1. The three organisms B. pyocyaneus, B. fluorescens-liquefaciens and non-liquefaciens form a closely related group of organisms both biologically and also by reason of their behaviour towards disinfectants.
2. Bacteriological observations support the view that germicides can naturally be divided into two classes, viz. “chemical” and “physico-chemical,” their actions on micro-organisms depending respectively on
(1) Chemical reactivity with protoplasmic constituents.
(2) Denaturation or precipitation of the cell proteins.
3. B. fluorescens non-liquefaciens and related organisms are selectively attacked by “physico-chemical” germicides, e.g. phenol, whilst B. coli is less sensitive to their action, but is particularly susceptible to the “chemical” disinfectants, e.g. quinones.
The B. fluor. non-liq. group is also more readily affected by hot water than B. coli.
4. Substitution of various groups into the benzene nucleus in the case of aromatic disinfectants affects the germicidal power on B. coli and B. fluor. non-liq. unequally, with the result that the selective action and differentiation may be obscured or even reversed in the case of the substituted derivatives.
5. B. fluor. non-liq. is more sensitive than B. coli to the action of reducing agents, and this factor also affects the degree and nature of selective action, sometimes resulting in the classification of “chemical” germicides in the “physico-chemical” group.
6. Selective action is not affected by changes in experimental conditions, such as temperature and period of disinfection, nature of culture medium, number of bacteria present, or physical state of the disinfectant. The susceptibility of B. fluor. non-liq. to the action of hot water, and such germicides as phenol and alcohol, is thus probably associated in some way with its internal colloid state.
7. The analogy between the action of hot water and that of the alcohols and phenols is borne out by preliminary experiments with the thermophile bacteria (heat resistant). These organisms are somewhat less sensitive to phenol that B. coli, and more susceptible to p-nitrosodimethylaniline.