pH and Bacterial Growth
Name: Amy W.
Can you describe in detail how pH affects bacterial
growth, and what pH level allows minimum or maximum growth?
Bacteria need a physiological pH inside their cells,
just like all other living organisms. Their ability to
survive in extreme pH (either high or low) depends on
their ability to correct for the difference between
inside and out. One example of a bacterium that can
live in acidic environments is Helicobacter pylori
which lives in the stomach. It produces high amounts
of urease which is an enzyme that degrades urea, and
by doing so decreases the acidity (raises the pH).
Imagine the bacteria produce a 'cloud' of neutral pH
around them to protect them from the acidic
There are other bacteria that are specialized to live
in basic pH, for instance near black smokers,
geological fountains of minerals that shoot highly
alkaline minerals into the ocean.
In conclusion, which pH is lethal for the bacteria
depends on the species. Their defense is to keep the
protons or OH- ions out. Would they not succeed, then
their proteins would rapidly denature. That is the
lethal toxicity of non-physiological pH.
This would take about a thousand pages to give some of the details. VERY
briefly, the pH affects the ionization and therefore the binding and
interaction of a myriad of molecular processes...this includes very basic
things such as nutrient availability. For example, depending on the pH
certain metals will take on different ionization states and therefore will
or will not be able to be utilized. PH also affects the solubility of many
sunstances that bacteria need. There is also no certain pH level for
maximum growth for bacteria in general since they all differ slightly in
their evolution. Some grow in a very acid environment eg. H. pylori while
others do not..eg. many Pseudomonas. Species... nothing is simple in
Peter Faletra Ph.D.
Office of Science
Department of Energy
pH affects bacteria the same way it affects all living things. Extremes of
pH affect the function of enzyme systems by denaturing them. However,
bacteria become adapted over time to their surroundings. Just like we have
enzymes that are adapted to the pH of our stomachs (very acidic) or to the
small intestine (basic), bacteria that live in acid conditions are adapted to
them. If they are moved to an environment that is neutral or basic they will
probably die. So my point is, it depends on the bacteria and what its
natural environment is. Bacteria that are human or animal pathogens are
generally adapted to a pH of about 7.4 which is slightly basic.
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Update: June 2012