Procaryotic cell structure

When compared with the profusion of elaborate organelles encountered inside a typical eucaryotic cell, the interior of a typical bacterium looks rather empty. The only internal structural features are:

• a bacterial chromosome or nucleoid, comprising a closed loop of double stranded, supercoiled DNA. In addition, there may be additional DNA in the form of a plasmid

• thousands of granular ribosomes

• a variety of granular inclusions associated with nutrient storage.

All of these are contained in a thick aqueous soup of carbohydrates, proteins, lipids and inorganic salts known as the cytoplasm, which is surrounded by a plasma membrane. This in turn is wrapped in a cell wall, whose rigidity gives the bacterial cell its characteristic shape. Depending on the type of bacterium, there may be a further surrounding layer such as a capsule or slime layer and/or structures external to the cell associated with motility (flagella) or attachment (pili/fimbriae). Figure 3.3 shows these features in a generalised bacterial cell. In the following pages we shall examine these features in a little more detail, noting how each has a crucial role to play in the survival or reproduction of the cell.

Genetic material

Although it occupies a well defined area within the cell, the genetic material of procaryotes is not present as a true nucleus, as it lacks a surrounding nuclear membrane (c.f. the eucaryotic nucleus, Figure 3.12). The nucleoid or bacterial chromosome comprises a closed circle of double stranded DNA, many times the length of the cell and highly folded and compacted. (The common laboratory

Not all bacteria conform to the model of a single circular chromosome; some have been shown to possess two with genes shared between them, while examples of linear chromosomes are also known.

Plasma membrane

Outer membrane

Mesosome

Mesosome

Procaryotic Cell

Ribosome

Figure 3.3 Structure of a generalised procaryotic cell. Note the lack of complex internal organelles (c.f. Figure 3.12). Gram-positive and Gram-negative bacteria differ in the details of their cell wall structure (see Figures 3.7 & 3.8)

Ribosome

Figure 3.3 Structure of a generalised procaryotic cell. Note the lack of complex internal organelles (c.f. Figure 3.12). Gram-positive and Gram-negative bacteria differ in the details of their cell wall structure (see Figures 3.7 & 3.8)

bacterium Escherichia coli is around 3-4 ¡m in length, but contains a DNA molecule some 1400 ¡m in length!) The DNA may be associated with certain bacterial proteins, but these are not the same as the histones found in eucaryotic chromosomes. Some bacteria contain additional DNA in the form of small, self-replicating extrachromosomal elements called plasmids. These do not carry any genes essential for growth and reproduction, and thus the cell may survive without them. They can be very important however, as they may include genes encoding toxins or resistance to antibiotics, and can be passed from cell to cell (see Chapter 12).

Ribosomes

Apart from the nucleoid, the principal internal structures of procaryotic cells are the ribosomes. These are the site of protein synthesis, and there may be many thousands of these in an active cell, lending a speckled appearance to the cytoplasm. Ribosomes are composed of a complex of protein and RNA, and are the site of protein synthesis in the cell.

Although they carry out a similar function, the ribosomes of procaryotic cells are smaller and lighter than their eucaryotic counterparts. Ribosomes are measured in Svedberg units (S), a function of their size and shape, and determined by their rate of sedimentation in a centrifuge; procaryotic ribosomes are 70S, while those of eucaryotes are 80S. Some types of antibiotic exploit this difference by

Plasmids are small loops of DNA independent of the chromosome. They are capable of directing their own replication.

Table 3.3 Comparison of procaryotic and eucaryotic ribosomes

Procaryotic Eucaryotic

Overall size 70S 80S

Large subunit size 50S 60S

Small subunit size 30S 40S

Small subunit RNA 16S 18S

targeting the procaryotic form and selectively disrupting bacterial protein synthesis (see Chapter 14).

All ribosomes comprise two unequal subunits (in pro-caryotes, these are 50S and 30S, in eucaryotes 60S and 40S: Table 3.3)). Each subunit contains its own RNA and a number of proteins (Figure 3.4). Many ribosomes may simultaneously be attached to a single mRNA molecule, forming a threadlike polysome. The role of ribosomes in bacterial protein synthesis is discussed in Chapter 11.

A polyribosome (polysome) is a chain of ribosomes attached to the same molecule of mRNA.

Inclusion bodies

Within the cytoplasm of certain bacteria may be found granular structures known as inclusion bodies. These act as food reserves, and may contain organic compounds such as starch, glycogen or lipid. In addition, sulphur and polyphosphate can be stored as inclusion bodies, the latter being known as volutin or metachromatic granules. Two special types of inclusion body are worthy of mention. Magnetosomes, which contain a form of iron oxide, help some types of bacteria to orientate themselves downwards into favourable conditions, whilst gas vacuoles maintain bouyancy of the cell in blue greens and some halobacteria.

Figure 3.4 The bacterial ribosome. Each subunit comprises rRNA and proteins. The nucleotide sequence of small subunit (16S) rRNA is widely used in determining the phylogenetic (evolutionary) relationship between bacteria (see Chapter 7)

Endospores

Endospores of pathogens such as Clostridium botulinum can resist boiling for several hours. It is this resistance that makes it necessary to autoclave at 121 °C in order to ensure complete sterility.

