Medical Microbiology Lecture Notes PDF

Medical Microbiology 2 Lecture nd  Microbial metabolism  Microbial metabolism is the means by which a microbe obtains the energy and nutrients (e.g. carbon) it needs to live and reproduce.  There are two types of metabolism: anabolism and catabolism. Anabolism p...

Medical Microbiology 2 Lecture nd  Microbial metabolism  Microbial metabolism is the means by which a microbe obtains the energy and nutrients (e.g. carbon) it needs to live and reproduce.  There are two types of metabolism: anabolism and catabolism. Anabolism processes require energy to produce proteins and nucleic acid while catabolism processes create or release energy 2  3 3 Microbial growth  Microbes grow via binary fission, resulting in exponential increases in numbers  Bacterial “growth” means an increase in the number of cells, not an increase in cell size.  One cell becomes colony of millions of cells  Bacteria grow by binary fission to produce identical offspring, which cannot be distinguished as a parent or offspring  Generation time; is the time it takes for a single cell to grow and divide  Average for bacteria is (1-3 hours)  Escherichia coli: 20 minutes……20 generations (7 hours), 1 cell becomes 1 million cells!  Mycobacterium much slower: (12-24h) 4  5 5 Phases of Growth :Four main growth phases 1. Lag phase 2. Exponential (Log) phase 3. Stationary phase (Post-exponential) 4. Decline (death )phase 6  7 7  8 8  The log phase (sometimes called the logarithmic phase or the exponential phase)  is a period characterized by cell doubling.  The number of new bacteria appearing per unit time is proportional to the present population.  If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles with each consecutive time period. 9  10 10  Exponential growth cannot continue indefinitely, because the medium is soon depleted of nutrients and enriched with wastes  The stationary phase is often due to a growth limiting factor such as the depletion of an essential nutrient, and/or the formation of an inhibitory product such as an organic acid.  Stationary phase results from a situation in which growth rate and death rate are equal. 11  12 12  Mutations can occur during stationary phase. DNA damage is responsible for many of the mutations arising in the genomes of stationary phase or starving bacteria.  Endogenously generated reactive oxygen species appear to be a major source of such damages  At death phase (decline phase), bacteria die. This could be caused by lack of nutrients, environmental temperature above or below the tolerance band for the species, or other injurious conditions 13  14 14 Bacterial Genetics Bacteria have only one copy of their genome DNA (i.e., they are haploid). In contrast, eukaryotic cells have two copies of their genome DNA (i.e., they are diploid). Bacterial DNA is circular; human nuclear DNA is linear Genes are functional units of heredity as they are made of DNA. The chromosome is made of DNA containing many genes. Every gene comprises of the particular set of instructions for a particular function or protein-coding. Genes are responsible for heredity. 46 million bacterial genes :The researchers found nearly 46 million bacterial genes—about 24 million in the mouth microbiome and 22 million in the gut microbiome.  There are numerous bacteria found on planet earth. They divide quickly by binary fission producing identical daughter cells. Thus, the genetic information is transferred from the mother to the offspring and is known as vertical transmission. The mutations are transferred from one bacteria to another through horizontal transmission. There are three different types of horizontal transmission for the transfer of genetic information. 1. Conjugation 2. Transduction 3. Transformation Bacterial Conjugation Conjugation is the method of transfer of genetic material from one bacteria to another placed in contact. The F-factor can move between E.coli cells. Mechanism of Bacterial Conjugation 1-Pilus Formation The donor cells (F+ cells) form a sex pilus and begin contact with an F- recipient cell. 2-Physical Contact between Donor and Recipient Cell.The pilus forms a conjugation tube and enables direct contact between the donor and the recipient cells. 3-Transfer of F-Plasmid The F-factor opens at the origin of replication. One strand is cut at the origin of replication, and the 5’ end enters the recipient cell. 4-Synthesis of Complementary Strand The donor and the recipient strand both contain a single strand of the F-plasmid. Thus, a complementary strand is synthesized in both the recipient and the donor. The recipient cell now contains a copy of F plasmid and becomes a donor cell. Bacterial Transduction Transduction is the process of transfer of genes from the recipient to the donor through bacteriophage. 