This is the type of structure found in L. The cleavage sites of the different classes of PG hydrolases are indicated with arrows. Although a given bacterial species has a basic, characteristic PG structure, the PG layer remains in a dynamic state throughout a bacterium's life, and PG structure is the result of complex biosynthetic, maturation, and degradation reactions, which will be described below. Structural analysis of PG muropeptides using HPLC and mass spectrometry has allowed the identification of the nature of peptide cross-bridges, the degree of cross-linking, and the frequency of maturation and hydrolysis events.
It has also revealed the existence of covalent PG modifications, such as O -acetylation, N -deacetylation, or amidation; these modifications may play essential roles in bacterial physiology. PG synthesis can be divided in three general steps: the first step takes place in the cytoplasm and leads to the synthesis of lipid II, the second step involves the transfer of lipid II to the extracellular side of the membrane, and the third step results in the polymerization of the synthesized subunits into a macromolecule [ 24 ] Figure 2.
Schematic representation of the main steps of peptidoglycan and wall teichoic acid biosynthesis. Grey arrows denote the steps of PG biosynthesis, and brown arrows indicate the steps of WTA biosynthesis. The membrane-embedded undecaprenyl-phosphate carrier is represented by dark grey curved lines The glycerol-phosphate units are represented with green circles.
The pre-existing PG is highlighted in gray. In the schematic, D-Asp is added to the lipid precursors; however, depending on the bacterial species, it may also be added to soluble precursors. In addition, in L. Modification of the last residue of the stem peptides of PG precursors has been shown to result in significant changes to PG structure and cell morphology [ 27 ].
The UDP-MurNAc-pentapeptide is then attached with a pyrophosphate link to the lipid transporter, bactoprenol undecaprenyl-phosphate , by the membrane translocase MraY, a process that yields undecaprenyl-pyrophosphoryl-MurNAc-pentapeptide, or lipid I Figure 2.
Finally, the glycosyl-transferase MurG adds GlcNAc to lipid I, forming undecaprenyl-pyrophosphoryl-disaccharide-pentapeptide, or lipid II, which is the basic subunit used in PG assembly [ 28 ].
Another important enzymatic step that takes place in the cytoplasm is the assembly of peptide side chains that are added either to the nucleotide MurNAc-pentapeptide or the lipid precursors, depending on the species [ 29 ]. D-Asp is produced from L-Asp by the aspartate racemase encoded by racD , which is located in the same operon as the aslA gene in L. Lipid II with or without a side chain is then translocated outside the cytoplasmic membrane by a flippase Figure 2.
The integral membrane protein FtsW has been shown to transport lipid-linked PG precursors across the membrane and is proposed to act at the septum level. The RodA homologous protein appears to be involved in lateral PG synthesis during cell elongation in ovococci and bacilli [ 33 ]. In the last step of PG synthesis, PG monomer units are polymerized via transpeptidation and transglycosylation reactions, which take place outside the cytoplasmic membrane Figure 2.
The major proteins involved in PG assembly are called penicillin-binding proteins PBPs because they are targets for penicillin and other beta-lactam antibiotics [ 34 ]. Class A PBPs contain both transglycosylation and transpeptidation domains located at the N- and C-terminals of the protein, respectively, whereas class B PBPs are exclusively involved in transpeptidation. During transglycosylation, lipid II's disaccharide is bound to the pre-existing PG chain; the bactoprenol loses one inorganic phosphate and is recycled to the inner side of the cytoplasmic membrane to initiate another round.
To create a solid PG mesh around the bacterial cell, newly extended chains must be connected to neighboring chains by transpeptidation. A covalent bond is created between the carbonyl group of the D-Ala in position four of one pentapeptide chain donor chain and the free amine of either the diamino acid in position three of a second peptide chain or the attached side-chain amino acid acceptor chain.
Analysis of the genome of L. Ovococci display both septal and peripheral growth, which results in the slight longitudinal expansion that generates their ovoid shape.
