Osmotic Requirements of Bacteria

Put this in a subpage for those who do not undestandeRecall your general science experiments with osmosis. If you place corn syrup, or salt, or sugar in a semi-permeable membrane and tie it closed. and place it in tap water. It swell up and it may burst. That is because water will enter the area where it is lower concentration. An easy example is a whole raw egg. It contains proteins in high concentration and some salt. If you place an egg in water, water will enter the egg. If you had some way to measure, you would find pressure inside the egg rises. If are able to insert a glass or plastic tube into a raw egg and seal the joint with parafin then water will rise in the tube. Another simple demonstration is to place an egg so only a small amount of the small end of the egg is above water. After you have it firmly located, poke a small hole in the small end of the egg and let it stand for a few hours. If I have said is true, then water should enter the egg through its membrane and force contents of egg out the small hole.

A similar thing happens when you put any cell into a liquid medium. If the medium is high in salt or sugar High osmotic index., the water flows from the cell into the saltwater until the water concentration on both sides of the cell mebrane is equal. Thus, if you drop a cell into high osmotic fluid the cell will shrink (lose water). If you drop a cell into pure water, the cell will swell and perhaps burst. Therefore, it makes sense that if you are going to grow bacteria, yeasts, human cells, the osmotic pressure of the medium should be similar to that inside the cell.

Bacteria species differ in ability to grow in high osmotic media. Long before man developed the the osmotic theory discussed above, he learned by experience that high osomotic media are one one to preserve foods by preventing growth of microganism. Perhaps, he noticed honey doesn't rot readily. For example, ripe fruits which would rot quickly may sometimes be preserved for winter food, if one drops the fruit into honey. It may not rot. That is why we make jelly. Fruit juice is an ideal food for many bacteria, fungi, and yeasts, but if you centrate the juice by boiling and add lots of sugar, spoilage is less likely. In fact, usually only molds will develop in jelly. Recall that candy doesn't rot unless it gets very wet. Indians ground meat and fat with dried fruit. This pemmican resisted spoilage somewhat. However, fresh meat stored in saturated salt brine resists spoilage unless a salt loving bacterium contaminates the stored meat. Milk spoils quickly, but make it into cheese containing high salt and it can be taken on long journeys. Or place meat in pure dry salt and it will be preserved more or less. Ask anyone who is an expert canner, and she can tell you that in addition to the salt or sugar. you need to wash the produce carefully and Usually boil in some in water to kill the bacteria on the surface. For example, you can't preserve hambuger because the bacteria are thourgly mixed into the meat. Yet some saugasges are made with salt and bacteria to keep at room temperatures and are called summer sausages.

SauerKraut is a good example. Cabbage rots quite readily. But cut it finely so salt is in good contact and certain bacteria lactic acid bacteria. are able to grow. They produce acid from the sugars and the acid plus salt preserve the sauerkraut [German: sauer=acid; kraut=cabbage. Another example is pickles. Huge open silos are filled with salt brine and cucumbers outdoors. Only a few bacteria can grow and they generatre acids. The process is not really very clean. Wine is make similarly, but here alcohol is formed and it kills many organism. Hover, cetain bacteria can oxidize the wine to vinegar (acid).

Some of the above example demonstrate the importance of high external osmotic pressure in food preser`vation. In this experiment we shall study the effect of high osmotic pressue on the growth of different species of bacteria.

The follow bacteria are very suitable for these experiments: Escherichia coli, Beneckea Harveyi MAV, Staphylococcus epidermidis, Halobacterilum halobium R1, Pseudomonas fluorescens, Lactobacillus casei, Photobacterium phosphoreum, and Bacillus subtilis 168.

can use an inoculating loop to measure out the bacteria. .

Build a table like that on b062; except here the table gives info on the high osmotic and normal bacteria. Bergey gives salt data, do I fhave have any sugar data. ?

To be continued


Written by Harold Eddleman, Ph. D., President, Indiana Biolab, 14045 Huff St., Palmyra IN 47164
Last modified 1999 January 23.
Suggestions, corrections, and comments are appreciated: Contact Harold Eddleman (indbio@disknet.com).