Medical Microbiology
 
Bacterial Morphology

Bacterial Structure

:: Cytoplasmic Structures ::

      Gram positive and Gram negative bacteria have similar internal, but very different external structures. The cytoplasm of the bacterial cell contains the DNA chromosome, the mRNA, ribosomes, proteins, and metabolites). Unlike eukaryotes, the bacterial chromosome is a single, double-stranded circle that is contained not in a nucleus but in a discrete area known as the nucleoid. Histones are not required to maintain the conformation of the DNA, and the DNA does not form nucleosomes. Plasmids, which are smaller, circular, extrachromosomal DNAs, may also be present. Plasmids are most commonly found in Gram negative bacteria, and although not usually essential for cellular survival, they often provide a selective advantage: many confer resistance to one or more antibiotics.

Gram positive and Gram negative bacteria. A Gram positive bacterium has a thick layer of peptidoglycan (left). A Gram negative bacterium has a thin peptidoglycan layer and an outer membrane (right). Structures in () are not found in all bacteria.

      The lack of a nuclear membrane simplifies the requirements and control mechanisms for the synthesis of proteins. Without a nuclear membrane, transcription and translation are coupled; in other words, ribosomes can bind to the mRNA, and protein can be made as the mRNA is being synthesized and still attached to the DNA.
      The bacterial ribosome consists of 30S + 50S subunits, forming a 70S ribosome. This is unlike the eukaryotic 80S (40S + 60S) ribosome. The proteins and RNA of the bacterial ribosome are significantly different from those of eukaryotic ribosomes and are major targets for antibacterial drugs.
      The cytoplasmic membrane has a lipid bilayer structure similar to the structure of the eukaryotic membranes, but it contains no steroids (e.g., cholesterol); mycoplasmas are the exception to this rule. The cytoplasmic membrane is responsible for many of the functions attributable to organelles in eukaryotes. These tasks include electron transport and energy production, which are normally achieved in the mitochondria. In addition, the membrane contains transport proteins that allow the uptake of metabolites and the release of other substances, ion pumps to maintain a membrane potential, and enzymes. A coiled cytoplasmic membrane, the mesosome, acts as an anchor to bind and pull apart daughter chromosomes during cell division.

:: Cell Wall ::

      The structure, components, and functions of the cell wall distinguish Gram positive from Gram negative bacteria. The important differences in membrane characteristics are outlined in. The cytoplasmic membranes of most prokaryotes are surrounded by rigid peptidoglycan (murein) layers. The exceptions are Archaeobacteria organisms (which contain pseudoglycans or pseudomureins related to peptidoglycan) and mycoplasmas (which have no cell walls at all). Because the peptidoglycan provides rigidity, it also determines the shape of the particular bacterial cell. Gram negative bacteria are also surrounded by outer membranes.

Bacterial Membrane Structures
Structure
Chemical Constituents
    Plasma Membrane     Phospholipids, proteins, enzymes for energy, membrane potential, transport.
    Cell Wall     
       Gram +ve Bacteria     
          Peptidoglycan     Glycan chains of GlcNAc and MurNAc cross linked by peptide bridge.
          Teichoic Acid     Polyribitol phosphate or glycerol phosphate cross linked to peptidoglycan.
          Lipoteichoic Acid     Lipid linked teichoic acid.
       Gram -ve Bacteria     
          Peptidoglycan     Thinner version of that found in Gram positive bacteria.
          Periplasmic Space     Enzymes involved in transport, degradation, and synthesis.
          Outer Membrane     Phospholipids with saturated fatty acids.
             Proteins     Porins, lipoprotein, transport proteins.
             LPS     Lipid A, core polysaccharide, O antigen.
    Other Structures     
       Capsule     Polysaccharides (disaccharides and trisaccharides) and polypeptides.
       Pili     Pilin, adhesins.
       Flagellum     Motor proteins, flagellin.
    Proteins     M proteins of streptococci (for example).
GlcNac=N-Acetylglucosamine; MurNAc=N-acetylmuramic acid; LPS=lipopolysaccharide.


Functions Of The Bacterial Envelope
Function
Component(s)
   Structural Rigidity    All.
   Packaging Of Internal Contents    All.
   Permeability Barrier    Outer membrane or plasma membrane.
   Metabolic Uptake    Membranes and periplasmic transport proteins, porins, permeases.
   Energy Production    Plasma membrane.
   Adhesion To Host Cells    Pili, proteins, teichoic acid.
   Immune Recognition By Host    All outer structures.
   Escape From Host Recognition    Capsule, M protein.
   Antibiotic Sensitivity    Peptidoglycan synthetic enzymes.
   Antibiotic Resistance    Outer membrane.
   Motility    Flagella.
   Mating    Pili.
   Adhesion    Pili.

