Introduction to bacteria-II << Back

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Introduction to bacteria-II

  • Bacterial cell wall :Basic structure,Gram positive cell wall,Gram negative cell wall,Cell wall of archaebacteria
  • Functions of cell wall
  • Cell membrane: Basic structure, Functions of Plasma membrane
  • Internal membrane system: Mesosomes, Photosynthetic machinary in membranes
  • Cytoplasm
  • Nucleoid
  • Plasmids
  • Cellular reserve material
  • Dormant forms of Bacteria: Bacterial endospores, Bacterial exospores

Cell wall

  • Basic structure

All bacteria could be divided into two major groups based on Gram staining. Gram positive bacteria stain purple while gram negative stains pink. Cell wall of Gram Positive bacteria is  thick ( 20 to 80 nm)  while cell wall of Gram negative bacteria is thin (2 to 7 nm). Gram positive cell wall is made up of homogeneous peptidoglycan structure lying outside the plasma membrane. While cell wall of Gram negative is quite complex. It has a peptidoglycan layer bounded by a  outer membrane. Peptidoglycan is made up of the  alternating N-acetyl glucosamine  (NAG) and N-acetyl muramic acid (NAM) residue. A peptide chain of four alternating D and L aminoacids is connected to the carboxyl group of N-acetyl muramic acid. Chains of linked peptidoglycan subunits are joined together by cross links between the peptides. Peptidoglycan consist of  two sugar derivatives, N-acetylglucosamine and N-acetylmuramic acid, and few different amino acids. Three amino acids which are routinely presents are —D-glutamic acid, D-alanine, and meso-diaminopimelic acid. These  are not found in proteins. D-amino acids defend against peptidases.

 

Peptidoglycan consist of alternating  N-acetylglucosamine and N-acetylmuramic acid residues. A peptide chain of four alternating D- and L-amino acids is connected to the carboxyl group of N-acetylmuramic acid. Some bacteria substitute another diaminoacid, like  L-lysine, in the third position for meso-diaminopimelic acid. Chains of linked peptidoglycan are connected through crosslinks between the peptides.

  • Gram positive cell wall: Normally the thick  cell wall of gram-positive bacteria is made up of primarily of peptidoglycan, which consist of a peptide interbridge However gram-positive cell walls also contains more amounts of teichoic acids. The teichoic acids are connected to either the peptidoglycan or the lipids of the plasma membrane. The functions of teichoic acids are not clear it may be helpful in maintaining the cell structure. Gram negative bacteria does not contain the teichoic acid.

 

  • Gram negative cell wall : Cell wall of  gram-negative bacteria is more complex than Gram positive bacteria. It consist of the thin layer of peptidoglycan and outer membrane. The outer membrane lies outside the peptidoglycan layer. Braun’s lipoprotein found in plentiful in the cell wall. It is lipoprotein covalently connected to peptidoglycan and rooted in the outer membrane. The outer membrane and peptidoglycan are tightly connected with each other.

 

Lipopolysaccharides (LPSs) are found in the outer membranes. It is made up of the lipid and  carbohydrate. It consist of the three parts:  (1) lipid A, (2) the core polysaccharide, and (3) the O side chain. The lipid A region contains two glucosamine sugar derivatives. It is hidden in the outer membrane and the remaining part project from the surface. The polysaccharide is connected to lipid A. The O side chain is a polysaccharide chain. O side chain is  project outward from the core. It has several sugars. Composition of the O side chain varies in different strains of the Gram negative bacteria. This side chain act as a antigen. This side chain is recognized by the host antibody.

