BACTERIAL GROWTH

FYBSc.

Bacterial growth

4-2

Binary fission

Generation time

Phases of growth

Binary fission

Figure 4.2

1. Prokaryote cells grow

by increasing in cell

number (as opposed

to increasing in size).

2. Replication is by

binary fission, the

splitting of one cell

into two

3. Therefore, bacterial

populations increase

by a factor of two

(double) every

generation time.

Generation time

4-4

The time required to for a population to double (doubling

time) in number.

Ex. Escherichia coli (E. coli) double every 20 minutes

Ex. Mycobacterium tuberculosis double every 12 to 24

hours

Principles of Bacterial Growth

Growth can be calculated

Nt = N0 x 2n

(Nt ) number of cells in population

(N0 ) original number of cells in the population

(n) number of divisions

Example

N0 = 10 cells in original population

n = 12

4 hours assuming 20 minute generation time

Nt = 10 x 212

Nt = 10 x 4,096

Nt = 40,960

1. Bacteria growing in batch culture produce a growth curve with up to

four distinct phases.

2. Batch cultures are grown in tubes or flasks and are closed systems

where no fresh nutrients are added or waste products removed.

3. Lag phase occurs when bacteria are adjusting to them medium. For

example, with a nutritionally poor medium, several anabolic pathways

need to be turned on, resulting in a lag before active growth begins.

4. In log or exponential phase, the cells are growing as fast as they can,

limited only by growth conditions and genetic potential. During this

phase, almost all cells are alive, they are most nearly identical, and they

are most affected by outside influences like disinfectants.

5. Due to nutrient depletion and/or accumulation of toxic end products,

replication stops and cells enter a stationary phase where there is no

net change in cell number.

6. Death phase occurs when cells can no longer maintain viability and

numbers decrease as a proportion.

Growth in Batch Culture

Growth in Batch Culture

Mean Generation Time

and Growth Rate

The mean generation time (doubling time) is the amount of time required for the concentration of cells to double during the log stage. It is expressed in units of minutes.

Growth rate (min-1) =

Mean generation time can be determined directly from a semilog plot of bacterial concentration vs time after inoculation

timegeneration mean

1

Mean Generation Time

and Growth Rate

Basic Chemostat System

Lab Chemostat System

Environmental factors

4-12

Temperature

Oxygen requirement

pH

Water availability

Temperature

4-13

Enzymes, the machinery of the cell, are influenced by external factors and can be shown to have a range where they function that includes an optimal value that produces the highest activity.

The range of enzyme activity determines the range for growth of specific bacteria, analogously leading to a value for optimal growth rate.

In the case of temperature, bacteria are divided into categories based on the temperature range where they can grow and the temperature that provides optimal growth.

Temperature

Psychrophile

0o to 18o C

Psychrotroph

20°C to 30°C Important in food spoilage

Mesophile

25°C to 45°C More common

Disease causing

Thermophiles

45°C to 70°C Common in hot springs and hot water heaters

Hyperthermophiles

70°C to 110°C Live at very high temperatures, high enough where water threatens to become

a gas

Usually members of Archaea

Found in hydrothermal vents

Oxygen requirements

4-15

• Oxygen is a very reactive molecule and can affect cells in several ways. The effect of oxygen is often determined using thioglycollate broth, a special medium that contains a reducing agent (thioglycollate) that removes oxygen so that a gradient occurs within the tube.

• Obligately aerobic bacteria can obtain energy only through aerobic respiration and have to have oxygen available. Thus, they will grow only at the surface of thioglycollate broth.

• Obligately anaerobic bacteria die in the presence of oxygen and can only grow at the bottom of thioglycollate broth. Some anaerobes are so sensitive to oxygen that even thioglycollate broth is not anoxic enough to provide suitable anaerobic conditions.

• Microaerophiles require oxygen for growth but the 20% in air is too toxic. As a result, they grow near the top but beneath the surface of thioglycollate broth where the oxygen concentration is typically 4 – 10%.

• Facultative anaerobes can use oxygen for aerobic respiration but can switch to fermentative metabolism in the absence of oxygen. As a result, they will grow throughout thioglycollate broth. (Heavier growth at top.)

