Structural components of bacteria

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BACTERIAL STRUCTURE

Cell Envelope: Four(4) layers are

  1. Capsule/ Slime layer
  2. Outer membrane
  3. Cell wall
  4. Inner membrane

When the layer consists of a network of polysaccharides extending from the surface of the cell, it is referred to as the glycocalyx, a term that can encompass both capsules and slime layers because they usually are composed of polysaccharides.

Capsule

  • Outermost gelatinous Organized glycocalyx layer
  • It is hydrophilic -amorphous outer envelop
  • Polysaccharide layer + Polypeptide(sometimes)
  • They contain Capsular(K) antigen.
  • Not easily detachable
  • Not always present

Functn : Prevents drying as it is hydrophilic,Facilitate adherence, Decrease phagocytosis efficacy of WBC/Macrophages

  • Capsulated bacteria, e.g.: Pseudomonas
  • Bacillus anthracis produces polypeptide capsules: Poly-D glutamic acid.
  • Negative staining,India ink is used to visualize a capsule.

 

Slime Layer:

It is an irregularly diffused glycocalyx which can cover more than one bacteria together. E.g: Actinobacillus sp.

  • Outermost Unorganized layer
  • Only Polysaccharide layer
  • Easily removable(Loose)
  • Not always present

Functn : Adherence to cell surfaces, foreign subs (Endotracheal intubation, catheter), Protects bacteria from immune system & antibiotics.

Only bacteria having both capsule and slime: Streptococcus salivarius

S-layer

Many prokaryotes have a regularly structured layer called an S-layer on their surface. In bacteria, the S-layer is external to the cell wall. The S-layer has a pattern something like floor tiles and is composed of protein or glycoprotein. In gram-negative bacteria the S-layer adheres directly to the outer membrane; it is associated with the peptidoglycan surface in gram positive bacteria. It may protect the cell against ion and pH fluctuations, osmotic stress, enzymes, or the predacious bacterium Bdellovibrio. The S-layer also helps maintain the shape and envelope rigidity of some cells. It can promote cell adhesion to surfaces. Finally, the S-layer seems to protect some bacterial pathogens against host defenses, thus contributing to their virulence.

Outer membrane

  • Present only in Gram Negative bacteria
  • Phospholipid bi-layer
  • The most unusual constituents of the outer membrane are its lipopolysaccharides (LPS=Endotoxin). These large, complex molecules contain both lipid and carbohydrate, and consist of three parts: 
    • (1) lipid A- Endotoxic effect, induce WBC
    • (2) the core polysaccharide, and 
    • (3) the O side chain (“O” antigen)- Binds with antibody, used for serotyping

 

Cell wall

The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane. It is one of the most important prokaryotic structures for several reasons:

  1. it helps determine the shape of the cell; 
  2. it helps protect the cell from osmotic lysis; 
  3. it can protect the cell from toxic substances; and 
  4. in pathogens, it can contribute to pathogenicity.

The gram-positive cell wall consists of a single 20 to 80 nm thick homogeneous layer of peptidoglycan (murein) lying outside the plasma membrane (figure 3.17). In contrast, the gram-negative cell wall is quite complex. It has a 2 to 7 nm peptidoglycan layer covered by a 7 to 8 nm thick outer membrane.

Periplasmic space: Space between the plasma membrane and the outer membrane of gram-negative bacteria, and is sometimes observed between the plasma membrane and the wall in gram-positive bacteria.The substance that occupies the periplasmic space is the periplasm.Many enzymes resides here.

Beta lactamase enzymes present in the periplasmic space, it inhibits Penicillin.Beta-lactamases are present only in the gram negative bacteria.

Components of Gram Positive Cell Wall:

Gram-positive bacteria normally have cell walls that are thick and composed primarily of peptidoglycan. Peptidoglycan in gram positive bacteria often contains a peptide interbridge.In addition, gram-positive cell walls usually contain large amounts of teichoic acids, polymers of glycerol or ribitol joined by phosphate groups.The teichoic acids are covalently connected to either the peptidoglycan itself or to plasma membrane lipids; in the latter case they are called lipoteichoic acids.Teichoic acids are not present in gram-negative bacteria.The periplasmic space of gram-positive bacteria, when observed, lies between the plasma membrane and the cell wall and is smaller than that of gram-negative bacteria.Enzymes secreted by gram positive bacteria are called exoenzymes.

