Length: smallest bacteria = 0.5-1 μm; largest = up to a few μm

Basic Structures:  rod, coccus (round), and spiral

Shapes:  all variations of the basic structure exist (rods: short, long, curved, bent, pleomorphic; ends can be pointed, round, or flat -- cocci: regular, irregular)

Associations:  tandem, chains, clusters (regular, irregular)

Cocci:

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Rods:

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Spiral:

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Cytoplasmic Constituents

1.  Water: ~70%

2.  Macromolecues: ~26% (~90% of dry wt)

• Proteins (~50-60% of dry wt, up to 100 different enzymes, ribosomes--up to 25% of cell volume → target for may antibiotics)
• Nucleic Acids (DNA: 3% of dry wt, generally one chromosome + plasmids and phages, bacterial genomes contain high % of unmethylated CG dinucleotide sequences; RNA: 10-20% of dry wt, mRNA [mono- or polycistronic], tRNA [carries charged amino acids to growing peptide chains], rRNA [part of 30S and 50S ribosomal units])
• Storage granules (stored energy; used under metabolic stress; kinds: glycogen [polymers of glucose, good energy source, most common potential cariogenic trait], poly-beta-hydroxybutyric acid [lipophilic, very high energy], sulfur granules [common in some photosynthetic bacteria], and polyhexametaphosphate [AKA volutin granules, used to store phosphate which is often a limiting nutrient, are metachromatic])
• Endospores (small, survival structure produced in response to nutrition deprivation, lack of water, or severe temperature changes; resting structures [metabolically inactive]; resistant to heat, to desiccation, to disinfectants, to UV [major source of problems in infection control, basis for resistance: impermeable surface layer, low water concentration]; spores have high concentrations of calcium most of which is complexed with dipicolinic acid; 3 parts: Core--dehydrated cytoplasm containing DNA, ribosomes, enzymes, etc. everything that is needed to function once returned to the vegetative state, Cortex--modified cell wall/peptidoglycan layer that is not as cross-linked an in a vegetative cell, Coat--several protein layers outside of the cortex that are impermeable to most chemicals [the coat is responsible for the resistance to chemicals]; sporulation process takes 4-8 hrs; germination takes just minutes [can be triggered by heat in the presence of moisture, swelling of endospore, metabolic activity starts, loss of resistance to environmental stress, disintegration of exterior parts of the envelope, development into a fully functional vegetative cell])

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3.  Small molecules & ions: ~4% (~10% of dry wt)

Functions:

• Permeability barrier, prevents cell contents from leaking out (very impermeable to polar and charged molecules--most biomolecules, also small ions such as K+, H+; only water, lipid-soluble materials and gasses pass freely)
• Segregation of chromosomal and plasmid DNA
• Import and Export (transport proteins--bind specific substrates and transport then to cytoplasm; secretory proteins--involved in secretion of periplasmic and extracellular proteins; this system can play a role in resistance to antibiotics!)
• Energy Metabolism (proteins involved in energy metabolis - generation of electrochemical gradient of ions; ATPase--formaiton of ATP by use of proton motive force)

Composistion:

• 50-70% protein (for: transport, energy metabolis, cell wall synthesis, sensor for two component regulatory system)
• 30-50% phospholipid (form bilayer in which proteins are embedded; no sterols--except for those parasitic bacteria that use parts of the host cell's membrane; but: hopanoids)
• small amounts of carbohydrates

Can be a target for antibiotics (mechanism unknown)

Purpose: increase surface area of CM

Invaginations of the CM

Provide increased membrane surface for enzymes of electron transport

Might be involved with segregation of new replicated chromosomes

Located outside of CM

Functions:

• shape
• rigidity (=couters osmotic pressure; water wants to flow into the cell because of high concentrations of solutes inside the cell)
• platform for surface appendages

Clinically:

• contributes to the ability of bacteria to cause disease
• site of action of several antibiotics
• can trigger innate immune response

Composition:

• Common component is peptidoglycan (PG) = murein
• PG consists of a polysaccharide backbone with peptide cross-links
• Polysaccharide chain: alternating residues of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
• Peptide: tetrapeptide synthesized as pentapeptide; linked to polysaccharide chain at carboxyl group of NAM; alternating L- and D-amino acids; peptide chains are almost identical from organism to organism (exceptions where noted below)

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Unusual Properties:

• contains D-amino acids
• contains diaminopimelic acid
• contains muramic acid
• peptide chain is synthesised by a series of enzymes and NOT by ribosomes

Autolysins:

• group of enzymes that cleave PG in specific places
• necessary for cell wall synthesis
• Amidase = cleaves between NAM of polysaccharide chain and L-Ala of peptide chain
• Endopeptidase = cleaves cross-link between two peptide chains
• Lysozyme = cleave between NAM and NAG of the polysaccharide chain

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Bacteria without cell wall:

• Mycoplasma: group of bacteria which lack cell wall naturally; found in environment with high osmotic strength (e.g. moist mucosal surfaces as in lung, genital tract, etc.)

