Microbial Growth Microbiology

Microbial Growth 0 In microbiology growth is defined as an increase in the number of cells. 0 Knowledge of how microbial populations expand is useful when designing methods to control microbial growth.

Factors that Affect Microbial Growth 0 Physical Factors 0 Temperature 0 pH 0 Osmotic pressure 0 Chemical Factors

Physical Requirements 0 A. Temperature 0 Optimal growth temperature 0 Permissible range 0 human pathogens optimal = 37°C

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

Physical Requirements 0 B. pH 0 Acidophiles- “acid loving” 0 Acidity inhibits most microbial growth and is used frequently for food preservation (Ex: pickling) 0 Certain bacteria, such as those in sauerkraut and yogurt, prefer acidic environments of 6.0 or below. 0 Fungi tend to live in slightly acidic environments pH Molds and yeast grow in wider pH range, but prefer pH between 5 and 6. 0 Neutrophiles- 0 most organisms optimal pH (near neutral) 0 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) 0 Alkaliphiles- “alkali loving” 0 Alkalinity inhibits microbial growth, but not commonly used for food preservation. 0 Grow at pH of 7-12 or higher 0 Example: Vibrio cholerae optimal pH= 9

0 C. Osmotic pressure- Cells are % water 0 Most bacteria require isotonic solutions (no net flow of water in either direction of cell) 0 human blood = 0.9% NaCl, isotonic 0 human skin = ~3-6% NaCl, hypertonic Physical Requirements 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 Bacteria gain water and cell may burst This is an example of how antibiotics work

Chemical Requirements 0 A. Carbon- structural backbone of all organic compounds 0 B. Nitrogen, Sulfur and Phosphorus 0 Needed for synthesis of cellular material 0 (all above: to build organic molecules) 0 E. Trace elements: K +, Mg 2+, Ca 2+, Fe Used to make enzymes 0 F. Oxygen- can classify mo’s based on O 2 requirements Aerobe-requires O 2 Anaerobe-can’t use O 2 Facultative-with or w/o O 2 Pseudomonas- common nosocomial pathogen Clostridium bacteriaE. coli, staphylococcus, yeast, many intestinal bacteria

Microbial Growth 0 Growth of Bacterial Cultures 0 Logarithmic representation of Bacterial Growth 0 Phases of growth 0 Bacterial Growth Curve 0 When bacteria are inoculated into a liquid growth medium, we can plot of the number of cells in the population over time.

Bacterial Growth Curve Lag phaseLog phaseStationary phaseDeath phase Little to no growth Pop. doesn’t increase Bacteria acclimate to new environment Intense metabolic activity= growth in size Period of exponential growth with constant generation time Period of most rapid growth Cells are more susceptible to adverse environmental factors Example: antibiotics radiation Cell growth= cell death, stability Slow microbial growth influenced by limited nutrients Low O 2 Accumulation of toxic waste Cell death exceeds cell growth