Fig. 1. Magnified photograph of an Esch. coli colony grown on broth-agar 

 medium containing 1 [jig. of streptomycin per ml. and incubated at 37° 

 for 48 hours. Note the light background region of bacterial growth with a 

 number of faster-growing secondary colonies on top of it. Bacteria from 

 these secondary colonies are more resistant to streptomycin than bacteria 

 taken from the background region. 



[ facing page 55 



