Antisepsis, Disinfection, and Sterilization. Gerald E. McDonnell
Spirals
FIGURE 1.6 Basic structure of a bacterial cell, showing the cell surface in greater detail.
A key component of all bacterial cell walls is peptidoglycan, which is a polysaccharide (a polymer of sugar units) of two repeating sugars, N-acetylglucosamine and N-acetylmuramic acid, linked by (β-1,4 glycosidic (sugar-sugar) bonds (Fig. 1.8). The N-acetylmuramic acids have attached tetrapeptides (peptides of four amino acids), which are composed of amino acids such as L-alanine, D-alanine, D-glutamic acid, and lysine (usually in gram-positive bacteria) or diaminopimelic acid (DAP) (usually in gram-negative bacteria). These tetrapeptides cross-link the polysaccharide layers. The exact structure, extent of cross-linking, and thickness of the peptidoglycan vary among bacteria. For example, Escherichia coli (a gram-negative bacterium) tetrapeptides consist of L-alanine, D-glutamic acid, DAP, and D-alanine, and the peptidoglycan is only a minor component of the cell wall (~10%), which is loosely cross-linked. In contrast, the Staphylococcus aureus peptidoglycan has lysine instead of DAP in the tetrapeptide but is also indirectly linked to an adjacent tetrapeptide by a five-amino-acid (glycine) bridge. The peptidoglycan makes up ~90% of the staphylococcal cell wall and is highly cross linked. It is the dense nature of peptidoglycan in the gram-positive cell walls that allows differentiation in the Gram stain. Some archaea have been found to have a similar but distinct peptidoglycan structure present in their cell walls (see section 1.3.4.2).
Overall, the basic structure of a gram-positive bacterium’s cell wall consists of peptidoglycan; however, other proteins and polysaccharides have been described and can be specific to different bacterial species. These include polysaccharides (e.g., the A, B, and C streptococcal polysaccharides), teichoic acids, and teichuronic acids. The teichoic acids are found in the cell walls of many gram-positive bacteria, including those of Bacillus, Staphylococcus, and Lactobacillus. They are polysaccharides based on ribitol or glycerol, with attached sugars and amino acids, and are covalently linked to peptidoglycan. Some may also be bound to the cell membrane and are known as lipoteichoic acids. Other polysaccharides include the teichuronic acids (e.g., in Bacillus), which are also linked to peptidoglycan. Proteins and enzymes are also found attached to the peptidoglycan or otherwise associated with the cell wall; they may be involved in interaction with host tissues, peptidoglycan turnover, cell division, and nutrient acquisition. Finally, the actinomycetes typically stain gram positive, but with a different cell wall structure. Structurally, they resemble fungi, can form hyphae, and produce spores (sporophores) by filament fragmentation; however, the nucleic acid is free in the cytoplasm and the filaments and cell sizes are much smaller than in eukaryotic fungi (see section 1.3.2). Nocardia, as an example, has a tripartite cell wall structure similar to that of mycobacteria (see below), while Streptomyces has a more typical gram-positive cell wall structure consisting of an external peptidoglycan but also contains a major portion of fatty acids. Table 1.9 gives some common examples of gram-positive bacteria.
In general, the cell wall in gram-negative bacteria has a minor peptidoglycan layer directly bound to an external outer membrane by lipoproteins (Fig. 1.7). The area between the inner and outer membranes is known as the periplasm. The periplasm can contain a variety of proteins involved in cellular metabolism or in interactions with the extracellular environment. The outer membrane is essentially similar to the inner, cytoplasmic membrane but, in addition to phospholipids and integral proteins, also contains LPSs. LPS contains a lipid portion (known as lipid A) that forms part of the external surface of the outer membrane, linked to a polysaccharide (containing a core and an O-polymer of sugars); the types of fatty acids and sugars that make up LPS structure vary among gram-negative species. LPSs, in particular the lipid A portions, are also known as endotoxins, which are pyrogenic and play a role in bacterial infections (see section 1.3.7). Similar to the inner membrane, proteins can be found associated through or at the periplasmic or external surface of the outer membrane. An important group of integral proteins are the porins, which form channels to allow the transport of molecules through the outer membrane. Some common examples of gram-negative bacteria are given in Table 1.10.
FIGURE 1.7 Bacterial cell wall structures. The cell membranes are similar structures in all types. Gram-positive bacteria have a large peptidoglycan layer (shown as crossed lines) with associated polysaccharides and proteins. Gram-negative bacteria have a smaller peptidoglycan layer linked to an outer membrane. The mycobacterial cell has a series of covalently linked layers, including the peptidoglycan-, arabinogalactan-, and mycolic acid-containing sections.
FIGURE 1.8 Basic structure of peptidoglycan. Polysaccharides of repeating sugars are cross-linked by peptide bridges. Two different types of peptide bridges, which have been described in gram-positive and gram-negative bacterial cell walls, are shown.
TABLE 1.9 Examples of gram-positive bacteria
| General type | Key characteristics | Example(s) | Significance |
| Gram-positive cocci | Diverse group of gram-positive cocci; nonsporeformers | Enterococcus (e.g., E. faecalis, E. faecium) | Widely distributed in soil, water, and animals; normal flora in lower gastrointestinal tract; often identified as causing urinary tract diseases and wound infections. Vancomycin-resistant strains (VRE) are a concern in hospital-acquired infections. |
| Lactococcus Staphylococcus (e.g., S. epidermidis, S. aureus) | Found in plant and dairy products; can cause food spoilageCommon human and animal parasites. S. epidermidis is usually found on the skin and mucous membranes. S. aureus is commonly identified as a pathogen, including in skin, wound, gastrointestinal, and lower respiratory tract diseases. Methicillin-resistant S. aureus (MRSA) strains are a leading cause of hospital-acquired wound infections. | ||
| Streptococcus (e.g., S. pyogenes, S. pneumoniae) |
Common human and animal pathogens. S. pyogenes and S. pneumoniae are both associated with upper and lower respiratory tract diseases, including pharyngitis (sore throat), pneumonia, and scarlet fever. S. pyogenes can also cause a wide variety of other diseases, including skin and |