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Batterjee Medical College Batterjee Medical College Bacterial Introduction and Structure Dr. Manal El Said Head of Microbiology Department Batterjee Medical College Microbiology • Microbiology is the study of microorganisms, which are microscopic, unicellular, and cell-cluster organisms. •Medical microbiology deals with the study of microorganisms which are of medical importance and causing diseases in human beings. Batterjee Medical College Taxonomic Rank Batterjee Medical College Bacteria Compared with Other Microorganisms The agents of human infectious diseases belong to five major groups of organisms: 1-Bacteria : 2-Fungi Prokaryote kingdom : 3-Protozoa: Protists kingdom 4-Helminths: Animal kingdom NB. Taken together, helminths and protozoa are commonly called parasites. 5-Viruses: they are not cells but can replicate only within cells Batterjee Medical College Eukaryotes & Prokaryotes Cells have evolved into two types: - Eukaryotic - Prokaryotic Characteristic Prokaryotic Bacterial Cells Eukaryotic DNA within a nuclear membrane No Yes Mitotic division No Yes DNA associated with histones No Yes Chromosome number One More than one Membrane-bound organelles, such No as mitochondria and lysosomes Yes Size of ribosome 8OS 70S Cell wall containing peptidoglycan Yes No Batterjee Medical College Structure of Bacterial Cells Shape & Size • Bacteria have three shapes: - cocci (spheres) - bacilli (rods) - spirochetes (spirals) • Cocci are arranged in three patterns: - Pairs (diplococci) - Chains (streptococci) - Clusters (staphylococci) • The size of most bacteria ranges from 1 to 3 μm. • Mycoplasma, smallest bacteria (smallest cells) are 0.2 μ m. Batterjee Medical College Structure of Bacterial Cells Bacterial Cell Wall •All bacteria have a cell wall composed of peptidoglycan except Mycoplasma, which are surrounded only by a cell membrane. •Gram-negative bacteria have a thin peptidoglycan covered by an outer lipid-containing membrane •Gram-positive bacteria have a thick peptidoglycan & no outer membrane. •These differences explain why: -Gram-negative bacteria lose the stain when exposed to a lipid solvent in the Gram stain process - Gram-positive bacteria retain the stain and remain purple. Batterjee Medical College Structure of Bacterial Cells The outer membrane of gram-negative bacteria Peptidoglycan: • It is found only in bacterial cells. • It is a network that covers entire bacterium & gives organism its shape. • It is composed of a sugar backbone (glycan) and peptide side chains (peptido). • The side chains are cross-linked by transpeptidase (enzyme that is inhibited by penicillins & cephalosporins) Batterjee Medical College Structure of Bacterial Cells Comparison of Cell Walls of Gram-Positive and Gram-Negative Bacteria Batterjee Medical College Structure of Bacterial Cells Cell Walls of Acid-Fast Bacteria • Mycobacteria are acid-fast because it resist decolorization with acid–alcohol after staining with carbolfuchsin. • It is due to high concentration of lipids in cell wall. • The dyes used in the Gram stain do not stain mycobacteria. Batterjee Medical College Structure of Bacterial Cells The outer membrane of gram-negative bacteria Endotoxin (lipopolysaccharide, LPS): • The main inducer of septic shock. • Endotoxin consists of: - lipid A fever & hypotension in septic shock - polysaccharide(O antigen) use in laboratory identification Periplasmic space: •It lies between inner cell membrane & outer membrane of gram-negative bacteria (It is the location of β–lactamases-the enzymes that degrade β lactam antibiotics, such as penicillins and cephalosporins). Batterjee Medical College Structure of Bacterial Cells The outer membrane of gram-negative bacteria Lysozymes They kill bacteria peptidoglycan. by cleaving glycan backbone of The cytoplasmic membrane of bacteria : • It consists of phospholipid bilayer (without sterols) located just inside peptidoglycan. • It regulates: - Active transport of nutrients into cell. - Secretion of toxins out of cell. Batterjee Medical College Structure of Bacterial Cells Gram stain • Gram-positive bacteria stain purple, whereas gram-negative bacteria stain pink. • Gram-positive bacteria has ability to retain crystal violet– iodine complex in presence of lipid solvent as alcohol. • Gram-negative bacteria lose purple dye when treated with alcohol (have outer lipid-containing membrane & thin peptidoglycan) & become colorless & then stain pink when exposed to red dye such as safranin. • Not all bacteria can be visualized using Gram stain. Batterjee Medical College Structure of Bacterial Cells Gram stain Batterjee Medical College Structure of Bacterial Cells ZN stain Batterjee Medical College Structure of Bacterial Cells Bacterial DNA The bacterial genome It consists of a single chromosome of circular DNA located in nucleoid. Plasmids They are extrachromosomal pieces of circular DNA that encode both exotoxins & enzymes antibiotic resistance Transposons •They are small pieces of DNA that move chromosomal DNA and plasmid DNA. • They carry antibiotic-resistant genes. between Batterjee Medical College Structure of Bacterial Cells Structures External to the Cell Wall Capsules • They are antiphagocytic, i.e., they limit ability of neutrophils to engulf bacteria. • They are composed of polysaccharide (the polypeptide capsule of anthrax bacillus is exception). • Capsules are antigens in several vaccines, such as pneumococcal vaccine. • Antibodies against capsule neutralize antiphagocytic effect & allow bacteria to be engulfed by neutrophils. • Opsonization is process by which antibodies enhance phagocytosis of bacteria. Batterjee Medical College Structure of Bacterial Cells Structures External to the Cell Wall The glycocalyx • It is polysaccharide "slime layer" secreted by certain bacteria. • It attaches bacteria firmly to: - Surface of human cells - Surface of catheters - Prosthetic heart valves & hip joints. Pili •They are filaments of protein that extend from bacterial surface & mediate attachment of bacteria to surface of human cells. • A different kind of pilus, sex pilus, functions in conjugation. Batterjee Medical College Structure of Bacterial Cells Structures External to the Cell Wall Spores • They are highly heat resistant & are not killed by many disinfectants. • Boiling will not kill spores. • They are formed by certain gram-positive rods, especially Bacillus & Clostridium species. • They have a thick, keratinlike coat that allows them to survive for many years, especially in the soil. • Spores are formed when nutrients are in short supply • Spores germinate to form bacteria that can cause disease when nutrients are restored. • They are metabolically inactive but contain DNA, ribosomes & other essential components. Batterjee Medical College Structure of Bacterial Cells Structures External to the Cell Wall Spores