Bacterial 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.


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. 
           
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 

   psychrophiles or psychrophilic (0­-200 C)  32­-680 F 
   mesophiles or mesophilic (25­-450 C)  77­-1130 F 
   thermophiles or thermophilic (50­-700 C)  122­-1580 F 
           
a temperature of 750 C (1670 F) for 20 minutes will kill vegetative cells 
but not endospores 
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 
B. pH 
   most bacteria grow in the range of pH near neutrality; usually between 6.5 ­-75;
   as bacteria grow, their metabolic activities change the pH of their living 
   environment; i.e. the culture media; to neutralize these changes, BUFFERS are usually 
   added to the media to counteract or neutralize these metabolic changes and to keep 
   the pH constant; some bacteria are capable of living in what would be considered   
   extreme pH ranges, e.g. acidophiles ­ acid lovers can be found in acid runoff 
   with pH as low as 1.6; Thiobacillus thiooxidans; alkalophiles ­ live in 
   basic environments such as the ocean, pH about 8.2; similar to small intestine pH 

vast majority of BUFFERS are PHOSPHATE compounds because they are non-toxic; they work 
within the range of most bacteria and can serve as essential nutrients

pH Range				Solution Added to Reach that pH

 1.5-3.0				sulfuric acid
 4.0-6.0				citric or glutamic acid
 6.0-7.5				sodium or potassium phosphate
 8.0-9.0				"tris" or trihydroxymethylaminomethane
10.0-11.0				sodium silicate or sodium borate
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ 
C. Oxygen ­ respiratory requirements 
   microbes that use oxygen are called AEROBES; organisms that require oxygen in order 
   to live are called OBLIGATE AEROBES; some representative examples are: Bacillus, 
   Pseudomonas, Mycobacterium; microbes that CANNOT use oxygen or for whom oxygen 
   is actively toxic are called OBLIGATE ANAEROBES; some representative examples  
   are found in the genus Clostridium

   in both of the foregoing situations, regulating the amount of oxygen present 
   represents a means of controlling growth or the rate of growth of the microbes; 
   e.g. subjecting dead or NECROTIC tissue to oxygen under pressure (as in a hyperbaric 
   chamber) will force oxygen into these tissues and kill any anaerobic organism (but 
   not its endospores) present in these areas; the most dangerous organisms are those 
   most flexible in their oxygen requirements; organisms that can grow in the presence 
   of oxygen OR in its absence are called FACULTATIVE ANAEROBES; representative examples 
   include both Escherichia coli and Staphylococcus aureus  
         
Oxygen requirements fall along a continuum ranging from:          
OBLIGATE ANAEROBES­­­­­­­­­­­­­­­­­­­­­­­TO OBLIGATE ANAEROBES with  
FACULTATIVE ANAEROBES falling in the middle of this continuum.  
Additionally, some organisms can tolerate the presence of oxygen but not use it for growth; these are termed AEROTOLERANT ANAEROBES; other microbes can use oxygen, but only if the concentration is less than that found in the ambient air (20%); these microbes are called MICROAEROPHILIC; thus we have: 
           
OBLIGATE ­­ MICROAERO ­­ FACULTATIVE ­­ AEROTOLERANT ­­ OBLIGATE  
AEROBES               PHILIC                                                                    ANAEROBES          
 
THIOGLYCOLLATE BROTH is a liquid medium which can be used to grow all respiratory types.

Chemical Requirements
                                         
Microorganisms are cultured in water to which appropriate dissolved nutrients are added. 
        These nutrients fall into three categories:
	1. Energy sources
	2. Cell structural components (Elemental Requirements)
	3. Miscellaneous Growth factors

not all kinds of organisms require the same nutrients nor can any one organism use all kinds

1. Energy Sources 
   a. Organic energy sources ­ sugars, starches, fats, protein; glucose is the most 
      common; acetic, glutamic, lactic acid; used by most bacteria, all fungi and   
      protozoa 
           
   b. Inorganic energy sources ­ NH4+, nitrite, iron, H2S; only bacteria use these sources (sulfur bacteria, chemoautotrophs, 
      chemolithotrophs) 
           
   c. Light ­ photoautotrophs (cyanobacterium) 
           
2. Elemental requirements 
   Macro­ and micro­ or trace elements or nutrients; including 
   but not limited to: C, H, O, P, K, I, N, S, Ca, Fe, Mg 
           
3. Miscellaneous growth factors 
   required in small amounts and usually cannot be synthesized from other carbon 
   sources; varies by species 
   a. Vitamins ­ B1, biotin, pyroxidine (B6), B12, others may be needed 
           
   b. Amino Acids ­ get from protein digests; e.g. casein (milk          
      protein); peptone (meat protein) 
           
   c. Purines and Pyrimidines 
           
   d. Heme 
­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­