Pharmaceutical Microbiology Lecture 2 PDF

Pharmaceutical Microbiology 2024/2025 Lecture (2) 1.2. Glycopeptides: Two agents, vancomycin and teicoplanin, belong to this group. They are large molecules inhibit cell wall synthesis at the second and third st...

Pharmaceutical Microbiology 2024/2025 Lecture (2) 1.2. Glycopeptides: Two agents, vancomycin and teicoplanin, belong to this group. They are large molecules inhibit cell wall synthesis at the second and third stages. They bind directly to the D-alanyl-D-alanine portion of the pentapeptide leading to blocking transglycosylation and transpeptidation. The effect is the same as with β-lactams, prevention of peptidoglycan cross-linking. Glycopeptide antibiotics prevent glycosylation and transpeptidation. Both agents are not active against gram-negative bacteria because their molecules are too large to penetrate the outer membrane of gram-negative bacteria. They are bactericidal but are primarily active only against gram-positive bacteria. Their main use has been against serious infections by multi-resistant gram- positive bacteria including those caused by strains of staphylococci (MRSA/MRSE) or in individuals with serious allergy to penicillins. Both agents are not active orally, they are used orally in treatment of Clostridium difficile infections of the bowel. A group of lipoglycopeptides antibiotics are considered derivatives of the glycopeptide agents which comprises Telavancin (short acting) and more recently Dalbavancin and Oritavancin (Long acting). They have long half-lives (150-250 h) They are prescribed for the treatment of acute bacterial skin and soft tissue infection. Two doses per a week and once a week are the prescribed doses for these drugs respectively. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 1 Pharmaceutical Microbiology 2024/2025 1.3. Miscellaneous cell wall inhibitors: 1.3.1. Fosfomycin Inhibits the first step in peptidoglycan synthesis by inhibiting the formation of NAM from NAG, limited use for lower urinary tract infection (cystitis). Has high incidence of developing bacterial resistance. 1.3.2. Cycloserine Affects peptidoglycan synthesis by inhibiting enzymes responsible for the formation of D- alanine from L-alanine in the bacterial cytoplasm, thus preventing the formation of D-alanyl-D-alanine dipeptide. It is used as a second-line drug in the treatment of tuberculosis (i.e. its use is only considered if one or more first-line drugs cannot be used). One reason for limited use of this drug is the neurological side effects it causes, since it is able to penetrate into the central nervous system (CNS). 1.3.3. Bacitracin It prevents the dephosphorylation of the phospholipid (Bactoprenol) that carries the peptidoglycan subunit across the cell membrane. This blocks the regeneration of the lipid carrier and inhibits cell wall synthesis. Bacitracin is useful in the treatment of superficial skin infections but too toxic for systemic use (can cause kidney damage). Site of action of both cycloserine and bacitracin 1.3.4. Isoniazid Works by interfering with the synthesis of mycolic acid a necessary component of the cell wall of acid-fast organisms. It is used to treat infections with Mycobacterium tuberculosis. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 2 Pharmaceutical Microbiology 2024/2025 2. Antibacterial Drugs that Affect the Bacterial Cell Membrane Few antimicrobial compounds act on the cell membrane, because the structural and chemical similarities of bacterial and human cell membranes make it difficult to provide sufficient selective toxicity. 2.1. Polymyxins The clinically most useful compounds are polymyxin B and E (colistin). Polymyxins are cyclic peptides with hydrophobic tail; they have positively charged free amino groups, which act like a cationic detergent to disrupt the phospholipid structure of the cell membrane. Effect of polymyxin on the bacterial cell membrane Polymyxin binds to the cell membrane and forms abnormal openings that cause the membrane to become leaky. They are active against gram-negative bacteria due to their specificity for the negatively charged phosphate groups on lipopolysaccharide molecule that exists within many gram-negative outer membranes. Polymyxin antibiotics are relatively neurotoxic and nephrotoxic, so are usually used only as a last alternative if modern antibiotics are ineffective or are contraindicated. Typical uses are for infections caused by strains of multiple drug- resistant Pseudomonas aeruginosa or carbapenemase-producing Enterobacteriaceae. Polymyxins are not absorbed from the gastrointestinal tract. They are used in infantile diarrhea (colistin) and are used externally as a cream or drops to treat many infections as eye infections and otitis externa (swimmers’ ear). Polymyxins have less effect on gram-positive organisms and are sometimes combined with other agents (as with trimethoprim/polymyxin) to broaden the effective spectrum. 