The other PBPs appear to have redundant functions, acting in both biosynthetic pathways [ 36 ]. Furthermore, alteration of PBP2x and PBP2b activity has been proposed to directly affect the coccus-to-rod transition and further filamentation observed in L. Only part of the PG stem peptides are connected by transpeptidation, and the degree of cross-linking is a PG characteristic. During the exponential growth phase, the cross-linking index has been estimated to be Another important feature of PG that likely influences PG architecture is glycan chain length.
When a mutant without PSs on its surface was imaged, using a tip functionalized with the PG-binding LysM domain, PG was found to be organized in the form of cables running parallel to the short axis of the cells [ 39 ].
In most bacterial species, PG basic structure is partially modified--either the glycan chains undergo N- deacetylation or O -acetylation or the free carboxyl groups of the amino acids in the peptide chains are amidated Figure 1 Table 1 [ 40 ]. These structural modifications usually have functional consequences Table 2 ; for instance, they may modulate the activity of endogenous PG hydrolases PGHs as well as that of exogenous PGHs produced by eukaryotic organisms, such as lysozyme.
PG modifications have been shown to allow pathogenic bacteria to escape from the host's innate immune system [ 41 ]. Below, we will review PG modifications by chemical groups, given that wall TA or PS polymers that covalently attach to PG may also be considered to be modifications; they can even be linked to the same sites on PG see text below.
In many Gram-positive pathogens, O -acetylation of MurNAc is associated with resistance against lysozyme [ 42 ]. The enzyme is composed of two domains: the N-terminal domain contains 11 predicted transmembrane helices, whereas the C-terminal domain appears to contain a catalytic acetyltransferase domain.
The donor of the acetyl group is probably acetyl-CoA [ 46 ]. The acetyl group is likely added to the newly polymerized PG outside the cytoplasmic membrane since O -acetylation of lipid precursors has not been observed [ 43 ] and the OatA acetyltransferase domain is predicted to be located outside the membrane. O -acetylation of MurNAc residues has been detected in the different LAB species for which structural analysis of PG has been performed; estimated levels of O -acetylation vary, from rather low in L.
It has been proposed that the first lactococcal response to treatment with lysozyme is the activation of the two-component system TCS CesSR, which then activates the transcription of several genes belonging to the cesSR regulon, among which is spxB [ 47 ], which belongs to the family of global transcriptional factors found in Gram-positive bacteria [ 48 ]. SpxB activates oatA expression; OatA activity increases PG resistance to lysozyme and thus counteracts cell wall stress [ 45 ].
Interestingly, while increased PG O -acetylation makes L. The regulon has also been shown to be induced in an L. The addition of the acetyl group to GlcNAc is performed by a second, specific O -acetyltransferase--OatB--that shares a similar two-domain structure with L. It is noteworthy that, until now, the presence of two Oat proteins has only been found in a very limited number of bacterial species, including two other LAB species, Lactobacillus sakei and Weissella paramesenteroides [ 23 ].
PG O -acetylation has an impact on L. In contrast, in this species, O -acetylation of MurNAc has been shown to activate autolysis through the activity of the putative N -acetylmuramoyl-L-alanine amidase LytH [ 23 ]. The N -deacetylation of GlcNAc, which leads to the presence of glucosamine GlcNH 2 on Figure 1 in the PG backbone, is performed by PG-deacetylase PgdA, which was first identified in Streptococcus pneumoniae thanks to its sequence homology with chitin deacetylases [ 53 ].
A pgdA homolog is present in the L. These modifications are catalyzed by specific enzymes and take place intracellularly; PG precursors, either UDP-MurNAc-pentapeptide or lipid intermediates, are amidated before the molecules are translocated through the cytoplasmic membrane [ 29 ].
Amidation of D-Asp cross-bridges has been observed in L. D-Asn and D-iso-Asn are not substrates for aspartate ligase, as has been shown in L. As a result, amidation of the alpha-carboxyl group of D-Asp takes place after D-Asp has been added to the PG precursor and is performed by an asparagine synthase AsnH , which was identified in L.
PGH activity is affected by D-Asp amidation. Indeed, an L. D-Asp amidation also decreases L. In this bacterium, amidation has also been shown to be mediated in the cytoplasm by an amidotransferase named AsnB1, the first enzyme to be associated with such activity [ 57 ].
