Microbial GrowthMicrobiology

Microbial Growth

0 In microbiology growth is defined as an increase in the number of cells.

0Knowledge of how microbial populations expand is useful when designing methods to control microbial growth.

Factors that Affect Microbial Growth

0Physical Factors

0 Temperature0 pH0 Osmotic pressure

0Chemical Factors

Physical Requirements

0A. Temperature

0 Optimal growth temperature0 Permissible range0 human pathogens optimal = 37°C

Physical Requirements

0Temperature

0 Psychrophile: cold loving0Range: 0C-20C

0 Mesophile: moderate temp. loving0Range: 20C-40C

0 Thermophile: heat loving0Range: 40C-100C

Physical Requirements0B. pH

0Acidophiles- “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 5-6. 0 Molds and yeast grow in wider pH range, but prefer pH between 5 and 6.

0Neutrophiles- 0 most organisms optimal pH 6.5-7.5 (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)

0Alkaliphiles- “alkali loving”0 Alkalinity inhibits microbial growth, but not commonly used for

food preservation.0 Grow at pH of 7-12 or higher0 Example: Vibrio cholerae optimal pH= 9

0C. Osmotic pressure- Cells are 80-90 % water

0 Most bacteria require isotonic solutions (no net flow of water in either direction of cell)

0 human blood = 0.9% NaCl, isotonic0 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 Requirements0A. Carbon- structural backbone of all organic compounds

0 B. Nitrogen, Sulfur and Phosphorus0 Needed for synthesis of cellular material0 (all above: to build organic molecules)

0 E. Trace elements: K+, Mg2+, Ca2+ , Fe2+ ...0 Used to make enzymes

0 F. Oxygen- can classify mo’s based on O2 requirementsAerobe-requires O2 Anaerobe-can’t use O2 Facultative-with or

w/o O2

Pseudomonas- common nosocomial pathogen

Clostridium bacteria E. coli, staphylococcus, yeast, many intestinal bacteria

Microbial Growth0Growth of Bacterial Cultures

0 Logarithmic representation of Bacterial Growth

0Phases of growth

0 Bacterial Growth Curve

0When 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 phase Log phase Stationary phase Death 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 O2

• Accumulation of toxic waste

• Cell death exceeds cell growth