Bacterial cell walls:
Why do these tiny organisms need a cell wall?
Bacterial cells are so tiny that they are bestowed with great protection which they need to protect themselves against harmful substances and also foreign particles that may kill them. Bacterial cells are also surrounded by plasma-membrane just like the Eukaryotic cells, which is usually a double layered one. Many bacteria also have external structures like flagella, pilli, fimbriae which help them move and also to attach to the surfaces such as rocks or the host cells in case of infection. There are several layers surrounding the bacterial cell which all when put together are called as the cell Envelope. The cell has a fluid like cytoplasm with several cell organelles like the Nucleoid, ribosomes, inclusions and many others within it. The cytoplasm is given protection and a shape by the plasma membrane surrounding it and also the transport of nutrients and important substances occurs through it which allows the passage of only some substances thereby preventing the others. The cell wall differs in Gram positive and Gram negative bacteria which accounts for The structure and function of bacterial cell wall are as follows:Cell wall:
Bacterial cell walls are the layers surrounding the plasma membrane which help in protection of the cell from outer harmful sources and also account for their pathogenicity. Because of this reason, they are regarded as the main target for antibiotic action in preventing their formation. The main component of Bacterial cell walls is a polymer named, Peptidoglycan which has many sugar sub units and also amino-acids. The main sugar derivatives of this mesh like layer are N-acetylglucosamine(NAG) and N-acetylmuramic acid(NAM) respectively. There are D and L-amino acids like the D-alanine, meso-diaminopimelic acid etc, which form a short peptide that is connected to the carboxyl group of the NAM. The presence of D-amino acids, protects the bacteria from being attacked by most peptidases(the enzymes that degrade the peptide bonds). A framework of the sugars and the amino acids, called peptidoglycan sacculus is formed in which the sugars link to one another to form the strands and these strands are in turn connected to each other through the covalent bonds between the peptides coming out from each strand. Many bacteria have cross-links between their strands of the peptidoglycan sacculus and few other have peptide interbridge in place of the cross links. The presence of the cross-links makes the peptidoglycan sacculus layer elastic and it is also strong, flexible and porous allowing few globular high molecular weight proteins to pass through it.
There are differences in the cell walls of Gram positive and Gram negative bacteria which makes it possible to distinguish them through the Gram staining procedure. The cell walls of the Gram positive bacteria have thick peptidoglycan layer which retains the primary stain and is not easily washed by acetone. Hence they do not take up the counter stain and appear purple whereas the Gram negative bacteria because of having thin peptidoglycan layer get decolourised easily and take up the secondary stain, safranin and appear pink. That means Gram negative bacteria such as E-coli appears Purple in this method and species of Gram positive bacteria such as the the Bacillus species, appear purple. Apart from the peptidoglycan layer, many other components are also present in the Gram positive cell wall such as the teichoic acids which are polymers of glycerol or ribitol joined by phosphate group and help in the pathogenicity of the bacteria. When these teichoic acids are covalently connected to plasma membrane, they are called as lipoteichoic acids. There is a space between the plasma membrane and the cell wall called the periplasmic space which contains some enzymes called the exoenzymes. They degrade the large polymeric nutrients to make them pass through the plasma membrane. There are also some other proteins on the surface which help in the interaction of the bacteria with its external environment.
ptidoglycan layer resting within it. The periplasm has many proteins which participate in various activities of the cell. Some of these proteins participate in the electron transport which is an important energy conserving process in the denitrifying bacteria that convert nitrate to nitrogen gas and help in maintaining the nitrogen cycle. Some periplasmic proteins also help in the peptidoglycan synthesis. There is an outer membrane lying outside the periplasm which is connected to it by the Braun's lipoprotein. The outer membrane consists of several lipopolysaccharides (LPS) made up of three main components namely the lipid A, The core polysaccharide and the O side chain. The core polysaccharide consists of ten sugars which is joined to the lipid A. Lipid A is made up of two glucosamine sugar units, each of which in turn contains three fatty acids and a phosphate or pyrophosphate attached. The components of the LPS play an important role such as,
1) The O antigen helps in elucidating immune response in the host cell.
