pension. If the formalin is very acid it seems preferable to neu- 

 tralize it with sodium hydroxide. The final suspension should show 

 a barely visible turbidity. 



To show the presence of pH-sensitive flagella the bacterial cul- 

 ture is divided into two tubes. Ten per cent dibasic potassium 

 phosphate is added to one tube to a concentration of 17c, and 

 10% monobasic potassium phosphate added to the other to a like 

 concentration. After mixing, formalin is added and the culture 

 washed as before. The flagella on some kinds of bacteria assume 

 the curly shape in the acid phosphate and the normal shape in the 

 alkaline phosphate. 



The author has encountered only one group of bacteria which 

 is injured by distilled water, namely the red halophiles. These 

 bacteria completely dissolve in distilled water. They may be 

 washed in 20% sodium chloride solution but very successful 

 flagella stains of these bacteria were not obtained. 



Preparation of Bacterial Smear 



Clean and grease-free sHdes are essential for good stains. In 

 emergencies powdered cleansers such as Bon Ami may be used. 

 The author uses concentrated sulfuric acid saturated with potas- 

 sium dichromate as cleaning solution. A strong solution of the 

 dichromate is first made in a relatively small quantity of water 

 and the sulfuric acid poured into this solution. If the cleaning so- 

 lution is kept at room temperature the slides may require several 

 days to a week before they are clean. Greater efficiency is obtained 

 with hot solution, e.g., 70° to 80° C. In this temperature range 

 most slides are satisfactorily cleaned overnight. A glass rack for 

 holding the slides in the cleaning solution saves time and trouble. 

 When the slides have been cleaned they must be thoroughly 

 washed, first in tap water and then in distilled water, to remove 

 every trace of the acid. After washing they are dried by being 

 placed upright against a clean surface, such as a large beaker 

 placed on a paper towel. The dry slides are stored in a clean slide 

 box. The fingers must never touch any part of the slide to be used 

 for staining. It is a good practice to indicate on the storage box 

 which end of the slide has been handled and use the other end for 

 the staining. Just before use the slide is heated in the colorless 

 flame of a Bunsen burner (the side to be used against the flame) 

 and then laid on a piece of paper, to prevent cracking, until cool. 



4 



