BACTERIA 19 



division the two new cells usually separate but in many types 

 they remain together, thus forming colonies (Fig. 8) of one 

 sort or another. If a coccus divides only in one plane, a chain of 

 cocci is formed (streptococci) ; if it divides in two planes, flat 

 sheets or masses are formed (staphylococci) ; if the divisions are 

 in three planes, cubical aggregates (sarcinae) result. 



Mass Action of Bacteria 



Obviously a single bacterium can not produce much of an 

 effect good or bad, but where colonies of millions of them are 

 involved their effects become very evident, and are beneficial or 

 harmful according to the kind of the bacterial colony and its 

 products. Thus in a can of improperly prepared vegetables or 

 fish, the bacterium causing ptomaine poisoning in man will mul- 

 tiply, although deprived of free oxygen, and the excretion product 

 of the multitudes of bacteria which result constitutes one of the 

 most powerful poisons known. In a similar way many types of 

 bacteria find a suitable environment in the organs and the cavities 

 of the human body, and the products of their activity, known as 

 toxins, produce various types of disease. Each toxin is different 

 from every other toxin and is specific in its effects. Streptococcus 

 pneumoniae causes about 95 percent of all lobar pneumonias; 

 Bacillus influenzae causes influenza; Bacillus typhosus, typhoid 

 fever; Bacillus tuberculosis, pulmonary tuberculosis; etc. 



Helpful Bacteria 



The mass action of bacteria, however, is by no means always 

 harmful, indeed the beneficial effects of most forms far out- 

 weigh the deleterious effects of the injurious species. The natural 

 decomposition of the dead bodies of animals and plants is brought 

 about mainly by bacteria of decay and many of the products of 

 their activities, usually accompanied by the production of mal- 

 odorous compounds, are often useful to other living things. They 

 also bring about the decomposition of protein to form relatively 

 simple nitrogenous compounds, such as amides, peptones, and 

 aromatic bodies, which are used directly as food by many types 

 of minute saprozoic animals and saprophytic plants.* Other 

 products of this bacterial activity escape as gases — carbon di- 

 * See "The Plant World," page 64, in this Series. 