2) The lipid A acts as the endotoxin and cause some of the symptoms of infection by these bacteria.
3) It provides a permeability barrier to the entry of various antibiotics and other pathogenic micro-organisms that may harm these bacteria.
4) It stabilizes the outer membrane and also provides a positive charge to the cell with the presence of the core polysaccharide.
Apart from all these constituents, there are also some proteins called porin proteins which form tube like clusters and allow the passage of small sugar molecules avoiding larger ones through it.
References:
Joanne M. Willey, Linda M. Sherwood, Christopher J. Woolverton. Prescott's Microbiology Ninth Edition. United states of America: McGraw-Hill, 2014.Print.
There are differences in the cell walls of Gram positive and Gram negative bacteria which makes it possible to distinguish them through the Gram staining procedure. The cell walls of the Gram positive bacteria have thick peptidoglycan layer which retains the primary stain and is not easily washed by acetone. Hence they do not take up the counter stain and appear purple whereas the Gram negative bacteria because of having thin peptidoglycan layer get decolourised easily and take up the secondary stain, safranin and appear pink. That means Gram negative bacteria such as E-coli appears Purple in this method and species of Gram positive bacteria such as the the Bacillus species, appear purple. Apart from the peptidoglycan layer, many other components are also present in the Gram positive cell wall such as the teichoic acids which are polymers of glycerol or ribitol joined by phosphate group and help in the pathogenicity of the bacteria. When these teichoic acids are covalently connected to plasma membrane, they are called as lipoteichoic acids. There is a space between the plasma membrane and the cell wall called the periplasmic space which contains some enzymes called the exoenzymes. They degrade the large polymeric nutrients to make them pass through the plasma membrane. There are also some other proteins on the surface which help in the interaction of the bacteria with its external environment.
What makes the Gram negative cell walls different?
The Gram negative bacteria such as E-coli have a complex cell wall arrangement and they do not have a thick peptidoglycan layer which is usually 2-7 nm thick. The width of the periplasmic space is about 30-70 nm and has the peptidoglycan layer resting within it. The periplasm has many proteins which participate in various activities of the cell. Some of these proteins participate in the electron transport which is an important energy conserving process in the denitrifying bacteria that convert nitrate to nitrogen gas and help in maintaining the nitrogen cycle. Some periplasmic proteins also help in the peptidoglycan synthesis. There is an outer membrane lying outside the periplasm which is connected to it by the Braun's lipoprotein. The outer membrane consists of several lipopolysaccharides (LPS) made up of three main components namely the lipid A, The core polysaccharide and the O side chain. The core polysaccharide consists of ten sugars which is joined to the lipid A. Lipid A is made up of two glucosamine sugar units, each of which in turn contains three fatty acids and a phosphate or pyrophosphate attached. The components of the LPS play an important role such as,
1) The O antigen helps in elucidating immune response in the host cell.
2) The lipid A acts as the endotoxin and cause some of the symptoms of infection by these bacteria.
3) It provides a permeability barrier to the entry of various antibiotics and other pathogenic micro-organisms that may harm these bacteria.
4) It stabilizes the outer membrane and also provides a positive charge to the cell with the presence of the core polysaccharide.
Apart from all these constituents, there are also some proteins called porin proteins which form tube like clusters and allow the passage of small sugar molecules avoiding larger ones through it.
Conclusion:
The cell is surrounded by a plasma membrane which provides a barrier to it and plays an important role. In spite of that, the cell wall surrounding the bacteria helps it against other infectious organisms which may harm them and also poisonous chemicals, antibiotics and many other substances whose presence may be a threat to them. The role of cell wall cannot be replaced by any other organelle and hence it becomes very important for the bacteria to have them. They account for the pathogenicity of the bacteria and hence are targeted by the antibiotics in killing them. There are many differences within the Gram positive and the Gram negative cell walls which makes them differ from each other and also helps in differentiating them through the Gram staining procedure. There are several components in the bacterial cell walls which have different functions.
References:
Joanne M. Willey, Linda M. Sherwood, Christopher J. Woolverton. Prescott's Microbiology Ninth Edition. United states of America: McGraw-Hill, 2014.Print.
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