Schematic illustration of the cell envelope of gram positive and gram negative bacteria. NAG-NAM stands for N-acetylglucosamine and N-acetylmuramic acid, which are the building blocks of the cell wall. TA and LTA stand for teichoic acid and lipoteicoic acid, respectively. These components stabilize the cell wall and anchors it in the plasma membrane of gram positive bacteria. LPS stands for lipopolysaccharide and BLP stands for Brown's lipoprotein, which binds the cell wall to the outer membrane of gram negative bacteria.
Image: Karl-Erik Johansson (BVF, SLU). - Click on the image to enlarge it.
The cell wall and the cell membrane are very important structures of bacteria because they form the bacterial contact surface with the environment. For bacteria, the cell wall together with the cell membrane and also the outer membrane (if present), for the cell envelope. Differences in cell envelope structures can be used to differentiate different groups of bacteria e.g. by gram staining. For some antibiotics, the fact that there is a large difference between the bacterial cell envelope and the cell envelope of higher organisms (eukaryotic cells) is exploited. There are two different main types of structures in the cell envelopes of bacteria and since it can be utilised in gram staining, these bacteria are said to be gram positive or gram negative. The differences between the gram positive and the gram negative cell envelopes are shown in the very simplified in figure.
In gram positive bacteria, the cell wall is 20-80 nm thick, while in gram negative bacteria it is only 5-10 nm thick. The cell wall consists of peptidoglycan, which is made up of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). The peptidoglycan chains are cross-linked to each other through peptide bridges and form a continuous network around the bacterium. This network can be considered a giant molecule. Gram-negative bacteria also have an outer membrane containing lipopolysacharide (LPS) to protect against the environment. Both the plasma membrane and the outer membrane are about 8 nm thick and contain proteins that are important for the bacterial functions. Some authors include both peptidoglycan and outer membrane of gram negative bacteria in the term cell wall. The periplasm is the space that exists between the cell wall and the plasma membrane in gram negative bacteria. Some authors define the periplasm as the space between the plasma membrane and the outer membrane. The periplasm is gelatinous and contains enzymes which are important for the metabolism of the bacteria.
In addition to the above components, bacteria may have flagella, fimbriae and capsules on the cell surface (not shown in the figure). In the lecture on chlamydia, it is stated the chlamydia lack the peptidoglycan, which is not correct according to current knowledge.
The cell envelope is, of course, vital for the bacterium, since its surroundings can be very harsh and thus the envelope provides protection for the bacterium. There are various enzyme systems that are bound to the plasma membrane, e.g. those that are part of the elektron transport chain and some of the enzymes that participate in the synthesis of the cell wall. The molecular motor, which consists of various protein subunits and drives the movement of flagella, is anchored in the plasma membrane of the bacterium. Many bacteria have so-called secretion systems, which are of great importance for their pathogenic ability. The secretion systems also consist of a number of different membrane-bound protein subunits. Porins are membrane-bound proteins that can transport substances through the plasma membrane and the outer membrane if present.
There are bacteria that have a completely different structure of the cell envelope than described in the figures above.
Bacteria belonging to the phylum Tenericutes, which includes the genus Mycoplasma, are lacking a cell wall because they are unable to produce the peptidoglycan. These bacteria usually live in physiological environments where the mechanical protection of the cell against osmotic lysis is not needed. More information about mycoplasmas can be found in a series of lectures.
The Chlamydiaceae family
It was long believed that bacteria within the Chlamydiaceae family have no peptidoglycan, but instead cysteine-rich proteins, which stabilize the outer membrane, through disulfide bonds. However, a few years ago, a research group showed that the reticular bodies of chlamydia have a thin peptidoclycan, which is important in cell division, which in chlamydia occurs in host cell inclusions. In the lecture on chlamydia, it is stated that chlamydias lack the peptidoclycan, which is not correct according to current knowledge,
The order Corynebacteriales, to which the families Corynebacteriaceae, Mycobacteriaceae and Nocardiaceae belong, have a cell envelope, most similar to that of gram-positive bacteria. These bacteria thus have a plasma membrane, peptidoglycan and outside the peptidoglycan a polysaccharide layer (arabinogalactan) as well as mycolic acid, which with its hydrocarbon chains binds to a single layer of lipids. Mycobacteria are said to be acid-resistant because they have an unusually stable cell envelope.