Controlling Microbial Growth in the Body: Antimicrobial Drugs

Chapter 12 Topics - Antimicrobial Therapy - Selective Toxicity - Survey of Antimicrobial Drug - Microbial Drug Resistance - Drug and Host Interaction

Antimicrobial drugs an intro: In 1900, 1 in 3 children were expected to die of an infectious disease before the age of 5. The introduction of modern drugs in the 1930 s was a medical revolution, they were regarded as miracle drugs. However in some parts of the world mortality rates from infectious diseases are as high as before they were before antimicrobial drugs.

The goal of antimicrobial chemotherapy is simple: administer a drug to an infected person that destroys the infective agent without harming the hosts cells. The ideal drug should be: easily administered yet able to reach the infectious agent anywhere in the body; be absolutely toxic to the infectious agent while simultaneously being nontoxic to the host, it should remain active in the body as long as needed, yet be safely and easily broken down and excreted. All while not easily available to antimicrobial resistance.

Interaction between drug and microbe The ideal antimicrobial is: Easily administered Selectively toxic Highly potent Stable Soluble in the body s tissues and fluids, Does not disrupt the immune system or micro flora of the host Is exempt from drug resistance.

Where did antimicrobials come from? - Can be naturally occurring or synthetic - Common metabolic products of aerobic bacteria and fungi - Normal function is to inhibit growth of other microorganisms in the same habitat which results in antibiotic producers having less competition for nutrients and space - Most come from bacteria in genera Streptomyces and Bacillus and molds in genera Penicillium and Cephalosporium

Some Terminology Chemotherapeutic drug any chemical used in the treatment, relief or prophylaxis of a disease Prophylaxis- use of a drug to prevent imminent infection of a person at risk Antimicrobial chemotherapy the use of chemotherapeutic drugs to control infection

Antimicrobials all inclusive term for any antimicrobial drug, regardless of origin Antibiotics substances produced by the natural metabolic processes of some microorganisms that can inhibit or destroy other microorganisms Semisynthetic drugs drugs chemically modified in the lab after being isolated from natural sources Synthetic drugs the use of chemical reactions to synthesize antimicrobial compounds in the lab.

Narrow spectrum (limited spectrum) Antimicrobials effective against a limited array of microbial types. Example: a drug effective on mainly Gram (+) bacteria. Broad spectrum (extended spectrum) Antimicrobials effective against a wide variety of microbial types. Example: a drug effective against both gram (+) and gram (-) bacteria

Figure 10.8 Spectrum of action for selected antimicrobial agents

Figure 10.9 Zones of inhibition in a diffusion susceptibility (Kirby-Bauer) test Bacterial lawn Zone of inhibition

Drug Administration The goal: to get an effective amount of drug to the site of infections before it is broken down and excreted. External therapy Local/topical therapy for skin surface infections drug is applied directly to infected area. Systemic therapy Intravenous (IV) administration introducing the drug directly into a vein via needle or catheter. Fastest administration of a high level of drug, but painful and can result in added infection.

Intramuscular administration introduces drug directly into muscle via injection. Drug reaches peak level in blood in 15 minutes, but painful and done by a professional. Oral administration (PO per os) drug is swallowed. Absorbed into bloodstream through GI. Common, simple, painless administration but slow and inefficient. Only a fraction of drug reaches bloodstream, and must be administered often. This leads to dosage errors and failure to comply.

Elimination of drugs from the body Two methods of drug elimination, metabolic conversion or excretion. Metabolic conversion into a different compound occurs in the liver. This metabolic product is usually inactive. Excretion occurs through the kidneys and into the urine. A few pass through the liver into bile, and then into the feces Important to know elimination route when dealing with someone with impaired liver or kidney function

Mechanism of Drug Action Goal of antimicrobial drugs: is to disrupt the cell processes, or structures of the microbe. Most interfere with the function of enzymes, or destroy structures already there. Most importantly drugs should be selectively toxic, they should kill or inhibit microbial cells without damaging host cells.

Antimicrobial drugs are divided into categories based on which cell targets they affect: 1. Inhibition of cell wall synthesis. 2. Interference with cell membrane structure or function. 3. Inhibition of protein synthesis. 4. Inhibition of nucleic acid (DNA or RNA) structure and function. 5. Inhibition of folic acid synthesis.

