Study of Pole Assembly in
Bacillus subtilis
by Computer Reconstruction of Septal Growth Zones Seen in Central, Longitudinal Thin Sections of Cells
Authors: Burdett I. D. J. , Higgins M. L. 
Published: January 2020

The septal growth of
Bacillus subtilis
168/s has been studied by making a number of observations from thin sections of cells from exponentially growing cultures. The process was initiated by the formation of a new cross wall under a preexisting layer of cylindrical wall. An annular notch appeared to cut through the overlying wall and presumably allowed the cross wall to split into two layers of peripheral wall. During this initial notching process, two raised bands of wall material were produced which resembled those previously observed in morphological studies of
Streptococcus faecalis
. Through an improved fixation technique, it was possible to preserve the bands seen in
B. subtilis
to the extent that they were used as markers to study the subsequent stages of septal growth. These stages included (i) the continued displacement of the two bands from the cross wall (as the two nascent polar surfaces enlarged and as the diameter of the cross wall decreased), (ii) the closure of the cross wall, and (iii) the final severance of the common cross wall connection between two completed poles. To study this process in a more quantitative manner, three-dimensional reconstructions of the envelope observed between pairs of the raised bands were made from axial thin sections of cells. The process of reconstruction was based on a technique by which
x, y
coordinates were taken from thin sections and were rotated around the cell's central axis. These reconstructions were used to estimate the surface area or volume of the reconstructed zones or their parts. A round of septal growth was then simulated by arranging 118 reconstructions in order of increasing surface area or volume. The topology of the process was studied by noting how various measurements of septal thickness, length, surface area, and volume varied as a function of increasing septal zone size. This analysis was based on several assumptions, of which three of the most important are: (i) the bands produced by the initial notching process are markers which separate septal from cylindrical wall growth; (ii) a septal zone observed between pairs of bands is made up of two nascent poles and a single cross wall; and (iii) as septal zones develop in terms of relative age they increase in size (volume or surface area) or amount of wall. The data suggested that the
S. faecalis
model of surface growth (in which polar growth occurs through a regulated constrictive separation and expansion of a cross wall) also seems applicable to the pattern of septal growth observed here for
B. subtilis
. This was indicated from measurements which showed that increases in the size of nascent polar surfaces were correlated with decreases in cross wall diameter. An explanation of these observations may be that decreases in cross wall diameter were due to a progressive splitting of the cross wall that removed surface from the outer circumference of the cross wall and converted it into new polar surface. Calculations further suggested that if the poles of
B. subtilis
were made by this model a sizeable and variable increase in surface area of the cross wall would also be required to convert these separating cross wall layers into two curved polar structures. Measurements of wall thickness taken from various locations within septal zones indicated that while the thickness of the polar wall of
B. subtilis
was constant over its surface, the width of the cross wall varied considerably during a round of synthesis. Again, one of the simplest explanations compatible with these observations and those previously made in
S. faecalis
is that the
B. subtilis
cross wall is brought to a constant thickness (possibly by remodeling or precursor addition) before or during separation. Although most observations made from the reconstruction of the septal zones of
B. subtilis
may fit the
S. faecalis
model of surface growth, differences in the pattern of septal growth were seen when the two organisms were compared. These have been discussed in terms of differences in the regulation of their respective septal growth sites and basic mechanisms of wall assembly and modification.