The Gram-Positive Envelope

Peptidoglycan: Peptidoglycan(Peptide+Glycan), or murein, is an enormous meshlike polymer composed of many identical subunits. The polymer contains two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid (the lactyl ether of N-acetylglucosamine), and several different amino acids. Three of these amino acids are not found in proteins: D-glutamic acid, D-alanine, and meso diaminopimelic acid. The presence of D-amino acids protects against degradation by most peptidases, which recognize only the L-isomers of amino acid residues.Tetrapeptide chain, a peptide chain of four alternating D- and L-amino acids is connected to the carboxyl group of N-Acetylmuramic(NAM) acid [NAM-o-o-o-o-NAM] .

N-acetylglucosamine-

N-acetylmuramic acid-

Penta-Peptide interbridge-

Teichoic acids-

Components of Gram Negative Cell Wall:

  1. Outer membrane [Present only in Gm-ve]
  2. Thin Peptidoglycan

The thin peptidoglycan layer next to the plasma membrane and bounded on either side by the periplasmic space may constitute not more than 5 to 10% of the wall weight.

The outer membrane lies outside the thin peptidoglycan layer.

Possibly the most unusual constituents of the outer membrane are its lipopolysaccharides (LPSs), of gram negative bacteria released only after cell lysis.These large, complex molecules contain both lipid and carbohydrate, and consist of three parts: (1) lipid -A [Endotoxin part], (2) the core polysaccharide, and (3) the O side chain [Antigenic part].

The core polysaccharide part is strain specific and varies from strain to strain.It is an immunogenic but not pathogenic component.

The lipid A region contains two glucosamine sugar derivatives, each with three fatty acids and phosphate or pyrophosphate attached. The fatty acids attach the lipid A to the outer membrane, while the remainder of the LPS molecule projects from the surface.It is same in all gram negative bacteria, producing endotoxemia by similar pathogenic machanism. It is not very immunogenic but very much pathogenic. Lipid A is responsible for septic shock.  The core polysaccharide is joined to lipid A.

Importantly, the lipid -A portion of LPS often is toxic; as a result, the LPS can act as an endotoxin and cause some of the symptoms that arise in gram-negative bacterial infections. If the bacterium enters the bloodstream, LPS endotoxin can cause a form of septic shock for which there is no direct treatment.

The O side chain or O antigen is a polysaccharide chain extending outward from the core.LPS has many important functions. Because the core polysaccharide usually contains charged sugars and phosphate, LPS contributes to the negative charge on the bacterial surface.Lipid A also helps stabilize outer membrane structure. LPS may contribute to bacterial attachment to surfaces and biofilm formation. A major function of LPS is that it aids in creating a permeability barrier(restrict the entry of bile salts, antibiotics, and other toxic substances).The O side chain of LPS is also called the O antigen (Somatic anitgen) because it elicits an immune response and production of strain specific antibodies,it is used for serotyping.However, many gram negative bacteria are able to rapidly change the antigenic nature of their O side chains, thus thwarting host defenses.

Presence of porin proteins at the outer membrane permits the passage of small molecules like glucose and other monosaccharides.

The Gram-Negative Envelope

Inner membrane/ Cytoplasmic membrane

It is present in both Prokaryotes & Eukaryotes.The membrane contains special receptor molecules that help prokaryotes detect and respond to chemicals in their surroundings.

The most widely accepted model for membrane structure is the fluid mosaic model of Singer and Nicholson – membranes are lipid bilayers within which proteins float. Two types of proteins are there- loosely attached Peripheral soluble proteins(20-30%) & Integral Insoluble proteins(70-80%).Integral proteins are amphipathic. 

Bacterial membranes are similar to eukaryotes but their phospholipids are not Sterols, rather it is sterol like molecules called Haploids.Plasma membrane infoldings are common in various bacteria like Cyanobacteria,nitrifying bacteria etc.These invaginations of plasma membranes are called Mesosomes.The cytoplasmic membranes of bacterial cells are flexible structures composed of phospholipids and proteins. They can be observed only by electron microscopy and are structurally similar to the plasma membranes of eukaryotic cells. However, bacterial cytoplasmic membranes, with the exception of those present in mycoplasmas, do not contain sterols. The outer faces of cytoplasmic membranes are hydrophilic while the interior is hydrophobic, forming a barrier to most hydrophilic molecules. Only a limited range of small molecules such as water, oxygen, carbon dioxide and some lipid – soluble compounds can enter bacterial cells by passive diffusion. 