1. The bacteriophage attaches itself to the host bacterial cell. 2. The bacteriophage then injects its DNA into the host cell. 3. Enzymes from the bacteriophage break down the host DNA. 4. Replication of the bacteriophage occurs, and new ones are created which may include some of the host's DNA. Bacterial Transformation 1. Transformation is the process of DNA uptake by the bacteria from the surrounding environment. The cells that have the ability to uptake DNA are known as competent cells. This process was first reported in Streptococcus pneumonia. Antibiotic and chemotherapy Antibiotics are medicines that fight bacterial infections in people and animals. They work by killing the bacteria or by making it hard for the bacteria to grow and multiply. Antibiotics can be taken in different ways: Orally (by mouth). This could be tablets , capsules, or liquids. Injections ( IM, IV ….) or locally (skins ). For an antibiotic to be clinically useful, it must exhibit selective toxicity (i.e., it must inhibit bacterial processes significantly more than it inhibits human cell processes). There are four main targets of antibacterial drugs: cell wall, ribosomes, cell membrane, and nucleic acids. Human cells are not affected by these drugs because our cells do not have a cell wall, and our cells have different ribosomes, nucleic acid enzymes, and sterols in the membranes. Bactericidal drugs kill bacteria, where as bacteriostatic drugs inhibit the growth of the bacteria but do not kill. 1-Inhibition of Cell Wall Synthesis Penicillins and cephalosporins act by inhibiting transpeptidases, the enzymes that cross-link peptidoglycan. Trans-peptidases are also referred to as penicillin-binding proteins.. Exposure to penicillins activates autolytic enzymes that degrade the bacteria. If these autolytic enzymes are not activated the bacteria are not killed and the strain is said to be tolerant. Penicillins kill bacteria when they are growing (i.e., when they are synthesizing new peptidoglycan). Penicillins are therefore more active during the log phase of bacterial growth than during the lag phase or the stationary phase. Penicillins and cephalosporins are β-lactam drugs (i.e., an intact β-lactam ring is required for activity). β-Lactamases (e.g., penicillinases and cephalosporinases) cleave the β-lactam ring and inactivate the drug. Hypersensitivity to penicillins, especially IgE-mediated anaphylaxis, remains a significant problem Inhibition of Protein Synthesis Aminoglycosides and tetracyclines act at the level of the 30S ribosomal subunit, whereas chloramphenicol, erythromycins, and clindamycin act at the level of the 50S ribosomal subunit. Aminoglycosides inhibit bacterial protein synthesis by binding to the 30S subunit, which blocks the initiation complex. No peptide bonds are formed, and no polysomes are made. Aminoglycosides are a family of drugs that includes gentamicin, tobramycin, and streptomycin. Tetracyclines inhibit bacterial protein synthesis by blocking the binding of aminoacyl tRNA to the 30S ribosomal subunit. The tetracyclines are a family of drugs; doxycycline is used most often. Chloramphenicol inhibits bacterial protein synthesis by blocking peptidyl transferase, the enzyme that adds the new amino acid to the growing polypeptide. Chloramphenicol can cause bone marrow suppression. Erythromycin inhibits bacterial protein synthesis by blocking the release of the tRNA after it has delivered its amino acid to the growing polypeptide. Erythromycin is a member of the macrolide family of drugs that includes azithromycin and clarithromycin. Clindamycin binds to the same site on the ribosome as does erythromycin and is thought to act in the same manner. It is effective against many anaerobic bacteria. Clindamycin is one of the antibiotics that predisposes to pseudomembranous colitis caused by Clostridium difficile and is infrequently used. Inhibition of Nucleic Acid Synthesis Sulfonamides and trimethoprim inhibit nucleotide synthesis, quinolones inhibit DNA synthesis, and rifampin inhibits RNA synthesis. Sulfonamides and trimethoprim inhibit the synthesis of tetrahydrofolic acid—the main donor of the methyl groups that are required to synthesize adenine, guanine, and thymine. Sulfonamides are structural analogues of p-aminobenzoic acid, which is a component of folic acid. Trimethoprim inhibits dihydrofolate reductase—the enzyme that reduces dihydrofolic acid to tetrahydrofolic acid. A combination of sulfamethoxazole and trimethoprim is often used because bacteria resistant to one drug will be inhibited by the other. Quinolones inhibit DNA synthesis in bacteria by blocking DNA gyrase (topoisomerase)— the enzyme that unwinds DNA strands so that they can be replicated. Quinolones are a family of drugs that includes ciprofloxacin, ofloxacin, and levofloxacin. Rifampin inhibits RNA synthesis in bacteria by blocking the RNA polymerase that synthesizes mRNA. Rifampin is typically used in combination with other drugs because there is a high rate of mutation of the RNA polymerase gene, which results in rapid resistance to the drug. Additional Drug Mechanisms Isoniazid inhibits the synthesis of mycolic acid—a long-chain fatty acid found in the cell wall of mycobacteria. Isoniazid is a prodrug that requires a bacterial peroxidase (catalase) to activate isoniazid to the metabolite that inhibits mycolic acid synthesis. Isoniazid is the most important drug used in the treatment of tuberculosis and other mycobacterial diseases. Metronidazole is effective against anaerobic bacteria and certain protozoa because it acts as an electron sink, taking away the electrons that the organisms need to survive. It also forms toxic intermediates that damage DNA. Antimicrobial drugs are used to prevent infectious diseases as well as to treat them. Chemo prophylactic drugs are given primarily in three circumstances: to prevent surgical wound infections, to prevent opportunistic infections in immunocompromised patients, and to prevent infections in those known to be exposed to pathogens that cause serious infectious diseases. Thank you 32