Bacterial Multiplication 145 



tremely reliable for determining total bacterial counts. Various 

 modifications in lighting and in staining of the bacteria have been 

 advocated, and interested students are referred to advanced text- 

 books for further information relative to these procedures. 



The three methods just discussed involve direct counting tech- 

 nics, but there are also some indirect procedures available for the 

 determination of total bacterial counts, and a few of these will be 

 mentioned briefly. 



OPACITY METHOD 



As bacteria grow in liquid media their protoplasm produces an 

 opaqueness, or turbidity. A given strain of organism of known 

 numbers will impart a standard opacity to the medium. There- 

 fore, by terminating bacterial growth at a given level, and by kill- 

 ing the cells to prevent further multiplication, this suspension of 

 cells can be employed as a standard to which can be compared 

 unknown suspensions of bacteria. If we have a series of known 

 standards of different densities, it is possible to match an unknown 

 with one of the opacity standards and to determine with reasonable 

 accuracy the number of cells in the unknown suspension. As with 

 the other technics previously discussed, the opacity method has its 

 limitations. For one thing, not all organisms are of the same size, 

 and one large cell may produce an opaqueness greater than one 

 small cell. But in a culture containing millions, or even billions, 

 of organisms this factor is probably compensated for. Trying to 

 match densities introduces the human error. But if more accurate 

 determinations are desirable, such aids as photometers (light- 

 measuring devices) or photoelectric densitometers (density- 

 measuring devices) are available. 



Bacterial suspensions, even though inactivated with heat, may 

 undergo physical change upon prolonged storage. To circumvent 

 this condition, chemical opacity standards may be prepared. 

 When certain chemicals (such as barium chloride and sulfuric 

 acid) are mixed together in controlled proportions, a precipitate 

 (milkiness) will form in a predictable quantity. This turbidity 

 can be standardized to correspond to known numbers of mor- 

 phological types of bacteria in suspension. Such nephelometers 



