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Effect of pH
on Growth Rate

H err Gustav Druer, the Brno wine merchant has a problem and Brother Gregory has been asked to help.

You are to become his research assisitants and help him carry out a research investigation into the properties of microbes.


Brother Gregory has been given a series of the microbes and been asked to determine the growth properties of each species to see where it can grow best.

He wants you, his research assistants, to try growing these microbe under a number of different environmental conditions and find out how fast they reproduce.

This investigation involves a determination of the effect of hydrogen ion concentration (pH) on the growth of microbial cultures.

Microbes, even if they are supplied with all the necessary nutritional requirements, still may not grow.

Bacteria, single celled eukaryotes and other microbes, can only live and reproduce within a certain range of environmental conditions. Factors that can influence if or how microbes can grow are temperature, pH, dissolved gases, osmotic pressure and water availability.

Microbes, such as bacteria are sensitive to the hydrogen ion concentration they find in their environment. Large proteins, such as enzymes, are affected by pH. Their shape changes (they denature) and the very often brings about an alteration of the ionic charges on the molecule. Usually, the catalytic properties of the enzymes are lost and metabolism is halted.

Upper and Lower pH Values
Most bacteria grow best around neutral pH values (6.5 - 7.0), but some thrive in very acid conditions and some can even tolerate a pH as low as 1.0. Such acid loving microbes are called acidophiles. Even though they can live in very acid environments, their internal pH is much closer to neutral values.

Some bacteria produce acid as they grow. This acid is excreted and lowers the pH or the surrounding environment. This eventually brings bacterial growth to a halt unless something else in the environment neutralizes the bacterial acid.

When grown in broth, a buffering agent can be used to mop up the excess acid, and keep the pH of the growing culture at optimum values.

Each species of microbe has its own characteristic range of pH values in which it grows and reproduces best.

Tools of the Trade
In these investigations, a tiny group of each microbe species are placed into a liquid, nutrient filled broth that has been sterilized (so no other bacteria will compete!). Usually this is in a special flask (called an "Erlenmeyer flask"), which is slowly shaken (to keep the microbes and nutrient at uniform distributions).

Each growing culture is carefully buffered at the appropriate, and constant, pH for the length of the experiment.

At regular intervals of time, small samples of the growing culture are taken from the flask and all reproduction of the microbes stopped by some poison or inhibitor (they can also be chilled or frozen). The size of the population at each time point is then determined.

Mendel's Mother shows you --- -- how bacteria grow.

Recording Results

print out, and use this
Table of Results
to record your data

The results of each of your investigations should be recorded as a table (a Table of Results). In these tables you should indicate the name of the microbe being studied, the pH of the experimental broth, and make an accurate record of either the growth data (growth curve), or the value of the generation time (generations per minute), as required.

The logatithmic value of the generations per hour should also be recorded on your table of results.


print out, and use this
Presenting the Results
sheet to graph your data

The results of each investigation should then be presented as a graph.

The horizontal axis of the graph should be the intervals of the pH values at which the microbes were grown. The vertical axis should represent the logarithmic value of the generations per hour determined for that sample.

The shape of these graphs or plots is characteristic for each species of microbe, but each organism will show an optimum pH where growth proceeds most rapidly, and as the pH values either exceed, or fall below that optimum, growth slows down. Above or below the maximum and minimum permissive pHs, all growth stops.

------ begin growing cells.


Each investigation is carried out under a specific set of growth conditions.

A species of microbe is chosen first.

It is then necessary to chose a pH value. Use the sliding scale to set the chosen pH. The value chosen will appear in the box.

For each temperature, click on "GROWTH" and record your results. In some investigations you will need to record the entire growth curve (data on extreme right), but for most investigations you only need to record the "generations per minute" and "log. value of the generations per hour".

Record all the pHs and all the values where you see that the microbial species could grow at all. It is not necessary to record those values that occur when there is not microbial growth.

Science at a Distance
© 2000, Professor John Blamire