Babylon university Dr.Ilhamalsaedi
College of Medicine Assist. Proff.Dept.of Microbiology
Eukaryotes and Prokaryotes:
Cell have evolved into tow different types,eukaryotic
d to the high concenandprokaryotic,which can distinguished on the basis of thire structure and
1-The eukaryotic cell has a true nucleus with multiple chromosomes surrounded by anuclear membrane and uses amitotic apparatus to ensure equal allocation of the chromosomes to progeny cells.
2-The nucleoid of prokaryotic cell consists of a single circular molecule of loosely organized DNA lacking a nuclear membrane and mitotic
The other diffrences:
1-Eukaryotic cells cotainOrganelles,such as mitochondria and lysosome,and larger (80S) ribosomes,wheres prokaryotes contain no organelles and smallar(70S) ribosome.
2-Most prokaryotes have a rigid external cell wall that contains Peptidogltcan,a polymer of aminoacids and sugars, asit unique structure component.Eukaryotes,on the other hand, do not contain peptidoglycan. Either they are bound by a flexible cell membrane or,in the cases of Fungi,they have a rigid cell wall within chitin. 3-The eukaryotic cell membrane contain sterole,wheres no prokaryote except the wall-less Mycoplasma,has sterol in its membrane. Another criteria by which these organisms can be contrasted is motility. Most protozoa and some bacteria are motile, wheres Fungi and Viruses are non motile.The protozoa are heterogeneous group that have three different organs of locomotion:Flagella,Cilia,andPsudopods. The motile bacteria move only by means of flagella.
Structure of bacterial cells
A-Shape and Size
Bacteria are classified by shape into three basic groups:Cocci,Bacilli,andSpirochetes.Some bacteria are variable in shape and are said to be pleomorphic(many shaped).The shape of a bacterium is determined by its riged cell wall.The microscopic appearance of bacterium is one of the most important criteria used in its identification.
In addition to their characteristic shapes,the arrangement of bacteria is important.Forexample,certaincocci occur in pairs (diplococcic),some in chains(Streptococci),and others in grapelike clusters (Staphylococci). The arrangments are determined by the orientation and degree of attachment of the bacteria at the time of cell division.Bacteria range in size from about 0.2 to 5µ m.
B-Structure:
1-Cell wall:
The cell wall is the outermost component to all bacteria (except mycoplasma species,which are bounded by a cell membrane,not a cell wall).Some bacteria have surface features external to the cell wall,such as capsule, flagella and pili,which are less commen components.
cytoplasmicmembrane.It composed of an inner layer of(peptidoglycan)srounded by an outer membrane that varies in thikness and chemical composition depending upon the bacterial type.
The peptidoglycan provide the characteristic shape of the cell, protection of internal structure and confers rigidity. a-Cell walls of Gram-positive and Gram-negative bacteria
The structure,chemicalcomposition,andthikness of the cell wall differ in G+ve and G-ve bacteria:
1-The peptidoglycan layer is much thicker in G+ve than in G-ve bacteria.
Some G+ve bacteria also have a layer of teichoic acid on the outside of the peptidoglycan,wheres G-ve bacteria have no techoic acid.
2-Incontrast,the G-ve organisms have a complex outer layer consisting of lipopolysaccharide,lipoprotein,and phospholipid.
Lying between the outer-membrane layer and the cytoplasmic membrane in G-ve bacteria is the (periplasmic space) which is the site,in some species,of enzymes(e.g,beta lactamases) that degrade pencillins and other beta lactam drugs.
Thecell wall has several other important properties:
1-In G-veorganisms,it contains endotoxins,lipopolysaccharide.
2-Its polysaccharide and proteins are antigens that are useful in laboratory identification.
3-Its porin proteins play a role in regulating the passage of molecules in to the cell.
b-Cell walls of acid-fast bacteria
Mycobacteria,e.g.Mycobacterium tuberculosis ,have an unusual cell wall,resulting in their inability to be gram-stained. These bacteria are said to be((acid fast)) this property is relatetration in the cell wall of lipids called((mycolic acid)).
c-Peptidoglycan
Peptidoglycan is a complex polymer that srounded the entire cell.It is also called mucopeptide or murein,found in all bacterial cell walls.A layer of peptideoglycan consist of two types of alternating joined molecules called N-acetylglucose amine and N-acetylmuramic acid and a set of identical tetrapeptide side chain.
The ? 1,4glycosidic bond between N-acetylglucose amine and N-acetylemuramicacid,which cleaved by the bacteriolytic enzyme (Lysozyme).The lysozyme which is present in human tears, mucus and saliva,can cleave the peptidoglycan backbone by breaking its glycosylbonds,thereby contributing to the natural resistance of the host to microbial infection.
d-Lipopolysaccharide (LPS):
The LPS of the outer layer of the cell wall of G-ve bacteria is endotoxin.It is responsible for many of the features of disease, such as fever and shock,caused by these organisms.It is called endotoxin because it is an integral part of the cell wall,in contrast to exotoxins which are freely released from the bacteria.The LPS is composed of three distinct units:
(1)-A phospholipid called Lipid A,which is responsible for the toxic effects.
