the bacterial cell wall provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan (poly- N -acetyl glucosamine and N -acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. intracellular parasites such as Mycoplasma ) contain peptidoglycan, not all cell walls have the same overall structures. Since the cell wall is required for bacterial survival, but is absent in eukaryotes, several antibiotics (penicillins and cephalosporins) stop bacterial infections by interfering with cell wall synthesis, while having no effects on human cells.
Prokaryotes (Bacteria)• Eubacter "True" bacteria – human pathogens – clinical or environmental – one kingdom• Archaea – Environmental organisms – second kingdom 4
Introduction• Bacteria despite their simplicity, contain a well developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteria and are not found among eukaryotes.
Cell Morphology• The most elemental structural property of bacteria is cell morphology (shape).• Bacteria are classified by shape into three basic groups: • cocci (spherical) • bacilli (rod-like) • spirochetes (spiral)• Some bacteria are variable in shape and said to pleomorphic.• The shape of a bacteria is determined by its rigid cell wall.• The microscopic appearance of a bacterium is one of the most important criteria used in identification.
• The arrangement of bacteria is important. Example: • Cocci in pairs (diplococci) • Cocci in chains (streptococci) • Cocci in clusters (staphylococci)
Bacteria Size• Bacteria range in size from about 0.2 to 5 um.• The smallest bacteria (Mycoplasma) are the same size as a large virus.• The longest bacteria rods are the same size as some yeasts and human red blood cells.
CLASSIFICATION OF BACTERIA• ON BASIS OF SHAPE• ON BASIS OF ARRANGEMENT• ON BASIS OF STAINING• ON BASIS OF OXYGEN REQUIREMENT 10
Cell envelope– The cell envelope: Consists of cell membrane and cell wall plus an outer membrane if present.– Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative based on Gram staining characteristics that reflect major structural differences 21 between the two groups
Cell wall• Cell wall consists of the peptidoglycan layer attached structures (eg: capsule, pili etc) 26
PEPTIDOGLYCANS ( murein/ mucopeptide)– Multilayered in gram positive bacteria.– Surrounds the bacterial cell membrane.– Provides rigidity. It is huge (billions in molecular weight)– Consists of a glycan (polysaccharide) backbone with peptide side chains.– PG is found in all bacteria except Chlamydia and Mycoplasma. 27
Gram positive cellenvelope:Thick peptidoglycanwith covalently boundteichoic acid. These negativelycharged moleculesconcentrate metalions from thesurroundings. Lipoteichoic acid isprimarily associatedwith the cellmembrane. 28
Gram negative cell envelope• Outer membrane present ( lipopolysachride, lipoprotein &phospholipid) --- endotoxin Lipid A --- toxic Polysaccharide core Outer polysaccharide -- antigenic• Thin peptidoglycan layer• No teichoic acid• Periplasmic space : b/w two membranes 29
CELL MEMBRANE .Cell Wall Cytoplasm Cell membrane•Oxidative phosphorylation (no mitochondria).•Synthesis of cell wall precursor.•Active transport•Synthesis of enzymes. 32
CYTOPLASM• Mesosome Cytoplasmic membrane invagination. Cell division-----DNA binding site. Ribosomes 70S in size (50S & 30S) Protein synthesis. Nucleoid• No nuclear membrane.• DNA---Single, circular, about 2000 genes.• No introns in DNA, no mitotic spindle, no 33 nucleolus, no histones
Capsules and slime layers (Glycocalyx)• well defined: capsule• outside cell envelope• usually polysaccharide (in B. anthracis– D glutamate)• Gives virulance (antiphagocytic).• Antigenic (vaccine formation).• Adherence.• Quellung reaction (Identification).2. not well defined-- slime layer or glycocalyxo Adherence 35o S.mutans (Plaque formation)
FLAGELLA• For motility – long & whip like• Subunits—Flagellin• Respond to food/poison –chemotaxis• Never present in cocci.• Use ATPs – proton motive force• Number and location of flagella. (Peritrichous, monotrichous, lophotrichous, axial). 36
• Flagella –embedded in cell membrane –project as strand –Flagellin (protein) subunits –move cell by propeller like action –*axial filament –*flagellar antigen 37
Axial filaments– spirochetes– similar function to flagella– run lengthwise along cell– snake-like movement 39
Pili (fimbriae)•Short , hair-like projections of the cell•Protein--pilin•Mostly on gram-ve bacteria•adhesion to host epithelium•Two types– ordinary pili sex pili•sexual conjugation (sex pili) 40
Endospores (spores)• Metabolically dormant cell• Produced when adverse conditions eg starved – sporulation – bacterial DNA, cytoplasm, cell membrane, peptidoglycans, water & keratin- like coat• Resistant to adverse conditions - high temperatures (not killed by boiling), - radiation, dehydration - organic solvents• Killed by autoclaving (121 C for 30 min)• contain dipicolinic acid• Bacillus (central) and Clostridium (terminal)• Survival for many years 41
CYTOPLASM Granules – Storage of nutrients – Stained with dyes Transposons Jumping genes – within DNA or between DNAs of bacteria, plasmids & bacteriophages Pieces of DNA No independent replication. Can be more than one in 1 DNA. For toxins, enzymes, antibiotic resistance etc. 4 domains– inverted repeats, transposase, repressor, resistance. 42
Plasmids Multiple copies in number Extra-chromosomal DNA, double stranded, circular Can be incorporated in DNA Coding pathogenesis and antibiotic resistance, heavy metal & U-V light resistance, toxins, pili etc. Replication independent of bacterial chromosome.Transmissable & Non transmissable. 43
Making Wall-less forms• Result from action of: – enzymes lytic for cell wall (eg lyzozyme) – antibiotics inhibiting peptidoglycan biosynthesis• Wall-less bacteria that don’t replicate: – spheroplasts (with outer membrane) gram - ve – protoplasts (no outer membrane) gram +ve.• Wall-less bacteria that replicate – L forms ( relative resistance to antibiotics) 44
Procedure of Gram Staining• Developed by Christian Gram• In 1884• 4 steps: Stain with Crystal voilet (primary stain) Then pour Gram,s iodine (mordant) Decolourize Pour Safranin (counter stain) 48