Lecture 1 Bacterial Structure PDF

General Bacteriology Bacterial structure Dr. Ayat Mostafa Lecturer of Microbiology and Immunology Faculty of Pharmacy- Assiut University  Microbiology: micro = very small, bio = living organisms, and ology...

General Bacteriology Bacterial structure Dr. Ayat Mostafa Lecturer of Microbiology and Immunology Faculty of Pharmacy- Assiut University  Microbiology: micro = very small, bio = living organisms, and ology = the study of or science. Microbiology: the science which deals with tiny "small" microbes (ex. Bacteria, fungi, viruses) Classification of organisms: 1-Before Electron Microscope: Protista included (bacteria, fungi, protozoa and algae) 2-After E.M: a) Prokaryotes includes (Bacteria, Rickettsia and Chlamydia) b) Eukaryotes includes (Fungi, protozoa and plant and animal cells). c) Viruses.  Feature of structure Prokaryotes Eukaryotes Nuclear structure Nuclear membrane Absent Present Nucleolus Absent (nuclear body) Present Chromosome Single Multiple Histone associated with DNA No Yes Organelles Mitochondria Absent Present Compare between Chlorophyll Absent Present Golgi apparatus Absent Present prokaryotes and Endoplasmic reticulum Absent Present Eukaryotes Mesosomes Present Absent =Compare between Cell wall Present (except in Absent in animal cells, bacteria and fungi Mycoplasma) Plants have Cellulose and fungi have chitin Cytoplasmic membrane Contains no sterol, Contains sterol except Mycoplasma Ribosomes 70S (30S, 50S) 80S (40S, 60S) Division Simple binary fission Mitosis, budding and a sexual reproduction Size Smaller Larger Examples Bacteria, Rickettsia, Fungi, protozoa, plant Chlamydia and and animal cells Mycoplasma Classification of bacteria: According to 1-Morphology: bacteria are unicellular organisms a) Spherical → called Cocci b) Rod shaped →called Bacilli, may be twisted (coma shaped ex. Vibrio) c) Spiral→ex. Spirochates Classification of bacteria Arrangement: bacteria can be arranged in special arrangements. Ex: a) Pairs → called Diplococcic b) Chains → called Streptococci. c) Clusters → called Staphylococci 2-Staining: according to Gram stain into gram +ve and gram –ve. 3-Metabolism and nutrition 4- Orders→Families→Genera→Species→Groups →Types.  Structure of the Bacterial Cell A-Essential (Basic or primary) They are the target for antibiotics Essential for life 1-Cell wall 2-Cytoplasmic membrane 3-Protoplasm (cytoplasm+ Nuclear material) B-Accessory (secondary) Not-essential for life but increase the virulence (pathogenicity) 1-Capsule and slime 2-Fimbriae (pili) 3-Flagellum 4-Spores 5-Plasmids 1- Cell wall Characters, Function and medical importance: 1-Rigid structure thus maintains the shape of bacteria. 2-Osmotic insensitive thus protect the cell from osmotic rupture or lysis (especially hypotonic solutions). 3-Plays a role in cell division. 4-Virulent factor: a) LPS (lipo-polysaccharide) in Gram –ve bacteria contains the lipid A fraction which is highly toxin and called the endotoxin. b) Teichoic acid in Gram+ve bacteria is responsible for adhesion to host cells. c) Protein A in Staph and protein M in Strept are anti-phagocytic. Medial importance: 1- The cell wall is considered the Major antigen "called the O Ag", which stimulates the immune response to produce antibodies. 2- Used for Classification of the bacteria according to gram stain to gram +ve and Gram-ve 3-It is a good target for antibiotics. Ex. β-lactam antibiotics that include (penicillin and cephalosporins) inhibits cell wall synthesis → death of bacteria.  Gram +ve Gram –ve Composition: 1-Peptido glycan (50%): 1-Outer membrane layer: which contain the Peptidoglycan: Thick, 40 sheets, it is the outer layer. lipo-polysaccharide (LPS) (%,number of 2-Teichoic acid (50%): 2- Inner layer: Peptidoglycan (5-10%): sheets, location) Wall teichoic acid: linked to peptidoglycan Thin, 1 or 2 sheets (Inner layer). -formed of poly-ribitol and poly-glycerol 3-Lipoprotein to connect outer and inner layer. phosphates. Periplasmic space is a space that is filled with a gel. Importance: - In some species It stores enzymes called β -lactamase a- Considered major surface antigen. that degrade β-lactam antibiotics (ex.penicillin) b- Attachment to host cells.  Gram +ve Gram -ve Chemical Peptidoglycan (murein): Lipo-polysaccharide (LPS) consists of: composition: a) Back bone of NAG (N-acetyl glucose A- polysaccharides: amine) linked to NAM (N-acetyl - It is considered an antigen (called the O Ag) muramic acid) by glycosidic bonds. b- A core polysaccharide. b) Layers are crosslinked by peptide C- Inner Lipid A (endotoxin): cross bridge which is formed by -It is highly toxic. Transpeptidase enzyme (this enzyme -It cause fever, hypotension and shock. attacked by penicillin antibiotics) -It is released when the cell is lysed. Notes: -Porins: are proteins in Gram –ve cell wall that form pores or channels. -Function: It allows passage of hydrophilic molecules (nutrients) and prevents passage of large molecules & harmful hydrophobic compounds (such as bile salts).  