(2)-A core polysaccharide of 5 sugars linked through ketodeoxyctulonate to lipid A;and
(3)-An outer polysaccharide consisting of up to 25 reapeating units of 3-5 sugars.This outer polymer is the important somatic or O antigen of several G-ve bacteria that is used to identify certain organisms in the clinical laboratory.
The functions of LPS to gram-negative bacteria:-
1.It is essential for bacterial growth and survival.
2.Helpe in the adherence (Colonization) of bacteria.
3.Preventdistrubtion by phagocytosis.
4.It is permeable to only low molecular weight,hydrophobic molecules of less than 700 daltons.Thes prevent the penetration of bile salts and other toxic molecules from gastrointestinal tract from pentaration in to these types of bacteria especially(Enterobacteriaceae).
*Bacteria are classified as gram positive and gram negative according to their response to the gram staningprocedure.Gram stain involves 4 steps:-
-The cells are first staind with crystal violet dye and iodin then washed with aceton or alcohol then stained with safranin.Theaceton or alcohol decolarize G-ve bacteria(because its cell wall contains littele peptidoglycan and a lotof LPS) but not G+ve bacteria which contain a lotof of peptidoglycan that take the stain and not dissolved or affected by aceton or alcohol,so it appear blue purple in color on examination under microscop,while the G-ve bacteria often loss this coler by decolorize;it will stain with(counter stain)safranin that give it the red or pink color.2-Cytoplasmic membrane:
Just inside the peptidpglycanlayer,which is composed of phospholipid bilayer, the cytoplasmic membrane has 4 important functions:-
1.active transport of molecules in to the cell.
2.energy generation by oxidative phosphorylation.
3.synthesis of precursors of the cell wall.
4.secretion of enzymes and toxins.
3-Cytoplasm:the cytoplasm contain:
A.Nucleoid
The nucleoid is the area of the cytoplasm in which DNA is located, the DNA of prokaryotes is a single,circular,closedmolecule,contain the genetic informations needed by the bacterium.The absence of a membrane surrounding the nucleoid is one of the main charactristics used to distinguish prokaryotic cells from eukaryotic cells.
B.Plasmids
Are extrachromosomal,double-stranded,circular DNA molecules.Plasmids are not essential for the life of cell.They may confer certain properties like toxigenicity,virulence and drug resistance.
C.Ribosomes
Are the site of protein synthesis as in eukaryotic cells, ribosomes the site of action of many antibiotics that inhibit bacterial,but not human,proteinsynthesis.Consist of two subunits,the protein and RNA.D-Intracellular inclusions
They are considered to represent areserve material like,polysaccharides,lipids,phosphate,sulfer or other materials needed for the biochemistry.
Specialized structures outside the cell wall:-
1_Capsule
The capsule is a gelatinous layer covering the entire bacterium.Its composed of polysaccharide.The capsule gives the bacteria agreater opportunity for survival and thus a better chance to cusediseas as it help in
a.Adherence to human tissues.
b.Invasiveness of the pathogen.
c.Protection from phagocytosis.
2_Glycocalyx(Slim layer):
The glycocalyx is a polysaccharide coating that is secreted by many bacteria.It covers surface like afilm and allowes the bacteria to adher to various structures ,e.g.skin,heart valves.
3_Flagella:
Are protein appendages for locomotion, consist of three basic parts;basalbody,hook and long filament composed of polymerized protein called ((Flagellin)).
Som bacteria has singie flagellum called monotrichus or has flagellum at each end of the cell called amphitrichus,or multiple flagellae at one end called lophotrichus or flagella distributed over the entire body called peritrichus bacteria.
Flagella are important for:-
*They are the means by which bacteria move towared nutrients or away form harmful substances.
*Some species of bacteria,e.g.Salmonellaspecies,are identified in the clinical labatory by the use of specific antidies against flagellar proteins. 4_Pili(Fambriae):
Are interchangeable terms used to designate hairlike,short structure that extend from the cell surface.They are shorter and straighter than flagella and composed of protein called((Pillin)).
Pili have 2 important roles:
1.They mediate the attachment of bacteria to specific receptors on the human cell surface, which is anecessary step in the intiation of infection for some organisms.
2.Aspecialized kind of pilus, the sex pilus, forms the attachment between the male(donor) and the female(recipient) bacteria during conjugation.
5_Endospores:
Are formed as survival response to certain nutritional conditions such as depletion of certain source.
Are metabolically inactive bacterial cells that are highiy resistant to heat,and various chemicals.Possess a core that contain many cell components, a spore wall, a cortex,a coat and exosporium.
Contain calcium dipicolinate,which aids in heat resistance whithin the core germinate under favorable nutritional conditions after an activation process that involves damage to the sporecoat.Endospors are helpful in identifying some species of bacteria(e.g. Bacillusand Clostridia).