Antimicrobial Therapy, Resistance, and Resistance Testing PDF 2025

Introduction to antimicrobial therapy Mechanisms of attacking bacteria Dr I. Gobe Dr I. Gobe SoAHP, UB  Definition of Antimicrobial Agents ◼ Antimicrobial agents: o Agents that kill micro-organisms or suppress multiplication or...

Introduction to antimicrobial therapy Mechanisms of attacking bacteria Dr I. Gobe Dr I. Gobe SoAHP, UB  Definition of Antimicrobial Agents ◼ Antimicrobial agents: o Agents that kill micro-organisms or suppress multiplication or growth micro-organisms (Medical dictionary) o chemicals or medicines used to treat infections caused by microbes. o include anti-infective agents i.e. antibacterial medicines/antibiotics, antifungal agents, antiviral agents (and antiparisitic medicines). o May be derived from natural or synthetic sources. o Some are derived from micro-organisms e.g. Actinomyces – Strepotomyces spp. (erythromycin, amphotericin B, tetracycline, gentamycin, streptopmycin or fungi (Cephalosporium spp, Penicillum spp (penicillin, griseofulvin) o Antibiotics/antibacterial medicines – agents used to treat bacterial infections. What are the targets of action of antimicrobial agents? ◼ OR, “5 ways to attack bacteria” Dr I. Gobe SoAHP, UB  Modes of Action Fig. 13-2 Dr I. Gobe SoAHP, UB  Inhibition of Cell Wall Synthesis ▪Mode of action? bind to enzymes that cross-link cell wall components transpeptidases aka “Penicillin Binding Proteins” ▪Faulty cell wall forms → collapses → cell death due to Role of Cell wall: osmotic shock structural support prevents cell lysis Can Penicillin affect a cell wall that is already formed? Dr I. Gobe SoAHP, UB Inhibitors of Cell Wall Synthesis ▪The β-lactams Penicillins Cephalosporins Monobactams Carbapenems ▪Glycopeptides: Vancomycin binds peptide side chains in Peptidoglycan→ prevents wall formation Too large to cross OM pores → Gram negatives are intrinsically resistant ▪Bacitracin blocks export of PG subunit across cell membrane inhibits “bactoprenol action Dr I. Gobe SoAHP, UB Antimicrobials that disrupt cell membranes: ▪Polymyxins Bind phospholipids in cell membranes → disrupts both cell membrane AND outer membrane → cell death ▪Lipopeptides (Daptomycin) insert into plasma membrane→ cell death Dr I. Gobe SoAHP, UB  What does polymyxin do? ▪ Binds to phospholipids in cell membranes ▪ Disrupts both cell membrane AND outer membrane ▪ Gram negative bacteria! ▪ Disrupt barrier → cell Fig. 4-7 b) death Bactericidal antimicrobials cause cell death ▪ Topical use only! Bacteriostatic antimicrobials only inhibit cell metabolism and growth Dr I. Gobe SoAHP, UB  Sulfanilamide: ▪ How does sulfanilamide show Selective toxicity? ▪ Ie) why does it affect bacteria but not us??? Dr I. Gobe SoAHP, UB Inhibition of Metabolism: blocking Folate Synthesis ▪Sulfonamides & Trimethoprim Inhibit action of enzymes in folic acid synthesis pathway → block purine formation → block DNA synthesis. Dr I. Gobe SoAHP, UB Antimicrobial resistance Dr I. Gobe School of Allied Health professions University of Botswana  What is Antibiotic Resistance? ◼Ability of a microorganism to remain unharmed by an antimicrobial agent ◼ How do bacteria become antibiotic resistant? ◼Intrinsic resistance independent of previous antibiotic exposure, ◼Acquired resistance: adaptation process or gene transfer  Mechanisms of Antibiotic Resistance ◼Intrinsic Mechanisms of ◼Acquired Mechanisms of Resistance are more predictable Resistance → (less ◼Impermeability predictable) ◼Biofilms ◼Target Site modification: ◼Efflux chromosomal mutations ◼Horizontal gene ◼Enzymatic Inactivation transfer/uptake of resistance gene  Enzymatic Inactivation ▪ e.g Resistance to beta lactam antibiotics Production of Beta-lactamases; gram negative: ESBL extended-spectrum β-lactamases (ESBLs) β-lactamases that hydrolyse penicillin, oxyimino- Enzymatic modification of cephalosporins, and monobactams antibiotic → inactivation ESBLs are often plasmid- mediated enzymes. Klebsiella pneumoniae, K. oxytoca and E. coli also Acinetobacter baumannii, Proteus spp, Pseudomonas aeruginosa and Salmonella spp Altering Target of the antibiotic: MRSA MRSA methicillin-resistant S. aureus due to expression of