146 Microbes and You 



when sealed to prevent evaporation, can be kept almost indefinitely. 

 Just before use, the precipitate can be resuspended by shaking to 

 distribute the milkiness evenly throughout the test tube. When 

 adjusting the density of unknown bacterial suspensions, it is im- 

 portant to remember that tubes of the same diameter as the stand- 

 ard comparison tubes must be employed. 



This method is accepted by some workers as a good index of 

 bacterial mass but not as an accurate means of determining num- 

 bers of bacteria. Leise in 1926 reported that opacity of spherical 

 cells is determined by the surface area of the cells and thus varies 

 with the radius. Vaccines, especially autogenous vaccines, are 

 commonly standardized as to numbers by resorting to the opacity 

 technic. 



CENTRIFUGE METHOD 



While not too accurate a method for indirectly measuring total 

 numbers of cells, the centrifuge technic is a rather ingenious ap- 

 proach to the problem. By centrifuging (spinning down) a broth 

 culture in a special capillary tube and measuring the volume of 

 packed cells, the total number of cells in the packed mass can be 

 calculated, if you know the diameter of a single cell. The volume 

 occupied by an individual cell can be divided into the volume of 

 packed cells in the centrifuge tube, and an estimate of the total 

 number of bacteria can be obtained. 



A miscellaneous group of methods available for these determi- 

 nations of total numbers of cells, including the calculation of the 

 total nitrogen content of the cells, the amovmt of acid produced by 

 a culture, direct weighing of bacterial masses, etc., are interesting, 

 but they are not as commonly used as the ones discussed above. 



TECHNICS FOR DETERMINING VIABLE BACTERIAL COUNTS 



BROTH DILUTION METHOD 

 The dilution method discussed under isolation technics for bac- 

 teria may also be put to use in determining the number of living 

 bacteria in a suspension. This is also known as the most probable 

 numbers method, and it has application in water and sewage analy- 

 sis where the determination of the most probable numbers of coli- 



