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Bacterial Growth

Bacterial are very diverse and quite adaptable, but they still need certain factors in order to thrive.  The physical requirements for bacterial growth include pH range, temperature, oxygen concentrations, moisture levels, hydrostatic pressure, osmotic pressure, and radiation levels.

When considering the pH level, bacteria are classified as either acidophiles (acid-loving), neutrophiles (neutral ph range), or alkaliphiles (alkali-loving).  The one that causes disease in humans would be the neutrophiles, which have an ideal pH range of 5.4 to 8.0.  There are exceptions, however, like Alcaligenes faecalis and Vibrio choleae, which are both alkaliphiles and can infect humans.

Bacteria can also be classified as to the temperature range in which it grows best.  Psychrophiles are cold-loving and can live in temperatures from 0 - 20 C.  Mesophiles are where the majority of bacteria fall, living between 25 - 40 C.  Thermophiles are heat-loving and grow starting at 50 C all the way up to 110 C.  The human body is about 37 C, which means that mesophiles  are the bacteria that cause infection to humans.

Oxygen requirements classify bacteria as aerobes which need oxygen to grow, or anaerobes, which do not.  While both aerobes and anaerobes can cause disease in humans, aerobes are limited by the amount of oxygen they receive. 

Moisture and hydrostatic pressure,are also factors affecting bacterial growth.  The bacterial cells need moisture to live and most would die after only a few hours in a dry climate.  Hydrostatic pressure is the amount of pressure exerted by water as you travel deeper.  Bacteria can live at all different pressure ranges but there are some, called barophiles, that can only live at very high pressures.  These cells need the pressure to keep them intact.  When the pressure is released, they tend to lose their shape and die.  

Osmotic pressure plays a part in cell growth also.  Most bacteria can regulate this pressure due to a selectively permeable cell wall which allows movement and equalization.  Sometimes, however, the cells can distend and die because the pressure has become too much and they fill with water. 

The last physical factor affecting bacterial growth is radiation energy.  This consists of gamma rays and UV light.  These can both cause mutations or death to the cell. 

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