The asnB1 gene has been found to play an essential role in L. In a mutant strain with a mDAP amidation defect, growth and cell morphology were strongly affected; filamentation and long-chain formation were observed, suggesting that mDAP amidation may play a critical role in controlling the septation process. The genes responsible for D-Glu amidation have been identified in S.
Lipid precursors, but not soluble UDP-MurNAc-pentapeptide precursors, are substrates for this enzymatic complex [ 59 ]. The murT and gatD genes are grouped in an operon and play an essential role in S. Past research has found that inhibition of amidation results in a markedly reduced bacterial growth rate, which suggests that amidated PG may serve as a better substrate for proteins that catalyze PG biosynthesis and cell division; furthermore, resistance to beta-lactam antibiotics and increased sensitivity to lysozyme have been observed [ 58 ].
PGHs are enzymes that can hydrolyze specific bonds in bacterial cell wall PG. Among them are bacterial autolysins and phage endolysins. Autolysins are endogenous bacterial PGHs whose activity may lead to autolysis, in particular when cells experience stressful conditions.
Moreover, the cleavage of PG strands is required to insert newly synthesized PG subunits during bacterial cell growth and to separate daughter cells following cell division [ 60 , 61 ]. Bacteriophage genomes encode PGHs called endolysins that, in association with holins, are responsible for host cell destruction after the viral particles have multiplied during phage dissemination [ 62 ].
They may also encode PGHs that are tail-associated lysins involved in phage entry into the host bacteria [ 63 ]. From a technological cheese making point of view, highly focused, applied studies have sought to understand and control LAB lysis, with the aim of being able to release the intracellular pool of enzymes of starter bacteria to improve cheese flavor development [ 11 , 64 ]. Bacterial PGHs as well as phage endolysins usually exhibit modular organization and have a catalytic domain associated with a cell wall binding domain CWBD.
The availability of complete genome sequences allows the full PGH complement of a given bacterial species to be analyzed and identified using amino acid sequence similarity searches that employ representative sequences of all known classes of PGHs.
Generally, a given bacterial species produces several PGHs that have various hydrolytic specificities, although not necessarily all the specificities listed above.
Five PGHs were initially identified in L. AcmA has a modular structure; its N-terminal catalytic domain demonstrates N -acetylglucosaminidase specificity [ 69 ], and its C-terminal domain is made up of three LysM sequences. The LysM repeats have been shown to bind to PG, and binding appears to be hindered by other cell wall constituents, which results in localized binding of AcmA to the cellular septum [ 70 ]. In contrast, Cse in S. It is worth noting that these enzymes are, respectively, the major autolysins of the aforementioned bacterial species and they are involved in daughter cell separation.
They illustrate the diversity that exists among bacterial species in cell-separating enzymes, a point highlighted in past research [ 73 ]. In each species, inactivation of the corresponding genes led to defects in daughter cell separation and long-chain formation.
In agreement with their role, all these PGHs were located at the cell septum. Also, AcmA and Acm2 are the major autolysins involved in the bacterial cell autolysis that is observed during the stationary phase or after bacteria are transferred to buffer solution. Other PGHs have been characterized in L. O- glycosylation of p75 Msp1 appears to confer protection against proteolytic degradation [ 77 ]. Furthermore, L.
In this species, O -glycosylation has been shown to modulate Acm2 PG-degradation activity see section 1. Very recently, the glycosyltransferases involved in Acm2 O -glycosylation were identified [ 79 ].
A strong PG mesh is needed to maintain cell shape and to counteract both high turgor pressure and cell wall stress related to environmental factors. At the same time, the growth and separation of bacterial cells also require a high degree of PG elasticity. These two opposing demands require the coordinated and balanced action of PG synthetic and degradation enzymes. The loss of this equilibrium may cause growth arrest and cell lysis.
In bacteria, such equilibrium is achieved mostly by regulating activities of potentially lethal autolytic enzymes that are PGHs. PGH regulation can take place at the transcriptional level but may also be mediated by mechanisms involving post-transcriptional modifications of PGHs or modification of their substrate, PG [ 60 , 61 , 80 ].