:: Gram Positive Bacteria ::

      A Gram positive bacterium has a thick, multilayered cell wall consisting mainly of peptidoglycan (150 to 500 A) surrounding the cytoplasmic membrane. The peptidoglycan is a meshlike exoskeleton similar in function to the exoskeleton of an insect. Unlike the exoskeleton of the insect, however, the peptidoglycan of the cell is sufficiently porous to allow diffusion of metabolites to the plasma membrane. The peptidoglycan is essential for the structure, for replication, and for survival in the normally hostile conditions in which bacteria grow. During infection, the peptidoglycan can interfere with phagocytosis, is mitogenic (stimulates mitosis of lymphocytes), and has pyrogenic activity (induces fever).
      The peptidoglycan can be degraded by treatment with lysozyme. Lysozyme, an enzyme in human tears and mucus, is also produced by bacteria and other organisms. Lysozyme degrades the glycan backbone of the peptidoglycan. Without the peptidoglycan, the bacteria succumb to the large osmotic pressure differences across the cytoplasmic membrane and lyse. Removal of the cell wall produces a protoplast that lyses unless it is osmotically stabilized.
      The Gram positive cell wall may also include other components such as teichoic and lipoteichoic acids and complex polysaccharides (usually called C polysaccharides). Proteins such as the M protein of streptococci and R protein of staphylococci also associate with the peptidoglycan. Teichoic acids are water-soluble polymers of polyol phosphates, which are covalently linked to the peptidoglycan. Lipoteichoic acids have a fatty acid and are anchored in the cytoplasmic membrane. These molecules are common surface antigens that distinguish bacterial serotypes and promote attachment to other bacteria as well as to specific receptors on mammalian cell surfaces (adherence). Teichoic acids are important factors in virulence. Lipoteichoic acids are shed into the media and host and, although weaker, can initiate endotoxic-like activities.

:: Gram Negative Bacteria ::

      Gram negative cell walls are more complex than Gram positive cell walls, both structurally and chemically. Structurally, a Gram negative cell wall contains two layers external to the cytoplasmic membrane. Immediately external to the cytoplasmic membrane is a thin peptidoglycan layer, which accounts for only 5% to 10% of the Gram negative cell wall by weight. There are no teichoic or lipoteichoic acids in the Gram negative cell wall. External to the peptidoglycan layer is the outer membrane, which is unique to Gram negative bacteria. The area between the external surface of the cytoplasmic membrane and the internal surface of the outer membrane is referred to as the periplasmic space. This space is actually a compartment containing a variety of hydrolytic enzymes, which are important to the cell for the breakdown of large macromolecules for metabolism. These enzymes typically include proteases, phosphatases, lipases, nucleases, and carbohydrate-degrading enzymes. In the case of pathogenic Gram negative species, many of the lytic virulence factors such as collagenases, hyaluronidases, proteases, and beta-lactamase are in the periplasmic space. This space also contains components of the sugar transport systems and other binding proteins to facilitate the uptake of different metabolites and other compounds. Some binding proteins can be components of a chemotaxis system, which senses the external environment of the cell.

Comparison of the Gram positive and Gram negative bacterial cell walls. A, a Gram positive bacterium has a thick peptidoglycan layer that contains teichoic and lipoteichoic acids. B, a Gram negative bacterium has a thin peptidoglycan layer and an outer membrane that contains lipopolysaccharide, phospholipids, and proteins. The periplasmic space between the cytoplasmic and outer membranes contains transport, degradative, and cell wasll synthetic proteins. The outer membrane is joined to the cytoplasmic membrane at adhesion points and is attached to the peptidoglycan by lipoprotein links.

      As mentioned previously, outer membranes are unique to Gram negative prokaryotes. The outer membrane is like a stiff canvas sack around the bacteria. The outer membrane maintains the bacterial structure and is a permeability barrier to large molecules (e.g., proteins such as Lysozyme) and hydrophobic molecules. It also provides protection from adverse environmental conditions such as the digestive system of the host (important for Enterobacteriaceae organisms). The outer membrane has an asymmetric bilayer structure that differs from any other biologic membrane in the structure of the outer leaflet of the membrane. The inner leaflet contains phospholipids normally found in bacterial membranes. However, the outer leaflet is composed primarily of an amphipathic molecule (meaning that it has both hydrophobic and hydrophilic ends) called lipopolysaecharide (LPS). Except for those LPS molecules in the process of synthesis, the outer leaflet of the outer membrane is the only location where LPS molecules are found.
      LPS is also called endotoxin, a powerful stimulator of immune responses. LPS activates B cells and induces macrophage and other cells to release interleukin-I and interleukin-6, tumor necrosis factor, and other factors. LPS causes fever and can cause shock. The Shwartzman reaction (disseminated intravascular coagulation) follows the release of large amounts of endotoxin into the blood stream. LPS is shed from the bacteria into the media and host. Neisseria meningitidis sheds large amounts of a related compound, lipooligosaccharide (LOS), resulting in fever and symptoms.
      The variety of proteins found in Gram negative outer membranes is limited, but several of the proteins are present in high concentration, resulting in a total protein content higher than that of the cytoplasmic membrane. Many of the proteins traverse the entire lipid bilayer and are thus transmembrane proteins. A group of these proteins is known as porins because they form pores that allow the diffusion of hydrophilic molecules less than 700 Da in mass through the membrane. The outer membrane and the porin channel allow passage of metabolites and small hydrophilic antibiotics, but the outer membrane is a barrier for large or hydrophobic antibiotics and proteins such as 1ysozyme.
      The outer membrane also contains structural proteins and receptor molecules for bacteriophages and other ligands. The outer membrane is connected to the cytoplasmic membrane at adhesion sites and is tied to the peptidoglycan by lipoprotein. The lipoprotein is covalently attached to the peptidoglycan and is anchored in the outer membrane. The adhesion sites provide a membranous route for the delivery of newly synthesized outer membrane components to the outer membrane.
      The outer membrane is held together by divalent cation (Mg+2 and Ca+2) linkages between phosphates on LPS molecules and hydrophobic interactions betwecn the LPS and proteins. These interactions produce a stiff, strong membrane that can be disrupted by antibiotics (e.g., polymyxin) or by the removal of Mg and Ca ions (chelation with ethylenediaminetetraacetic acid [FDTA]). Disruption of the outer membrane weakens the bacteria and allows the permeability of large, hydrophobic molecules. The addition of lysozyme to cells treated in this manner produces spheroplasts, which, like protoplasts, are osmotically sensitive.

