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 –Semi-synthetic antibiotics: use natural antibiotic as base, but modified chemically; most of our new “antibiotics” –Antibiotics are “small molecules”, 500 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 –Since the most resistant bacteria are often Gram negative, because of their OM, we often think of broad spectrum antibiotics being those that also work against many G- –Broad spectrum antibiotics can sometimes cause problems because of damage to normal microbiota of host –“Superinfection” may result from this situation

3 Selective Toxicity: the key to antibiotic therapy Basic pharmacology: any biologically active substance will have a level at which there is no effect, a level where it will do what you want (kill microbes), and a level that will be toxic. 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 Constant toxicity, not influenced by host condition or behavior Non-allergenic Stable in vivo, slowly broken down and excreted Difficult to become resistant to Long shelf life low $

7 HOW resistance is Acquired Non-genetic: Evasion, bacteria may hide in cells or organelles; L-forms, temporary forms of cell w/o cell walls, removing a target. Mutations: change in transport protein, ribosome, enzyme, etc. Normally harmful mutations are selected FOR in the presence of antibiotic. Plasmids: through conjugation, genetic information allowing cell to overcome drug. images/Stryer/Stryer_F32-13.jpg

8 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.

9 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) –Full time course needed; 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

10 Fighting antibiotic resistance Use all drug at sufficiently high concentration –Don’t allow the least sensitive bacteria to survive Drugs in combination –Odds of mutating to resist 2 drugs: 1 in 10 6 x 10 6 –Synergism: e.g. amoxicillin and clavulanic acid Limit antibiotic use –>50% of infections are viral; not affected by antibiotics –Constant exposure breeds resistance New drugs

11 MRSA: how it does it Methicillin-resistant Staph. aureus –Acquired in hospitals and the community –Now only treatable with vancomycin Only injected More likely to cause side effects Beta lactam antibiotics bind to transpeptidases –“PBPs”, build peptidoglycan; –Mutant protein PBP 2a has altered structure including altered active site; methicillin cannot bind –One good transpeptidase keeps bacterium alive