penicillin- binding protein 2a (PBP2a), a transpeptidase that catalyzes cell-wall crosslinking in the face of the challenge by β-lactam antibiotics. resistance to all beta-lactam antibiotics.. The mecA gene encodes for an alternative penicillin binding protein 2a (PBP-2a) with low affinity to β-lactam mecA is acquired and transmitted through a mobile genetic element,  Altering target of the antibiotic : MDR TB The rpoB gene codes for the RNA polymerase b subunit, which is the target of rifampicin, an essential drug in the treatment of tuberculosis Mutations in the rpoB gene alter the structure of this protein and cause drug resistance. 95% of rpoB mutations occur within a small 81-bp region named the rifampicin- resistance determining region (RRDR) molecular detection of rifampicin resistance is used as a surrogate marker of MDR-TB in many countries, since over 90% rifampicin resistant cases are also resistant to isoniazid. MDR- TB 1. resistance to rifampin is caused by mutations leading to a change in the structure of the beta subunit of RNA polymerase. MDR TB is caused by TB bacteria that are resistant to at least isoniazid and rifampin, the two most potent TB drugs. Biofilms clusters of microorganisms that stick to non-biological surfaces, such as rocks in a stream, as well as to surfaces on plants (roots) or in animals (epithelium). Bacterial biofilms are resistant to antibiotics, disinfectant chemicals and to phagocytosis E.g persistence of staphylococcal infections related to foreign bodies is due to biofilm formation  Efflux pumps ▪Decreased Uptake and/or increased efflux Net effect: reduce antibiotic concentrations inside cell Ex) active transport of quinolones out of cell (Staphylococci) Efflux: active transport of antimicrobial agents out of cell → resistance  Gene transfer. ▪Resistance gene transfer Conjugation transfer of plasmids or transposons between bacteria Transformation uptake of DNA fragments from environment Transduction bacteriophage transfer Mobile genetic elements include genes from one cell to plasmids & transposons another End of lecture  Antibiotic Susceptibility Testing ▪ Determine antibiotic susceptibility or resistance  Bacterial Culture Grow and isolate bacteria ▪ Pros Various media Identify the isolate Identify a pathogen Very specific ▪ Cons Lengthy Normal flora Biochemical Testing: Identification Identification Checking for antibiotic resistance in the lab ▪Aka Kirby-Bauer test ▪Principle of Test: Diffusion of antibiotic from disk → concentration gradient → measure zone of growth inhibition Conventional methods: Disk Diffusion ▪Interpretation Looks for Zone of inhibition CLSI standards ▪Susceptible, Intermediate or Resistant Clinical lab standards institute CLSI Case 1: Report  A question of selective toxicity…. ◼ The antibiotic Amphotericin B disrupts plasma membranes by combining with sterols; it will affect all of the following cells except: ◼ a) animal cells ◼ b) bacterial cells ◼ c) fungal cells ◼ d) Mycoplasma cells Dr I. Gobe SoAHP, UB Factors contributing to antibiotic resistance Environmental factors Drug related factors Patient related factors Prescriber related factors  Thank you Questions??? Slides that follows are reading assignment. Dr I. Gobe SoAHP, UB  Ideal Properties of Antimicrobial Agents ◼ Selective toxicity against pathogen (and not host and environment) ◼ Spectrum activity – broad spectrum vs narrow spectrum ◼ Bactericidal vs bacteriostatic ◼ Fast action ◼ Effectiveness – good penetration at site of action (including solid sites) ◼ Favourable pharmacokinetics – delivered to site of action in effective concentration  Ideal Properties of Antimicrobial Agents ◼ Minimal risk of toxicity/adverse drug effects ◼ Favourable therapeutic index = large ratio of effective dose to toxic dose ◼ Limited risk for developing resistance ◼ Cost-effective ◼ Stable during storage (no loss of potency & no degradation) No antimicrobial agent is ideal  Some Basic Features of antimicrobial Agent activity ◼Antibiotic must be able to reach the appropriate concentration at target location in body ◼Agent must attach to bacterial cell → enter & bind target  Anatomic distribution of some common antibacterial agents (examples) serum CSF urine Ampicillin + + + Vancomycin + +/- + Ciprofloxacin + +/- + Gentamicin + - + Clindamycin + - - Nitrofurantoin - - + Certain agents have anatomic limitations: knowledge of sites of infection → drug selection  Bacteriostatic Versus Bactericidal ◼ Bacteriostatic ( Growth inhibition): cause a reversible inhibition of growth, with bacterial growth restarting after elimination of the drug ◼ Bactericidal (cell death); kill their target bacteria. ◼ The decision of whether to use a bacteriostatic or bactericidal drugs depends on the type of infection and the immune status of the patient. ◼ when a patient is immunocompromised, a bactericidal drug is essential for the successful treatment of infections. ◼ Regardless of the immune status of the patient, life- threatening infections such as acute endocarditis require the use of a bactericidal drug.  spectrum of activity ◼ relates to diversity of targeted bacteria. ◼ A narrow-spectrum antimicrobial targets only specific subsets of bacterial pathogens eg Gram pos bacteria only ◼ A broad-spectrum antimicrobial targets a wide variety of bacterial pathogens, including both gram-positive and gram- negative species  Dosage and Route of Administration ◼ The amount of medication given during a certain time interval is the dosage, and it must be determined carefully to ensure that optimum therapeutic drug levels are achieved at the site of infection without causing significant toxicity ◼ the goal is to select the optimum dosage that will minimize the risk of side effects while still achieving clinical cure. ◼ Body mass ◼ Metabolism and elimination ◼ Oral/intramuscular/intravenous  Drug Interactions ◼ synergistic interaction: two antibacterial drugs may be administered together:better than the efficacy of either drug alone. E.g trimethoprim and sulfamethoxazole (Bactrim). Individually, these two drugs provide only bacteriostatic inhibition of bacterial growth, but combined, the drugs are bactericidal. ◼ antagonistic interactions: may cause loss of drug activity, decreased therapeutic levels due to increased metabolism and elimination, or increased potential for toxicity due to decreased metabolism and elimination.E.g: some antibacterials are absorbed most effectively from the acidic environment of the stomach. If a patient takes antacids, however, this increases the pH of the stomach and negatively impacts the absorption of these antimicrobials https://bio.libretexts.org/Courses/Portland_Community_College/Cas cade_Microbiology/17%3A_Antimicrobial_Drugs/17.2%3A_Propertie s_of_Antimicrobial_Drugs ◼ Explain the difference between synergistic and antagonistic drug interactions. ◼ List five factors to consider when determining the dosage of a drug. ◼ Name some typical side effects associated with drugs and identify some factors that might contribute to these side effects.  Classification of antibiotics ◼ Antibiotics are classified by: ◼ Chemical structure ◼ Mechanism of action ◼ Spectrum of activity ◼ Mode of action  Chemical structure Β-Lactams Penicillins, Cephalosporins, Carbapenems and Monobactams Aminoglycosides Gentamycin, Streptomycin, Amikacin, Kanamycin Macrolide antibiotics Erythromycin, Clarithromycin, Azythromycin Sulphonamides Sulphamethoxazole, Sulphadizine, Dapsone Diaminopyrimidines Trimethoprim Quinolones Nalidixic acid, ciprofloxacin, levofloxacin, ofloxacin Tetracyclines Tetracycline, Doxycycline Nitrofuran derivatives Nitrofurantoin Nitroimidazoles Metronidazole Nitrobenzene derivatives Chloramphenicol Nicotinic acid Isoniazid, Pyrazinamide, Ethionamide derivatives Others Rifampicin, Ethambutol, Vancomycin  Spectrum of activity Broad spectrum Narrow spectrum ◼ Cephalosporins ◼ Benzyl penicillin ◼ Ampicillin/Amoxy ◼ Cloxacillin cillin ◼ Vancomycin ◼ Erythromycin ◼ Chlorampenicol ◼ Tetracycline  Broad and narrow spectrum antibiotics Antibiotic Gram Gram Rickettsia Mycobacteria positive negative & Chlamydia Narrow Benzyl penicillin spectru m Isoniazid Broad Ampicillin spectru Chloramphenicol m Tetracycline Streptomycin  Mode of action Bactericidal Bacteriostatic ◼ Penicillins ◼ Erythromycin ◼ Cephalosporins ◼ Chloramphenicol ◼ Aminoglycosides ◼ Tetracycline ◼ Cotrimoxazole