Bacteriology

(2) Anatomy of Bacterial cell

Essential structures which are present in all bacteria -include : Cell wall Cytoplasmic membrane Cytoplasm Nuclear material

Cell wall Site : Outermost layer , common to all bacteria except mycoplasma Function : - It is a complex rigid structure with some elasticity which gives bacteria their definite shape -It is a permeable to passage of liquid nutrient material into the cell and to outward passage of substances produced within the cell

Gram positive bacteria –Composition of Cell wall -Cell wall of gram +ve is thicker than gram –ve -Peptidoglycan : strength of cell wall is due to its presence , constitutes 50-90% of the wall -Teichoic acid : responsible for the pathogenecity (normal bacterial flora is deficient in teichoic acid) -polysacchrides

Gram negative bacteria -More complex than gram positive -Peptidoglycan : consistutes 5-10% of the cell wall , thin layer -Lipopolysacchride (LPS): -the outer most layer , it is called endotoxin , released only after cell death and lyses . Responsible for endotoxic shock (fever , hypotension , hypoglycaemia and DIC ) - All the toxicity of LPS is due to lipid A

Cytoplasmic membrane -Site : It lies just inside the cell wall -composition: It is thin (0.5-10 nm) , elastic and consists mainly of lipoprotein -Function : 1) it acts as semi-permeable membrane controlling the inflow and outflow of metabolites to and from the protoplasm 2)it permits the passive diffusion of water in and out 3)Selective transport of specific nutrients into the cell and that of waste products out of it

Cytoplasm -It is a viscous watery solution of soft gel , containing a variety of organic and inorganic solutes and numerous small granules called ribosomes -The cytoplasm of bacteria differ from the eukaryote in not containing endoplasmic reticulum , Golgi apparatus , mitochondria and lysosomes

Ribosomes -It is the site of protein synthesis -Composition : ribosomal RNA (rRNA) and ribosomal protein -They are designated by their sedimentation coefficient ( S or Svedberg unit) -They have sedimentation coefficient of 70S -There are differences between bacterial and host cell ribosomes (70S & 80S respectively) -This allows us to use antibiotics such as streptomysin which interferes with bacterial metabolism at ribosomal level without upsetting human ribosomal function

Mesosomes -They develop by invagination of cytoplasmic membrane into the cytoplasm -Function:- 1-principal sites of respiratory enzymes in bacteria 2-They are corresponding to mitochondria in eukaryotic cells

Intracytoplasmic inclusions -they are not permanents or essential structures and may be absent under certain conditions of growth -they serve as storage of nutrients material e.g. Volutin granules -They are present in large number when there is excess of energy yielding nutrients and diminish or disappear when there is energy source starvation

Bacterial Nucleus -The genetic information of a bacterial cell is contained in a single , circular double –stranded molecule of DNA -It is 1000 times the length of bacterial cell tightly coiled -The nucloid contains no nuclear membrane , nucleolus and doesn’t divide by mitosis

Extracellular structure of bacterial cell →Capsule or Slime layer → Flagella →Fimbriae or pilli →Spores

Capsule or Slime layer -Site :it is the outermost layer , lies outside the cell wall in capsulated bacteria -It is gelatinous covering layer of the entire surface -Nature : Polysacchride , polypeptide or protein -Slime layer : easily washed off -Capsule : discrete thickened gel around the cell , cannot be stained by ordinary stain but by India ink ( by ordinary stain it appears as hallow around the bacteria) -It can be visualized by reaction with specific antibody which cause a characteristic swelling of the capsule . It is known as Quellin reaction Functions : 1- protect the bacteria from antibacterial agents such as lytic enzymes found in nature 2-They inhibit phagocytosis , thus contributing to the virulence of the bacteria 3-Loss of the capsule render the bacteria to be avirulent

Flagella -Definition : long thread-like appendage -Nature : it is protein in nature , formed of subunits known as flagellin -Function :responsible for the motility of few coccal forms , half of bacilli , and almost all of sprilla and vibrios -Flagella can be visualized by ordinary light microscope by special stains or by electron microscope -The flagellum consists of 3 parts : the filament , the hook , and the basal body

-Types : according to their number and distribution around the cell , they may be : →Monotrichous : single polar flagellum →Lophotrichous: they have a tuft of flagella at one pole →Amphitrichous: they have single polar flagella or tuft of flagella at both poles

→Peritrichous : flagella are distributed all round the cell

Fimbriae or Pili -Definition : they are hair like filaments , they are shorter , thinner and much more numerous than flagella , present in flagellated and non flagellated bacteria -They can be seen by electron microscope -Function : It is necessary for initiation of infection , it mediates attachement of bacteria to human cell through specific receptors -Sex pili : Certain bacteria possess specialized pili which are linger than the common type -These appear to be hollow and constitute tubes through which DNA is transferred from one organism to another during conjugation

