Module 4 Microbial Populations

Bacterial Growth

defined as an increase in the number of bacteria in a population

Geometric or exponential manner

pattern of growth of a bacterial population

1. Lag Phase

2. Exponential or Log Phase

3. Stationary Phase

4. Phase of Decline

5. Phase of Readjustment

Phases of Bacterial Growth

Lag Phase

• Bacteria adapt to a new environment

• Inability of bacterial population to double

  the population (population is uniform)

• Bacteria are deficient in enzymes and

  co-enzymes that support bacterial

  metabolism and cell division

Exponential or Log phase

• A period of maximal cell growth

• Cells divide steadily fast at a constant rate

• Chemical composition of cells, metabolic activity and other physiological characteristics of the population are uniform

Stationary phase

• Gradual tapering of the log phase

• Cessation of bacterial growth

• Exhaustion of some nutrients

• Population remains constant (cessation of cell division)

Phase of decline

Death rate is faster than cell division

Phase of readjustment

Bacteria are transferred to a new environment

• Sources of energy

• Temperature requirements

• Oxygen requirements

• pH requirement

• Osmotic pressure

• Radiation

• Vitamin requirements

• Mineral requirements

• Other growth factors

Factors that Influence the Growth of Bacteria

1. phototrophs

2. chemotrophs

3. autotrophs

4. heterotrophs

5. obligate

Sources of energy

5 to 30 °C

Psychrophiles Temperature requirements

30 to 45 °C

Mesophiles Temperature requirements

45 to 60 °C

Thermophiles Temperature requirements

above 60 °C

Stermothermophiles Temperature requirements

obligate anaerobes

live in a strict anaerobic environment (high reducing intensity)

a. Boiling the culture medium for a few minutes to drive off dissolved oxygen

b. Use of anaerobic chamber

c. Use of reducing agents (ex. Cysteine) that lower oxygen content

d. Addition of hydrogen in order to produce water

Methods of maintaining anaerobiosis

Facultative anaerobes

grow in either the presence or absence of oxygen

Aerobes

require oxygen for growth

Microaerophiles

utilize a low amount of oxygen

acidophiles

bacteria that grow best below pH 5.5

neutrophiles

bacteria that grow best at pH 5.5 to 7.9

alkalophiles

bacteria that grow best at pH 8.0 and above

Halophiles

bacteria that require 1 to 4% NaCl for optimal growth

Extreme halophiles

bacteria that grow in a very salty environment (15 to 30% NaCl)

Osmophiles

bacteria that grow in an environment with high sugar content

Xerophiles

bacteria that grow in a very dry environment

Vitamin B

integral part of a variety of bacterial enzymes and co-enzymes

Nicotinic acid and riboflavin

for hydrogen function and energy production systems

• phosphorus

• sulfur

• magnesium

• molybdenum

Mineral requirements

phosphorus

energy metabolism and fat storage

sulfur

forms amino acids

magnesium

activates enzymatic reactions and RNA synthesis

molybdenum

for nitrogen fixation

1. yeast extracts

2. whole blood

3. amino acids

4. glucose

Other growth factors

Binary fission

• Cell division doubles the starting size of a population

• A cell splits in two producing 2 daughter cells (cell division has to take place at the right time, in the right place, and a provision of a complete copy of the essential genetic material to each daughter cells)