2.2. Daptomycin It is a lipopeptide antibiotic used in the treatment of systemic and life- threatening infections caused by gram-positive bacteria only. Daptomycin has a distinct Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 3 Pharmaceutical Microbiology 2024/2025 mechanism of action, disrupting multiple aspects of bacterial cell membrane function. It aggregates into the cell membrane and alters the curvature of the membrane, which creates holes that leak ions. This causes rapid depolarization and cell death. It is not an orally absorbed drug, and it cannot penetrate the outer membrane of gram- negative bacteria. Mechanism of action of daptomycin Daptomycin is approved for use in adults for skin and skin structure infection caused by gram-positive infections, S. aureus bacteremia, and endocarditis. It has a unique spectrum against MRSA and Vancomycin-Resistant E. faecalis and E. faecium (VRE). 3. Antibacterial Drugs that Interfere with Protein Synthesis Several drugs inhibit protein synthesis in bacteria without significantly interfering with protein synthesis in human cells. This selectivity is due to the differences between bacterial and human ribosomal proteins, RNAs, and associated enzymes. Bacteria have 70S ribosomes with 50S and 30S subunits, whereas human cells have 80S ribosomes with 60S and 40S subunits. They are generally broad- spectrum antibacterial drugs, this class includes: aminoglycosides, tetracyclines, chloramphenicol, macrolides/ketolides, lincosamides, oxazolidinones, and streptogramins. The protein synthesis (translation step) is divided into four phases. a- Initiation: The formation of a complex between mRNA, two ribosomal subunits and the tRNA carrying the initiating amino acid. b- Elongation: A repeated cycle of amino acyl tRNA binding, addition of an amino acid to the newly formed chain, and release of tRNA. c- Translocation: Movement of the ribosome through the mRNA. d- Termination: Stop of protein synthesis and release of the complete polypeptide. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 4 Pharmaceutical Microbiology 2024/2025 Sites of inhibition on the prokaryotic ribosome and major antibiotics that act on these sites. All have the general effect of blocking protein synthesis. Blockage actions are indicated by × Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 5 Pharmaceutical Microbiology 2024/2025 Mode of action of antibiotics that inhibit protein synthesis Antibacterial agent Ribosomal Mode of action Effect on subunit bacteria Aminoglycosides 30S Blocks function of Bactericidal initiation complex and causes misreading of mRNA Tetracyclines 30S Blocks tRNA Bacteriostatic binding to ribosome Chloramphenicol 50S Blocks Both* peptidyltransferase Macrolides /ketolides 50S Blocks Bacteriostatic translocation Lincosamides 50S Similar to Bacteriostatic macrolides Oxazolidinones 50S Blocks early step in Both* ribosome formation Streptogramins 50S Causes premature Both* (Quinupristin/Dalfopristin)! release of peptide chain * Can be either bactericidal or bacteriostatic, depending on the organism and the drug concentration. ! Dalfopristin binds to the 23S portion of the 50S ribosomal subunit, and changes the conformation of it, enhancing the binding of quinupristin by a factor of about 100. Quinupristin binds to a nearby site on the 50S ribosomal subunit and prevents elongation of the polypeptide, as well as causing incomplete chains to be released. Spectrum of activity of antibacterials that inhibit protein synthesis*** Agents Activity Comments 3.1. Aminoglycosides : Streptomycin (O, IV) Tuberculosis, Ototoxic and nephrotoxic. brucellosis Gentamycin (T) and Staphylococci; Most widely used topically tobramycin (IV, T) enterobacteriaceae and as eye preparations. including Pseudomonas aeruginosa Neomycin (O, T) Preoperative bowel Too toxic to be used preparation systemically; use orally since not absorbed. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 6 Pharmaceutical Microbiology 2024/2025 3.2. Tetracyclines: (tetracyclin, minocycline, gram-positive and Orally absorbed but chelated and doxycyclin) (O, T) negative rods and cocci by some foods. aerobics and anaerobics Limit use in pregnancy and and some intracellular children because of calcium bacteria (Mycoplasma, chelation, impairment of fetal Rickettsia, Chlamydia) long bone growth and dental discoloration. Tigecycline (IV) Wide spectrum as others As other tetracyclines (Tygacil) ® including those of (category D for pregnant multidrug resistant, as women) infection caused by Carbapenems Resistant Enterobacteriaceae (CRE) 3.3. Chloramphenicol: (O, I, T) Broad spectrum as Readily absorbed and tetracyclines may use in diffused into most body Haemophilus influenzae, compartments including CSF. meningitis, typhoid Bone marrow suppression fever, and Bacteroides and aplastic anemia limit use fragilis. to sever infections. 3.4. Macrolides/ketolides: (Erythromycin; Most gram-positive and Erythromycin was the first azithromycin; (O, IV) some gram-negative agent. clarithromycin (O)) including Neisseria, Others have enhanced gram- Bordetella, negative spectrum and have Campylobacter and longer half-life specially Legionella but not azithromycin. Clarithromycin enterobacteriaceae. used in combinations with other drugs to treat H pylori. Telithromycin (ketolides) Used for community Many bacteria that are (O)(Ketek)® acquired pneumonia resistant to macrolides are caused by various susceptible to telithromycin. bacteria including multi- drug resistant pneumococci. 3.5. Lincosamide: (Clindamycin) (O) Chemically unrelated to Pseudomembranous colitis* the macrolides but has is a major side effect. similar mode of action Common topical treatment of and spectrum, has great acne. Also used in dental Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 7 Pharmaceutical Microbiology 2024/2025 activity against infections. Penetrates bone anaerobes, both gram- better than most antibiotics positive as Clostridium so, used in osteomyelitis. perfringens and gram- negative as Bacteroides fragilis. 3.6. Oxazolidinones (Linezolid) (O) (Zyvox)® Clinically useful in High oral absorption even pneumonia and other soft with food. tissue infections, mainly Relatively safe drug when those caused by resistant taken for short time. No strains of staphylococci; cross- resistance due to pneumococci; and unique site of action. enterococci. 3.7. Streptogramins: (Quinupristin/dalfopristin) Used in fixed No cross-resistance between (IV) (Synercid)® combination, used for streptogramins and other treatment of infection by drugs that inhibit protein vancomycin-resistant synthesis. Enterococcus Faecium (VRE) and vancomycin resistant Staphylococcus aureus (VRSA) O; oral IV; Intravenous T; Topical *pseudomembranous colitis is colitis (inflammation of the large intestine) resulting from infection with Clostridium difficile. C. difficile releases toxins that may cause bloating and diarrhoea, with abdominal pain, which may become severe. 4. Antibacterial Drugs that Inhibit DNA or RNA 4.1. Quinolones: The quinolones are synthetic broad-spectrum antibacterial agents. They have a core of two fused six-member rings (quinolone ring) that when substituted with fluorine become fluoroquinolones which are now the main quinolones for bacterial infections. They inhibit DNA replication by inhibiting two enzymes involved in bacterial DNA synthesis, which are DNA topoisomerase II (or DNA gyrase) and topoisomerase IV that human cells lack and that are essential for bacterial DNA replication, thereby enabling these agents to be both specific and bactericidal. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 8 Pharmaceutical Microbiology 2024/2025 General Structure of Fluroquinolones Mechanism of action of fluoroquinolones The quinolones can be classified into four generations based on antimicrobial activity. Generation Antimicrobial spectrum Clinical use First: gram- negative organisms Uncomplicated urinary tract Nalidixic acid (not Pseudomonas species) infections, little used Cinoxacin (carcinogenic) Second: As the first plus Uncomplicated and complicated Norfloxacin, Pseudomonas species, UTI, pyelonephritis, sexually lomefloxacin, some gram-positive but transmitted diseases, prostatitis, Enofloxacin, ofloxacin, not S. pneumoniae skin and soft tissue infection. ciprofloxacin Third: Same as for second plus Acute Exacerbation of Chronic Levofloxacin, expanded gram-positive Bronchitis (AECB), community- Sparfloxacin. coverage including acquired pneumonia (CAP). penicillin sensitive and resistant S. pneumoniae Fourth: Same as for third Same as previous generations Moxifloxacin generation plus broad (excluding UTI) plus abdominal anaerobic coverage. infections, nosocomial pneumonia, pelvic infections. