Unit Cell Hypothesis for Streptococcus faecalis

Abstract

The mass doubling times of exponential-phase cultures of Streptococcus faecalis were varied from 30 to 110 min by omitting glutamine from a defined growth medium and providing different concentrations of glutamate (ranging from 300 to 14 ,Ig/ml). After Formalin fixation, cells were dried by the critical point method, and carbon-platinum replicas were prepared. The surface area and volume of cell poles seen in these replicas were estimated by a computer-assisted, three-dimensional reconstruction technique. It was found that the amount of surface area and volume of poles seen in these replicas were independent of the growth rate of culture from which the samples were taken. These observations were consistent with the unit cell model hypothesis of Donachie and Begg, in which a small number of surface sites would produce a constant amount of new cell surface regardless of the mass doubling time of the culture. However, measurements of the thickness of the cell wall taken from thin sections of the same cells showed that the cell wall increased in thickness as a function of the increase in cellular peptidoglycan content which occurs when the growth rate of this organism is slowed down by a decrease in glutamate concentration. Thus, it would seem that although the size of polar shells made by S. faecalis is invariant with growth rate, the amount of wall precursors used to construct these shells is not. MATERIALS AND METHODSCell growth. Cells of S. faecalis ATCC 9790 were allowed to go through at least six mass doublings in a chemically defined medium (14) (reaching the equivalent of 100 ,ug of dry mass per ml) before being used for study. Mass doubling times (TD) between 30 and 110 min were obtained by removing glutamine and providing a range of concentrations of glutamate (between 300 and 14 ug) (16). Electron microscopy. Formaldehyde prepared from the hydrolysis of paraformaldehyde was added 499 on August 1, 2020 by guest http://jb.asm.org/ Downloaded from 500 EDELSTEIN ET AL.

The different shapes of cocci

“…The drift could be a random diffusion, with the machineries reaching the equator being trapped by the presence of FtsZ. The possibility of additional layering synthesis is attractive, as it would also explain the fact that the cell shape, cell volume and cell surface are constant at various growth rates, but that the amount of synthesized peptidoglycan is not (Edelstein et al, 1980). The thicker cell wall of slow-growing cells would result from the longer time available for the PBPs to relocate at the equator of ovococci.…”

Initiation of wall assembly sites in Streptococcus faecium

“…This process occurs in discrete growth sites which are found between pairs of raised bands of wall material (8). By using electron microscopy to study replicas of exponential-phase cells which had doubled in mass between 30 and 110 min, Edelstein et al showed that both the average surface area and volume of the poles of these cells are constant and invariant with growth rate (4). These results suggest that, similar to chromosome replication, a round of envelope synthesis in S. faecium produces a constant amount of new product (i.e., two new poles) which is invariant with growth rate.…”

“…Electron microscopy. Sample preparation was previously described for existing cell populations used in this analysis (4). Additional samples were fixed by the same method; however, they were suspended in distilled water and placed on polylysine-coated mica to minimize cell aggregation.…”

“…Photography was done with a Hitachi model H-600 electron microscope at an instrumental magnification of x6,000. Quantitation of electron micrographs was described previously (4,6).…”

Analysis of initiation of sites of cell wall growth in Streptococcus faecium during a nutritional shift

“…6) indicate that this constant-size model also holds for cells during a nutritional shift. Thus, according to this scheme, once a new site is created it grows to produce two poles which do not vary in size with growth rate (6). Thus as a site approaches this maximum size (that of two finished poles) new sites must be created for the cell to continue to grow (8,15).…”