Certain bacteria such as Bacillus and Clostridium produce endospores. They are dormant forms of the cell that are highly resistant to extremes of temperature, pH and other environmental factors, and germinate into new bacterial cells when conditions become more favourable. The spore's resistance is due to the thick coat that surrounds it.

The plasma membrane _

The cytoplasm and its contents are surrounded by a plasma membrane, which can be thought of as a bilayer of phospholipid arranged like a sandwich, together with associated proteins (Figure 3.5). The function of the plasma membrane is to keep the contents in, while at the same time allowing the selective passage of certain substances in and out of the cell (it is a semipermeable membrane).

Phospholipids comprise a compact, hydrophilic (= water-loving) head and a long hydrophobic tail region (Figure 2.27); this results in a highly ordered structure when the membrane is surrounded by water. The tails 'hide' from the water to form the inside of the membrane, while the heads project outwards. Also included in the membrane are a variety of proteins; these may pass right through the bilayer or be associated with the inner (cytoplasmic) or outer surface only. These proteins may play structural or functional roles in the life of the cell. Many enzymes associated with the metabolism of nutrients and the production of energy are associated with the plasma membrane in procaryotes. As we will see later in this chapter, this is fundamentally different from

Figure 3.5 The plasma membrane. Phospholipid molecules form a bilayer, with the hydrophobic hydrocarbon chains pointing in towards each other, leaving the hydrophilic phosphate groups to face outwards. Proteins embedded in the membrane are known as integral proteins, and may pass part of the way or all of the way through the phospholipid bilayer. The amino acid composition of such proteins reflects their location; the part actually embedded among the lipid component of the membrane comprises non-polar (hydrophobic) amino acids, while polar ones are found in the aqueous environment at either side. Singleton, P: Bacteria in Biology, Biotechnology and Medicine, 5th edn, John Wiley & Sons, 1999. Reproduced by permission of the publishers

Integral protein

— Phospholipid bilayer

Figure 3.5 The plasma membrane. Phospholipid molecules form a bilayer, with the hydrophobic hydrocarbon chains pointing in towards each other, leaving the hydrophilic phosphate groups to face outwards. Proteins embedded in the membrane are known as integral proteins, and may pass part of the way or all of the way through the phospholipid bilayer. The amino acid composition of such proteins reflects their location; the part actually embedded among the lipid component of the membrane comprises non-polar (hydrophobic) amino acids, while polar ones are found in the aqueous environment at either side. Singleton, P: Bacteria in Biology, Biotechnology and Medicine, 5th edn, John Wiley & Sons, 1999. Reproduced by permission of the publishers

Peripheral protein

— Phospholipid bilayer eucaryotic cells, where these reactions are carried out on specialised internal organelles. Proteins involved in the active transport of nutrients (see Chapter 4) are also to be found associated with the plasma membrane. The model of membrane structure as depicted in Figure 3.5 must not be thought of as static; in the widely accepted fluid mosaic model, the lipid is seen as a fluid state, in which proteins float around, rather like icebergs in an ocean.

The majority of bacterial membranes do not contain sterols (c.f. eucaryotes: see below), however many do contain molecules called hopanoids that are derive from the same precursors. Like sterols, they are thought to assist in maintaining membrane stability. A comparison of the lipid components of plasma membranes reveals a distinct difference between members of the Archaea and the Bacteria.

The bacterial cell wall

Bacteria have a thick, rigid cell wall, which maintains the integrity of the cell, and determines its characteristic shape. Since the cytoplasm of bacteria contains high concentrations of dissolved substances, they generally live in a hypotonic environment (i.e. one that is more dilute than their own cytoplasm). There is therefore a natural tendency for water to flow into the cell, and without the cell wall the cell would fill and burst (you can demonstrate this by using enzymes to strip off the cell wall, leaving the naked protoplast).

The major component of the cell wall, which is responsible for its rigidity, is a substance unique to bacteria, called peptidoglycan (murein). This is a high molecular weight polymer whose basic subunit is made up of three parts: N-acetylglucosamine, N-acetylmuramic acid and a short peptide chain (Figure 3.6). The latter comprises the amino acids l-alanine, d-alanine, d-glutamic acid and either l-lysine or diaminopimelic acid (DAP). DAP is a rare amino acid, only found in the cell walls of procaryotes. Note that some of the amino acids of peptidoglycan are found in the d-configuration. This is contrary to the situation in proteins, as you may recall from Chapter 2, and confers protection against proteases specifically directed against l-amino acids.

