METHODS FOR MICROSCOPIC STUDY OF BACTERIA 247 
differences of greater refinement than the accuracy of the method 
permits of, and for this reason the descriptions of bacterial colonies 
should not be carried to extremes. In general, bacterial growths on 
solid media are described as solids in space— the average size, form, 
color, luster and texture. This applies equally well to colonies, slant 
and stab cultures. The really valuable information gleaned from a 
study of bacterial growths is the recognition of types of growth. P'or 
example, spore-forming bacteria (aerobic) produce rather heavy, 
opaque, flocculent colonies; members of the alcaligenes— dysentery, 
typhoid, paratyphoid group— grow characteristically as rather small, 
round, transparent colonies. 
Arkwright' and others^ have found that under certain circumstances, 
pure cultures of many pathogenic bacteria as typhoid, paratyphoid and 
dysentery may yield two distinct types of colonies upon plating. These 
are designated as smooth (S) and rough (R) colonies respectively. 
The S colony is typical; thin, round, translucent, with regular edges. 
The R colony, on the other hand, is rather thicker and more opaque, 
somewhat irregular in outline, and with uneven edges. In broth the 
S colony emulsifies to a uniform, quite stable turbidity, and agglutinates 
characteristically with homologous sera. The R colonies on the 
contrary, are clumped in normal saline suspensions and settle out 
quickly. Salt solutions of 0.2 to 0.5 per cent, however, usually yield 
fairly stable suspensions. The agglutination reactions are frequently 
quantitatively at variance with the typical (S) strain. The same 
tendencies are noted in broth cultures. The R variant retains its 
characteristics tenaciously, although Jordan- and other investigators 
have succeeded in transforming the R variant of paratyphoid bacilli 
and certain other organisms back to the more typical S type. Loss of 
virulence quite commonly accompanies the change from the typical to 
the R type. Hadley^ has studied this phenomenon of microbic disso- 
ciation very carefully, and his monograph contains a full discussion 
and bibliography upon this highly significant phase of bacterial varia- 
tion and mutation. 
(6) The Emnneration of Bacteria.— A very practical application of 
the plating method for the isolation of bacteria is the enumeration of 
bacteria in water, milk and other similar substances. The principle 
involved depends upon the development of colonies of bacteria from 
single cells. If a definite volume of water, 1 cc. for example, is dis- 
tributed in melted agar, thoroughly mixed in the tube by rotation 
between the hands, and poured carefully into a sterile Petri dish, the 
number of colonies which develop within a definite period of incu- 
bation may be regarded as a measure of the number of living bacterial 
cells in 1 cc. of water. Experience has shown that the accuracy of the 
method is influenced somewhat by the number of organisms in the 
1 Jour. Pathol, and Bacteriol., 1921, 24, 36. 
- Jour. Am. Med. Assn., 1926, 86, 177. 
3 Hadley: Microbic Dissociation, Jour. Infec. Dis., 1927, 40, 1-312, for very com- 
plete discussion. 
