Bacterial Cell Wall: Morphology and Biochemistry

The exterior structure of the unicellular microorganism contains and protects its intracellular structure, while regulating solute ingress and egress. This exterior structure is called the cell envelope. Within the domain of the archaea the structure of the cell envelope is a proteinaceous surface layer. ^{1 – 3} Within the domain of the bacteria, different cell envelope structures are found. ^{4 – 7} A key difference is whether the bacterium has a single membrane (is monoderm) or has two membranes (is diderm). ⁸ A key similarity for most monoderm and diderm bacteria is the presence of a structural peptidoglycan polymer that surrounds the entire bacterium. ^{9 – 14} This polymer is also called the bacterial cell wall. Although the terms cell envelope and cell wall often are used interchangeably, the terms are distinct. The cell envelope is the multilayer barrier that separates and protects the cytoplasm of the bacterium from its external environment. It includes the peptidoglycan cell wall, the membrane(s), the periplasmic space between the two membranes of the diderm bacteria, and the macromolecules that are associated with all of these structures. The location of the cell wall within the cell envelope is a fundamental distinction. The cell wall is the exoskeleton of the Gram-positive bacteria, albeit with extensive integration of a second glycopolymer, the teichoic acids ^{15 – 17} as well as capsular polysaccharides. ¹⁸ This peptidoglycan cell surface overlays an interstitial space, itself above the single bilayered, cytoplasmic membrane. ¹⁹ The cell wall of Gram-negative bacteria is an endoskeleton that is located within the periplasm, an interstitial space that separates the cytoplasmic membrane from the outer membrane on the surface of the bacterium. Both of its membranes are bilayered. ²⁰ The dominant structural entity of the surface layer of the Gram-negative bacterium is the lipopolysaccharide. ^{4 , 21} The nonmotile, strictly aerobic, nonfermenter Gram-negative pathogen Acinetobacter baumannii is an excellent example, from the dual perspectives of structure and pathogenicity, of the overall organization of the Gram-negative cell envelope. ²² The peptidoglycan cell wall of the mycobacteria is likewise an endoskeleton that is located above the cytoplasmic membrane. ^{23 – 27} The mycobacterial peptidoglycan is extensively modified on its outward face by a galactofuran polysaccharide outer surface, to which in turn are attached oligoarabinofuran strands that themselves have mycolic acid lipids. In numerous cartoon representations of the mycobacterial cell envelope, these mycolic lipids are represented as the inner leaflet of a second membrane (mycomembrane), with the exterior surface of the mycobacterium cell envelope a structurally heterogeneous (lipid, protein) “capsule.” ²⁸ These three envelopes are represented in cartoon form in Figure 18.1A. Bacteria further diversify their cell envelopes. The cell wall is absent in the phylogenically Gram-positive Mycoplasma (within the class Mollicutes), ^{11 , 29 , 30} and appears in cryptic structural form in the “cell-wall-less” and obligate intracellular pathogens of the class Chlamydiae. ^{31 – 33} Figure 18.1B shows in cartoon form the proteinaceous Z-ring that initiates septal peptidoglycan synthesis in Gram-negative bacteria, Gram-positive bacteria, and mycobacteria in contrast with the peptidoglycan ring of the cell-wall-less Chlamydiae. Other unusual adaptations with respect to the peptidoglycan cell wall are encountered. The symbiotic relationship between the mealybug and its Moranella bacterial symbiont derives from the horizontal transfer of genes for peptidoglycan biosynthesis from the genome of the bacterium to the genome of the insect. ^{34 , 35}