Precursor molecules for peptidoglycan are synthesised inside the cell, and transported across the plasma membrane by a carrier called bactoprenol phosphate before being incorporated into the cell wall structure. Enzymes called transpeptidases then covalently bond the tetrapeptide chains to one another, giving rise to a complex network (Figure 3.7); it is this cross-linking that gives the wall its mechanical strength. A number of antimicrobial agents exert their effect by inhibiting cell wall synthesis; j-lactam antibiotics such as penicillin inhibit the transpeptidases, thereby weakening the cell wall, whilst bacitracin prevents transport of peptidoglycan precursors out of the cell. The action of antibiotics will be discussed further in Chapter 14. Although all bacteria (with a few exceptions) have a cell wall containing peptidoglycan, there are two distinct structural types. These are known as Gram-positive and Gram-negative. The names derive from the Danish scientist Christian Gram, who, in the 1880s developed a rapid staining technique that could differentiate bacteria as belonging to one of two basic types (see Box 1.2). Although the usefulness of the Gram stain was recognised for many years, it

A protoplast is a cell that has had its cell wall removed.

Proteases are enzymes that digest proteins.

N-acetylmuramic N-acetylglucosamine acid residue residue

N-acetylmuramic N-acetylglucosamine acid residue residue

COOH

L-Alanine

D-Glutamic acid

C O NH2

H — C— CH3 D-Alanine meso-Diaminopimelic acid

CH3 C H

Figure 3.6 Peptidoglycan structure. Peptidoglycan is a polymer made up of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A short peptide chain is linked to the NAM residues (see text for details). This is important in the cross-linking of the straight chain polymers to form a rigid network (Figure 3.6). The composition of E. coli peptidoglycan is shown; the peptide chain may contain different amino acids in other bacteria. Partly from Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Figure 3.6 Peptidoglycan structure. Peptidoglycan is a polymer made up of alternating molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A short peptide chain is linked to the NAM residues (see text for details). This is important in the cross-linking of the straight chain polymers to form a rigid network (Figure 3.6). The composition of E. coli peptidoglycan is shown; the peptide chain may contain different amino acids in other bacteria. Partly from Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services

Figure 3.7 Cross-linking of peptidoglycan chains in E. coli. (a) The D-alanine on the short peptide chain attached to the N-acetylmuramic acid cross-links to a diaminopimelic acid residue on another chain. In other bacteria, the precise nature of the cross-linking may differ. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services. (b) Further cross-linking produces a rigid network of peptidoglycan. The antibiotic penicillin acts by inhibiting the transpeptidase enzymes responsible for the cross-linking reaction (see Chapter 15)

Figure 3.7 Cross-linking of peptidoglycan chains in E. coli. (a) The D-alanine on the short peptide chain attached to the N-acetylmuramic acid cross-links to a diaminopimelic acid residue on another chain. In other bacteria, the precise nature of the cross-linking may differ. From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permission of Thomson Publishing Services. (b) Further cross-linking produces a rigid network of peptidoglycan. The antibiotic penicillin acts by inhibiting the transpeptidase enzymes responsible for the cross-linking reaction (see Chapter 15)

was only with the age of electron microscopy that the underlying molecular basis of the test could be explained, in terms of cell wall structure.

Gram-positive cell walls are relatively simple in structure, comprising several layers of peptidoglycan connected to each other by cross-linkages to form a strong, rigid scaffolding. In addition, they contain acidic polysaccharides called teichoic acids; these contain phosphate groups that impart an overall negative charge to the cell surface. A diagram of the gram-positive cell wall is shown in Figure 3.8.

Gram-negative cells have a much thinner layer of peptidoglycan, making the wall less sturdy, however the structure is made more complex by the presence of a layer of lipoprotein, polysaccharide and phospholipid known as the outer membrane

Murein Und Mesosom

Figure 3.8 The Gram-positive cell wall. Peptidoglycan is many layers thick in the Grampositive cell wall and may account for 30-70% of its dry weight. Teichoic acids are negatively charged polysaccharides; they are polymers of ribitol phosphate and cross-link to peptidoglycan. Lipoteichoic acids are teichoic acids found in association with glycolipids. From Henderson, B, Wilson, M, McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-Host Interactions in Health and Disease, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Figure 3.8 The Gram-positive cell wall. Peptidoglycan is many layers thick in the Grampositive cell wall and may account for 30-70% of its dry weight. Teichoic acids are negatively charged polysaccharides; they are polymers of ribitol phosphate and cross-link to peptidoglycan. Lipoteichoic acids are teichoic acids found in association with glycolipids. From Henderson, B, Wilson, M, McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-Host Interactions in Health and Disease, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

(Figure 3.9). This misleading name derives from the fact that it superficially resembles the bilayer of the plasma membrane; however, instead of two layers of phospholipid, it has only one, the outer layer being made up of lipopolysaccharide. This has three parts: lipid A, core polysaccharide and an O-specific side chain. The lipid A component may act as an endotoxin, which, if released into the bloodstream, can lead to serious conditions such as fever and toxic shock. The O-specific antigens are carbohydrate chains whose composition often varies between strains of the same species. Serological methods can distinguish between these, a valuable tool in the investigation, for example, of the origin of an outbreak of an infectious disease. Proteins incorporated into the outer membrane and penetrating its entire thickness form channels that allow the passage of water and small molecules to enter the cell. Unlike the plasma membrane, the outer membrane plays no part in cellular respiration.

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