A statistical study of the form and growth of a diphtheroid bacillus

Abstract

In a previous communication 1 I described changes in the size of the cells of Bacillus megatherium during the growth of a culture, and variability in size, particularly the bimodality of the frequency curves in the early stages of growth. Further observations of this organism, varying the number and age of the cells in the inoculum showed that these changes are constant but vary in degree, the increase in size, and variation in size, and the tendency to bimodality being greater with smaller and older seedings; and that, while these changes take place during the vegative phase of the culture, there is apparently no actual correlation between the variations in size and the rate of cell division. It is noteworty that the coefficient of variation increases and decreases with the size of the cells.

I have made a similar study of a chromogenic diptheroid bacillus isolated from lake water. It is larger than most members of this group of bacteria, but like the rest of the group decreases in size during the vegetative stages of growth and increases during the resting period; the curve for size is therefore just the reverse of that for B. megatherium. The decrease in size began after a latent period and during the logarithmic growth phase. As the cells decreased in size the frequency curves became more symmetrical and the variation decreased; as the size increased again the curves became more extended and showed skewness. There was no tendency to bimodality the single mode gradually shifting. The coefficient of variation decreased with the actual decrease in size, i.e., during the period of miost rapid cell divisions. Therefore the coefficient of variation cannot be used as an index of rate of reproduction with organisms such as these, where the entire population is subject to fluctuations in size independent of the growth of the individuals from youth to maturity.