Characterization of degradation products of the cell wall released during growth and sporulation of Bacillus megaterium

Lipoteichoic acid from Bacillus subtilis subsp. niger WM: isolation and effects on cell wall autolysis and turnover

Lipoteichoic acid (LTA) was extracted by means of hot aqueous phenol from Bacillus subtilis subsp. niger WM cells grown under various conditions in chemostat culture. The extracts were partially purified by nuclease treatment and gel permeation chromatography. Chemical analyses revealed a composition consistent with a polyglycerol phosphate polymer. The influence on autolysis of the LTAs thus obtained was studied with both whole cells and autolysin-containing native walls of B. subtilis subsp. niger WM. Lysis rates of phosphate-limited cells could be reduced to about 40% of the control rate by the addition of LTA, whereas lysis of cells grown under phosphate-sufficient conditions was affected to a much lesser extent. The lysis of native walls prepared from variously grown cells proved to be fairly insensitive to the addition of LTA. The effect of LTA on wall turnover was studied by following the release of radioactively labeled wall material during exponential growth. The most obvious effect of LTA was a lowered first-order rate of release of labeled wall material; calculations according to the model for cell wall turnover in Bacillus spp. formulated by De Boer et al. (W. R. De Boer, F. J. Kruyssen, and J. T. M. Wouters, J. Bacteriol. 145:50-60, 1981) revealed changes in wall geometry and not in turnover rate in the presence of LTA.

Cell wall metabolism in Bacillus subtilis subsp. niger: accumulation of wall polymers in the supernatant of chemostat cultures

Cell wall polymers were measured both in the cells and in the cell-free medium of samples from steady-state chemostat cultures of Bacillus subtilis, growing at various rates under magnesium or phosphate limitation. The presence of both peptidoglycan and anionic wall polymers in the culture supernatant showed the occurrence of wall turnover in these cultures. Variable proportions of the total peptidoglycan present in the culture samples were found outside the cells in duplicate cultures, indicating that the rate of peptidoglycan turnover is variable in B. subtilis. Besides peptidoglycan, anionic wall polymers were detected in the culture supernatant: teichoic acid in magnesium-limited cultures and teichuronic acid in phosphate-limited cultures. In several samples, the ratio between the peptidoglycan and the anionic polymer concentrations was significantly lower in the extracellular fluid than in the walls. This divergency was attributed to the occurrence of direct secretion of anionic polymers after their synthesis.

Cell wall turnover in batch and chemostat cultures of Bacillus subtilis

Wall turnover was studied in Bacillus subtilis. The loss of radioactively labeled wall polymers was followed during exponential growth in batch and chemostat cultures. Turnover kinetics were identical under all growth conditions; pulse-labeled wall material was lost with first-order kinetics, but only after exponential growth for 1 generation time after its incorporation. Similarly, continuously labeled cells showed an accelerating decrease in wall-bound radioactivity starting immediately after removal of the labeled precursor and also reached first-order kinetics after 1 generation time. A mathematical description was derived for these turnover kinetics, which embraced the concept of "spreading" of old wall chains (H. M. Pooley, J. Bacteriol. 125:1127-1138, 1976). Using this description, we were able to calculate from our experimental data the rate of loss of wall polymers from cells and the fraction of the wall which was sensitive to turnover. We found that about 20% of the wall was lost per generation time and that this loss was affected by turnover activity located in the outer 20 to 45% of the wall; rather large variations were found with both quantities and also between duplicate cultures. These parameters were quite independent of the growth rate (the specific growth rate varied from 1.3 h-1 in broth cultures to 0.2 to 0.3 h-1 in chemostat cultures) and of the nature of the anionic polymer in the wall (which was teichoic acid in cultures with an excess of phosphate and teichuronic acid in phosphate-limited chemostat cultures). Some implications of the observed wall turnover kinetics for models of wall growth in B. subtilis are discussed.

Cell wall teichoic acid as a reserve phosphate source in Bacillus subtilis

Although exponential growth of Bacillus subtilis 168 in a phosphate-limited medium halted with the exhaustion of inorganic phosphate, the bacteria continued to grow at a slower rate for a further 3 to 4 h at 37 degrees C. This postexponential growth in the absence of an exogenous phosphate supply was accompanied by a loss of teichoic acid from the cell walls of the bacteria. Quantitative analysis of walls and culture fluids showed that the phosphate loss from the walls could not be accounted for by an increase in phosphate-containing compounds in the medium, which implied that the cells were using their own wall teichoic acids to supply phosphate necessary for growth. Addition of exogenous teichoic acid to phosphate-starved cultures resulted in stimulation of growth and in the simultaneous disappearance of teichoic acid phosphate from the medium. It is proposed that teichoic acids, which can contain more than 30% of the total phosphorus of exponential-phase cells, can be used as a reserve phosphate source when the bacteria are starved for inorganic phosphate.

Relation between cell wall turnover and cell growth in Bacillus subtilis

The kinetics of cell wall turnover in Bacillus subtilis have been examined in detail. After pulse labeling of the peptidoglycan with N-acetylglucosamine, the newly formed peptidoglycan is stable for approximately three-quarters of a generation and is then degraded by a process that follows first-order kinetics. Deprivation of an auxotroph of amino acids required for protein synthesis results in a cessation of turnover. If a period of amino acid starvation occurs during the lag phase of turnover, then the initiation of turnover is delayed for a period of time equivalent to the starvation period. During amino acid starvation, new cell wall peptidoglycan is synthesized and added to preexisting cell wall. This peptidoglycan after resumption of growth is also subject to degradation (turnover). It is suggested that cell wall turnover is dependent on cell growth and elongation. Several possible control mechanisms for cell wall autolytic enzymes are discussed in light of these observations.