Two major functions of the cytoplasmic membrane, (1) the active transport of nutrients into the cell and (2) the elimination of waste metabolites, require the expenditure of energy. The energy required by permeases and other carrier molecules for active transport of nutrients derives from adenosine triphosphate. The cytoplasmic membrane is also the site of electron transport for bacterial respiration, of phosphorylation systems and of enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers and membrane lipids.

Appendages

  • Flagella
  • Fimbrae
  • Pilus

Flagella:

Most motile procaryotes move by use of flagella (s., flagellum), threadlike locomotor appendages extending outward from the plasma membrane and cell wall.Bacterial flagella are slender, rigid structures, about 20 nm across and up to 15 or 20 m long. Flagella are so thin they cannot be observed directly with a bright-field microscope, but must be stained with special techniques designed to increase their thickness.The filament is a hollow, rigid cylinder constructed of subunits of the protein flagellin.Flagellar filament synthesis is an excellent example of self assembly.

  • Motility, Powered by ATP
  • 3parts- Basal body,Hook & Filament
  • H-antigen: Used in Widal test

Bacterial species often differ distinctively in their patterns of flagella distribution and these patterns are useful in identifying bacteria

Types of flagella:

  1. Monotrichous-  Vibrio
  2. Lophotrichous- Pseudomonas
  3. Amphitrichous-
  4. Peritrichous- E.coli

Types of motility:

    1. Tumbling- Listeria
    2. Darting- Vibrio
    3. Swarming-Proteus
    4. Falling leaf- Giardia
  • Gliding– Cyanobacterium

 Fimbriae & Pilli:

Many prokaryotes have short, fine, hairlike appendages that are thinner than flagella. These are usually called fimbriae (s., fimbria).They can be up to 1000 per cell.They are slender tubes.

Func:Attachment,Motilityt,

Many bacteria have about 1-10 sex pili (s., pilus) per cell.

Fimbria(Fimbriae) Pilus(Pili)
Solid, Shorter & Thinner Hollow, Longer & Thicker
More in numbers(~200-400) Less in numbers(~10)
Made by- Fimbrillin protein

Transcribed from Bacterial nucleoid

 Made by- Pilin protein

Transcribed from Bacterial Plasmid
Func: Adherence to host cell surface Func: Gene transfer(Conjugation-Sex pilli) and attachment
Present in both Gm+ & Gm-

Salmonella,Shigella

 More particularly in seen in Gm-

E.coli, Pseudomonas
No role in mobility Type IV pili show twitching motility

 

Endospore/Spore:

A number of gram-positive bacteria can form a special resistant, dormant structure called an endospore. Endospores develop within vegetative bacterial cells of several genera: Bacillus and Clostridium (rods), Sporosarcina (cocci), and others. These structures are extraordinarily resistant to environmental stresses such as heat, ultraviolet radiation, gamma radiation, chemical disinfectants, and desiccation. In fact, some endospores have remained viable for around 100,000 years.Endospores often survive boiling for an hour or more; therefore autoclaves must be used to sterilize many materials.Endospores are impermeable to most stains, they often are seen as colorless areas in bacteria treated with methylene blue and other simple stains; special endospore stains are used to make them clearly visible.Endospore position in the mother cell (sporangium) frequently differs among species, making it of considerable value in identification. Endospores may be centrally located(Bacillis cereus), close to one end (subterminal-Most common), or definitely terminal(Clostridium tetani). Sometimes an endospore is so large that it swells the sporangium.The spore often is surrounded by a thin, delicate covering called the exosporium. A spore coat lies beneath the exosporium, is composed of several protein layers, and may be fairly thick. It is impermeable to many toxic molecules and is responsible for the spore’s resistance to chemicals. The coat also is thought to contain enzymes that are involved in germination.The cortex, which may occupy as much as half the spore volume, rests beneath the spore coat. It is made of a peptidoglycan that is less cross-linked than that in vegetative cells. The spore cell wall (or core wall) is inside the cortex and surrounds the protoplast or spore core. The core has normal cell structures such as ribosomes and a nucleoid, but is metabolically inactive.As much as 15% of the spore’s dry weight consists of dipicolinic acid complexed with calcium ions, which is located in the core.Endospore formation, also called sporogenesis or sporulation, normally commences when growth ceases due to lack of nutrients.

Sporogenesis:

  • An axial filament of nuclear material forms (stage I)
  • An inward folding of the cell membrane to enclose part of the DNA and produce the forespore septum (stage II). 
  • The membrane continues to grow and engulfs the immature endospore in a second membrane (stage III). 
  • Next, the cortex is laid down in the space between the two membranes, and both calcium and dipicolinic acid are accumulated (stage IV). 
  • Protein coats then are formed around the cortex (stage V), 
  • and maturation of the endospore occurs (stage VI).
  •  Finally, lytic enzymes destroy the sporangium releasing the spore (stage VII).

