BROWNIAN MOVEMENT 11 



The chemical composition of bacteria varies much, not 

 only in different species, but even in the same species 

 when grown on different nutrient media. All bacteria 

 contain proteins, lipoid substances, and salts. Nencki 

 termed bacterial protein " mykoprotein," and asserted 

 that it differs from ordinary protein matter in not being 

 precipitated by alcohol and in not containing sulphur. 

 This does not appear to be the case with the proteins 

 obtained by grinding bacterial cells, which seem to agree 

 with other proteins in heat -coagulation, etc. 



The proteins are mainly globulins and nucleo-proteins. 

 The cell- wall is relatively insoluble, and consists for the 

 most part of a material like chitin, and not of cellulose ; 

 in this respect bacteria resemble animal rather than 

 vegetable cells. Carbohydrates are generally scanty. 

 Spores differ from the parent cells in containing a larger 

 proportion of solids and less water. 



All species of bacteria, but especially the smaller ones, 

 when suspended in a fluid exhibit what is known as 

 Brownian movement, consisting of an oscillation with 

 some amount of rotation about a fixed point, but there 

 is little actual movement of translation, unless due to 

 flotation. This Brownian movement is physical and 

 not vital in origin, and occurs with all fine particles sus- 

 pended in a fluid, arid must be clearly distinguished from 

 a true vital motility. 1 Some bacteria are always motion- 

 less, others are more or less motile, but these, too, have a 

 resting stage. For motility to occur the cells must be 

 young, and the conditions favourable to growth and 

 development. Motility is due to delicate protoplasmic 

 threads termed " flagella " connected with the outer layer 

 of the cell protoplasm ; these vibrate to and fro and 



1 Brownian movement is due to " the incessant movements of the mole- 

 cules of the liquid which, striking incessantly the observed particles, drive 

 them about irregularly through the fluid " (Perrin). 



