Bactericidal Effect of an Extract of a Soil Bacillus on Gram Positive Cocci

Abstract

An unidentified spore-bearing bacillus, capable of causing the lysis of living gram-positive cocci, has been isolated from a soil sample to which suspensions of these cocci had been added over a long period of time. Autolysates of cultures of the soil saphrophyte have yieded a soluble factor which lyses living staphylococci, pneumococci (R and S forms, irrespective of typederivation), hemolytic, green and indifferent streptococci (all types so far tested). The active principle is not volatile, does not dialyze through collodion membranes, and is heat-labile. It is very stable at alkaline reactions, but is rapidly inactivated at reactions more acid than pH 5.5, even at room temperature. When maintained at 0°C, the active principle can be precipitated quantitatively at pH 4.2-4.4, and the precipitate, redissolved in a neutral medium, exhibits the lytic activity of the original solution.

Several tests have been used to investigate the effect of this soluble extract upon the living cells of a number of bacterial species.

1. Lytic activity. 1 mg (dry weight) of the bacterial extract, added to 10⁹ pneumococci in a medium at pH 7.5, causes complete lysis of the bacterial cells after 1 hour at 377°C.

Staphylococci undergo solution under the same conditions, but somewhat more slowly. Although lysis of streptococci can also be obtained, it is always less complete and requires larger amounts of extract.

2. Bactericidal efect. Pneumococci, streptococci, and staphylococci incubated at 37OC with the extract, are rapidly killed. For instance, 1 mg of extract (dry weight) is enough to kill 10¹⁰ pneumococci or streptococci in 2 hours at 37OC. Similar results have been obtained with staphylococci. The cocci, however, do not lose their viability when the mixtures of extract and bacterial cells is maintained at O°C even for several hours.