Spore to Vegetative form:

The transformation of dormant spores into active vegetative cells seems almost as complex a process as sporogenesis. It occurs in three stages: (1) activation, (2) germination, and (3) outgrowth

** EXOSPORE

Exospores are produced in the eukaryotic cells of fungi, algae and cyanobacteria. Endospores are formed inside the mother cell, whereas, exospores are formed towards the end of the mother cell and released as bud.

Protoplast:

The protective nature of the cell wall is most clearly demonstrated when bacterial cells are treated with lysozyme or penicillin. The enzyme lysozyme attacks peptidoglycan by hydrolyzing the bond that connects N-acetylmuramic acid with N-acetylglucosamine (figure 3.18). Penicillin works by a different mechanism. It inhibits peptidoglycan synthesis. If bacteria are treated with either of these substances while in a hypotonic solution, they will lyse. However, if they are in an isotonic solution, they can survive and grow normally

  • When gram positive bacteria treated with lysozyme or penicillin results in the complete loss of the cell wall, and the cell becomes a protoplast.

Protoplast= Plasma membrane + Content within it

  • When gram-negative bacteria are exposed to lysozyme or penicillin, the peptidoglycan layer is lost, but the outer membrane remains. These cells are called spheroplasts.

Because they lack a complete cell wall, both protoplasts and spheroplasts are osmotically sensitive. If they are transferred to a dilute solution, they will lyse due to uncontrolled water influx

L-phase variant or L-forms:

The mycoplasmas comprise an important group of bacteria without cell walls. Conventional bacteria, exposed to the action of antibiotics such as penicillin, or other substances which interfere with the synthesis of peptidoglycan, cannot produce cell walls and are termed L forms. 

Involution forms: 

–an irregular or atypical bacteria formed under unfavorable conditions (as in old cultures)

Cell/Plasma Membrane: 

 It is present in both Prokaryotes & Eukaryotes.The membrane contains special receptor molecules that help prokaryotes detect and respond to chemicals in their surroundings.

The most widely accepted model for membrane structure is the fluid mosaic model of Singer and Nicholson – membranes are lipid bilayers within which proteins float. Two types of proteins are there- loosely attached Peripheral soluble proteins(20-30%) & Integral Insoluble proteins(70-80%).Integral proteins are amphipathic. 

Bacterial membranes are similar to eukaryotes but their phospholipids are not Sterols, rather they are sterol-like molecules called Haploids.

Mesosomes:

Plasma membrane infoldings are common in various bacteria like Cyanobacteria,nitrifying bacteria etc.These invaginations of plasma membranes are called Mesosomes.

Structure in bacteria equivalent to mitochondria.

Cytoplasm:

The cytoplasmic matrix is the substance in which the nucleoid, ribosomes, and inclusion bodies are suspended. It lacks organelles bound by lipid bilayers (often called unit membranes), and is largely water (about 70% of bacterial mass is water).

Cytoplamsic Matrix: 

In bacteria, it lacks organelles bound by lipid bilayers.

Inclusion Bodies: IB,that often are clearly visible in a light microscope, are granules of organic or inorganic materials.These bodies usually are used for storage of Carbon compounds, inorganic substances and energy, and also reduce osmotic pressure by tying up molecules in particulate forms.

Inclusion bodies, granules of organic or inorganic material that often are clearly visible in a light microscope, are present in the cytoplasmic matrix. These bodies usually are used for storage (e.g., carbon compounds, inorganic substances, and energy), and also reduce osmotic pressure by tying up molecules in particulate form

Some organic IB are Glycogen, Poly-B-hydroxy-butyrate (PHB) and gas vacuoles. Cyanobacteria contain IB like Cyanophycin granules and Carboxysomes.

Some inorganic IBs are: Polyphosphate granules(Volutin) and Sulfur granules.Sometimes they are called metachromatic granules as they appear different in color when stained. Magnetosomes are the inorganic IBs used by some bacteria to orient in the Earth’s magnetic field.

Stain used : Ponder’s stain, Albert stain, Neisser stain

Ribosome:  Ribosomes are composed of Protein and ribonucleic acid(rRNA).They are the site of protein synthesis.Prokaryotic ribosomes are smaller than eukaryotic ribosomes.These are 70S (50S + 30S ) ribosomes. (“S” stands for Svedberg unit,sedimentation coefficient ).

