154 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



1. Typical morphology, nonsporing bacillus, relatively small and often quite thick. 



2. Motility in young broth cultures. (This is at times unsatisfactory, as some 

 strains of the colon bacillus do not show it even in young bouillon cultures). 



3. Gas formula in dextrose broth. Of about 50% of gas produced, one-third 

 should be absorbed by a 2% solution of sodium hydrate (CO 2 ). The remaining gas 

 is hydrogen. (Later views indicate that the gas formula is exceedingly variable and 

 should not be depended upon. To carry out this test one fills the bulb of a fermenta- 

 tion tube with the caustic soda solution, holding the thumb over the opening or with 

 a rubber stopper, the bouillon culture and the soda solution are mixed by tilting the 

 fermentation tube to and fro. The total amount of gas is first recorded and then 

 that remaining after the CO 2 has been absorbed is reported as hydrogen.) 



4. Nonliquefaction of gelatin. 



5. Fermentation of lactose with gas production. 



6. Indol production. 



7. Reduction of nitrates to nitrites. 



To these may be added the acidifying and coagulation of litmus milk without 

 subsequent digestion of the casein. The production of gas and fluorescence in glu- 

 cose neutral red bouillon is also a very constant function of the colon bacillus. B. 

 coli aerogenes is similar to B. coli with the exception of nonmotility, formation of gas 

 in starch media (bubbles on potato slant) and frequent lack of indol production. 

 It is often, especially in milk cultures, provided with a capsule and rarely forms 

 chains. It is a member of the Friedlander group but differs from the typical pneu- 

 mobacillus by producing acid and gas in lactose broth and by its coagulation of 

 milk. 



B. coli anaerogenes is also similar to B. coli but does not produce gas in glucose and 

 lactose. This latter organism is not usually recognized by American authorities 

 but I have found on Endo plates an organism showing the red colony with metallic 

 luster which failed to produce gas in either glucose or lactose. 



NOTE. The reduction of neutral red with a greenish-yellow fluorescence is very 

 striking and has been suggested as a test for the colon bacillus. Many other organ- 

 isms, especially those of the hog cholera group, have this power. It is convenient, 

 however, to color glucose bouillon with about i% of a %% solution of neutral red. 



On the plates made for the detection of colon bacillus may be found 

 certain organisms which have origin in fecal contamination. The more 

 important of these are those of the paratyphoid, cloaca and proteus 

 groups. In addition, the B. fecalis alkdigines has not rarely been 

 isolated. Among natural water bacteria there may be present either 

 the liquefying or the nonliquefying B. fluorescens. These colonies 

 have a yellowish-green fluorescence. 



Certain chromogenic cocci and bacilli are found in uncontaminated waters as 

 B. indicus or B. violaceus. From surface washings we obtain certain soil bacteria as 

 B. mycoides, B. subtilis, B. megatherium. One of the higher bacteria which shows 

 long threads, Cladothrix dichotoma, is common, and is characterized by a brown halo 

 around its gelatin plate colony. 



