BIOLOGICAL PROPERTIES 



621 



Virulence. The virulence of the staphylococci is subject to variation which 

 cannot be anticipated. As a rule, virulence is markedly lowered in old 

 cultures, and to maintain it, it is necessary to sow sub-cultures every 5 or 6 

 days and to pass the culture through a rabbit from time to time by intra- 

 peritoneal inoculation. 



FIG. 2Q2.Staphylo- 

 coccus aureus. Surface 

 culture on agar (5 days). 



FIG. 2Q3.Staphylo- 

 coccus albus. Surface 

 culture on agar (5 days). 



FIG. 294Staphylo- 

 coccus citreus. Surface 

 culture on agar (5 days). 



Staphylococci recovered from the circumambient media are often avirulent ; 

 sometimes even the staphylococcus isolated from a septic infection in man 

 is absolutely devoid of virulence for laboratory animals. 



The addition of glucose to culture media increases the virulence of staphy- 

 lococci (Budjwid, Nicholas). 



2. Bio-chemical reactions. 



[Gordon from an investigation of a number of staphylococci obtained from 

 various sources (air, skin, saliva, scurf, urine, sputum and pus) showed that 

 " comparison between various staphylococci in regard to nine selected actions 

 has revealed differences not merely of degree, but of kind, and has shown 

 that a differentiation far more elaborate than has yet been supposed to exist 

 naturally obtains amongst staphylococci." 



[The tests used by Gordon were as follows : 



1. The action on gelatin (12 per cent.) with regard to liquefaction when 

 incubated for one week at 22 C. 



2. The clotting of litmus milk when incubated for one week at 37 C. 



3. The peptonization of milk under the same conditions. 



4. The reduction of nitrate to nitrite during incubation at 37 C. for three days. 



5. The reduction of neutral red in a broth medium when incubated at 27 C. 

 anaerobically for two days. 



6. The production of acid in a slightly alkaline litmus broth containing 1 per 

 cent, lactose when incubated at 37 C. for one week. 