Inhibition of Cell wall synthesis Perfect mode of action since peptidoglycan is unique to bacteria. Active cells are constantly making new peptidoglycan Penicillin and Cephalosporin block the final step of protein cross links. Vancomycin and Cycloserine interfere with synthesizing the NAG and NAM strands.

Interference of Cell membrane Not the best site of action, every cell has some kind of membrane, so selective toxicity is reduced. Often specificity is for particular types of lipids within cell membranes. Best if used topically If used systematically, it comes with serious side effects, fever chills, vomiting and kidney failure.

Polymyxins interact with membrane phospholipids and distort the cell surface. Polyene antifungal antibiotics (Amphotericin B and Nystatin) form complexes with sterols on fungal membranes. These complexes form passageways. Both of these agents cause seepage of liquids, proteins, and ions.

Inhibition of Protein Synthesis The site of action is at the ribosome-mrna complex. The selective toxicity is achievable because of the differences between prokaryotic and eukaryotic ribosomes (70S vs 80S respectively, S refers to Svedberg units).

Streptomycin and gentamicin cause the misreading of the mrna = abnormal proteins. Tetracyclines block attachment of trna to the A site, stops protein synthesis. Chloramphenicol prevents peptide bond formation. Erythromycin prevents movement of the ribosome along mrna.

Targeting Nucleic acids Both prokaryotic and eukaryotic cells contain nucleic acids. Some enzymes are different enough that we can have enough selective toxicity to be effective. Rifampin inhibits bacterial RNA polymerase, the Quinolones inhibit a microbial topoisomerase.

Inhibiting folic acid synthesis Through competitive inhibition Sulfonamides and trimethoprim inhibit folic acid synthesis. Given together they provide a synergistic effect (an additive effect by using both drugs, which in turns requires less of each drug). Good selective toxicity, because we don t manufacture folic acid.

Penicillins - Antibiotic/synthetic: Antibiotics & semisynthetics - Mode of action: Inhibits cell wall - Spectrum: mostly gram +, some gram Penicillins share the same basic structure and differ in the R group attached.

- All are relatively well tolerated - Problems include - allergies - resistant pathogens beta lactamase enzymes destroy the beta lactam ring Clavulanic acid inhibits these enzymes. Often added to these drugs. (Clavamox aka Augmentin)

Figure 10.16 How beta-lactamase (penicillinase) renders penicillin inactive Lactam ring Penicillin β-lactamase (penicillinase) breaks this bond Inactive penicillin

Cephalosporins Antibiotic/synthetic: Antibiotics & semisynthetics Mode of action: Inhibits cell wall Spectrum: mostly gram +, some gram -

- Closely related to Penicillins but different in its six carbon ring instead of the 5 carbon ring of penicillin. - Very effective with gram + s and with each generation (there are 4) become more effective against gram s. - More resistant against to beta lactamases - Can still cause some allergic reaction like Penicillins

Vancomycin Antibiotic/synthetic: Antibiotic Mode of action: Inhibits cell wall Spectrum: mostly gram + Most effective in treating Staph in cases of resistance (penicillin, methicillin) or in those allergic to Penicillins. It is very toxic (kidneys), must be administered intravenously.

Streptomyces synthesizes many different antibiotics such as aminoglycosides, tetracycline, chloramphenicol, and erythromycin. Fig. 12.9 A colony of Streptomyces

Aminoglycosides Antibiotic/synthetic: Antibiotics & a few semisynthetics Mode of action: Inhibits protein synthesis Spectrum: Gram Poor absorption when taken orally so must be injected and can be very toxic. Resistance can be developed very easily. Often given in combination with other drugs.

Streptomycin - oldest but still the drug of choice for the bubonic plague and considered a good anti-tuberculosis agent. Gentamicin - is less toxic and is widely administered for infections caused by gram s

Chloramphenicol Antibiotic/synthetic: Antibiotic now made synthetically Mode of action: Inhibits protein synthesis Spectrum: gram +, gram -, chlamydiae, rickettsiae, mycoplasmas

Once thought to be ideal: Doesn t cause allergies, penetrates tissues, effective when taken internally, can be stored w/out refrigeration and few side effects. **However in rare cases can cause aplastic anemia. bone marrow stops producing blood cells. (fewer than 1 in 30,000) - Now, only used to treat seriously ill hospitalized patients

Tetracyclines Antibiotic/synthetic: Antibiotics & semisynthetics Mode of action: Inhibits protein synthesis Spectrum: gram +, gram -, chlamydiae, rickettsiae, mycoplasmas Discovered in the 1950 s one of the first to be categorized broad spectrum. Well absorbed orally, few allergies occur. However they do not penetrate the blood/brain barrier.