Cross-linking usually occurs between the third amino acid of one peptide chain and the forth amino acid of the other peptide chain.

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Gram-negative bacteria usually have a direct cross-link while gram-positive bacteria often have peptide bridges between cross-linked peptide chains (most commonly peptide bridge consists of 5 gly).

In gram-negative bacteria, only about 50% of the potential cross-links are formed; the remainder are free tetra- or pentapeptides or no peptide at all.

In gram-positive bacteria, all peptide chains are linked, often with additional peptide bridge joining distant glycan chains (PG of gram-positive bacteria is much stronger because it takes advantage of all peptide links).

1.  PG cross-linking

2.  Thickness of PG

• Gram-negative = 2 to 3 layers
• Gram-positive = as many as 40 layers
• The Gram-stain is based on the difference in cell wall thickness which determines the ability of the bacteria to retain a crystal violet-iodine complex.

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3.  Outer membrane?

• Gram-negative = yes
• Gram-positive = no

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• thick (20-80 nm) and relatively homogenous
• PG comprises 40-80% of total cell weight and is extensively cross-linked in three dimensions
• components other than PG: teichoic acids or teichuronic acids, polysaccharides, proteins

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Teichoic acids and surface proteins are used in the identification and serological classification of Gram-positive bacteria.

Potential roles of surface proteins:

• can act as adhesins (lipoproteins of oral streptococci)
• can be antiphagocytic (M protein of group A streptococci)
• antigenic variation to evade host immune system
• mimicking of host antigens

• Strengthen and stabilize then PG layer
• highly charged (neg. charge)
• long acidic plymers of alditols (ribitol or glycerol) linked through phosphodiester bridges

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• free hydroxyl groups (R) are substituted with amino acids, amino sugars, or mono-, di-, and trisaccharides

Two types of teichoic acids:

• Wall teichoic acid - ribitol teichoic acids linked to NAM in PG
• Lipo(or membrane)teichoic acid - glycerol teichoic acid linked to a glycolipid in CM; is present in all Gram-positive bacteria; R = often D-Ala

Teichuronic Acids

• highly charged polymers with glucuronic acid as a backbone
• contains no phosphorus
• Production depends upon the availability of phosphate; phosphate available = teichoic acid; NO phosphate = teichuronic acid

Cell wall contains equal amounts of PG and arabinogalactans complexed with lipids.

50% of these lipids are mycolic acids (C₃₂-C₉₀ fatty acids) which are attached to arabinogalactans and form a water-insoluble complex (= wax D).

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Periplasm = space between CM and OM

• may occupy as much as 30% of cell volume
• contains many specialized proteins which ar often involved in nutrient acquisition (binding proteins = bind nutrients [sugars, aa, vitamins] that enter through porins; enzymes = cleave organic molecules to provide nutrients [e.g. phosphatases, proteases, endonucleases])
• Peptidoglycan: 2 to 3 layers; cross-linked in two dimensions only; attached to outer membrane (OM)
• Murein lipoproteins: small (7.5 kDa); function: anchors OM to PG and maintains structure and organization of OM; protein portion is covalently attached to ~10% of the peptide chains in PG; lipid portion consists of three fatty acids which are embedded in OM

Outer Membrane (OM):

• Composition: phospholipids (35%; on the inner side of OM next to PG; composition similar to CM), protiens (15%; porins - create pores, allow small hydrophilic molecules to pass freely; receptors - for nutrients such as vitamins, siderophores [small molecules that bacteria release to capture iron and bring it back to the cell], and phosphate), and lipopolysaccharides (LPS; 50%)

 Complex unique glycolipid which is composed of three covalently linked regions. 

