∗ Bactericidal Action of Sea-Water

It has been shown by the author 1 that sea-water inhibits the multiplication of freshwater bacteria while marine bacteria require sea-water media for their initial isolation, although these differences rapidly disappear following the laboratory cultivation of the bacteria. Similar observations on the selective bacteriostatic action of sea-water have been reported by Berkeley, 2 Lipman, 3 and Korinek. 4

It is generally known that intestinal and other bacteria of public health interest do not survive very long when emptied directly into the sea unless there is appreciable organic matter present or considerable freshwater dilution. Colon bacilli are rarely recovered from the open sea and they occur far less frequently in the vicinity of sewage effluents than can be accounted for by dilution or oceanic circulation. The failure of freshwater bacteria to survive in the sea has been attributed by various investigators to predacious protozoa, lack of nutrients, bacteriophage, 5 or to the lethal effect of sunlight. The following experiments reveal that sea-water per se is bactericidal.

In the first series of experiments sewage was appropriately diluted with “formula C”† and 1.0 cc. quantities of the resultant dilutions were transferred to 99 cc. water blanks consisting of different kinds of sea-water as well as a “formula C” control. After thorough mixing by shaking, 1.0 cc. quantities of the final dilutions were plated with nutrient agar. About one minute elapsed from the time the final dilutions were made in sea-water until the agar was poured. Additional 1.0 cc. samples were plated after 15 minutes and again after 30 minutes, and at 30-minute intervals thereafter. The plates were incubated 4 days at 37°C. and the colonies counted.