Bacterial nuclear apparatus:

Nucleoid: 

Prokaryotes lack a membrane-delimited nucleus. The prokaryotic chromosome is located in an irregularly shaped region called the nucleoid (other names are also used: the nuclear body, chromatin body, nuclear region).Usually prokaryotes contain a single circle of double-stranded deoxyribonucleic acid (DNA), but some have a linear DNA chromosome and some, such as Vibrio cholerae and Borrelia burgdorferi (the causative agents of cholera and Lyme disease, respectively), have more than one chromosome. Chemical analysis of purified nucleoids reveals that they are composed of about 60% DNA, 30% RNA, and 10% protein by weight.Bacteria do not use histone proteins to package their DNA.

PLASMIDS:

In addition to the genetic material present in the nucleoid, many prokaryotes (and some yeasts and other fungi) contain one or more extrachromosomal DNA molecules called plasmids.Plasmids are small, double-stranded DNA molecules that can exist independently of the chromosome. Both circular and linear plasmids have been documented, but most known plasmids are circular.Plasmids are able to replicate autonomously.Some plasmids are able to integrate into the chromosome and are thus replicated with the chromosome. Such plasmids are called episomes. Plasmids are inherited stably during cell division, but they are not always equally apportioned into daughter cells and sometimes are lost. The loss of a plasmid is called curing. It can occur spontaneously or be induced by treatments that inhibit plasmid replication but not host cell reproduction. Some commonly used curing treatments are acridine mutagens, UV and ionizing radiation, thymine starvation, antibiotics, and growth above optimal temperatures.

Types: 

  • Conjugative plasmids are of particular note. They have genes for the construction of hairlike structures called pili and can transfer copies of themselves to other bacteria during conjugation.
  • Resistance factors (R factors, R plasmids) are another group of important plasmids. They confer antibiotic resistance on the cells that contain them. R factors typically have genes that code for enzymes capable of destroying or modifying antibiotics. Some R plasmids have only a single resistance gene, whereas others have as many as eight. Often the resistance genes are within mobile genetic elements called transposons, and thus it is possible for multiple-resistance plasmids to evolve. R factors usually are not integrated into the host chromosome.
  • Col plasmids contain genes for the synthesis of bacteriocins known as colicins, which are directed against E. coli.

Bacteriocins are bacterial proteins that destroy other bacteria.Bacteriocins are also encoded into Plasmids.

  • Virulence plasmids encode factors that make their hosts more pathogenic. For example, enterotoxigenic strains of E. coli cause traveler’s diarrhea because they contain a plasmid that codes for an enterotoxin.
  • Metabolic plasmids carry genes for enzymes that degrade substances such as aromatic compounds (toluene), pesticides (2,4-dichlorophenoxyacetic acid), and sugars (lactose).

1 Angstrom Å 10-10 meter

Bacterial Antigens:

There are two main groups-

A.Soluble antigens: Some soluble substances produced by the bacteria, which are excreted into the environment. For example, toxins, enzymes etc.

B.Cellular antigens: They are the structural units of bacterial cells. Common bacterial antigens are:

  1. Somatic (O) antigen: In gram negative bacteria (Salmonella, E. coli, Brucella etc.), somatic antigens are composed of lipopolysaccharide (LPS)–protein complex, which are good antige
  2. n and produce good immune response. But O antigens are highly variable (LPS consists of highly variable Oligosaccharides) and thus immunity against one ‘O’ antigen will not confer immunity against bacteria bearing other ‘O’ antigens. But a common core antigen (under laying core polysaccharide) has a potential use as a vaccine.
  3. Capsular (K) antigen: A variety of bacterial species have capsules (e.g. Bacillus anthracis, E. coli, Salmonella spp. etc) which is antigenic. Capsules commonly consist of polysaccharides (e.g. K antigen of E.coli) but some are composed of polypeptides (e.g. Poly-D-glutamic acid in case of B.anthracis).
  4. Flagellar (H) antigen: Motile bacteria have flagella (e.g. Salmonella spp., E.coli, Proteus spp.). These Flagella are composed of protein (flagellin), which is antigenic.
  5. Fimbrial (F ) antigen: Fimbriae or Pili antigen are present on the surface of bacteria .
  6. Spore antigen: Bacterial spores (e.g. Bacillus spp., Clostridium spp. etc.,) especially the exosporium is antigenic.
  7. Other significant bacterial antigens include the porins, heat shock proteins, the exotoxins etc.