Download Introduction to bacteria-II

Introduction to bacteria-II
Outlines:

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.
Plasma membrane:
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 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 generall 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.