APPEARANCE OF BACTERIAL COLONIES 57 



frequently employed in industrial processes. According to the various 

 phenomena they produce they may be classified as follows: a. Zymo- 

 gens producing fermentation; b. Aerogens producing gas; c. Photogens 

 producing light; d. Chromogens producing color; e. Saprogens, produc- 

 ing putrefaction; f. Pathogens, producing disease. 



Physical Appearance of Bacterial Colonies and Individual Forms 



Because of their minute size — a space the size of a pinhead may hold 8 billion 

 of them — the student commences his study of bacterial growths in colonies or cul- 

 tures, each kind possessing characteristics by which they may be distinguished and 

 differentiated. 



The individuals in the colony, depending upon the kind of bacteria under ex- 

 amination, may be globular, rod-shaped, or spiral. Bacteria are classed according 

 to shape, as 



Cocci (singular coccus), globular or berry-shaped. 

 Bacilli (singular, bacillus — a httle rod), rod-shaped. 

 Spirilla (singular, spirillum), spiral or corkscrew-shaped. 



Sporulation. — A large number of bacteria possess the power of developing into 

 a resting stage by a process known as sporulation or spore formation. Sporulation 

 is regarded as a method of resisting unfavorable environment. This is Ulustrated 

 by the anthrax bacilli which are readily killed in twenty minutes by a lo per cent, 

 solution of carbolic acid, and able, when in the spore condition, to resist the same 

 disinfectant for a long period in a concentration of 50 per cent. And, while the 

 vegetative forms show little more resistance against moist heat than the vegetative 

 form of other bacteria, the spores will withstand the action of hve steam for as long 

 as ten to twelve minutes or more. 



Whenever the spores are brought into favorable condition for bacterial growth, 

 as to temperature, moisture and nutrition, they return to the vegetative form and 

 then are capable of multiplication by fission in the ordinary way. 



Reproduction. — Bacteria multiply and reproduce themselves by cleavage or 

 fission. A young individual increases in size up to the limits of the adult form, 

 when by simple cleavage at right angles to the long axis, the cell divides into two 

 individuals. 



Morphology Due to Cleavage. — According to limitations imposed by cleavage 

 directors, the cocci assume a chain appearance, or a grape-like appearance, or an 

 arrangement in packets or cubes having three diameters. This gives rise to the 



Staphylococcus (plural, staphylococci), from a Greek word referring to the 

 shape of a bunch of grapes. 



Streptococcus (plural, streptococci), from a Greek word meaning chain- 

 shaped. 



Sarcina, package shaped or cubical. 



Form of Cell Groups after Cleavage.- — The individual bacteria after cleavage 

 may separate, or cohere. The amount of cohesion, together with the plane of cleav- 

 age, determines the various forms of the cell groups. Thus among the cocci diplo- 

 or double forms may result giving rise to distinguishing morphological character- 



