Microbial growth requirements

An understanding of what and how microbes do things depends upon our ability to grow

the organisms in the laboratory. In order to culture them, we must provide them with
proper food and environmental conditions, isolate them and maintain them in
pure culture (clone).

In microbiology, the term growth refers to an increase in the number of cells
 (colony or culture), not an increase in the size of the individual cells.

Microbial growth requirements,

1.    Proper nutrients must be available

2.    Oxygen or other gases must be available.

3.    Moisture is necessary.

4.    Proper pH.

5.    Proper Temperature

6.    The medium must be free of interfering dis-burdens(growth Inhibitors).

7.    Contamination must be preventive.

Requirements for microbial growth are divided into two categories, physical

and chemical. 

Physical aspects include temperature, pH, and osmotic pressure.

Chemical requirements include water, sources of carbon and nitrogen, minerals,

oxygen,and organic growth factors.

1.   Physical Requirements          

A.  Temperature :

most microbes live within restricted ranges of temperature with a range of Tolerance (minimum ­ maximum) which includes an optimum  temperature (most rapid growth in the shortest period of  time); while  the optimum is usually closer to the maximum temperature, this temperature may not be best for all cellular activities.

Temperature  based 3 types of organisms:

1.    Thermophilic (heat loving) : growing bacteria grater than 45ºC

2.    Mesophilic (middle temperature loving) :Growing bacteria 25-40ºC (optimal temperature), Optimum temperature commonly 37ºC.

3.    Psychrophilic (cold loving):  Growing bacteria below 20ºC

Sterilization  temperature of 121º C (250º F) for 15 minutes will kill vegetative cells  but not endospores.

 Human pathogenic Organisms are generally optimal temperature (mesophiles or mesophilic) growing bacteria.

­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ B.  pH :

·         most bacteria can grow in the range of pH 6.5 ­-7.5.

·         Fungi (molds and Yeast )grow in wider range pH range, but prefer pH between 5 and 6.

pH  based 3 types of organisms :

1.    Acidophiles: (Acid loving) Grow at very low pH (0.1 to 5.4)

2.    Neutrophiles: grow at pH 5.4 to 8.5.

3.    Alkaliphiles: (alkali loving) Grow at higher pH range (7 to 17 or more)

C. Osmotic Pressure:

       Cells are 80% to 90% water.

1.    Hyper-tonic solution : High osmotic pressure removes water from cell, causing shrinkage of cell membrane (plasmolysis).

2.    Hypo-tonic solution: Low osmotic pressure causes water to enter the cell. In most cases cell wall prevents excessive entry of water. Microbe may lyse or burst if cell wall is weak.

2.   Chemical Requirements :

A. Energy source: Carbon,Nitrogen,Sulfur and phosphorus.

B. Other elements : Potassium, magnesium, and calcium are often required as enzyme       cofactors.  

Calcium is required for cell wall synthesis in Gram positive bacteria.

C. Trace Elements: Many are used as enzyme co-factors.

                         Commonly found in tap water.

                          Iron

                         Copper

                         Molybdenum

                         Zinc

D. Oxygen:

Microbes that use oxygen are called Areobes.

1.    Obligate Areobes: Organisms that require oxygen in order to live are called “obligate areobes”. examples: Bacillus,  Pseudomonas, Mycobacterium.

2.    Obligate Anaerobes: Microbes that cannot use oxygen or for whom oxygen  is actively toxic are called Obligate anaerobes.Examples are found in the genus Clostridium, Bacteroids vulgatus, Porphyromonas, Fuso bacterium, Propinobacterium Acnes.

3.    Facultative Anaerobes: oxygen in its absence are called facultative anaerobes include both Escherichia coli,Salmonella  and Staphylococcus aureus.