The biosynthesis of wall teichoic acid by toluenised cells of Bacillus subtilis W23

A continuum of anionic charge: structures and functions of D-alanyl-teichoic acids in gram-positive bacteria

Teichoic acids (TAs) are major wall and membrane components of most gram-positive bacteria. With few exceptions, they are polymers of glycerol-phosphate or ribitol-phosphate to which are attached glycosyl and D-alanyl ester residues. Wall TA is attached to peptidoglycan via a linkage unit, whereas lipoteichoic acid is attached to glycolipid intercalated in the membrane. Together with peptidoglycan, these polymers make up a polyanionic matrix that functions in (i) cation homeostasis; (ii) trafficking of ions, nutrients, proteins, and antibiotics; (iii) regulation of autolysins; and (iv) presentation of envelope proteins. The esterification of TAs with D-alanyl esters provides a means of modulating the net anionic charge, determining the cationic binding capacity, and displaying cations in the wall. This review addresses the structures and functions of D-alanyl-TAs, the D-alanylation system encoded by the dlt operon, and the roles of TAs in cell growth. The importance of dlt in the physiology of many organisms is illustrated by the variety of mutant phenotypes. In addition, advances in our understanding of D-alanyl ester function in virulence and host-mediated responses have been made possible through targeted mutagenesis of dlt. Studies of the mechanism of D-alanylation have identified two potential targets of antibacterial action and provided possible screening reactions for designing novel agents targeted to D-alanyl-TA synthesis.

Polymerization of mycobacterial arabinogalactan and ligation to peptidoglycan

The cell wall of Mycobacterium spp. consists predominately of arabinogalactan chains linked at the reducing ends to peptidoglycan via a P-GlcNAc-(alpha1-3)-Rha linkage unit (LU) and esterified to a variety of mycolic acids at the nonreducing ends. Several aspects of the biosynthesis of this complex have been defined, including the initial formation of the LU on a polyprenyl phosphate (Pol-P) molecule followed by the sequential addition of galactofuranosyl (Galf) units to generate Pol-P-P-LU-(Galf)1,2,3, etc. and Pol-P-P-LU-galactan, catalyzed by a bifunctional galactosyltransferase (Rv3808c) capable of adding alternating 5- and 6-linked Galf units. By applying cell-free extracts of Mycobacterium smegmatis, containing cell wall and membrane fragments, and differential labeling with UDP-[14C]Galp and recombinant UDP-Galp mutase as the source of [14C]Galf for galactan biosynthesis and 5-P-[14C]ribosyl-P-P as a donor of [14C]Araf for arabinan synthesis, we now demonstrate sequential synthesis of the simpler Pol-P-P-LU-(Galf)n glycolipid intermediates followed by the Pol-P-P-LU-arabinogalactan and, finally, ligation of the P-LU-arabinogalactan to peptidoglycan. This first time demonstration of in vitro ligation of newly synthesized P-LU-arabinogalactan to newly synthesized peptidoglycan is a necessary forerunner to defining the genetics and enzymology of cell wall polymer-peptidoglycan ligation in Mycobacterium spp. and examining this step as a target for new antibacterial drugs.

Ligation of arabinogalactan to peptidoglycan in the cell wall of Mycobacterium smegmatis requires concomitant synthesis of the two wall polymers

To study the late events of cell wall assembly in Mycobacterium smegmatis, specific in vivo radiolabelling of exponentially growing liquid cultures over periods of less than one cell generation were carried out. N-Acetyl-[(14)C]glucosamine was used to label peptidoglycan and [(14)C]glucose to label arabinogalactan and arabinomannan. Over periods of several generations, radioactive cell wall material was turned over as soluble autolysis products into the culture fluid. However, turnover of newly synthesized and labelled cell wall was delayed for about one cell generation, implying inside-to-outside growth of the wall as observed in Bacillus. Little radioactive wall material was released into the culture fluid during the first generation of labelling in growing cultures, but the addition of amoxicillin plus the beta-lactamase inhibitor clavulanic acid, at the minimum inhibitory concentration of amoxicillin, led to the release of radioactive peptidoglycan that could be isolated by gel filtration chromatography and contained nearly 3 mol alanine per glutamic acid residue, indicating that it was linear, un-crosslinked peptidoglycan that had never been substantially cross-linked to the cell wall due to inhibition of transpeptidation by amoxicillin. This peptidoglycan had no covalently attached arabinogalactan. Radioactive arabinogalactan was synthesized and released from the amoxicillin-treated bacteria without attachment to peptidoglycan. The results indicate that during growth, incorporation of arabinogalactan into the cell wall requires its ligation to newly synthesized peptidoglycan and that the peptidoglycan must be undergoing concomitant cross-linking to the inner surface of the cell wall. Inhibition of peptidoglycan transpeptidation prevents ligation of arabinogalactan to peptidoglycan and its consequent incorporation into the wall.