Comparison between pili and sex pili

Pili Sex pili

Short . thin , numerous One long pilus Number

Allow attachment of bacterial cell to cell surfaces

Mediate conjugation Function

Spores -Definition : they are dormant forms of bacteria and highly resistant (can remain for many years ) -They are formed in response to unfavourable environmental condition -Sporulation of bacteria is a method of preservation and not of reproduction . They are formed outside human body -Formation of spores occurs after 4-8 hrs after stop of bacterial growth -They cannot be sterilized by ordinary stains -The most common bacteria which form spores are gram positive bacteria e.g. Bacillus sp , Clostridia sp -Their killing requires moist heat at 100-120Oc form10 min , while other vegetative cells can be killed by heating at 60 oC for 10 min

SPORE FORMATION

1- Vegetative growth stops , DNA is duplicated 2-Septum forms , divide bacteria asymmetrically 3-The larger compartment engulfs the smaller compartment , forming a fore spore within the mother cell 4-Peptidoglycan containing material is laid down between the 2 membranes that now surround the fore spore 5-The mother cell is degraded and the endospore released

continued –Spores -shape : round , oval or elongated -site : occupying a terminal , subterminal , or central position -size :they may be narrower than the width of the bacilli or broader and bulging

Key facts -All living cells are either prokaryotic or eukaryotic -Prokaryotes such as bacteria are simple cells -Eukaryotes have a nucleus , organelles such as mitochondria , and complex internal membranes e.g. Fungi , human cells -Peptidoglycan is present in the cell wall of both gram positive and gram negative but , it is thicker in gram positive . It gives rigidity and shape of organism -Lipopolysacchrides (LPS) is the outer most layer of outermembrane of gram negative (not in gram positive ) bacteria -LPS is endotoxin and therefore gram positive bacteria do not produce endotoxin -Bacterial cytoplasm contains chromosomal , nuclear material , ribosomes , mesosomes , and inclusion / storage granules -Structures external to the cell wall of bacteria are flagella , pili or fimbriae , capsule and slim layer -Flagella are used for movement , pili for adhesion and capsule for protection of bacteria against antibiotics and phagocytosis (virulence of bacteria) -sporulation is a response to starvation in Bacillus spp and Clostridium spp

Important Questions 1- Enumerate the essential structures of bacteria 2-What is the difference between gram positive and gram negative cell wall 3-Mention the function of the following structures a-cytoplasmic membrane b-ribosomes c-mesosomes d-nucleus 4-enumerate extra-structures of bacterial cells 5-Mention the function of each of the following a-capsule b-flagella c-fimbriae d-spores 6-choose : ... is not permanent structure ( cytoplasm – DNA-RNA-intracytoplasmic inclusions) 7-complete a-The cell wall of ... contains about 90% peptidoglycan b-lipopolysacchride layer of ... can lead to endotoxic shock c-The size of prokaryote cell is .. and cells in eukaryote cells is ... 8-State True or False 1-The Capsule is the inner most layer of bacterial cell 2-Mobility of bacteria is one of the functions of capsule 3-There are 4 types of flagella according to its number and arrangement 4-Sporulation is a method of reproduction 5-Fimbriae help bacteria to preserve itself during unfavourable conditions 6-Spores can be killed by boiling only

Answers 1- Cell wall Cytoplasmic membrane Cytoplasm Nuclear material

2-Gram positive bacteria -Cell wall of gram +ve is thicker than gram –ve -Peptidoglycan : strength of cell wall is due to its presence , constitutes 50-90% of the wall

-Teichoic acid : responsible for the pathogenecity (normal bacterial flora is deficient in teichoic acid) -polysacchrides

Gram negative bacteria -More complex than gram positive -Peptidoglycan : consistutes 5-10% of the cell wall , thin layer -Lipopolysacchride (LPS): -the outer most layer , it is called endotoxin , released only after cell death and lyses . Responsible for endotoxic shock (fever , hypotension , hypoglycaemia and DIC ) - All the toxicity of LPS is due to lipid A 3-a-1) it acts as semi-permeable membrane controlling the inflow and outflow of metabolites to and from the protoplasm 2)it permits the passive diffusion of water in and out 3)Selective transport of specific nutrients into the cell and that of waste products out of it b) It is the site of protein synthesis c) 1-principal sites of respiratory enzymes in bacteria 2-They are corresponding to mitochondria in eukaryotic cells d) The genetic information of a bacterial cell is contained in a single , circular double –stranded molecule of DNA 4) →Capsule or Slime layer → Flagella →Fimbriae or pilli →Spores 5)a-1- protect the bacteria from antibacterial agents such as lytic enzymes found in nature 2-They inhibit phagocytosis , thus contributing to the virulence of the bacteria 3-Loss of the capsule render the bacteria to be avirulent b- responsible for the motility of few coccal forms , half of bacilli , and almost all of sprilla and vibrios c-: It is necessary for initiation of infection , it mediates attachement of bacteria to human cell through specific receptors d-a method of preservation 6-intracytoplamic inclusions 7-a-gram +ve bacteria b-gram negative bacteria c-70S-80S

8-1-F 2-F 3-T 4-F 5-F 6-F