70 OUTLINES OF BACTERIOLOGY 



namely, that there are some substances which, in a concentrated 

 solution, are able to abstract water out of the protoplasm of bacteria, 

 thereby causing their death. 



Suppose we place in a bucket of water a bottle full of alcohol, closed 

 at the top with a piece of parchment. After a time it will be found 

 that the parchment bulges outwards, showing that some water has got 

 into the bottle. It will also be found that some alcohol has gone out 

 into the water, and the bulging of the parchment is an expression of 

 the fact that less alcohol has gone out than water has passed into the 

 bottle. If we had placed a bottle of water, with a similar parchment 

 covering, in a bucket of alcohol, the parchment would be found to 

 bulge inwards. This interchange of liquids through a permeable 

 membrane is called Osmosis, and plays an important rdle in the 

 absorption of food material in the case of plants. In the case of 

 bacteria, the protoplasm represents the alcohol, the membrane the 

 parchment, and the nutrient medium the water of our illustration. 

 Under favourable circumstances of growth the protoplasm sucks in the 

 nutrient medium at a far greater rate than the latter sucks out the 

 water of the protoplasm, for the protoplasm itself cannot bodily pass 

 out. If, however, the concentration of the nutrient solution be 

 increased, the preponderance is reversed, and more water passes out 

 than nutrient solution passes in. This causes the 

 protoplasm, deprived of its water, to contract, 

 /^^ when it is said to be plasmolysed (Fig. 64). 



%>v Unless this is speedily remedied, a plasmolysed 



H cell is soon rendered incapable of growth, and 



\ A <S5> death ensues. Hence, in making a nutrient solu- 

 ^^S^ tion, care must be taken to prevent the food 



Fio. 64.— Plasmolysed material being too concentrated, in case some of 



cells. 



its compounds may have this plasmolysing power. 

 Thus, 5-10 per cent, cane sugar is a good nutrient material, but a 

 50 per cent, solution acts as an antiseptic. The high concentration 

 of the sugar in many jams is probably the cause of the comparative 

 immunity from the attacks of moulds which many jams and similar 

 articles enjoy. Again, common salt is used as a food material, but 

 if it is present in a concentrated solution bacterial growth is im- 

 possible. 



In considering the relation of water to bacterial life, we must also 

 bear in mind that absorption of food can be effected only when the 

 latter is in a state of solution, as no solids or gases can pass through 

 the membranes of bacteria. 



^ 



