the polar flagella were finally visualized but would have been 

 missed otherwise. Reliance cannot always be placed on spreading 

 growth in semisolid or motility agar. Polar flagellated bacteria 

 spread much less than the peritrichous flagellated, and if the 

 flagellation is poor and polar, the culture may appear entirely 

 nonmotile in the semisolid agar. 



The simplest technique for making a satisfactory moist prepara- 

 tion is to place a loopful of culture on a slide and observe directly 

 with low and high dry objectives. With proper adjustment of the 

 condenser most bacteria may be seen. Light through a ground 

 glass is better than light through a blue glass. The author uses a 

 20 X objective. For small bacteria, and poorly motile bacteria, a 

 disk with an opaque center sHpped into the condenser is very effec- 

 tive. Using the 10 or 20 X objective and the condenser all the way 

 up, a very nice dark field may thus be obtained and even the 

 slightest motion of the smallest bacteria detected. With low mag- 

 nification the observer is less likely to mistake Brownian motion, 

 and motion of convection currents, for vital motion. 



Staining of Flagella 



Bacterial Suspension 



If the bacteria are growing on a solid surface a Hght suspension 

 is made in distilled water, taking care that only bacterial growth 

 and none of tlie agar is carried into the suspension. For routine 

 diagnostic purposes staining may be made directly from this sus- 

 pension. Better preparations may be obtained by washing the 

 bacteria. If the bacteria are pathogenic, formalin should be added 

 to the suspension to a concentration of from 5 to 10%. The author 

 routinely adds formalin to every suspension. Washing is accom- 

 plished as described for broth cultures. 



If the bacteria are in broth culture, add 5-10% formahn, dilute 

 with distilled water, mix and centrifuge; pour off supernatant and, 

 wliile tube is still inverted, rinse lip of tube with distilled water to 

 remove any supernatant which clings; now add 1-2 ml. of distilled 

 water and shake to resuspend bacteria; dilute with distilled water, 

 mix and recentrifuge; pour off supernatant as before, rinse lip of 

 tube; suspend bacteria in 1-2 ml. of water and dilute to light sus- 



