Morphology of Nitrosomonas europaea and classification of the nitrifying bacteria.

This medium, dispensed in 100-mi amounts in 250-ml Erlenmeyer flasks, was inoculated with an agar block containing L colonies which had been growing 5 days following 15 passages on a staphylococcal L form maintenance agar (Marston, J. Infectious Diseases, 108, 75-84, 1961). Slight turbidity and a granular sediment developed in the liquid medium in 4 days at 37 C and serial transfer of the sediment resulted in more abundant growth in a shorter time. The sediment was composed of minute, morphologically indistinct elements and some large bodies; no cocci were observed. Many of the large bodies appeared to contain particles which showed Brownian movement. That the granular sediment contained L forms was demonstrated by transferring 0.1 ml to brain heart infusion agar plates with final concentrations of 5% NaCl, 10% horse serum and 1,000 units/ml penicillin G. This transfer resulted in appearance of typical L colonies. Fig. 1 shows the granular elements in liquid media and an L colony derived from them. The colonial morphology was, in all respects, identical to the L colonies used as the initial inoculum. No reversion of the L forms to cocci was observed following continued cultivation or repeated serial transfer to media free of penicillin and with a lower salt concentration. Serological and biochemical studies of staphylococcal L forms cultivated in this manner are rendered more tenable and should contribute to the expanding knowledge of them.