Maintenance of D-alanine ester substitution of lipoteichoic acid by reesterification in Staphylococcus aureus

Toluene-treated Staphylococcus aureus cells did not synthesize teichoic acid and lipoteichoic acid under the conditions used. The organism displayed, however, a high capacity of incorporating D-[14C]alanine into previously formed polymers. The reaction was dependent on ATP and enhanced by magnesium ions. The incorporation rate into lipoteichoic acid correlated with the rate of loss of alanine ester which occurred through transfer to teichoic acid and base-catalyzed hydrolysis. At pH 6.5 the loss (20% within 4 h) was completely compensated for by reesterification. At pH 7.5 the loss was 60%, but by accelerated incorporation it was reduced to 10%. Incorporation was also enhanced when the original substitution of lipoteichoic acid was lowered by previous growth of S. aureus at high salt concentration. The newly added alanine was randomly distributed along the poly(glycerophosphate) chain. The decreased alanine substitution of lipoteichoic acid after growth at high salt concentration was shown to result from a direct inhibition of alanine incorporation.

Peptidoglycan synthesis by partly autolyzed cells of Bacillus subtilis W23

Partly autolyzed, osmotically stabilized cells of Bacillus subtilis W23 synthesized peptidoglycan from the exogenously supplied nucleotide precursors UDP-N-acetylglucosamine and UDP-N-acetylmuramyl pentapeptide. Freshly harvested cells did not synthesize peptidoglycan. The peptidoglycan formed was entirely hydrolyzed by N-acetylmuramoylhydrolase, and its synthesis was inhibited by the antibiotics bacitracin, vancomycin, and tunicamycin. Peptidoglycan formation was optimal at 37 degrees C and pH 8.5, and the specific activity of 7.0 nmol of N-acetylglucosamine incorporated per mg of membrane protein per h at pH 7.5 was probably decreased by the action of endogenous wall autolysins. No cross-linked peptidoglycan was formed. In addition, a lysozyme-resistant polymer was also formed from UDP-N-acetylglucosamine alone. Peptidoglycan synthesis was inhibited by trypsin and p-chloromercuribenzenesulfonic acid, and we conclude that it occurred at the outer surface of the membrane. Although phospho-N-acetylmuramyl pentapeptide translocase activity was detected on the outside surface of the membrane, no transphosphorylation mechanism was observed for the translocation of UDP-N-acetylglucosamine. Peptidoglycan was similarly formed with partly autolyzed preparations of B. subtilis NCIB 3610, B. subtilis 168, B. megaterium KM, and B. licheniformis ATCC 9945. Intact protoplasts of B. subtilis W23 did not synthesize peptidoglycan from externally supplied nucleotides although the lipid intermediate was formed which was inhibited by tunicamycin and bacitracin. It was therefore considered that the lipid cycle had been completed, and the absence of peptidoglycan synthesis was believed to be due to the presence of lysozyme adhering to the protoplast membrane. The significance of these results and similar observations for teichoic acid synthesis (Bertram et al., J. Bacteriol. 148:406-412, 1981) is discussed in relation to the translocation of bacterial cell wall polymers.

Activation and inactivation of synthesis of secondary wall polymers in Bacillus subtilis W23

The progress of activation and inactivation of synthesis of the wall polymers, teichoic acid and teichuronic acid, in response to changes in the phosphate content of the growth medium has been examined using toluenised cells of B. subtilis W23. Activation of teichoic acid synthesis from nucleotide precursors was independent of protein synthesis, but chloramphenicol prevented activation when DL-glycerol 3-phosphate and CTP replaced CDP-glycerol as one of the substrates of the reaction. Activation of teichuronic acid synthesis was dependent on synthesis of protein. Inactivation of synthesis of both polymers was slowed, but not prevented, by inhibition of protein synthesis. Evidence was obtained that a protein synthesised during phosphate starvation retards the activation of teichoic acid synthesis.

Relation between wall teichoic acid content of Bacillus subtilis and efficiency of adsorption of bacteriophages SP 50 and phi 25

Efficient adsorption of bacteriophages SP 50 and phi 25 occurred only to bacilli that contained wall teichoic acid and neither phage bound to phosphate limited bacilli that contained teichuronic acid instead of teichoic acid. Though both phages require the presence of teichoic acid, their receptors are not identical. Efficient binding of phage phi 25 required the presence of greater proportions of teichoic acid in the wall and the receptor for this phage was destroyed when bacteria or isolated walls were heated at pH 4 whereas the ability of these samples to bind phage SP 50 was unaffected by such treatment. Efficient binding of phage SP 50 was not highly dependent on the presence of glucosyl substituents on the teichoic acid. Such substituents were required for phage phi 25 binding though their anomeric configuration appeared to be unimportant since the phages bound well to both strains W23 and 168, the wall teichoic acids of which carry glucosyl substituents of opposite anomeric configuration. The differences in the nature of the receptors may be of value in the use of the phages as probes for the location and distribution of teichoic acid in the wall.

Teichoic acids from chemostat-grown cultures of Streptococcus mutans and Lactobacillus plantarum

We examined the effect of growth conditions in chemostat culture on the quantity and composition of the cell wall teichoic acids of Streptococcus mutans BHT and Lactobacillus plantarum NCIB 7220 and the membrane lipoteichoic acid from S. mutans Ingbritt. With the cell wall teichoic acids, which are covalently linked to peptidoglycan, the amount of teichoic acid is independent of the growth conditions employed. However, the extent of glucosyl substitution of the polymer from L. plantarum was dependent on growth conditions. S. mutans Ingbritt lipoteichoic acid, on the other hand, was little affected by growth conditions in terms of composition or serological activity, but the amount produced was markedly affected by changes in growth conditions.