Biological Properties of “Fresh” and “Stock” Strains of the Meningococcus

Abstract

A study has been made of the biological differences in freshly isolated and old stock strains of the meningococcus. The majority of freshly isolated strains present colonies on blood agar which are pearly in color, lenticular and flattened in form, round in outline, and distinctly moist. While single colonies measure up to 4 mm. in diameter after 24 hours' growth, there is a tendency to coalesce with the formation of a sheet-like growth. By the end of 36 hours the original growth tends to become transparent and glassy, sometimes with the formation of minute, glistening crystals on the surface; new and opaque, yellowish white growth appears at the margin usually in the form of papillae, and this tends to grow over and obliterate the original glassy colony. While, save rarely, transplantation from the original growth is unsuccessful, subculture can be made from the marginal growth during the next 48 or 72 hours. The colonies so obtained are usually more opaque and domed than in the original culture. Individual colonies are mostly smaller and show less tendency to coalesce, but there are some larger, flattened colonies resembling those of the original culture, giving the plate a characteristically uneven growth. Further subculture accentuates those characteristics noted for the secondary growth—the colonies become smaller and more domed with less tendency to coalesce, Most of the older strains show opaque white colonies but in occasional stock strains the colonies are transparent. They remain viable on blood agar for 48 or 72 hours. In many the formation of crystals becomes well-marked at the end of 48 hours and subcultures are no longer successful. Rough forms have appeared in only one strain,—a stock Type IV culture with considerable yellowish pigment formation.