Gram +ve Gram -ve Gram stain Violet pink Example Staphylococci E-coli  Cell wall deficient bacteria There are 2 types of bacteria that lack cell wall. These bacteria will be: -Pleomorphic- no defined shape. -Not affected by antibiotics acting on cell wall such as the B-lactam antibiotics that include (penicillins and cephalosporins). -Poorly stained or not stained with Gram.  Cell wall deficient bacteria Mycoplasma L-forms Origin Bacteria that lack the cell wall -Bacterial in which the cell wall is artificially removed genetically=naturally -They are formed in the body: a) Due to presence of antibiotics acting on cell wall (ex.penicillins) or enzymes that destroy the cell wall (ex.lysozymes). b) Due to spontaneous mutations during chronic infection or in high osmotic pressure (in urine). Formation They do not revert to the parental form. They revert to the parental form (having cell wall) when the cell wall of cell wall inhibitors are removed Sterol They contain sterols (cholesterol) in the cell No cholesterol membrane to stabilize it Types - Types: Protoplasts: Originate from gram +ve bacteria. Lack cell wall completely. Spheroplasts: Originate from gram –ve bacteria. "weakened cell wall " Differences between Gram-positive and Gram-negative bacteria Properties Gram-positive Gram-negative Colour of Gram-stained cell Violet Pink or red Effect of alcohol Not decolourized Decolourized Cell wall Less porous More porous Peptidoglycan 50% of cell wall (Thick) 5-10% (Thin) Techoic acid Present Absent Outer membrane Absent Present LPS Absent Present Periplasmic space Absent Present Porin proteins Absent Present Forms result from cell wall inhibitors Protoplast Spheroplast 2-Cytoplasmic membrane: Structure: semi-permeable delicate membrane around the cytoplasm. It maintains constant environment within the cell by controlled transport mechanisms. Chemical composition: phospholipid bilayer + protein Function: 1-Selective permeability to different molecules -by Passive diffusion, active transport of nutrients. It acts as osmotic barrier. 2-Excretion of hydrolytic enzymes and toxins 3-Stie of respiration and energy production: as it supplies the bacteria with energy through electron transport and oxidative phosphorylation. 4-Plays a role in DNA replication. 5-Biosynthesis: as it provides enzymes and lipid carriers for synthesis of cell wall. 2-Cytoplasmic membrane: Mesosomes: Definition: they are invaginations of the cell membrane. Function: they play a role in A) Respiration: they contain enzymes responsible for respiration B) Cell division: They are the origin of the transverse septum (called septal mesosome) that divides the cell into 2 halves. 3-Bacterial Cytoplasm viscous, watery, soft gel like Containing 1-cytoplasmic inclusion granules : ex.meta-polyphosphate as a source of energy. 2-Ribosomes: Concerned with protein synthesis. There are 3 kinds of cytoplasmic RNA. -Ribosomal RNA (r RNA): 70s in size, with 30s and 50s subunits. They are the site of protein synthesis. -Transfer RNA (t RNA). -Messenger RNA (m RNA).  4-Bacterial Nucleus: It is in the form of single circular double stranded DNA that is supercoiled (it is 1mm long if unfolded) Bacteria multiply by binary fission in which: chromosome attaches to mesosome that forms the transverse septum. Then each strand forms complementary stand forming 2 copies which are identical B-Accessory structures 1-Capsule and slime Definition: Thick, gelatinous, layer that surround the cell wall in some species (from outside). Types: a) Capsule is firmly attached to the cell wall. It is condense, closed and well-defined. b) Slime layer (Glycocalyx) is loosely attached to the cell wall. It can detach. Capsule The capsular material is responsible for the mucoid appearance of the colonies. Composition ; -Capsular material may be polysacharides as in pneumococci -polypeptide as in B. anthracis -hyaluronic acid as in streptococci They are formed only In vivo because its formation is influenced by environmental and cultural conditions. Its presence lead to mucoid appearance of colonies. Functions of capsules: 1- Virulence factor of many bacteria as it protects bacteria against phagocytosis. 2- The capsule may play a role in the adherence of bacteria to human tissue. 3- It is immunogenic and used for specific identification of organisms by using antiserum against the capsular polysaccharide e.g quelling reaction-→ serotypes ex. There are 80 types of pneumococci differ in only capsular structure (K-antigen) 4- Capsular polysaccharide used as antigens in certain vaccines.  Demonstration: 1) Not stained with Gram or Methylene blue (MB)→so appear as unstained Halo (uncolored) around the organism 2) Can be stained with Indian ink stain or polychrome MB. The capsule appear pink around the blue bacilli (MacFadyean reaction). 