Observations with Oerskov's Milk Bacillus

Abstract

Oerskov 1 reported that in the cultures of certain fluorescens strains isolated from milk, if the media contains saccharose, masses of tiny non-stainable granules develop which multiply independently from the bacteria. The cultures on ordinary agar plates correspond in every respect to the cultures of other fluorescens strains; on saccharose agar plates the colonies consist mostly of an amorphous substance in which strands of bacteria are embedded. By direct examination under the microscope or by the usual bacterial staining methods in the amorphous substance, no form elements are visible. In dark background preparations it seems to consist of innumerable tiny granules small enough to pass a Berkefeld filter. If the development of the colonies is examined under the microscope, it is seen that the bacilli and the amorphous substance start to grow separately. The amorphous substance grows in tiny round transparent colonies with a more refractile zone at the center. Most of these colonies coalesce later with the colonies of the bacillus and those which remain free grow only to a size of 0.1–0.2 mm. Oerskov did not succeed in growing the granules separately from the bacteria but when a bacillus emulsion was killed by moderate heat and planted, the granule colonies grew between the dead bacilli. The conclusion of Oerskov is that a virus-like organism is growing in symbiosis with the bacillus.

We succeeded at the first attempt in isolating Oerskov's bacillus and were able to verify his most important observations. In the following points our observations supplement those of Oerskov or are at variance with them.

The granules can be cultivated separately from the bacillus. The granule colonies are of 2 distinct types; one is smaller, perfectly transparent and gives no growth in transplants; the other is somewhat larger and, as already indicated by Oerskov, it contains a more refractile central zone.