One of the main differences between Gram-negative and Gram-positive organisms is the presence or absence of an outer membrane.
In Gram-negative organisms, the outer membrane protects the organism from the environment.
It filters out toxic molecules and establishes a compartment, the periplasm , that retains the extracytoplasmic enzymes required for cell wall growth and degradation.
It also serves as a scaffold to which proteins and polysaccharides that mediate interactions between the organism and its environment are anchored.
Also, in ways that are not fully understood, the outer membrane works in conjunction with a thin layer of peptidoglycan to help stabilize the inner membrane so that it can withstand the high osmotic pressures within the cell.
Gram-positive organisms, on the other hand, lack an outer membrane and a distinct periplasm. The peptidoglycan layers are consequently very thick compared to those of Gram-negative organisms.
These thick layers of peptidoglycan stabilize the cell membrane and also provide many sites to which other molecules can bind.
Gram-positive peptidoglycan is highly modified with carbohydrate-based anionic polymers that play an important role in membrane integrity.
These anionic polymers appear to perform some of the same functions as the outer membrane:
They influence membrane permeability, mediate extracellular interactions, provide additional stability to the plasma membrane, and, together with peptidoglycan, act as scaffolds for extracytoplasmic enzymes required for cell wall growth and degradation.
An important class of these cell surface glycopolymers are teichoic acids (AT), which are phosphate-rich molecules found in a wide range of Gram-positive, pathogenic and non-pathogenic bacteria alike.
There are two types of teichoic acids:
Lipoteichoic acids (ALT) : which anchor to the plasma membrane and extend from the cell surface to the peptidoglycan layer.
Wall Teichoic Acids (ATP) : which bind covalently to peptidoglycan and extend through and beyond the cell wall.
Together, lipoteichoic acids and wall teichoic acids create what has been aptly described as a “negative charge continuum” that extends from the bacterial cell surface beyond the outermost layers of peptidoglycan.
Neuhaus and Baddiley extensively reviewed both lipoteichoic acids and wall teichoic acids in 2003.
Since then, however, new roles for wall teichoic acids have been discovered in pathogenesis and it has been suggested that the biosynthetic enzymes that produce these polymers are targets for new antibacterial agents.
Structure of wall teicoic acid
Wall teichoic acids are anionic glycopolymers that are covalently attached to peptidoglycan through a phosphodiester bond to the C6 hydroxyl of N-acetylmuramic acid sugars.
They can represent up to 60% of the total cell wall mass in Gram-positive organisms.
The chemical structures of wall teichoic acids vary between organisms, as described in detail by Neuhaus and Baddiley.
But the most common structures are composed of a disaccharide of N-acetylmannosamine (ManNAc) (β1 → 4) N-Acetylglucosamine (GlcNAc) with one to three glycerol phosphates attached to the C4 hydroxyl of the N-acetylmannosamine residue (the “unit link »).
Followed by a much longer chain of glycerol phosphate or ribitol repeats (the “backbone”).
B. subtilis, the Gram-positive model organism, produces poly (glycerol phosphate) or poly (ribitol phosphate) wall teichoic acids depending on the strain, whereas S. aureus strains produce mainly poly (ribitol phosphate) teichoic acids from Wall.
The hydroxyls in the glycerol or ribitol phosphate repeats are matched with cationic d-alanine esters and monosaccharides, such as glucose or N-acetylglucosamine.
The presence of wall teichoic acids and the particular adaptive modifications found in them have profound effects on the physiology of Gram-positive organisms, impacting everything from cation hom eostasis to antibiotic susceptibility to survival in a host.
Roles of teichoic acids in bacterial physiology
The roles of teichoic acids in bacterial physiology are incompletely understood, but the evidence for their importance is overwhelming.
Mutants of B. subtilis and S. aureus deficient in lipoteichoic acid biosynthesis can be obtained, but only if they are grown under a narrow range of conditions; they are sensitive to temperature and show severe growth defects.
