Bacterial Structures and Functions Flashcards
Size of bacteria
They are small in size, usually in the order of 1μm in diameter
Morphological features of bacteria
Bacteria is composed of:
• Nuclear material called nucleoid
• Cytoplasm which consist of enzymes, granules, ribosomes e.t.c
• Cell envelope composed of:
• plasma membrane which bond the cytoplasm
• external to which is the cell wall
• with or without capsule
• Appendages: Pilli and flagella may be present
Discuss THE NUCLEOID,and the differences between the DNA structure in bacteria and eukaryotes
Function of mesosomes
• The nuclear material are not membrane bond but scattered all over the cytoplasm
• They are composed of one or more double stranded circular DNA in haploid form
• Eubacteria has no non coding region called introns and are not associated with histones
• DNA is similar to that of eukaryotes as a polymer of 4 deoxyribonucleotides, joined together by phosphodiester bonds at the 3’ and 5’ position of the component sugar atom
• The bases are linked to the second strand by hydrogen bond
The DNA are kept in a super coiled state through covalent bonding by the action of DNA gyrase
• DNA replication is aided by DNA polymerase
• The nuclear material is also composed of RNA transcribed from DNA by DNA dependent RNA polymerase enzyme. Extrachromosomal DNAs i.e plasmid, transposomes are often present.
• The nuclear material associates with an invagination at the cell membrane called mesosome
Discuss the cytoplasm
Discuss cytoplasm in relation to osmotic pressure
Role of the osmotic pressure
The cytoplasm is composed of:
• Ribosomes
• Granules: glycogen, starch, sulphur, fat, polyphosphate (metachromatic/ volutin)
• Enzymes: hydrolytic, polymerases etc
• Water and ions
• Macromolecules: lipids, structural proteins, carbohydrate e.t.c
• All these are present freely in the cytoplasm
• The various macromolecules, ions, carbohydrate are present free in the cytoplasm and so exact a high osmotic pressure which range from 5 atmosphere in Gram negative to 20 atmosphere in gram positive bacteria
• The high osmotic pressure in turn necessitates a rigid cell wall that tends to prevent the cell from osmotic lyses when the organism is located in normal osmotic environment
Discuss Ribosomes
The difference in the nature of ribosomes in pro- and eukaryotes (use)
• Ribosomes are globular in shape
• They are the site of protein synthesis
• The complete unit in bacteria is 70’s while in eukaryotes, it is 80’s unit.
• Ribosomes can be separated by ultra centrifugation into two subunits of 50’s and 30’s in bacteria and 40’s and 60’s subunits in eukaryotes.
• The difference in the nature of ribosomes in pro- and eukaryotes has been used in antibacterial chemotherapy e.g gentamycin, tetracyclins e.t.c
Discuss the CELL ENVELOPE and differences in gram positive and negative envelope
- The layers that surround the prokaryotic cell are collectively referred to as the cell envelope
- The structure and organization of the cell envelope differ in gram-positive and gram negative bacteria.
- Gram positive envelope consists of cytoplasmic membrane and cell wall (basically peptidoglycan layer) with or without a capsule or an S-layer
- Gram positive bacteria cell wall also contains other substances such as techoic acid, teichuronic acid and various polysaccharides
Discuss The cell membrane
The cell membrane is the limiting external border of the cytoplasm & borders the cell wall internally
• It is a dynamic fluid of lipids containing structural and functional proteins
• It appears in fixed section as a lipid bi-layer intercepted by integral and structural proteins with the hydrophobic ends arranged inwards while the hydrophilic ends are arranged outwards
• Bacteria cell membrane is similar to that of eukaryotes but contains some other components e.g phosphatidylethanolamine in E. coli
Cell membrane are important for
1 Selective permeability
2 Electron transport chain in energy generation
3 Receptor site for antigen binding
4 Location of enzymes
5 Mitosis
Discuss the bacterial cell wall
The significance of peptidoglycan in determining gram positive and negative
• The cell wall of bacteria contains peptidoglycan.
Peptidoglycan:
• Gives the shape and rigidity of the cell
• Maintain the high osmotic pressure of the cell
• Forms the basis of the major classification of bacteria into gram positive and gram negative as its quantity in the cell wall determines the staining reaction
• Gram-positive bacteria have a relatively thick layer of peptidoglycan (mucopeptide or murein) while murein layer in Gram-negative bacteria is thin.
• Gram negative bacteria, also contain periplasmic space and lipopolysaccharide among others
• The wall is given its strength by cross-links that form between adjacent murein strands.
• Some bacteria lack peptidoglycan e,g Mycoplasma spp, protoplast, spheroplast of L-forms
Discuss the Gram negative cell wall
Gram negative cell wall can be divided into, Inner cell wall, outer cell wall separated by a periplasmic space
• Inner membrane is the cytoplasmic limiting membrane
• The periplasmic space contains a few murein or peptidoglycan layers, gel like solution of structural and enzymatic proteins and polymers of glucose.
• The periplasmic space occupies about 20% of cell volume
Linking the outer membrane and peptidoglycan layer is the lipoproteins which is numerically the most abundant protein in the gram negative cell and it stabilizes and anchors the outer membrane to the peptidoglycan layer
• External to the outer membrane is the lipopoly-saccharides (Endotoxin)
• GN endotoxin is highly toxic and its elaboration can result in septicaemia manifesting with high fever, generalised vasodilatation resulting in shock
Characteristics of EXOTOXIN
- Released from the cell before or after lysis
- Protein
- Heat labile
- Antigenic and immunogenic 5. Toxoids can be produced
- Specific in effect on host
- Produced by gram-positive and gram-negative organisms
Characteristics of ENDOTOXIN
- Integral part of cell wall
- Endotoxin is LPS; Lipid A is toxic component
- Heat stable
- Antigenic; ??immunogenicity 5. Toxoids cannot be produced 6. Many effects on host
- Produced by gram-negative organisms only
Discuss the APPENDAGES
Function, classification and importance
• These may be in form of pilli (fibrae) or flagella
• They are made of protein and are highly antigenic
• They usually arise from cytoplasmic membrane
• They consist of basal body, hook and filament
• They can be polar, (uni- or bi) or peritrichous in arrangement.
• The flagella are important for motility
• Pilli are important for attachment and
conjugation
• Appendages are very important antigenically
Discuss the CAPSULE
• There may be a well defined layer external to the cell wall called the capsule
• They are composed of polysaccharides, except in Baccilus anthracis which is composed of polypeptides
• When the capsular material are arranged in a loose meshwork form, it is called slime layer
• If arranged as fibrils extending outward from the cell, it is called a glycocalyx
• Capsule is important in bacterial invasiveness and resistance to phagocytosis
Discuss SPORES
• Some bacteria contain spores which are very resistant to adverse conditions such as heat
• Dipicolinic acid content determines its resistance to heat
• Spores can be killed at 120 degrees Celcius after 30 mins while vegetative bacteria will be killed at 60 degrees celcius in 30 mins
• Pathogenic bacteria with spores include Clostridium spp, Bacillus anthracis
