Cell wall turnover in lyt mutants of Bacillus subtilis

Analysis of the autolysins of Bacillus subtilis 168 during vegetative growth and differentiation by using renaturing polyacrylamide gel electrophoresis

The autolysins of Bacillus subtilis 168 were analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with substrate-containing gels. Four bands of vegetative autolytic activity of 90, 50, 34, and 30 kDa (bands A1 to A4) were detected in SDS and LiCl extracts and in native cell walls by using B. subtilis 168 vegetative cell walls as the substrate incorporated in the gel. The four enzyme activities showed different substrate specificities and sensitivities to various chemical treatments. The autolysin profile was not medium dependent and remained constant during vegetative growth. During sporulation, band A4 greatly increased in activity just prior to mother-cell lysis. No germination-associated changes in the profile were observed, although a soluble 41-kDa endospore-associated cortex-lytic enzyme was found. By using insertionally inactivated mutants, bands A1 and A2 were positively identified as the previously characterized 90-kDa glucosaminidase and 50-kDa amidase, respectively. The common filamentous phenotype of various regulatory mutants could not be correlated to specific changes in the autolysin profile.

Zonal turnover of cell poles of Bacillus subtilis

Turnover of cell walls of Bacillus subtilis occurs in three distinct phases: a lag phase, a relatively rapid phase persisting for 2-3 generations and a much slower phase continuing for several additional generations. A lectin probe revealed that cell pole material was lost during the slow phase of turnover and that the loss of wall occurred in zones, beginning at the cylinder-pole junction and continuing to the cell tip. This is in contrast to cell wall turnover in cylinders where turnover occurs randomly at many surface sites.

Partition of autolysins between the medium, the internal part of the wall, and the surface of the wall of gram-positive rods

Autolysins are exoenzymes formed by bacteria to aid in wall expansion and in cell division. For the Gram-positive rod, these enzymes function predominately on the periphery of the wall. As they destroy their substrate they rebind to peptidoglycan when the cell density is high. Only then is the concentration of autolysins high enough on the external surface to favor cell division and prevent the formation of chains of cells (filaments). It is suggesting that circumstance is important for the kinetics of turnover of the wall and the splitting of the septum.

The functions of autolysins in the growth and division of Bacillus subtilis

Some bacteria, such as streptococci, exhibit growth from discrete and well-defined zones. In Streptococcus faecalis, growth zones can be observed in the electron microscope, and the position of the zone can be used as a marker for cell cycle events. Growth of the cell surface of Bacillus subtilis appears to be by a much different mechanism from that of streptococci. Cell elongation takes place by the insertion at many sites in the cell cylinder of peptidoglycan components. The insertion occurs on the inner face of the wall, and upon cross linking, the new wall material becomes stress bearing and older wall is pushed to the surface. When old wall reaches the surface, it becomes susceptible to excision by autolysins, resulting in wall turnover; cell elongation, due to the stretching of the cross-linked peptidoglycan, therefore, accompanies turnover and does not require a specialized growth zone.