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 9 Pharmaceutical Microbiology 2024/2025 4.2. Rifampicin and rifabutin They are bactericidal antibiotics from rifamycin group, they inhibit RNA by inhibiting bacterial RNA-polymerase, thereby preventing the production of mRNA, without affecting RNA polymerase of human cells. Rifampicin or rifampin is used primarily for the treatment of tuberculosis in combination with other drugs and for prophylaxis in close contacts of patients with meningitis caused by either N. meningitidis or H. influenzae. Rifampicin is given in combination with other drugs because resistant mutants appear at a high rate when it is used alone. It is red, and the urine, saliva, and sweat of patients taking rifampicin often turn orange, it is excreted in high concentration in saliva, which accounts for its success in the prophylaxis of bacterial meningitis since the organisms are carried in the throat. 5. Antibacterial Drugs that Affect Metabolic Pathways Sulfonamides and trimethoprim are synthetic drugs that act by competing the normal substrate of an enzyme in a process called competitive inhibition, they interfere with folate metabolism by blocking enzymes required for the synthesis of tetrahydrofolate, which is needed by bacterial cells for the synthesis of folic acid and the eventual production of DNA and RNA and amino acids. Trimethoprim and sulfonamides are orally active and often given simultaneously to achieve a bactericidal synergistic effect. How sulfa drugs and trimethoprime block the metabolic pathway bacteria use to synthesis folic acid Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 10 Pharmaceutical Microbiology 2024/2025 The selective toxicity of these compounds is explained by the fact that mammals derive folic acid from their diet and so do not possess this enzyme system. Sulfonamides are not used systemically without other drugs, but it is highly soluble and penetrates exceptionally well into the eye, so it is still widely used in the form of eye drops and eye ointment. An oral combination of trimethoprim-sulfamethoxazole is clinically useful in treatment of UTIs, Pneumocystis pneumonia, and shigellosis. Anti-mycobacterial (Anti-tuberculous) drugs Tuberculosis, the disease caused by Mycobacterium tuberculosis, is one of the world’s most challenging infections. Tuberculosis and other mycobacteria diseases are difficult to treat for several reasons 1. Mycobacteria replicate more slowly than “typical” bacteria. 2. Mycobacteria can also exist in a dormant state. 3. They live inside human cells. 4. The outermost layer of mycobacteria consists of phospholipids and mycolic acids that make a waxy layer that resists penetration from antibiotics. 5. Low defense mechanism of the host. Combinations of drugs are always given for patients with active disease to minimize the development of resistance and shorten the duration of therapy The most widely used drugs are: 1- First-line drugs: Are used in the beginning of the treatment, all are given orally. a- Rifampicin: The most potent anti-tuberculous drug available. Liable to cause hepatitis. b- Isoniazid: Also highly effective, liable to cause hepatitis or peripheral neuritis. c- Ethambutol: In low dosage a useful supporting drug. Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 11 Pharmaceutical Microbiology 2024/2025 d- Pyrazinamide: Has good penetration into cerebrospinal fluid and is thus particularly useful in tuberculous meningitis. 2- Secondary drugs: Are less frequently used because of their side effects. They include: a- Streptomycin: Principal reserved drug after the first line drugs. It has the disadvantage that it has to be injected. b- Thiacetazone: limited use due to its side effects which include hepatitis and hemolytic anemia used mainly to prevent resistance to more powerful drugs. Principles of management of tuberculosis a) Duration of treatment must be adequate. Short-course chemotherapy using 4 drugs is effective in respiratory tuberculosis (isoniazid, rifampicin, pyrazinamide and ethambutol) for two months followed by isoniazid and rifampicin for further 4 to 6 months. b) The effectiveness of therapy must be assured. Therapy renders the patient noninfectious within 2-3 weeks. c) Therapy must lead to alleviating cough and reducing bacterial numbers in sputum. d) All contacts of the patient must be screened for gaining of the disease (x-ray and microbiological tests). Faculty of Pharmacy- The British University in Egypt (FP-BUE). Page 12