Feb. 14, 1916 
Longevity of Soil Micro-organisms 
939 
organisms at any time as did the clay loam, although its oxygen supply 
in consequence of these same conditions must have been somewhat 
greater. . 
We therefore perceive that the optimum condition for microbial 
activity in soil is a proper adjustment of these previously mentioned 
factors. With regard to longevity, fewer factors are concerned, the data 
so far obtained indicating that it is a function of both grain size (and 
therefore amount of hygroscopic moisture) and content of organic sub¬ 
stances. 
The influence of soil type was made evident not only in the numerical 
counts but also in the varieties of organisms persisting in the different 
soils throughout the two months during which they were in the air-dry 
state. As the condition of the sand had been such as to favor the develop¬ 
ment of organisms with high oxygen requirements, plates of high dilu¬ 
tion always showed a predominance of those types. Such of these as 
were spore bearers became a larger and larger proportion of the total 
number, as the period of desiccation extended and the non-spore-bearing 
species died out. Among the spore bearers most frequently found were 
Bacillus mycoides and aerobes of similar morphological and cultural 
characters. Of the non-spore-formers an organism found in larger num¬ 
bers than any other single species showed the greatest longevity. The 
characteristics of this organism are as follows: 
It is a rod with rounded ends, 0.6/i by 1.3 to 1.5^. It is actively motile, non-spore¬ 
forming and non-capsule-forming. It is frequently observed in pairs. It stains 
readily with aqueous alcoholic fuchsin. In nutrient broth it produces a decided 
turbidity, some sediment, and a soft surface scum. The growth on agar is glistening, 
translucent, grayish white, and very abundant. On a gelatin stab there is a white 
surface growth, with a filiform growth in the stab, but not liquefaction. Litmus milk 
becomes bluer after 48 hours; some peptonization in 30 days. No indol from Dun¬ 
ham's peptone solution. Ammonia produced from Dunham's solution and nitrates 
reduced. Facultative anaerobe. Optimum temperature, 25 0 C. Habitat, soil. 
Physical conditions in the clay had somewhat inhibited the extensive 
multiplication of strongly aerobic types, but permitted the development 
of facultative bacteria. Since anaerobic organisms could not be secured 
by the method of plating used, no mention of them is possible. As the 
non-spore-bearing types declined, the plates showed more evidence of 
spore-bearing, strictly aerobic varieties similar to those met with in the 
sand. The fact that such colonies were not found until their diminishing 
numbers necessitated the use of lower dilutions suggests their develop¬ 
ment from spores which had merely remained latent in the clay without 
passing through a process of multiplication and subsequent destruction 
like the majority of the facultative non-spore-bearing species. The non¬ 
spore-forming organism showing greatest endurance of desiccation was a 
type identical with that persisting in the sand. 
