MORPHOLOGY AND RELATIONSHIPS OF MICROORGANISMS 27 



MORPHOLOGY OF BACTERIA 



Shape of Bacteria. Bacteria may be classified according to 

 shape into spheres, straight rods, bent or spiral rods, and filaments. 



A spherical form is called a coccus (pi. cocci). Although the 

 coccus is theoretically spherical, there are many that appear some- 

 what flattened or ovoid when in groups or chains. 



Fig. 2. Types of bacteria: Cocci, bacilli, and spirilla (Jordan). 



A straight rod is called a bacillus (pi. bacilli). 



A curved or spiral rod is called a spirillum (pi. spirilla). 



The filamentous bacteria are those in which the organism is 

 greatly elongated. No specific name has been given to this form. 

 Frequently the filamentous type exhibits branching and in other 

 ways resembles the mycelium of the higher fungi or molds. 



Fig. 3. Involution forms of bacteria: 1, Bacillus radidcola a, Normal 

 rods; b, bacteroids. 2, Bacillus tuberculosis. 3, Bacillus subtilis. 4, Bacillus 

 aceti. 5, Bacillus pestis. 6, Actinomyces sp. 7, Spirillum sp. 8, Micro- 

 coccus aureus a, Normal forms; b, involution forms. 



When grown under favorable conditions, as a result of the 

 action of certain stimulation, many bacteria assume unusual and 

 abnormal shapes. Cells of this type are called involution forms. 

 Such cells are not necessarily incapable of continued growth and 