Mutants deficient in wall teichoic acid biosynthesis are also compromised and exhibit increased sensitivity to temperature and certain components of the buffer, including citrate; they also tend to aggregate in cultivation.
Furthermore, B. subtilis strains that do not express wall teichoic acids show profound morphological aberrations.
Bacterial strains in which the expression of lipoteichoic acids and wall teichoic acids are prevented are not viable, an observation that suggests that these polymers have overlapping functions and may partially compensate for each other.
In fact, this could be expected for some functions as both contain phosphate-linked repeating units with similar adaptive modifications.
One of the adaptive modifications, d-alanylation, is carried out with the same machinery, so there is even some overlap in the biosynthetic pathways.
This fact makes it difficult to dissect the functions of individual anionic glycopolymers, but is consistent with the idea that lipoteichoic acids and wall teichoic acids are partially redundant.
Some of the functions attributed to wall teichoic acids are described in the following paragraphs.
Lipoteichoic acids are outside the scope of this review, but will be mentioned where relevant to the discussion of wall teichoic acids.
Morath et al. and Rahman et al. each of them has written recent reviews on the structure and biosynthesis of lipoteichoic acids.
Cation Bond Functions
Wall teichoic acids form a dense network of negative charges on Gram-positive cell surfaces.
To alleviate the resulting repulsive electrostatic interactions between neighboring phosphates, teichoic acids bind to cationic groups, including monovalent and divalent metal cations.
The cation networks coordinated by wall teichoic acids affect the general structure of polymers, and this in turn influences the porosity and rigidity of the cell envelope.
Wall-wall teichoic acids are important for cation homeostasis in Gram-positive organisms, and provide a deposit of ions near the cell surface that might be necessary for enzymatic activity.
Furthermore, the ion gradient could somehow mitigate the osmotic pressure change between the inside and outside of the cell.
The amount of bound cations can be modulated by d-alanylation, an adaptive modification that introduces positively charged amines.
Wall-wall teichoic acids lacking d-alanyl esters can bind up to 60% more Mg2 + ions than analogous polymers containing this modification.
The importance of cation binding is highlighted by the observation that B. subtilis strains positively regulate their production of teichoic acids in the presence of low concentrations of Mg2 + and produce other negatively charged polymers (teichuronic acid) in the presence of concentrations of limiting phosphate.
Recent structural studies have focused on elucidating modes of cation attachment by wall teichoic acid polymeric phosphate groups.
And researchers have suggested that a clear understanding of the three-dimensional structure of wall teichoic acids and their bound cation groups could provide insights that facilitate the design of new antimicrobials.
In addition to providing binding sites for cations, wall-wall teichoic acids serve as scaffolds or receptors for a wide range of other molecules.
In S. aureus, for example, they function as receptors necessary for phage infection. Depending on your tailoring modifications, they could also promote the adhesion of lytic enzymes produced by neutrophils.
They are also believed to serve as scaffolds for endogenously produced cell wall hydrolases (autolysins) involved in cell growth and division.
In general, the molecular interactions between wall teichoic acids and other biomolecules are not well understood, but could provide crucial insights into the function of the cell envelope.
Adaptation of Modification Dependent Functions
The major chain hydroxyl groups in both glycerol and ribitol phosphate wall teichoic acid polymers are subject to further derivatization by adapted enzymes.
There are two classes of tailoring enzymes: those that catalyze the addition of d-alanyl esters and those that add glucosyl groups.
The degree to which these modifications occur in teichoic acid polymers is stress dependent and can also be affected by environmental conditions.
Efforts have been made to understand the role (s) of these modifications in bacterial physiology, and some of these studies are highlighted below.
The adaptive modification d-alanylation has been more thoroughly investigated than glycosylation and is much better understood at this point.
Perego et al. were the first to characterize the genetic pathway responsible for this modification (dlt operon) in B. subtilis.
Briefly, the biosynthetic pathway begins intracellularly with the activation of d-alanine to its corresponding aminoacyl adenylate by DltA.
