Bacterial growth and physiology

Bacterial growth definition

Bacterial growth: The processes of both the increase in number and the increase in mass of bacteria.

Growth has three distinct aspects: 

• biomass production, 
• cell production, and 
• cell survival.

 

 

Bacterial growth

• Reproduction is the main function of bacteria
• Need  suitable nutritious environment and appropriate physical condition to grow.
• After manufacturing twice the component material it starts to divide.  

 

Factors affecting bacterial growth

1. Nutrition

2. Temperature:

• Mesophilic bacteria:20-40°C (most pathogens)
• psychrophilic: 0-10°C (not pathogenic)    
• Thermophilic:  ˃ 40°C (few pathogens)

3. PH: neutral PH (most pathogens)

• Some bacteria can survive in acidic environment (Helicobacter pylori)
• some can survive in alkaline media (vibrio)

Nutritional requirements

• inorganic ions: phosphate, calcium, magnesium 
• vitamins: required by some bacteria.
• Oxygen: bacteria are divided into groups according to O2 utilization

1. obligate aerobes: need O2 to survive (ambient levels ~ 20%)
2. obligate anaerobes: can not survive in the presence of oxygen
3. facultative anaerobes: can survive in the presence or absence of O2.
4. micr-oaerophilic: require small amount of O2.
5. capnophilic: require higher CO2 levels

 

Standard Bacterial growth curve

lag phase:

• adaptation and preparation
• no increase in number.

exponential (log) phase:

• logarithmic increase in number              
• doubling time 

stationary phase:

• depletion of nutrients, accumulation of toxic metabolic products and change in PH
• no increase in number
• cell growth &division = cell death

Decline phase:

• number of dead bacteria ˃ living

 

Bacterial Growth Curve

 

Bacterial Growth Curve

Doubling time:

the time needed for one bacteria to become two

e.g. Vibrio cholerae needs only 13 minutes (fast grower)

Mycobacterium tuberculosis needs 24 hours (slow grower)

different incubation periods

different growth time in diagnostic laboratories

Classification of bacteria

    Based on 3 major criteria:

1. Morphology
2. Gram reaction
3. Oxygen requirement

e.g. gram positive facultative anaerobic cocci,

gram positive obligate anaerobic bacilli