116 A MANUAL OF BACTERIOLOGY 



A desiccation of four months at 32 C. (dry heat) l 

 does not destroy the vitality of M. prodigiosus; 

 but when exposed to the action of ozone the microbe 

 is killed. 2 From these facts one can readily under- 

 stand how it is that M. prodigiosus (as well as other 

 aerial microbes) is always present in the air of 

 towns, villages, etc. ; but is never in the air at sea, 

 for the ozone present in sea-air destroys the microbes. 



Micrococcus luteus. This is another chromogenic 

 aerial microbe. It is found as single cells, dumb- 

 bells, or in packets. The cells are 1 '2 p in diameter ; 

 and they grow rapidly on nutrient gelatine plates 

 (plate-cultivations) giving rise to a yellow pigment. 

 The colonies, so produced, are round and slightly 

 granular in appearance. M. luteus grows in nutrient 

 agar-agar, bouillon ; on steamed potatoes ; and as 

 drop -cultures. The pigment produced by this microbe 

 is insoluble in water, and is unchanged by sulphuric 

 acid and alkalies. It is also destroyed by the action 

 of ozone. 



Micrococcus chlorinus. This microbe produces a 

 yellowish green pigment when grown on sterilised 

 white of egg (see Fig. 21) and fluid media. The 

 cells are about 1 ft in diameter. The pigment is 

 soluble in water, and is decolourised by acids. 



Micrococcus aurantiacus. The cells of this aerial 

 microbe are 1*5 //, in diameter ; and they occur 

 singly, in pairs, or in zooglea. On plate-cultivations 

 they form orange-coloured drops and spots, which 



1 Griffiths in Proceedings of Royal Society of Edinburgh, vol. 

 xvii. p. 262. 



2 See Griffiths' Researches on Micro-Organisms, p. 184. 