 

  • Cell wall of archaebacteria: Cell wall of archaebacteria greatly varies with chemical composition. They contain protein, glycoprotein or polysaccharides as the major cell wall constitute. They do not contain peptidoglycan. They contain pseudomurein. They do not have the  muramic acid and D-amino acids characteristic of bacterial peptidoglycan due to which all  archaebacteria resist attack by lysozyme and beta lactam antibiotics such as penicillin. Some methanogens contains  pseudomurein which is a a peptidoglycan like polymer that has L-amino acids in its cross-links, N-acetyltalosaminuronic acid instead of N-acetylmuramic acid, and beta 1-3 glycosidic bonds instead of beeta 1-4 glycosidic bonds.
  • Functions of cell wall:  1) Prevents lyses of bacteria due to osmotic difference., 2) To maintain the shape of bacteria.3) Provide support to flagella. 4) It serve as a  site for bacterial virus attachment.

 

 

 

 

 

 

 

 

 

Plamamembrane:

 

Plasma membranes of bacteria contains proteins and lipids. Bacterial plasma membranes consist of higher concentration of protein than eucaryotic membranes. Lipids associated with the plasmamebranes are asymmetric with polar and nonpolar ends and are known as  amphipathic. The polar ends are

hydrophilic while nonpolar ends are hydrophobic. They form a bilayer in membranes like the eukaryotic membranes. Hydrophilic ends are outside while hydrophobic ends are inside. Most of these lipids are phospholipids. Cell membranes of bacteria does not contain the cholesterol but sterol like molecule called hapanoids are found in the many bacterial membranes. Bacterial cell membranes are very thin i.e. 5 to 10 nm thickness. The most widely accepted structure of bacterial membrane is the fluid mosaic model. It consist of the peripheral proteins and integral proteins. Basic structure of the membranes is same in all bacteria but there are certain variations are observed in the structure and function of the membranes. It acts as a selectively permeable barrier. Bacterial plasma membrane is the site for the many metabolic activities like  respiration, photosynthesis, the synthesis of lipids and cell wall constituents Bacterial membranes also consist of the receptor molecules which help them to respond to the surrounding environment.

 

 

 

Internal Membrane Systems

Prokaryotic cells does not contain the membrane bound organelles. Commonly found structure is the mesosomes.

 

Mesosomes: These are the invaginations of the plasma membrane and forms the shape of vesicles, tubules, or lamellae. They are found  in both gram positive and gram-negative bacteria. Sometimes it was all attached with the bacterial chromosome. It is believed that it may be involved with the formation of cell wall as well as they also play an important role in the chromosome replication.

 

Cytoplasm:

Prokaryotic cytoplasm does not contains the membrane-bound organelles.  Cytoplasmic substance lying between the plasma membrane and the nucleoid which are also known as cytoplasmic matrix.  The matrix is largely composed of the water. Bacterial cells do not have cytoskeleton but they contain  cytoskeleton like system of proteins in their cytoplasmic matrix. Variety of inclusion bodies are observed in the matrix.

 

 

 

Inclusion bodies:

 

A variety of inclusion bodies are present in the cytoplasmic matrix. These are used for storage. Some are not bound by the membranes and found free in the  cytoplasm like polyphosphate granules, cyanophycin granules, and some glycogen granules. Some are enclosed by a membrane like polyhydroxybutyrate granules, some glycogen and sulfur granules, carboxysomes, and gas vacuoles.

 

 

 

 

Ribosomes

 

Ribosomes are made up of proteins and ribose nucleic acid. In bacteria 70 S ribosomes are found.  Prokaryotic ribosomes are smaller than eukaryotic ribosomes. They are made up of 50 S and 30 S subunit. Here s stands for the Svedberg unit. Both prokaryotic and eukaryotic ribosomes are made up of small and large subunits. Bacterial ribosomes are found in the cytoplasmic matrix as well as inside the membrane. Ribosomes found in the cytoplasm synthesize the protein which are destined to remain in the cells while membrane ribosomes synthesize the protien which are destined to transport outside the cell.

 

Nucleoid

 

Prokaryotes consist of a primitive nucleus. It does not have membrane bound nucleus. Bacterial chromosome is located in an unevenly shaped region called the

nucleoid. Bacteria consist of a single circular  double-stranded deoxyribonucleic acid. The DNA is looped and coiled with the support of RNA and nucleoid proteins. These proteins are not the histone proteins but they are histone like proteins.

