BACTERIAL GROWTH

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Refers to an increase in bacterial cell

number (multiplication).

Results from bacterial reproduction

(binary fission)

parameter called generation time

(the average time required for cell

numbers to double).

Bacterial Reproduction

Bacteria reproduce a sexually by simple binary

fission.

1) Elongation of the bacterial cell.

2) Duplication of the chromosome.

3) The two sister chromosomes attach to the

mesosome.

4) Cell membrane and cell wall will form a

transverse septum that divides the mother cell

into 2 equal daughter cells.

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Bacterial Reproduction

o The number of bacteria (n) increases logarithmically.

1 2 4 8 16 32 64 128 etc

=2n

The doubling (generation) time: The time required by the bacteria to double its number,

varies from one species to another.

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The generation time varies from one species to

another e.g.

1-V. cholera has a generation time 17 minutes.

2-Escherichia coli has a generation time 20 minutes.

By contrast,

3-Mycobacterium tuberculosis has a generation time of 24

hours.

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Growth bacteria

.

Most bacteria will grow on artificial culture media prepared from

extract from animal or plant tissues, which supply pre-formed

nutrients and vitamins

Some bacteria, cannot grown in vitro;

e.g.

1- Mycobacterium leprae (leprosy)

2-Treponema pallidum (syphilis),

Other bacteria, only replicate intracellular within host cells

and are therefore grown in tissue culture. e.g.

1-Chlamydia spp.

2-Rickettsia spp.,

Growth Requirements

o In order to grow bacteria have certain requirements:

Nutrients

Growth factors

Suitable temperature

Suitable pH

Oxygen

Carbon dioxide

Moisture.

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Nutrients

o Nutritional requirement for growth include:

1- Macromolecules: needed in larger amounts.

Examples:

- CHNOPS

- Mineral salts: Ca, Fe, Mg, k,..

2- Micromolecules: required in trace amounts.

- Mn, Zn, Co, Ni

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Bacteria are classified into:

1- Autotrophic bacteria: (auto= self/ troph=

feeding)

Autotroph= use carbon dioxide as a source of

carbon and ammonium salts as a source of

nitrogen, from which they synthesize organic

substances, e.g protein, CHO,…

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Nutritional requirements

2- Heterotrophic bacteria: (hetro= other)

Use preformed organic compounds (made by

others)

they derive their energy by oxidation or

fermentation of organic compounds (glucose).

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Nutritional requirements

Most bacteria of medical importance

are heterotrophic bacteria

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Growth factors

oMany pathogenic species of bacteria require for

growth essential substances, which they can not

synthesize, like vitamins, amino acids,…

Fastidious bacteria: are those which require

unusually complex nutrients.

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Temperature

Different microbial species vary widely in their

optimal temperature ranges for growth:

Psychrophilic:

grow best at low temp (15 -20°C)

Mesophilic:

grow best at 30 -37° C.

Thermophilic forms:

grow best at 50 - 60°C

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Temperature

Hydrogen ion concentration (pH)

Most bacteria of medical importance can grow

at a pH of 6-8 (neutralophiles).

Some forms (acidophiles) have optima as low

as pH 3.0

,e.g lactobacilli.

Others (alkaliphiles) have optima as high as pH

10.5,

e.g. Vibrio cholerae.

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pH

Acidophiles

Neutralophiles

(most bacteria)

Alkalophiles

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Examples Definition Class

Mycobacterium

tuberculosis Grow only in the

presence of O2

Obligate (strict) aerobe

Clostridium

tetanus

Can not grow in

the presence of O2

Obligate (strict)

anaerobe

Most pathogenic

bacteria of

medical

Can grow in the

presence or

absence of O2

Facultative anaerobe

Campylobacter Require low O2

tension

Microaerophilic bacteria

Oxygen

CO2

Most bacteria require CO2 in low concentration.

Certain bacterial species (Carboxyphilic) require

higher concentrations of CO2, e.g.:

- Neisseria species require 5-10%.

- Brucella abortus require 20% CO2.

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Moisture

Four-fifth of bacteria cell weight consists of

water, and moist is necessary for growth.

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Bacterial Growth Curve

If a small number of bacteria are inoculated into

a liquid nutrient medium and the bacteria are

counted at frequent intervals and the results

plotted, a characteristic growth curve with 4

phases is obtained:

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Bacterial Growth Curve

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Bacterial Growth Curve

1- Lag phase:

-Metabolite-depleted cells adapt to new environment.

-No cell division (constant number).

- The bacteria form the enzymes and molecules needed for replication.

2- Logarithmic (exponential) phase:

- Rapid cell division occurs.

- The number of living bacteria increases by time.

- Clinical significance: this phase = symptoms and signs of the disease.

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3- Stationary phase:

- Nutrients are exhausted.

- Waste products are accumulated (toxic products).

- The number of dying cells = number of new cells

- (The number of living bacteria remains

constant).

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Bacterial Growth Curve

4- Decline phase:

- Nutrients are more exhausted.

- Waste products are more accumulated.

- The number of dying cells > number of new

cells.

The number of living bacteria decreases by

time.

o Clinical significance: this phase = recovery

and convalescence.

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Bacterial Growth Curve

Bacterial Metabolism

Many bacteria secretes enzymes(lipases,

nucleases, proteinases,..) that break down the

nutritive material in to simpler molecules.

Theses molecules are then oxidized by bacteria

to yield energy, and the degradation products

are used to build up structural components and

essential molecules for cell metabolism.

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Depending on the biochemical mechanisms

used, bacterial metabolism can be categorized in

to :

OXIDATION

- Aerobic respiration.

- An aerobic respiration.

Fermentation.

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Bacterial Metabolism

Bacterial Metabolism

Oxidation:

Removal of electrons and hydrogen ions.

The substrate to which hydrogen ions are released is

called H acceptor.

In aerobic respiration the hydrogen acceptor is oxygen.

This results in the production of toxic substances e.g

hydrogen peroxide (H2O2) and (superoxide).

Aerobes and facultative anaerobes; contain certain

enzymes , e.g. catalase and superoxide dismutase which

degrade these toxic compounds and protect bacteria

from their effect.

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Bacterial Metabolism

In the anaerobic respiration the ultimate

hydrogen acceptor is an inorganic compounds

(not hydrogen), because these bacteria does not

contain protective enzymes.

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Fermentation: - Refers to the break down of sugar to pyruvic

acid and then usually to lactic acid.

- Fermentation is also called glycoltic cycle, and

this is the process by which facultative bacteria

generate ATP in absence of oxygen.

- If oxygen present, the pyruvate produced by

fermentation enters the Krebs cycle bacteria

and generates CO2 and water.

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Bacterial Metabolism

The fermentation of certain sugars is the basis of

the laboratory identification of some pathogens.

The acids produced lower the pH, and this can be

detected by the change in color of indicator dyes.

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Bacterial Metabolism