One of the factors that affects autolysin activity is proteolytic degradation. It has been shown that the main lactococcal autolysin, AcmA, is degraded by extracellular proteinase PrtP and that the autolysis of L.
Also, the cell wall-housekeeping protease HtrA has been shown to process lactococcal autolysin AcmA [ 82 ]. The activity of a given PGH can also be affected by its specific location in the bacterial cell. Depending on their role in bacterial physiology, PGHs may be distributed all along the cell periphery or located at the septum, as has been observed for PGHs involved in daughter cell separation.
By immunofluorescent labeling, the major LAB autolysins L. Also, as described above in the text above, structural variations in the PG substrate, such as the O -acetylation of glycan chains and the amidation of peptide chains, can contribute to the modulation of PGH activity.
Finally, glycosylation of the autolysin Acm2 has recently been shown to control the enzyme's activity [ 71 ]. Most studies have focused on the transcriptional regulation of the genes encoding endogenous PGHs in the context of the cell envelope stress response [ 86 ].
Interestingly, instead of affecting the transcription of genes that encode endogenous PGHs, the lactococcal TCS that responds to cell envelope stress, CesSR, increases expression of the oatA gene, and this gene encodes PG O -acetyltransferase, whose activity increases PG resistance to the PGH lysozyme [ 45 ] see text above.
Nod receptors are intracellular receptors expressed by both epithelial cells and immune cells, such as dendritic cells. For example, the PG of an L. The corresponding synthetic muropeptide has been shown to have a protective effect in a mouse model of intestinal inflammation Nod2-dependent.
These results show that PG originating from probiotic or commensal LAB may play an active role in the gut's immune balance. Furthermore, in the well-documented probiotic L. Furthermore, p40 has been shown to prevent and treat colonic epithelial cell injury and inflammation in mouse models of colitis through a mechanism that is dependent on the epidermal growth factor EGF receptor [ 93 ].
The non-catalytic N-terminal domain, which does not contain any characterized functional domains, appears to be responsible for the beneficial effects [ 94 ]. The cell wall of most Gram-positive bacteria contains TAs, which are anionic polymers made of alditol-phosphate repeating units [ 95 ].
They are classified into two groups: wall teichoic acids WTAs , which are covalently linked to the PG molecule, and lipoteichoic acids LTAs , which are anchored in the cytoplasmic membrane with a glycolipid moiety. WTAs may constitute up to half of cell wall total dry weight in certain bacterial species [ 96 ].
The length of the poly Gro-P or poly Rbo-P chains varies between species and strains, as does the substitution level. Remarkably, in L. Permissions Icon Permissions. Abstract The peptidoglycan murein sacculus is a unique and essential structural element in the cell wall of most bacteria. Open in new tab Download slide.
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More on this topic Uptake, recognition and responses to peptidoglycan in the mammalian host. The multifarious roles of Tol-Pal in Gram-negative bacteria. Gram-positive bacteria are stained dark blue or violet by Gram staining. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, thus forming Gram-variable and Gram-indeterminate groups as well.
Gram-positive bacteria : These bacteria stain violet by Gram staining. It is based on the chemical and physical properties of their cell walls. Primarily, it detects peptidoglycan, which is present in a thick layer in Gram-positive bacteria. The Gram stain is almost always the first step in the identification of a bacterial organism, and is the default stain performed by laboratories over a sample when no specific culture is referred.
In Gram-positive bacteria, the cell wall is thick nanometers , and consists of several layers of peptidoglycan. They lack the outer membrane envelope found in Gram-negative bacteria. Running perpendicular to the peptidoglycan sheets is a group of molecules called teichoic acids, which are unique to the Gram-positive cell wall.
Teichoic acids are linear polymers of polyglycerol or polyribitol substituted with phosphates and a few amino acids and sugars. The teichoic acid polymers are occasionally anchored to the plasma membrane called lipoteichoic acid, LTA , and apparently directed outward at right angles to the layers of peptidoglycan.
Teichoic acids give the Gram-positive cell wall an overall negative charge due to the presence of phosphodiester bonds between teichoic acid monomers. The functions of teichoic acid are not fully known but it is believed to serve as a chelating agent and means of adherence for the bacteria.
