microtubule‐binding proteins play an important part in the process (e.g. microtubule‐associated proteins).
3.1.6 Cell Walls
Some cell types are enclosed by a cell wall:
Bacterial cells are surrounded by a peptidoglycan layer (Figure 3.27). Gram‐positive bacteria (e.g. members of the Bacillus genus) have a thick cell wall, which borders immediately to the outside milieu, whereas in Gram‐negative bacteria (e.g. Escherichia coli), a thin cell wall is surrounded by a second lipopolysaccharide membrane as an outer shell vfd (Figure 1.1). The outer membrane has porin proteins that allow the entry of food molecules. The cell wall is an important target structure for antibiotics – penicillins and cephalosporins inhibit cross‐linking of linear glycopeptide strands. Bacitracin inhibits the synthesis of polyprenol, which is a prerequisite for the formation of a murein sacculus.
Fungal cells are surrounded by a chitin wall.
Plant cells have cell walls consisting of cellulose, hemicellulose, and pectin. They can be enzymatically digested by cellulases, producing protoplasts, which are useful for plant biotechnology.
Figure 3.27 Schematic view of bacterial cell walls. (a) Gram‐positive bacteria. (b) Gram‐negative bacteria.
Cell walls serve mostly to protect and stabilize cells. They also ensure against lysis from osmotic swelling, when have taken up too much water through osmosis.
3.2 Structure of Bacteria
Compared with eukaryotic cells, bacteria have a fairly simple structure (Figure 1.2a). The outside of Gram‐positive bacteria is shielded by a thick peptidoglycan cell wall, known as the murein sacculus. In Gram‐negative bacteria a larger periplasmic space, where some of the metabolic processes take place, lies between a thin cell wall and the cytoplasmic membrane (Figure 3.27). To the outside, they have an outer membrane containing lipopolysaccharide (LPS) and porin channels. The cytoplasmic membrane contains many membrane proteins, including transporters, ABC transporters, receptors, and enzymes. There is no compartmentalization in bacterial cells (i.e. they do not contain organelles). However, the cytoplasmic membrane is sometimes in‐folded, which makes it resemble a eukaryotic endomembrane system. On the surface, many bacteria carry flagella and pili, which enable bacteria to move and adhere to surfaces.
Contrary to earlier views, bacteria also contain various types of cytoskeletal structures, either based on FtsZ, Mreb/Mbl, or EF‐Tu proteins. FtsZ seems to be related to tubulin and Mreb/Mbl to actin, as found in eukaryotes. There seems to be no eukaryotic equivalent to EF‐Tu. All three forms may coexist in the same cell. Underneath their cytoplasmic membrane, bacteria have a cytoskeleton consisting of monomer proteins. Monomer EF‐Tu proteins, for example, can form protofilaments. There are also cytoskeleton‐like interconnections or fibers.
The proteins are synthesized in ribosomes that lie freely in the cytoplasm or are associated with the inside of the cytoplasmic membrane (Figure 1.2a).
Bacteria carry their genetic information on one single chromosome. This ring‐shaped DNA strand is also known as a nucleoid. There are additional ring‐shaped molecules called plasmids, which also carry genetic information and may include antibiotic resistance genes. Modified plasmids play an important role as cloning vectors in molecular biology and biotechnology (see Chapter 15).
Bacteria continue to be the favorite “pets” of molecular biologists and biotechnologists. Basic research on genetics, molecular biology, and biochemistry is often first carried out in bacteria such as E. coli. Some bacteria are even indispensable for the cloning and expression of DNA (see Chapters 15 and 16).
Infectious diseases are probably as old as mankind. They can be caused by bacteria, fungi, viruses, protozoa (Plasmodium causing malaria, Trypanosoma causing sleeping sickness), and a variety of intestinal worms. These organisms are collectively called pathogens. Only a small fraction of microbes (Chapter 6) play a role as pathogens. Bacterial infections are the cause of many diseases in humans, animals, and plants. Bacterial pathogens often carry specialized virulence genes, which can be passed on to other bacteria via horizontal gene transfer. Some bacteria damage the host through sophisticated toxins that interfere with signaling pathways. Tetanus toxins from Clostridium tetani act as proteases. In the synapses, they specifically hydrolyze SNARE proteins, thus blocking neuronal signal transduction. Cholera‐causing Vibrio cholerae produces cholera toxin, an enzyme that redirects the transfer of ADP‐ribose from NAD+ to the α‐subunit Gs of a G‐protein. This inhibits GTPase, and the once activated adenylyl cyclases remain permanently active, producing cAMP. As a consequence, intestinal cells secrete excessive amounts of Cl+ ions and water, resulting in diarrhea. Bordetella pertussis, the cause of whooping cough, produces enzymes that activate the α‐subunit Gi of the G‐protein, preventing Gi from regulating its target proteins. This also leads to an overproduction of cAMP. Anthrax (Bacillus anthracis) is an acute infectious disease of sheep and cattle but can also kill humans. The anthrax bacteria secrete two toxins; they differ in the composition of the A subunit, whereas their B subunits are identical. The B subunit binds to cell surface receptors, mediating the uptake of the A subunits (differentiated into lethal factorand edema factor) into the cell. The edema factor is an adenylyl cyclase that overproduces cAMP leading to edema in skin and lung. The lethal factor is a protease that degrades several activated members of the mitogen‐activated protein (MAP) kinase kinase, leading to a disruption of signal pathways and finally death.
The discovery and the development of new antibiotics from various Streptomyces and fungal species in the second half of the twentieth century was a milestone in medical history, saving millions of lives. Antibiotics were given as single compounds not only to millions of people but also to animals. This favored the development of resistance through target site modification (transporter, cell wall, ribosomal proteins, rRNA), enzymes that degrade antibiotics (β‐lactamase), enzymes that inactivate antibiotics, and ABC transporter (efflux pumps). Important resistance types are extended‐spectrum β‐lactamase (ESBL), New Delhi metallo‐β‐lactamase 1 (NDM‐1), AmpC (β‐lactamases), Klebsiella pneumoniae carbapenemase (KPC), vancomycin‐resistant enterococci (VRE), vancomycin‐resistant Staphylococcus aureus (VRSA), pristinamycin‐resistant S. aureus (PRSA), and methicillin‐resistant S. aureus
