Bacterial Metabolism and Growth Lecture 3 PDF

**[Bacterial Metabolism and Growth]** What affects the way bacteria produce disease? - Growth requirements and types of growth exhibited by pathogenic bacteria. Example: where and how quickly? Why is it important to know that specific bacteria have different growth requirements? - so tha...

**[Bacterial Metabolism and Growth]** What affects the way bacteria produce disease? - Growth requirements and types of growth exhibited by pathogenic bacteria. Example: where and how quickly? Why is it important to know that specific bacteria have different growth requirements? - so that specimens are collected, processed, and cultured appropriately. What is the process of bacterial growth? - Bacteria reproduce by binary fission: one cell divides to form 2 progeny cells. - Exponential (logarithmic) growth: One bacterial cell produces 16 cells after just 4 generations - Examples: - Some bacteria, like E. coli, which can cause diarrhea, have a generation time of 20 minutes....a short generation time + exponential growth. - Some bacteria, like Mycobacterium tuberculosis, have a longer generation time; this can also cause big trouble, but it takes longer. What are the different forms of bacterial growth in the laboratory? - As colonies on a solid nutrient medium. - In a broth culture in suspension. - In biofilms in which growth is spread over an inert surface and nutrients are obtained from a fluid that bathes the surface. What are the 4 phases of bacterial growth cycle? - Lag phase: Period of adaptation to environment and preparation for cell division with high metabolic activity, no cell division. - Log phase: Period of rapid cell division and production of cell constituents, including peptidoglycan. β--lactam drugs are most effective during this phase. - Stationary phase: Growth slows due to nutrient depletion and accumulation of toxic products; cell number in a steady state due to a balance of cell division and cell death. - Death phase: Number of viable bacteria declines. - Note: Gene expression can vary greatly during these different phases, e.g. cells in stationary phase are often more resistant to stresses than cells in log phase. What are the requirements for bacterial growth and reproduction? - Bacteria cannot reproduce without a source of energy and the building blocks of cellular constituents (e.g. amino acids, carbohydrates, lipids). - Minimum requirements for growth are a source of carbon and nitrogen, an energy source, water and various ions. Iron is also an important requirement for most bacteria. - Growth requirements and products of metabolism are often used to classify bacteria - Need to know these specific requirements to culture successfully for lab diagnostic tests. - Some bacteria are obligate intracellular pathogens and cannot be cultured in the absence of a eukaryotic cell host. - Growth of most medically important organisms is enhanced by the presence of oxygen (Aerobic and Anaerobic Growth) - Oxygen acts as the hydrogen acceptor in the final stages of energy production - Catalyzed by the flavoproteins and cytochromes of the electron transport system What is catabolism and anabolism? - Bacterial cells require a source of energy to survive. Processes of catabolism and anabolism are tightly coordinated and referred to together as intermediary metabolism. - Catabolism- Process of breaking down organic substrates (carbohydrates, lipids, proteins). - Anabolism- Process of synthesizing cellular constituents (cell walls, proteins, fatty acids, nucleic acids) What is the difference between Aerobic and Anaerobic I energy production? - Aerobic uses TCA and ETC to produce energy - Aerobic growth produces 2 toxic molecules that can generate hydroxyl radicals which can damage proteins, DNA, lipids, etc. - Cells have specific enzymes to deal with these toxic molecules: - Superoxide (O2-) is converted to oxygen and hydrogen peroxide by the enzyme, superoxide dismutase. - Hydrogen peroxide (H2O2) is broken down into water and oxygen by the enzyme, catalase. - Anaerobic uses fermentation to produce energy - Fermentation is the process by which pyruvic acid is converted to energy (ATP). - Both facultative and anaerobic bacteria ferment, but obligate aerobes do not. - Fermentation tests in the clinical laboratory are used to identify certain bacteria. - Fermentation of sugars results in production of ATP and pyruvic or lactic acids. - The acids lower the pH of the medium which is detected by a change in color of indicator dyes. What are the different types of aerobic and anaerobic bacteria? - Obligate aerobes: require oxygen for growth. Ex. Mycobacterium tuberculosis, Nocardia - Obligate anaerobes generally do not have catalase nor superoxide dismutase - Strict anaerobes: cannot grow in the presence of oxygen (usually \> 10%). Ex. Clostridium, Bacteroides - Facultative anaerobes: can grow in the presence or absence of oxygen. Ex. Staphylococcus, Streptococcus What is the process of bacterial biosynthesis? - Bacterial metabolism results in products that are used for the synthesis of cellular constituents, like peptidoglycan and LPS. - Since bacteria do not have a nuclear membrane, transcription and translation are coupled. - Bacterial cytoplasmic membrane is the site for important function like electron transport and energy production. How is peptidoglycan synthesized? - Synthesis of peptidoglycan occurs in 3 phases, each of which can be inhibited by various antibiotics: - Phase 1 (Inside the cell): Precursor subunits are synthesized and assembled. - Phase 2 (membrane): At the membrane the units are attached to the bactoprenol. - Phase 3 (Outside the cell): Peptidoglycan units are attached to and crosslinked into the peptidoglycan polysaccharide. How is peptidoglycan synthesis inhibited? - Crosslinking of the tetrapeptide chains between the NAM-NAG glycan chains of peptidoglycan is catalyzed by enzymes called transpeptidases. These enzymes are also called penicillin-binding proteins (PBP) because they are the target of β-lactam antibiotics, such as penicillin, which bind to them. Transpeptidases are bound to the cell membrane of Gram-positive and Gram-negative bacterial cells. - Antibiotic, vancomycin also inhibits the transpeptidation crosslinking reaction using a different mechanism than β-lactam antibiotics. - Bactoprenol is normally recycled; the antibiotic, bacitracin, inhibits the re-use of bactoprenol. How is LPS synthesized? - Lipid A and core portions are enzymatically synthesized at the inside surface of the cytoplasmic membrane. - Repeating units of the O antigen are assembled on a bactoprenol molecule and transferred to a growing O antigen chain. - Completed O antigen chain is transferred to the core lipid A structure. - LPS molecule is then translocated through adhesion sites to the outer surface of the outer membrane.