 

PLASMIDS:

These are extra chromosomal double-stranded DNA molecules. Plasmids are generally circular and replicate independently of the chromosome or it may be integrated with chromosomes. They passed on to the progeny. Plasmids are not joined with the plasma membrane and sometimes lost to one of the progeny cells during division. Plasmids does not contain the genes for the growth and development. They consist of the genes that gives the selective advantage like drug resistance, PAH degradation genes. Plasmids can move from one bacterium to another by transformation or conjugation due to which properties like drug resistance will spread to other population.

 

Bacterial Endospores:

 

Spores are the a special resistant dormant structures. Endospores are develops inside the vegetative cells. Spore forming genus are Bacillus, Clostridium, Sporosarcina , etc. Spores are resistant to extreme  environmental conditions like heat, ultraviolet radiation, gamma radiation, chemical disinfectants, and desiccation.  Endospores are the survival strategy in such condition. Bacteria generally forms the spore when the food is insufficient. Endospores structures consist of the exosporium, spore coat, cortex and core with its nucleoid and ribosomes. Resistant nature of the spores is still not understood. Spores consist of the higher concentration of dipicolinic acid and calcium. It is believed that dipicolinic acid along with the calcium is responsible for this resistant nature. Endospores may be located in centre (central spore), slightly away from the centre (subterminal spore) , at the end (terminal spore). Process of formation of spores is known as sporogenesis. Spores starts germinating when they found the favourable environment.

Question : 1 The plasma membrane of bacteria has the following function

1: It prevents movement of molecules out of the organism
2: It selectively allows some molecules to pass into the organism
3: It contains various enzymes involved in respiratory metabolism
4: All of the above

Question : 2 Which of the following statement is correct regarding gas vesicles in bacteria?

1: Store oxygen for growth in anaerobic condition
2: Are bounded by a lipid bilayer membrane
3: Are bounded by a membrane impervious to gas
4: Help move cells to optimal nutrient levels

Question : 3 Lipopolysaccharides (LPSs) does not contain

1: Protein A
2: Lipid A
3: Core polysaccharide
4: O side chain

Question : 4 In gram staining of gram positive cells, the following occurs

1: Periplasmic space traps the crystal violet
2: Crystal violet stain is trapped by capsule
3: The multiple layers of peptidoglycan help to retain the crystal violet stain
4: The outer membrane helps to retain the crystal violet stain

Question : 5 Choose the correct statement

1: Protoplasts can be prepared from Gram- positive bacteria
2: Spheroplasts can be prepared from Gram-positive bacteria
3: Protoplasts can be prepared from Gram-negative bacteria
4: Spiroplasma has cell wall

Question : 6 The presence of D-amino acids in the crosslinks of the peptidoglycan layer is because

1: D-amino acids fit the structural constrains of the cell wall better than L-amino acids
2: Most peptidases can only cleave L-amino acids
3: Most L-amino acids have already been used for protein synthesis
4: D-amino acids are easier to crosslink in the absence of ribosome

Question : 7 Which of the following is false regarding peptidoglycan

1: A peptide chain of 4 alternating D- and L-amino acids is connected to N-acetylglucosamine
2: Many bacteria have meso-diaminopimelic acid in their peptidoglycan
3: Cross linking pentapeptide interbridge is made of glycine
4: Another name of peptidoglycan is murein

Question : 8 Choose the incorrect statement

1: Prokaryotic cytoplasm does not contain membrane-bound organelles
2: Mesosomes are the invaginations of the plasma membrane
3: Inclusion bodies are present in the nucleoid
4: The spore is a metabolically dormant form

Question : 9 Bacterial endospores does not contain

1: Acid-soluble DNA-binding proteins
2: Dipicolinic acid
3: Calcium
4: Diaminopimelic acid

Question : 10 o-polysaccharides represents

1: Endotoxin in gram positive bacteria
2: Endotoxin in gram negative bacteria
3: Surface antigen in gram positive bacteria
4: Surface antigen in gram negative bacteria