312 MICROBIOLOGY OF WATER. AND SEWAGE 



indicus, B. rubescens and B. rubefaciens. Several yellow and orange 

 organisms are commonly found, such as B. aquatilis, B. ochraceus, B. 

 aurantiacus, B. fulvus, etc. 



At certain times, particularly in river and brook waters, organisms 

 producing violet pigment are quite common. B. molaceus or B. janthi- 

 nus, as it is sometimes called, is the prevailing type; others are B. limdus, 

 B. amethystinus, and B. coeruleus. 



The chromogenic cocci produce either orange or yellow pigment, and 

 as a rule are not numerous in water. Sarcina lutea is the most common 

 species. 



Non-chromogenic cocci (Group XV) are more frequent. M. candi- 

 cans, M. nivalis, M. aquatilis, are non-liquefying forms, and M. corona- 

 tus is the type of those which liquefy gelatin. 



SOIL BACTERIA FROM SURFACE WASHINGS. During times of flood, 

 high water, and after rains, numerous soil organisms are found in 

 natural waters; and occasionally certain species persist for a consider- 

 able time. Among the commonest species is B. mycoides, with its 

 characteristic rhizoid colony; also B. subtilis, B. megatherium, and B. 

 mesentericus vulgatus, with its allied varieties; likewise B. m.fuscus and 

 B. m. ruber all belonging to Jordan's Group VII, and having many 

 characters in common, such as characteristic colonies, followed by 

 liquefaction when growing in gelatin, production of spores, etc. 



^Cladothrix dichotoma, one of the thread bacteria, easily recognized 

 on gelatin plates by the brown halo that surrounds the colony, is often 

 found in fresh and stagnant water, and in most soils. It seems to 

 flourish wherever there is much organic matter. 



These are the soil organisms most often found when beef peptone 

 gelatin is used for isolating purposes; but if other media are used, a 

 different flora appears, and we find nitrifying organisms, yellow 

 chromogens, etc. 



INTESTINAL BACTERIA, USUALLY OF SEWAGE ORIGIN. Proteus 

 Group. There are several groups of sewage organisms found in impure 

 water; some of these are very abundant in crude sewage, but are not 

 found in such relatively large numbers in contaminated water. Jor- 

 dan's Group III contains the organisms belonging to the large proteus 

 group, the principal species being B. vulgaris, B. zenkeri, B. mirabilis, 

 B. zopfii, the sewage proteus of Houston, and B. cloaca. All these are 

 frequently found in impure water, and in sewage. In the latter Hous- 