3) On the addition of specific antisera (Antibodies)→ capsules swell (capsule swelling reaction or called quellung test). Glycocalyx (slime layer) It is formed of polysaccharide and secreted by some bacteria. It forms a layer that attaches the bacteria firmly to host surfaces (e.g. skin, heart valves and catheters) → This layer is called bacterial Biofilm. Bacteria in biofilms are protected from the immune system and are resistant to antibiotics. - Adherence of Viridans streptococci to teeth leads to formation of plaque and dental caries - Adherence of Viridans streptococci to heart valves leads to endocarditis 2-Flagella: Long, thin filaments arise from protoplasm and firmly attached to cell wall and cell membrane by a basal body. Composed of protein called flagellin Present in Gram + and –ve bacteria Function: 1-The organ of motility (locomotion) in motile bacteria. Move the bacteria to reach the site of infection. 2- Has different antigenic forms (called H-Ag) so can be used in serotyping ex. salmonella species can be identified by antibodies against flagella proteins 2-Flagella: Distribution of flagella: 1-Monotrichous: single polar flagellum. 2-Amphitrichous: one flagellum at both poles. 3-Lophotrichous: multiple flagella at one or both poles. 4-Peritrichous: around the bacterial cell. -Mono and peri are the most frequent in pathogenic species. 5-Atrichous: No flagella. 3-Pili (Fimbriae) Short Hair-like filaments arise from cell surface. They are shorter and thinner than the flagellum. They are more abundant. Composed of protein called pilin Present only in Gram –ve bacteria Function: It is the organ of adhesion (attachment) to the host cell so increase the pathogenicity. 3-Pili (Fimbriae) There are 2 types A-Ordinary pili (colonization antigen) Play a role in the adherence of the organism to the host cell surface so initiate the infection (i.e., it is a virulence factor) Ex. Neisseria gonorrhea attach to urinary tract by pilli. Mutants of Neisseria gonorrohea without pili are non-pathogenic B-Sex pili: important in the conjugation of the 2 bacterial cells. 4-Spores Definition: It is a highly resistant, resting stage, of some Gram +ve bacteria. Ex.Genus Clostridium& Bacillus. Spores are formed under unfavorable conditions Ex. Depletion of nutrients, heat or dryness → so they are formed only in vitro (not formed in vivo or in animal tissues) The marked resistance of spores to heat and chemicals is due to: 1. They are surrounded by many layers or coat (impermeable wall). 2. The presence of calcium dipicolinate which is present in core of spore. 3. They are metabolically inert (very low metabolic activity) 4. They are dehydrated and resist dryness. But they are destroyed by autoclave (steam at 121°C for 20 min) 4-spores 4-Spores Medical importance: 1- Spores are extra-ordinary resistance to heat & chemical methods of sterilization and boiling (does not destroy spores). #but they can be destroyed by autoclave (= heating by steam at 121°C for 20-30 minutes). 2- Spores are metabolically inactive (cryptobiotic) so can stay for years in soil and when they enter the body through wounds → they germinate →to give the vegetative form and cause disease. 3) Spores are used as indicator of sterilization process to ensure complete sterilization 4) Used as biological weapon 4-Spores Demonstration: They are not stained by Gram's stain because the wall is impermeable and appear as unstained or clear area (uncolored). Can be stained by spore stain or modified ZN stain. 5-Plasmid Definition: Plasmids are extra-chromosomal, circular, double stranded DNA. They replicate “autonomous” = independently of the host bacterial chromosome multiple copies of the same plasmid are present in the cell. Plasmids do not encode essential functions for the bacterial cell. They are not required for the life of the cells (Dispensable). Plasmids Classification of Plasmids: 1)Conjugative plasmids are large plasmids that present as one or two copies per cell. They carry the genetic information that are required for the conjugal transfer. Examples: F factor which promotes conjugation and transfer of the plasmid DNA from one cell to another. 2)Non conjugative plasmids are small plasmids that lack the ability for their own transfer, but can be transferred by a conjugative plasmid. They present as multiple copies per cell.  Plasmids Properties Conjugative plasmids Non-conjugative plasmids Size Large Small Number of copies 1–5 10 – 200 F factor Present Absent Formation of sex pili Yes No Mechanism of transfer By conjugation By the aid of conjugative plasmids Common in Gram-negative bacilli Gram- positive cocci Plasmids Functions of Plasmids: Bacterial properties carried on plasmids include: 1. F plasmid or F factor (fertility factor): carry the tra gene and mediates the transfer of plasmid by conjugation (conjugative plasmids). 2. Virulence plasmids: carry genes for exotoxin production. Ex.Clostridium tetani produces a neurotoxin which is encoded by a plasmid. 3. R-plasmids (resistance plasmid): carry genes for resistance to antibiotics by releasing enzymes that destroy the drugs. Ex. Genes for production of B-lactamase enzymes that destroy the B- lactam antibiotics. 4. Production of enzymes responsible for metabolizing some substance. 5. Genes for production of bacteriocins (ex. Col factor by E coli) N.B: Bacteriocins are substances release by one bacteria to kill another 6. Used as cloning vectors to implant a specific gene into a certain cell.