82 ELEMENTS OF BIOLOGY 



dead tissues of all sorts, and are commonly present in all natural 

 environments unless special effort is made to remove them. 



According to their shape, visible bacteria may be grouped into 

 the following (Fig. 26) : 



Coccus: These are spherical in shape. They may exist singly, 

 scattered about without any pattern or arrangement, the Micrococci; 

 doubled in pairs, the Diplococci; in chains, the Streptococci; or in 

 groups of eight arranged as two tiers of four each, the Staphylococci. 



Spirilli: These are motile spiral forms; in some types the spiral 

 is flat, in others open, as a corkscrew. 



Bacilli: Blunt motile rods, or longer non-motile rods. 



Bacteria are also classified according to their efforts on the host. 

 Those which cause disease are termed pathogenic, and those which 

 are without injurious effect are termed non-pathogenic. 



Pathogenic Bacteria. Bacteremia. The action of bacteria 

 in producing disease may be of two general types. In some dis- 

 eases the bacteria become scattered throughout the body, thus work- 

 ing a general destructive effect. This is bacteremia or septicemia. 

 It is invariably accompanied by the second type of effect, namely, 

 toxemia. In toxemia the distressing effect on the host is the result 

 of the poisonous metabolic products of the bacteria circulating in 

 the body of the host and having both general and specific toxic 

 effects on the tissues of the host. 



Toxemia. Bacteria, like all other living organisms, carry on 

 metabolism and in their metabolic processes produce waste sub- 

 stances. It happens that the waste products of some types of bac- 

 teria are poisonous, in some cases incredibly so. A toxemia is there- 

 fore the effect of a local growth of bacteria producing poisonous 

 substances that are distributed throughout the body of the host. In 

 some types of disease these toxins affect some tissues more than 

 others. For example, in the case of tetanus, or lock jaw, the growth 

 of bacteria is confined to the immediate neighborhood of the wound, 

 but the toxins there produced are carried about and appear to 



