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GENERAL

BACTERIOLOGY

Dr. Waleed Eldars

Lecturer of Medical Microbiology and

Immunology

Faculty of Medicine

Mansoura University

Growth requirements of Bacteria

A) Nutrition

Basic elements: Carbon and Nitrogen

Major elements: Phosphorus and Sulpher.

Minor elements: Mg ++, K+, Ca ++ and Iron.

Essential metabolites and growth factors like

Nucleotides and vitamins

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(B) Gases: (B) Gases:

Oxygen:

1. Obligatory aerobes: Grow only in the presence

of O2 e.g. Mycobacterium tuberculosis.

2. Facultative anaerobes: Grow well in the

presence and absence of O2 e.g. pathogenic

bacteria.

3. Obligatory anaerobes: they can not grow in the

presence of O2 e.g. Clostridium.

4. Microaerophilic: They grow best in the

presence of a minimal amount of

O2.(Helicobacter pylori)

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CO2:

• The normal atmospheric CO2 (0.03%) is

usually sufficient for growth of most

bacteria.

• Some organisms may require higher CO2

concentration e.g.: for stimulation of growth

or toxin production.

(C) Temperature: (C) Temperature:

• The optimum temperature for growth and

multiplication of most pathogenic bacteria is

37oC.

• They have a minimum temperature (10oC)

below which they can not grow and a

maximum temperature (42oC) above which

they can not grow.

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(D) Hydrogen ion concentration (pH): (D) Hydrogen ion concentration (pH):

Pathogenic bacteria grow at a narrow

range of pH with an optimum 7.5.

Some bacteria need alkaline pH e.g. Vibrio cholera.

Some bacteria need acidic pH e.g.

Lactobacillus acidophilus.

(E) Moisture. (E) Moisture.

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Bacterial Products

A)Bacterial enzymes:

Proteolytic enzymes:

act on proteins.

Saccharolytic enzymes:

act on carbohydrates.

Lipolytic enzymes:

act on lipids.

Respiratory enzymes.

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(B) Bacterial pigments:

Endopigment:

Localized inside the organism and giving color to the

colony.

Example: Golden yellow pigment of Staphylococcus

aureus.

Exopigment:

Diffuses from the organism giving

color to surrounding

medium.

Example: the bluish green pigment of

Pseudomonas aeruginosa.

Staphylococcus aureus

endopigment

Staphylococcus aureus

endopigment

Pseudomonas aeruginosa

exopigment

Pseudomonas aeruginosa

exopigment

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(C) Bacterial toxins:

Exotoxin Endotoxin

Diffusibility Diffusible Cell bounded

Antigenicity Strong Weak

Toxicity High Low

Specificity Specific action on

cells

Non specific

Nature Protein Lipopolysaccharide

Source Gram positive,

some Gram negative

Cell wall of Gram

negative

Heating Labile Stable

Effect of

formalin

Change to toxoid Not affected

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Bacterial Growth and Reproduction

Growth:

• Increase in the number of bacterial cells which result from

increased in biomass of bacteria.

Reproduction: Simple binary fission

Steps:

Increase in size.

Elongation.

Duplication of chromosome

Transverse septum.

Formation of new cells.

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Bacterial Growth Curve

• When bacteria are cultured on fluid media,

the viable bacterial number follows a

course called "growth curve".

• The growth passes into four stages when

we plot the logarithmic number of viable

bacteria against time in hours.

Steps:

A. The lag phase.

B. The logarithmic

phase.

C. The stationary

phase.

D. The decline phase.

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1- Lag Phase:

• No increase in bacterial numbers as bacteria

prepares themselves for active division.

• The duration of the lag phase may be few hrs to

few days depending on:

Type of organism: E. coli < 1hr, while T.B.

bacilli: few days.

Medium (suitable).

• It correspond to the incubation period of the disease

2- Logarithmic Phase:

• Division occurs at a maximum rate (active & regular

division).

• Represented as ascending straight line.

• Bacteria keep dividing rapidly till a point (saturation

point) which depends on type of organism and

environmental factors (suitability of the medium

and the growth conditions).

• This stage corresponds to the invasive period of diseases.

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3- Stationary Phase:

• In this stage the rate of division = rate of death so

the number of living organisms remains

stationary.

• Growth rate decrease due to:

1- Exhaustion of food.

2- O2 starvation.

3- Accumulation of toxic materials.

• This stage corresponds to the period of signs and symptoms of disease.

4- Decline phase:

• In this stage the rate of death > rate of growth

and at end, bacteria are completely died.

• Increased death rate is due to:

• Accumulation of toxic metabolites.

• O2 exhaustion.

• This phase corresponds to the convalescent period of the disease.

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Chemostat (from Chemical environment is static)

• It is a bioreactor to which fresh

medium is continuously added,

while culture liquid is

continuously removed to keep

the culture volume constant.

• By changing the rate with which

medium is added to the

bioreactor the growth rate of the

microorganism can be easily

controlled.

Mechanisms of pathogenesis

Microorganisms cause disease by two basic

mechanisms:

1. Invasiveness: Invasion of tissue.

2. Toxigenicity: Production of toxins.

Intracellular pathogens: generally produce chronic

disease; extracellular pathogens generally produce

acute disease.

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1) Invasiveness:

• The ability to invade host tissues.

• Capsules.

• Extracellular enzymes: Some bacteria produce enzymes

that degrade host tissues e.g. hyaluronidase,

collagenase, protease and phospholipase

• Attachment: fimbriae and surface proteins (adhesins).

•

2)Toxigenicity:

• The production of toxins

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