CLASSIFICATION 7 



When saprophytic bacteria gain access to meat, milk, ice cream, canned 

 vegetables, etc., they sometimes produce chemical changes which result in 

 poisoning when these articles are ingested. 



Parasitic bacteria occur chiefly . or exclusively upon living animal and 

 vegetable bodies. Some are entirely beneficent, others are injurious or de- 

 structive to certain species of animals or vegetables. 



Pathogenic bacteria are those which directly cause disease. Many of 

 them have the faculty of living either as saprophytes or parasites and hence 

 are almost ubiquitous, commonly present in air, water and soil. Others are 

 exclusively parasitic. Indeed, the requirements of some parasitic bacteria are 

 so exacting that their occurrence is limited to a single species of plant or animal 

 life. 



There are three groups of bacteria distinguished by their shape: bacilli, 

 which are rod-shaped, cocci, which are spherical, and spirilla, curved or spiral- 

 shaped organisms. Morphologically, each of these three groups show numerous 

 subdivisions. 



Cocci or micrococci when observed singly are spherical or nearly so and are 

 nearly equal in size, regardless of species. Six forms are observed: 



1 . Staphylococci arranged in irregular masses, said to resemble bunches of 

 grapes. Frequently the manipulations incident to placing them on a glass 

 slide and staining or otherwise preparing for microscopic study, destroys the 

 bunch of grape-like arrangement so that we see Staphylococci singly, in little 

 irregular clumps of two, three or more elements as well as in bunches. 



2. Streptococci occur in chains. These chains may be long or short, three, 

 four, five or more cocci in a row forming a short chain; these short chains are 

 usually straight. The longer chains, some of which are composed of 50 or more 

 cocci, may be straight or curved or tangled just like a piece of rope. 



3. Tetrads are cocci that appear in groups of four. 



4. Sarcina are cocci arranged in cubes showing four, eight, twelve or sixteen 

 elements on each side of the cube observed. 



5. Diplococci those arranged in pairs, the surfaces in apposition being 

 somewhat flattened, similar to coffee beans. 



6. Diplococci arranged in pairs, without flattening of adjacent surfaces, 

 elements being either spherical or lancet shaped. Some of these latter are 

 encapsulated. 



Pathogenic cocci do not possess the power of locomotion. Spore formation 

 by cocci is a mooted question; arthrospore formation may be a property of some 

 species. 



BACILLI 



Bacilli as a class are rod-shaped; some are so short and plump as to appear 

 ovoid, others are distinctly rod-shaped. They vary considerably in size. Ob- 

 serving the smallest, it is difficult or impossible to determine whether or not 

 their ends are square or round, but this can be noticed when observing large 

 bacilli. 



There are long, wide bacilli having rounded ends, long wide bacilli with 