This molecule is covalently linked, as a thioester, to a cofactor linked to the d-Ala transporter protein, DltC.
Although the precise functions of DltB and DltD have not been confirmed, they are believed to facilitate the transport of DltC across the membrane and the incorporation of d-Ala into both lipoteichoic acids and wall teichoic acids.
D-alanylation has been found to be affected by several factors, including growth media, pH, and temperature.
Binding of d-alanyl esters to hydroxyls in teichoic acids alters the net charge of the polymer by adding positively charged amines.
This modification reduces electrostatic repulsion between neighboring teichoic acid chains and possibly facilitates stabilization of ion pair formation between cationic esters and anionic phosphate groups.
Modification of d-alanine modulates cell envelope-environment interactions and has been implicated in many of the known scaffold / receptor functions of wall teichoic acids.
For example, the absence of d-alanyl esters in teichoic acid polymers has been shown to increase susceptibility to cationic antimicrobial peptides, possibly by increasing the density of negative charge on the cell surface.
Removal of alanine residues also increases bacterial sensitivity to glycopeptide antibiotics and the lytic activity of enzymes produced by neutrophils during host infection.
On the contrary, the activity of autolytic enzymes decreases, suggesting a role for teichoic acids in the scaffolding and / or the activation of bacterial enzymes involved in the synthesis and degradation processes of the cell wall.
Removal of d-alanyl esters of teichoic acids has also been shown to attenuate the binding of S. aureus to artificial surfaces, as well as to host tissue.
A recent study has illustrated the importance of the charge balance of wall teichoic acids in adhesion to artificial surfaces such as glass and polystyrene.
Since d-alanylation promotes better adhesion to host tissue and confers some resistance to lytic enzymes produced by the host, mutant strains lacking this modification have been studied in animal infection models.
For example, in a mouse tissue cage infection model, bacterial strains lacking d-alanylation were more susceptible to type 2 Toll receptor-dependent host defenses.
In a sepsis model , such strains were attenuated in their ability to establish infection, possibly because they were more easily cleared by neutrophils.
Based on these and other studies, it was proposed that d-alanine modification is a putative target for new antimicrobials that work by attenuating virulence.
In 2005, May et al. reported the synthesis and evaluation of a non-hydrolyzable aminoacyl adenylate analog of d-Ala as the first designed inhibitor of DltA, the enzyme that activates d-Ala.
The compound enhanced the activity of vancomycin against B. subtilis. This result is consistent with the inhibition of DltA, and supports the idea that small molecules that interfere with d-alanylation could provide a new antimicrobial strategy.
Glycosylation is a ubiquitous tailoring modification of wall teichoic acids but its functions are not well understood.
Glucose is commonly added to wall teichoic acid polymers in B. subtilis, while N-acetyl glucosamine (GlcNAc) is added in S. aureus.
Depending on the bacterial strain, the stereochemistry of the glycosidic bond can be β-, α-, or a mixture of the two anomers.
All sequenced strains of B. subtilis and S. aureus contain one or more putative glycosyltransferase genes clustered with the biosynthetic genes for wall teichoic acids.
For example, B. subtilis 168 contains a gene for a putative retained glucosyltransferase that could add α-Glu to glycerol phosphate polymers.
S. aureus strains contain two genes that encode putative reversing glycosyltransferases that can transfer β-GlcNAc to poly (ribitol phosphate) polymers.
Although some strains of S. aureus have been shown to contain α-glycosidic bound wall teichoic acids, no genes have yet been identified for any glycosyltransferases that can carry out this tailoring modification.
Furthermore, no studies have confirmed the enzymatic functions of any of the putative wall teichoic acid glycosyltransferases nor have they explored the effects of wall teichoic acid glycosylation prevention on growth, division, intercellular interactions, or pathogenesis. of bacterial cells.
