Introduction to bacteria-II << Back
Introduction to bacteria-II
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.
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.
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.
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.
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 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.
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.
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.
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
Question : 2 Which of the following statement is correct regarding gas vesicles in bacteria?
Question : 3 Lipopolysaccharides (LPSs) does not contain
Question : 4 In gram staining of gram positive cells, the following occurs
Question : 5 Choose the correct statement
Question : 6 The presence of D-amino acids in the crosslinks of the peptidoglycan layer is because
Question : 7 Which of the following is false regarding peptidoglycan
Question : 8 Choose the incorrect statement
Question : 9 Bacterial endospores does not contain
Question : 10 o-polysaccharides represents