Side effects include: - gastrointestinal pain and diarrhea - increased sensitivity to sunlight - can stain developing teeth Widely used in livestock feed, consequently resistant strains are plentiful.

Erythromycin Antibiotic/synthetic: Antibiotic Mode of action: Inhibits protein synthesis Spectrum: gram + - Discovered in 1952, widely used to treat strep throat and other respiratory infections. - However resistant S. pyogenes strains have emerged. - Easy oral administration, but side effects include nausea, vomiting, stomach pain.

Quinolones Antibiotic/synthetic: Synthetics Mode of action: Inhibits DNA replication (binds to topoisomerase) Spectrum: gram +, gram -, mycoplasmas, mycobacteria - Relatively new group of drugs, broad spectrum, easily administered orally, and few side effects - Drug resistance is not common. - Ciprofloxacin - used in the Anthrax scare

Antimycobacterial Drugs Antibiotic/synthetic: Synthetic & semisynthetics Mode of action: Inhibits protein synthesis (Rifampin), and inhibits mycolic acids in cell wall (Isoniazid, Ethambutol) Spectrum: Mycobacteria Rifampin Isoniazid Ethambutol

Mycobacteria are difficult to treat; 1. Mycolic acid layer nearly impermeable 2. Mycobacteria grow slowly 3. Resistant strains develop readily, combinations of drugs must be used. 4. Intracellular pathogen

Other types of antimicrobials; Antiprotozoan metronidazole Treat giardia Antimalarial Quinine malaria Antihelminthic mebendazole Tapeworms, roundworms

Antiviral Limited drugs available Difficult to maintain selective toxicity Effective drugs target viral replication cycle Entry Nucleic acid synthesis Assembly/release Interferon artificial antiviral drug

Antiviral drug structures and their mode of action. Table 12.5 Actions of selected antiviral drugs.

Antifungal agents Antibiotic/synthetic: Antibiotic & synthetics Mode of action: Inhibits cell membrane, inhibits protein synth., inhibits cell division. Spectrum: fungi - Due to fungi being eukaryotic they pose specific challenges. - Selective toxicity focuses on fungal sterols in the cell membrane

Nystatin usually topical, Candida albicans. Imidazoles & Triazoles can be used topically or systematically instead of Amphotericin B (serious side effects). Griseofulvin persistant ringworm infections.

Resistance Natural resistance the cell may lack the target the drug attacks, or naturally repel/block the drug. Acquired resistance when strains become drug resistant due to mutation and genetic exchange. 1940 s nearly all S. aureus were sensitive to penicillin, today 90% are resistant.

Mechansims of resistance Mechanisms of resistance fall into 3 categories: 1. Acquiring enzymes that inactivate or destroy the drug (Beta lactamase) 2. Changing the cellular target 3. Excluding the drug or removing it once it has entered (N. gonorrhea)

Selective process that leads to a change in genetic frequency Abuse and misuse of antibiotics leads to resistance, and leads to it more quickly.

How Does Resistance Develop? 1) Spontaneous mutations in critical chromosomal gene(s) - mutation is low frequency but occurs because of huge numbers of bacteria in a population 2) Acquisition of new genes or sets of genes from another species - originates from plasmids containing resistance (R) factors - transferred by conjugation, transformation, or transduction 3) Natural selection

Ways to slow resistance: 1. Limit non medical use of antibiotics ie, livestock feed. 2. Stop prescribing antibacterials for viral infections. 3. Stop selling antibiotics without prescription. 4. Stop prescribing too often, or indiscriminately. 5. Use combined therapy (two drugs at once to ensure effectiveness. 6. Encourage patient compliance.

Demonstration of how natural selection enables resistant strains to become dominant. Fig. 12.15 The events in natural selection for drug resistance