organisms optimal pH 6.5-7.5 (near neutral) • Since the pH of most human tissue is 7.0 to 7.2, these neutrophilic bacteria usually grow well in the body. (Most human pathogens) • Alkaliphiles- “alkali loving” • Alkalinity inhibits microbial growth, but not commonly used for food preservation. • Grow at pH of 7-12 or higher • Example: Vibrio cholerae optimal pH= 9

7. Physical Requirements • C. Osmotic pressure- Cells are 80-90 % water • Most bacteria require isotonic solutions (no net flow of water in either direction of cell) • human blood = 0.9% NaCl, isotonic • human skin = ~3-6% NaCl, hypertonic • Bacteria gain water and cell may burst • This is an example of how antibiotics work • High Osmotic pressure • In extremely salty solution, bacteria lose water through osmosis causing plamolysis (shrinking of cell mb. • Bacteria are in equilibrium with their environment

8. Chemical Requirements • A. Carbon- structural backbone of all organic compounds • B. Nitrogen, Sulfur and Phosphorus • Needed for synthesis of cellular material • (all above: to build organic molecules) • E. Trace elements: K+, Mg2+, Ca2+ , Fe2+ ... • Used to make enzymes • F. Oxygen- can classify mo’s based on O2 requirements

9. Microbial Growth • Growth of Bacterial Cultures • Logarithmic representation of Bacterial Growth • Phases of growth • Bacterial Growth Curve • When bacteria are inoculated into a liquid growth medium, we can plot of the number of cells in the population over time.

10. Bacterial Growth Curve