Antimicrobial Therapy

1. Antimicrobial Therapy • Chemotherapy: any treatment of patient with chemicals to treat a condition. • Now word associated with cancer treatment • Our focus is on antimicrobial agents • Antimicrobials: synthetic, antibiotics, or semi-synthetic • Antibiotics: natural products made by microbes, effective against other microbes. Some drugs are wholly synthetic. • Semi-synthetic antibiotics: use natural antibiotic as base, but modified chemically; most of our new “antibiotics” • Antibiotics are “small molecules”, 300 - 1500 MW, not to be at all confused with antibodies which are proteins (MW 150,000)

2. Spectrum • Some antibiotics are considered “broad spectrum” • By definition, these are effective against many types of bacteria, both Gram negative and Gram positive • Broad spectrum antibiotics can sometimes cause problems because of damage to normal microbiota of host • “Superinfection” may result from this situation • Overgrowth of “normal” microbes that cause disease

3. Selective Toxicity: the key to antibiotic therapy • 3 concentration ranges: ineffective, effective, and toxic. A drug needs to have a wide effective (therapeutic) range. Selective toxicity is the ability of the drug to harm the target without harming the host. Bacteria have many targets that are biologically different from us that the drugs can hit. As the target becomes more like us, there are fewer and fewer drugs that are selectively toxic: fungi, protozoa, worms, viruses, cancer.

4. Selective Toxicity and side effects • Drugs may fail to be selectively toxic and interfere with mammalian biochemistry. They may cause allergies. They may destroy too many normal bacteria.

5. Actions of antimicrobials • Drugs work against microbes by these basic mechanisms: • Inhibition of cell wall synthesis • Causes bacterium to commit suicide, but only during growth when cells are cutting their own PG. • Disruption of membrane function • Often toxic to humans because we have membranes too, cause leakage of vital molecules. • Inhibition of protein synthesis – many antibiotics • Bind to ribosomal RNAs, proteins. • Inhibition of nucleic acid synthesis • Attack transcription, DNA unwinding enzymes • Act as anti-metabolites – competitive inhibitors, inhibit function of enzymes, usually bacteriostatic.

6. Ideal Antibiotic • Good drug properties (e.g. soluble in body fluids) • Selectively toxic, obviously • Easily administered • Non-allergenic • Stable in vivo, slowly broken down and excreted • Difficult for microbe to become resistant to. • Long shelf life (chemically stable) • low $ http://catshospital.com/2095_160.gif

7. Measurement of Efficacy-1 • Disk diffusion assay • Paper disks with antibiotic applied to lawn in Petri dish • Zone of inhibition indicates susceptibility to drug http://faculty.mc3.edu/jearl/ML/20-17.jpg

8. Measurement of Efficacy-2 • Broth dilution test to measure MIC • Minimum inhibitory concentration • Drug is diluted in broth which is inoculated • Clear broth indicates no growth or bacteria killed. • That concentration of drug that first inhibits: MIC http://www.bmb.leeds.ac.uk/mbiology/ug/ugteach/icu8/images/antibiotics/mic.gif

9. Why bacteria might be resistant • Innate or behavioral: Evasion, bacteria may hide in cells or organelles; Gram – have OM; some bacteria have no cell walls. • Mutations: change in transport protein, ribosome, enzyme, etc. Mutations that are normally harmful are selected FOR in the presence of antibiotic. • New genetic information from elsewhere • Plasmids or other DNA from conjugation, transduction, etc. http://www.mun.ca/biochem/courses/3107/ images/Stryer/Stryer_F32-13.jpg

10. Mechanisms of drug resistance • Alteration of target: active site of enzyme changes, ribosome changes. • Alteration of membrane permeability: transport protein changes, drug no longer enters; drug that does enter is actively pumped out. • Enzymatic destruction of drug: penicillinases (beta lactamases) • “End around” inhibitor: bacteria learns to use new metabolic pathway, drug no longer effective.

11. Attack by penicillinase http://dwp.fcroc.nl/microbiologie/images/antibiotica/de_wer4.gif

12. Human behavior and antibiotic resistance • Bacteria once under control are making a comeback due to antibiotic resistance: • S. aureus, Enterococcus, M. tuberculosis, et al. • Human behavior: • Most diseases caused by viruses, non-cellular, not treatable with antibiotics (but Doctor, do something) • last bacteria left are the most resistant, if they aren’t killed, they become “normal”; don’t stop regimen because you feel better. • Social behavior • resistance in homeless/poor • growth stimulants in agriculture http://www.dkp-ml.dk/images/homeless.jpg

13. Fighting antibiotic resistance • Use drug as prescribed • Use drug at correct concentration, correct amount of time • Don’t allow the least sensitive bacteria to survive • Drugs in combination • Odds of mutating to resist 2 drugs: 1 in 106 x 106 • Synergism: e.g. amoxicillin and clavulanic acid • Limit antibiotic use • >50% of infections are viral; not affected by antibiotics • Constant exposure breeds resistance, selects for resist. • New drugs