These are essential to the viability of Gram-positive bacteria in the environment and provide chemical and physical protection. One idea is that they provide a channel of regularly-oriented, negative charges for threading positively-charged substances through the complicated peptidoglycan network.
Another theory is that teichoic acids are in some way involved in the regulation and assembly of muramic acid sub-units on the outside of the plasma membrane. There are instances, particularly in the streptococci, wherein teichoic acids have been implicated in the adherence of the bacteria to tissue surfaces and are thought to contribute to the pathogenicity of Gram-positive bacteria. Some bacteria lack a cell wall but retain their ability to survive by living inside another host cell.
For most bacterial cells, the cell wall is critical to cell survival, yet there are some bacteria that do not have cell walls.
Mycoplasma species are widespread examples and some can be intracellular pathogens that grow inside their hosts. This bacterial lifestyle is called parasitic or saprophytic. Cell walls are unnecessary here because the cells only live in the controlled osmotic environment of other cells. It is likely they had the ability to form a cell wall at some point in the past, but as their lifestyle became one of existence inside other cells, they lost the ability to form walls.
L-form bacteria : L-form bacterial lack a cell wall structure. Consistent with this very limited lifestyle within other cells, these microbes also have very small genomes. They have no need for the genes for all sorts of biosynthetic enzymes, as they can steal the final components of these pathways from the host. Similarly, they have no need for genes encoding many different pathways for various carbon, nitrogen and energy sources, since their intracellular environment is completely predictable.
Because of the absence of cell walls, Mycoplasma have a spherical shape and are quickly killed if placed in an environment with very high or very low salt concentrations.
However, Mycoplasma do have unusually tough membranes that are more resistant to rupture than other bacteria since this cellular membrane has to contend with the host cell factors. The presence of sterols in the membrane contributes to their durability by helping to increase the forces that hold the membrane together.
Other bacterial species occasionally mutate or respond to extreme nutritional conditions by forming cells lacking walls, termed L-forms. This phenomenon is observed in both gram-positive and gram-negative species. L-forms have varied shapes and are sensitive to osmotic shock. It is the space located between the outer surface of the cell membrane and the inner surface of the outer membrane, and it contains the gram negative peptidoglycan. Once the periplasmic enzymes have broken nutrients down to smaller molecules that can get past the LPS, they still need to be transported across the outer membrane, specifically the lipid bilayer.
Gram negative cells utilize porins , which are transmembrane proteins composed of a trimer of three subunits, which form a pore across the membrane.
Some porins are non-specific and transport any molecule that fits, while some porins are specific and only transport substances that they recognize by use of a binding site.
Once across the outer membrane and in the periplasm, molecules work their way through the porous peptidoglycan layers before being transported by integral proteins across the cell membrane. At one end the lipoprotein is covalently bound to the peptidoglycan while the other end is embedded into the outer membrane via its polar head. This linkage between the two layers provides additional structural integrity and strength.
Having emphasized the important of a cell wall and the ingredient peptidoglycan to both the gram positive and the gram negative bacteria, it does seem important to point out a few exceptions as well.
Bacteria belonging to the phylum Chlamydiae appear to lack peptidoglycan, although their cell walls have a gram negative structure in all other regards i. It has been suggested that they might be using a protein layer that functions in much the same way as peptidoglycan. Bacteria belonging to the phylum Tenericutes lack a cell wall altogether, which makes them extremely susceptible to osmotic changes.
They often strengthen their cell membrane somewhat by the addition of sterols , a substance usually associated with eukaryotic cell membranes. Many members of this phylum are pathogens, choosing to hide out within the protective environment of a host. Skip to content It is important to note that not all bacteria have a cell wall. Study Questions What are the basic characteristics and functions of the cell wall in Bacteria?
What is the Gram stain and how does it relate to the different cell wall types of Bacteria? What is the basic unit structure of peptidoglycan? What components are present and how do they interact? What is cross linking and why does this play such an important role in the cell wall? What different types of cross-linking are there? Why are D-amino acids unusual and how does having D-amino acids in the peptidoglycan keep this macromolecule stable?
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