Bacterial Structure PDF

Bacterial Introduction and Structure Dr. Mohamed Elmutasim Microbiology Department 26 Sep. 2022  Microbiology -Microbiology is the study of microorganisms, which are microscopic, unicellular, and cell-cluster organisms. -Medical microbiology deals with t...

Bacterial Introduction and Structure Dr. Mohamed Elmutasim Microbiology Department 26 Sep. 2022  Microbiology -Microbiology is the study of microorganisms, which are microscopic, unicellular, and cell-cluster organisms. -Medical microbiology deals with the study of microorganisms that are of medical importance and cause diseases in human beings.  Structure of Bacterial Cells Shape & Size The size of most bacteria ranges from 1 to 3 μm. Mycoplasma, the smallest bacteria (smallest cells) are 0.2 μ m. Bacteria have three shapes: - cocci (spheres) - bacilli (rods) - spirochetes (spirals) Cocci are arranged in three patterns: - Pairs (diplococci) - Chains (streptococci) - Clusters (staphylococci) The shape of a bacterium is determined by its rigid cell wall. The microscopic appearance of a bacterium is an important criterion used in its identification.  Bacterial Cell Wall All bacteria have a cell wall composed of peptidoglycan except Mycoplasma, which is 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. Comparison of Cell Walls of Gram-Positive and Gram-Negative Bacteria  Gram stain Gram-positive bacteria stain purple, whereas gram- negative bacteria stain pink. Gram-positive bacteria can retain crystal violet–iodine complex in the presence of lipid solvents such 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.  Peptidoglycan -It is found only in bacterial cells. -It is a network that covers the entire bacterium. -Function: a. gives the organism its shape. b. The site of action of penicillins & cephalosporins -It is composed of a sugar backbone (glycan) and peptide side chains (peptides). The side chains are cross-linked by transpeptidase (The site of action of penicillins & cephalosporins which inhibit transpeptidase enzyme)  The outer membrane of gram-negative bacteria Endotoxin (lipopolysaccharide, LPS): Function: The main inducer of septic shock. Endotoxin consists of: - lipid A fever & hypotension in septic shock - polysaccharide(O antigen) use in laboratory identification  The outer membrane of gram-negative bacteria Periplasmic space: -It lies between inner cell membrane & outer membrane of gram-negative bacteria Function: -It is the location of β–lactamases-the enzymes that degrade β -lactam antibiotics, such as penicillins and cephalosporins.  The membrane of bacteria Lysozymes -They kill bacteria by cleaving the glycan backbone of peptidoglycan. -The cytoplasmic membrane of bacteria : -It consists of a phospholipid bilayer (without sterols) located just inside peptidoglycan. -Function It regulates: - Active transport of nutrients into cells. - Secretion of toxins out of the cell.  Cell Walls of Acid-Fast Bacteria -Mycobacteria are acid-fast because they resist decolorization with acid–alcohol after staining with carbolfuchsin. -This is due to the high concentration of lipids in the cell wall. -The dyes used in the Gram stain do not stain mycobacteria. However, they are stained by Ziehl Neilson (ZN).  Bacterial DNA The bacterial genome: It consists of a single chromosome of circular DNA located in a nucleoid. Function: Carry genetic material. Plasmids: They are extrachromosomal pieces of circular DNA Function: encode both exotoxins & enzymes antibiotic resistance Transposons (Jumping genes): They are small pieces of DNA that move between chromosomal DNA and plasmid DNA. Function: They carry antibiotic-resistant genes.  Structures External to the Cell Wall Capsules: -They are composed of polysaccharides (the polypeptide capsule of anthrax bacillus is an exception). Function: a. They are antiphagocytic, i.e., they limit the ability of neutrophils to engulf bacteria. Antibodies against the capsule neutralize the antiphagocytic effect & allow bacteria to be engulfed by neutrophils. Opsonization is the process by which antibodies enhance the phagocytosis of bacteria. b. Capsules are antigens in several vaccines, such as pneumococcal vaccine. c. Specific identification made by using antiserum against the capsular polysaccharide.  Structures External to the Cell Wall The glycocalyx: It is a polysaccharide "slime layer" secreted by certain bacteria. Function: 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 the bacterial surface. Function: a. mediate attachment of bacteria to surface of human cells. b. A different kind of pilus, sex pilus, functions in conjugation.  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. Spores Thank You