In fact, as far as we know, there is only one information related to the functions of wall teichoic acid glycosyltransferases in the literature:
A rearrangement of mutagenesis into a putative glycosyltransferase in the S. aureus Newman strain showed attenuated virulence in a nematode kill assay, suggesting that glycosylation could play a role in the pathogenesis of S. aureus.
If glycosylation turns out to be important for bacterial pathogenesis, enzymes adaptable to glycosyltransferase, such as enzymes involved in d-alanylation (see above) would be possible targets for antimicrobials.
Roles in cell elongation and division
Recent studies have implicated lipoteichoic acids and wall teichoic acids in cell growth, division, and morphogenesis.
In the rod-shaped organism B. subtilis, teichoic acids have been shown to play different roles in bacterial morphogenesis.
The prevention of expression of the wall teichoic acids results in the production of round, severely defective progeny, while the prevention of lipoteichoic acid biosynthesis causes significant defects in septum formation and cell separation.
Separate multiprotein complexes are known to be involved in septation and elongation in B. subtilis, and Errington et al. Have suggested (based on localization studies using fluorescently labeled enzymes).
That the biosynthetic enzymes of the wall teichoic acids are associated with the machinery involved in elongation, while the lipoteichoic acid enzymes may associate with the machinery involved in the septation and cell division.
It was suggested that the spatial distribution of these two anionic glycopolymers determines their specific functions.
Defects in S. aureus after removal of wall teichoic acids are less pronounced than in B. subtilis, and specific functions in cell growth and division have not been proposed for wall teichoic acids in this organism.
However, Oku et al. reported that S. aureus strains devoid of lipoteichoic acids show significant defects in septum formation and cell separation and grow only under a restricted range of conditions, including reduced temperatures.
Functions in biofilm formation and adhesion of host tissue
As major components of the cell envelope, wall teichoic acids influence the interactions of bacterial cells with their environment in many ways.
We have already mentioned that S. aureus mutants lacking wall teichoic acids show reduced initial adhesion to artificial surfaces, including glass and polystyrene; they also have problems in their ability to form biofilms.
Null wall teichoic acid mutants that are altered in biofilm formation have been shown not to have reduced production of the exopolysaccharide poly-N-acetylglucosamine, which has been identified as an important factor for biofilm formation.
This finding highlights the independent role played by wall teichoic acids in biofilm formation.
Wall teichoic acids of S. aureus are also required for adhesion to host tissue.
Peschel et al. Have shown that S. aureus strains that do not express wall teichoic acids are severely impaired in their ability to adhere to nasal epithelial cells and cannot colonize the nasal passages of cotton rats.
They have also shown that wall-null teichoic acid mutants cannot colonize endothelial tissues derived from kidney and spleen.
The d-alanylation machinery was not affected in these strains, and d-alanylation could still have occurred in lipoteichoic acids; therefore, these results implicate wall teichoic acids as independent factors involved in cell adhesion.
Since wall teichoic acids are necessary for host infection and play an important role in biofilm formation, it was suggested that they are virulence factors, that is, factors necessary for the establishment and spread of infection in a host. .
Therefore, it was suggested that the enzymes involved in the biosynthesis of wall teichoic acids are targets for new antimicrobials that prevent host colonization by S. aureus.
Extensive work over several decades has illuminated many of the roles of teichoic acids in Gram-positive bacteria and has firmly established their importance in bacterial physiology.
Biochemical and genetic studies have contributed to a better understanding of the wall teicoic acid biosynthetic pathway, and most of the steps in the B. subtilis and S. aureus wall teichoic acid biosynthetic pathways have been reconstituted in vitro using the use of synthetic substrates.
A small molecule antibiotic was recently discovered that targets wall teichoic acid biosynthesis in S. aureus using recent genetic and biochemical advances in this field.
And it will enable studies to evaluate wall teichoic acid biosynthesis as a pathway for therapeutic intervention.
The positive results of these studies will validate this class of virulence factors as antibacterial targets and provide further impetus for their study and exploitation.