122 



Microbes and You 



It may seem incredible to a novice that it is possible to take a 

 loopful of culture containing hundreds or thousands of bacteria, 

 and by simply streaking that loop back and forth in an orderly 

 manner over the surface of nutrient agar in a 65 square centimeter 

 area of a petri dish, one can deposit in certain areas on that agar sur- 

 face single, well-isolated bacteria which are capable of growing into 



Fig. 25. When making streak platings, the lid of the petri dish 

 should be held in a position that will protect the surface of the agar 

 in the plate from outside contamination. 



distinct colonies. There are probably almost as many modifications 

 of technics for streaking plates as there are teaching institutions, 

 but most methods will produce satisfactory results if the prescribed 

 directions are carefully followed. Rather than outline any one 

 technic to the exclusion of others, it seems best to leave that spe- 

 cific detail to the individual instructor who undoubtedly has a 

 method he has previously found to be satisfactory. 



The streak plate is the most universally accepted procedure for 

 obtaining isolations from mixed cultures, and if a reasonable 