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Lipid A:  disaccharide linked to hydroxy fatty acids; fatty acid composition differes in bacterial genera but is consistent within species

Core Oligosaccharide (AKA Core Polysaccharide):  attached to lipid A; contains various sugars including two unusual ones - heptose and KDO (2-keto-3-deoxyoctolusonic acid)

O-specific Side Chain:  attached to sugar core; sticks out from bacterial surface; has variable number (0-50) of repeated oligosaccharide units of 3 to 5 sugars; major serological determinant of Gram-negative bacteria (O-antigen); varies within species and even between strains of a species

Biological activites of LPS:

• when cell-bound: strong negative charge assists in evasion of complement system phagocytosis
• when released (cell lysis): acts as endotoxin; toxicity resides in lipid A and is enhanced by polysaccharide; active only in sizeable amounts; toxin activates complement and stimulates release of cytokines - overstimulation of defense system results in dropping blood pressure (→ vascular collapse) and fever and septic shock

These structures may or may not be present depending on the species: 

• Flagella
• Fimbriae, Pili
• Glycocalyx (capsule, slime)

These structures can:

• contributed to virulence properties
• decrease susceptibility to host defense system
• trigger the innate immune response

Flagella (external)

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• on spirilla and most bacilli
• function:  locomotion (cells move by rotating flagella like a propeller, driving force is proton gradient between cytoplasm and periplasm or outside environment)
• Location:  1. Polar flagellation - flagella attached at one (monopolar) or both (bipolar) ends;  2. Peritrichous flagellation - completely surrounded by flagella
• Composistion: protein subunits of flagellin form a tube-like structure; hook and basal body are needed for anchorage and rotation

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• insturment of chemotaxis
• enables some pathogenic bacteria to penetrate  host defenses
• antigenic - serological determinant (H-antigen)
• can be object of phase variation: ability of some bacteria to alternate synthesis of different flagella → evasion of host defenses

• AKA axial filaments, endoflagella
• found in spirochetes
• internal structure
• 2 to >100 filaments extend from both ends of the cell between the OM and CM (located in the periplasm)
• Hypothesis: endoflagella rotate in the periplasmic space; this could cause the corkscrew-shaped outer membrane of the spirochete to rotate and propel the bacterium through the surrounding fluid

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Fimbriae

Short, rigid protein rods; not involved in motility; more numerous than flagella

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• Originate in CM and extend from cell surface
• function: attachment to surfaces and adherence to other cells (bacterial and animal) -- binding to specific sugar residues on glucoproteins and other surface components of target cell; associated with ability to cause disease
• are found in Gram-positive and Gram-negative oral bacteria

Pili (Fibrillae)

• longer than fimbriae, thin and flexible, only few per cell
• function: receptors for certain bacteriophages, attachment of some pathogens to their host, conjugation

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• external coating secreted onto the outside of the cell
• two types: capsule - closely associated with the cell; slime - secretions loosely adhering to the cell surface, w/o clearly definded border
• structure: principal constituent in water; contains polysaccharides and sometimes glycoproteins; structure varies in thickness and can be rigid or flexible; usually connected to cell wall by ionic bonding
• functions: protection, attachment (S. mutans produces slime layer in the presence of sucrose), virulence factor (evasion of host defense system--antiphagocutic), antigenic (used in serological classification--K-antigen)

• Cell wall (e.g. penicillin - inhibits cross-linking of PG)
• Cell membrane (e.g. polymyxin B - replaces cations on lipids → disrupts CM)
• DNA (e.g. quinolones - block DNA gyrase → block DNA replication)
• Ribosomes (e.g. gentamycin - 30S; clindamycin - 50S)

Some bacterial molecular sequences that are recognized by toll-like receptors and other pattern recognition receptors and initiate an immune respose:

• lipopolysaccharides
• peptidoglycan
• lipoteichoic acids
• mannose-rich glycans (glycoproteins)
• flagellin
• pilin
• bacterial DNA (due to methylation)
• phosphorylcholin and other lipids (bacterial membrane)

Bacterial Structures and Virulence

Bacterial cell walls and surface structures contribute to different stages of infection:

• Adhesion to biotic and abiotic surfaces (e.g. LPS, fimbriae, outer membrane proteins, glycocalix)
• Evasion of host defenses (e.g. capsule, flagella, teichoic acids)
• Invasion of tissues (as a consequence of adhesion and evasion of host defenses)
• Stimulation of proinflammatory activities → often resulting in tissue damage (e.g. PAMPs; Lipid A)