:: External Structures ::

      Some bacteria (Gram positive or Gram negative) are closely surrounded by loose polysaccharide or protein layers called capsules. In cases in which it is loosely adherent and nonuniform in density or thickness, the material is referred to as a slime layer. The capsule and slime layers are also called the glycocalyx. Bacillus anthracis, the exception to this rule, produces a polypeptide capsule. The capsule is hard to see in a microscope but can be visualized by the exclusion of India ink particles.

Transmission electrom micrographs of Porphyromonas (formerly Bacteroides) gingivalis and Pseudomonas aeruginosa revealing the surface associated capsule. Both strains were isolated from human patients, P. gingivalis from an adult with peridontitis and P. aeruginosa from a patieng with cystic fibrosis. C = capsule; OM = outer membrane; PG = peptidoglycan; CM = cytoplasmic membrane; R = ribosome; PP = polyphosphate. Bar = 0.1 um.

      Capsules and slimes are unnecessary for the growth of bacteria but are very important for survival in the host. The capsule is poorly antigenic and antiphagocytic and is a major virulence factor (e.g., Streptococcus pneumoniae). The capsule can also act as a barrier to toxic hydrophobic molecules, such as detergents, and can promote adherence to other bacteria or to host tissue surfaces. For Streptococcus mutans, the dextran and levan capsules are the means by which the bacteria attach and stick to the tooth enamel. Synthesis of the capsule takes energy and will not be effected by the bacteria after continued growth under laboratory conditions away from the selective pressures of the host.
      Flagella are ropelike propellers composed of helically coiled protein subunits (flagellin) that are anchored in the bacterial membranes through hook and basal body structures and that are driven by membrane potential. Bacterial species may have one or several flagella on their surfaces, and they may be anchored at different parts of the cell. Flagella provide motility for bacteria, allowing the cell to swim (chemotaxis) toward food and away from poisons. Bacteria approach food by swimming straight and then tumbling in a new direction. The swimming period becomes longer as the concentration of chemoattractant increases. The direction of flagellar spinning determines whether the bacteria swim or tumble. Flagella also express antigenic and strain determinants.
      Fimbriae (pill) (Latin for "fringe") are hairlike structures on the outside of bacteria; they are composed of protein subunits (pilin). Fimbriae can be morphologically distinguished from flagella because they are smaller in diameter (3 to 8 nm versus 15 to 20 nm) and usually are not coiled in structure. Generally, several hundred fimbriae are arranged peritrichously (uniformly) over the entire surface of the bacterial cell. They may be as long as 15 to 20 nm, or many times the length of the cell.
      Fimbriae promote adherence to other bacteria or to the host (alternative names are adhesins, lectins, evasins, and aggressins). As an adherence factor (adhesin), fimbriae are an important virulence factor for E. coli colonization and infection of the urinary tract, for Neisseria gonorrhoeae and other bacteria. The tips of the fimbriae may contain proteins (lectins) that bind to specific sugars (e.g., mannose). F pili (sex pill) promote the transfer of large segments of bacterial chromosomes between bacteria. These pill are encoded by a plasmid (F).

:: Bacterial Exceptions ::

      Mycobacteria have a peptidoglycan layer (slightly different structure), which is intertwined with and covalently attached to an arabinogalactan polymer and surrounded by a waxlike lipid coat of mycolic acid (large alpha-branched beta-hydroxy fatty acids), cord factor (glycolipid of trehalose and two mycolic acids), waxD (glycolipid of 15 to 20 mycolic acids and sugar), and sulfolipids. These bacteria are described as acidfast staining. The coat is responsible for virulence and is anti phagocytic. Corynebacterium and Nocardia organisms also produce mycolic acid lipids. The mycoplasmas are also exceptions in that they have no peptidoglycan cell wall and they incorporate steroids from the host into their membranes.