PROPERTIES USED FOR IDENTIFICATION 13 



Chap. V). Wlien a satisfactory growtli medium lias been 

 obtained, further assistance in identification can be obtained 

 from the investigation of colony form. The medium is mixed 

 with agar-agar, sterilised, and poured while hot into Petri 

 dishes ; on cooling, the medium solidifies as a sheet of nutrient 

 jelly. The organisms, in high dilution, are streaked on to the 

 surface of the solid medium, the dish covered, and then 

 incubated. Each organism on the medium proceeds to 

 multiply and to form a small pile or " colony " which, after 

 24-4:8 hours, is visible to the naked eye as a tiny stud or con- 

 vexity on the surface of the medium. Each single colony may 

 represent a pure culture in that it has arisen from a single 

 organism and, if the cells are far enough apart at inoculation, 

 then discrete and distinct colonies will appear on the plate. 

 Colonies of different organisms have different appearances: 

 Esch. coli gives smooth, round, translucent colonies; Strepto- 

 coccus faecalis on media containing glucose forms small, round, 

 white colonies; Staphylococcus aureus forms round, raised 

 colonies which turn golden-yellow or orange after 48 hours 

 incubation; Serratia marcesce^is gives blood-red colonies; 

 while Proteus vulgaris, which is highly motile, forms big, flat, 

 spreading colonies that look like mountain ranges on a contour 

 map. 



Oxygen requirement 



Bacteria fall into four main groups according to the oxygen 

 tension they can tolerate for growth: 



1. Strict aerobes: organisms which can multiply only in 

 the presence of oxygen. 



2. Facultative anaerobes : organisms which can live equally 

 well in the presence or complete absence of oxygen. 



3. Microaerophilic organisms: organisms which can live in 

 the absence of oxygen or in the presence of very low 

 oxygen tensions, high tensions being inhibitory. 



4. Strict anaerobes: organisms which can multiply only in 

 the complete absence of oxygen. 