• Aerotolerant anaerobes are anaerobic bacteria that can grow in the presence of air. (Not shown in diagram.)

pH

4-16

Neutrophiles grow best around neutral pH (7)

Acidophiles grow best at pH < 7

Alkophiles grow best at pH > 7

Acidotolerant grow best at pH 7 but can also grow at

lower pH

Alkotolerant grow best at pH 7 but can also grow at

higher pH

Water Activity

Liquid water is essential for life.

Aqueous solutions actually have different amounts of

water available, depending on how many solutes are

dissolved in it. As a very simple model, consider two

glasses, one full of pure water, the other containing the

same amount of water plus a sponge. Which one would

be easier to drink? On a much smaller scale, dissolved

solutes act like a sponge, making less water available.

Water activity (aw) can be decreased by the addition of

any soluble molecule although salt (NaCl) and sugars

are probably the most common.

Water Activity

Microbes that require a high water activity (near or at 1) are termed nonhalophiles. (Halophile = salt-loving)

Some bacteria require salt to grow and are called halophiles. If a very high concentration of salt is required (around saturation), the organisms are termed extreme halophiles.

A nonhalophile that can grows best with almost no salt but can still grow with low levels of salt (~ 7%) is called halotolerant.

In general, fungi are more tolerant of low water activity. (That’s why your jelly is more likely to get contaminated by fungi than bacteria.)

Nutritional Requirements

Growth of prokaryotes depends on nutritional

factors as well as physical environment

Main factors to be considered are:

Required elements

Growth factors

Energy sources

Nutritional diversity

Nutritional Requirements

Major elements (CHONPS + K, Mg, Fe, Ca) Carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus,

potassium, magnesium, iron, and calcium Essential components for macromolecules

Organisms classified based on carbon usage Heterotrophs

Use organism carbon as nutrient source

Autotrophs Use inorganic carbon (CO2) as carbon source

Trace elements (Co, Cu, Ni, Zn, Se, Mg, Wo) Cobalt, zinc, copper, molybdenum and manganese

Required in minute amounts Assist in enzyme function

Nutritional diversity Different organisms require the same nutrients but may require

different forms of the nutrients

Major elements

Element % dry wgt Source

Carbon 50 organic compounds or CO2

Oxygen 20 H2O, organic compounds, CO2, and O2

Nitrogen 14 NH3, NO3, organic compounds, N2

Hydrogen 8 H2O, organic compounds, H2

Phosphorus 3 inorganic phosphates (PO4)

Sulfur 1 SO4, H2S, So, organic sulfur compounds

Potassium 1 Potassium salts

Magnesium 0.5 Magnesium salts

Calcium 0.5 Calcium salts

Iron 0.2 Iron salts

Carbon Source

Organic molecules

Heterotrophs

Inorganic carbon (CO2)

Autotrophs

Nitrogen Source

Organic nitrogen

Primarily from the catabolism of amino acids

Oxidized forms of inorganic nitrogen

Nitrate (NO32-) and nitrite (NO

2-)

Reduced inorganic nitrogen

Ammonium (NH4+)

Dissolved nitrogen gas (N2) (Nitrogen

fixation)

Phosphate Source

Organic phosphate

Inorganic phosphate (H2PO4- and HPO4

2-)

Sulfur Source

Organic sulfur

Oxidized inorganic sulfur

Sulfate (SO42-)

Reduced inorganic sulfur

Sulfide (S2- or H2S)

Elemental sulfur (So)

Growth Factors

Some bacteria cannot synthesize some cell constituents These must be added to growth environment Referred to as growth factors

Organisms can display wide variety of factor requirements Some need very few while others require many These termed fastidious

Typical molecules Amino acids

Nucleotide bases

Enzymatic cofactors or “vitamins”

Culture Media

4-29

Complex (contains undefined components)

Chemically defined (all concentrations are known)

Selective (favors the growth of a particular organism or

group of organisms)

Differential (has reactions that give isolates different

appearance)

Anaerobic (oxygen-free)

4-30

Characteristics of Media