Antimicrobial Drugs and Antibiotics

Antimicrobial Drugs

• Antimicrobial drugs are therapeutic substances used to treat microbe-caused diseases.
• Antibiotics are antimicrobial chemical compounds, which are generated by microorganisms to inhibit or destroy other microorganisms in dilute solutions.
• Selective toxicity must be present in antimicrobial drugs.
  • This means that the drug must harm the microorganism without damaging the host.
• Chemotherapy refers to the use of chemicals to treat a disease.
• Chemotherapeutic index is the ratio of a drug's toxicity to the body versus its toxicity to an infectious agent and is the ratio of toxic dose to effective dose.

Antibiotic Sources

• More than half of antibiotics come from Streptomyces bacteria.
• A small portion of antibiotics comes from Bacillus bacteria.
• Molds (such as Penicillium and Cephalosporium spp.) are other sources for antibiotics.
• Synthetic drugs are synthesized in the laboratory.
• Semisynthetic drugs are partially made by microorganisms and partially in a laboratory.

Spectrum of Activity

• Spectrum of activity denotes the range of microbes an antimicrobial agent can act against.
• Broad-spectrum drugs target a wide range of microorganisms, including gram-negative and gram-positive bacteria.
  • Tetracycline is an example.
  • Broad-spectrum drugs are useful when the infection is from an unidentified organism.
  • Disadvantage: they can destroy normal microbiota.
• Narrow-spectrum drugs are effective against a small number of microorganisms or a single taxonomic group.
  • Penicillin G or Isoniazid are examples.
  • If the pathogenic organism is known, a narrow-spectrum drug is preferable as it is less likely to kill normal flora.
  • They also reduce the chance of developing drug resistance.
• Superinfection is an overgrowth of normal microbiota that is resistant to antibiotics.

Mechanisms of Antibacterial Action

• Antibacterial action mechanisms include inhibition of cell wall synthesis, protein synthesis, nucleic acid synthesis, and antimetabolite action; as well as disruption of the cell membrane.

Inhibiting Cell Wall Synthesis

• Antibiotics like penicillin and cephalosporin stop intact cell wall synthesis - meaning they prevent cell walls from completing cross-linking.
  • This causes the cell to weaken and undergo lysis.
• This weakening happens because bacterial cell walls contain peptidoglycan.

Inhibition of Protein Synthesis

• Differences between bacterial (70S) and animal (80S) ribosomes allow antimicrobial agents to attack bacterial cells without damaging eukaryotic cells
  • Eukaryotic mitochondria, however, have 70S ribosomes.
  • Aminoglycoside antibiotics like tetracyclines, chloramphenicol, and erythromycin can have adverse effects on the host.

Disruption of Cell Membrane Function

• Antimicrobials are selectively toxic due to differences in the phospholipid composition of human, bacterial, and fungal membranes.
• Polymyxins are polypeptide antibiotics that act as detergents and disrupt bacterial membranes.
  • As a result, the cells lose metabolites, and bind to bacterial membranes and alter the membrane permeability
• Amphotericin B is a polyene antibiotic that binds to unique sterols in fungal outer membranes.
• Ionophores antibiotics allow the uncontrolled movement of cations but are not used in humans.

Inhibition of Nucleic Acid Synthesis

• Differences between the enzymes used by bacterial and animal cells to synthesize nucleic acids cause selective toxicity.

Action of Antimetabolites

• Antimetabolites are substances affecting metabolite utilization and prevent a cell from carrying out metabolic reactions.
  • Sulfanilamide is one such antimetabolite, acting as a competitive enzyme inhibitors.
• Normal metabolic processes of microbial cells involve a series of intermediate compounds called metabolites, which are essential for cell growth.

Antibacterial Inhibitors of Cell Wall Synthesis

• Penicillin Groups are a group of approximately 50 chemically-related antibiotics with a core B-lactam ring structure.
• Penicillin's R group is a functional chemical group that is unique to each.
• Penicillin G is a narrow-spectrum antibiotic effective against staphylococci, streptococci, and some spirochetes,
  • As it is destroyed by stomach acid, it is given intravenously and has a short retention time in the body (3-6 hours).
• Penicillin V is similar to Penicillin G but can be taken orally.
• Molds like Penicillium notatum produce natural penicillins, including Penicillin G and Penicillin V.
  • Other penicillins are semisynthetic.
• Penicillin G and V are inactivated by penicillinases, or, bacterial enzymes that destroy penicillin, and have a narrow range of activity
• Semisynthetic penicillins are synthesized by adding a unique R group to the B-lactam nucleus and are made in an effort to prevent inactivation by penicillinases.
  • Methicillin and Oxacillin are penicillinase resistant.
  • Ampicillin is broad spectrum and acid resistant.
  • Bacteria have developed resistant genes to both Ampicillin and Amoxicillin (broad spectrum and acid resistant).
• Some penicillins are combined with potassium clavulanate (clavulanic acid), a noncompetitive inhibitor of penicillinase, to protect the antibiotic.

Penicillin Resistance

• Gram-negative bacteria are innately resistant because earlier penicillins could not penetrate the outer membrane.
• Bacteria has developed a B-lactamase gene, which cleaves the B-lactam ring of penicillins & cephalosporins.
• Semisynthetic penicillins & the cephalosporins are effective on gram-negative bacteria.

Monobactams

• Monobactams are a group of synthetic drugs with only a single-ring, as opposed to the double ring of B-lactam antibiotics.
  • An example is aztreonam, which affects some gram-negative bacteria and has low toxicity.

Cephalosporins

• Cephalosporins are broad-spectrum antibiotics, which have a similar mode of action and a similar structure to penicillin.
  • They are resistant to penicillinases but susceptible to other B-lactamases.
• Cephalosporins are commonly prescribed as they have a broad spectrum of activity, rarely cause side effects, and can be used prophylactically for surgery.
• There are more than 70 and most are semisynthetic.
  • Examples include cephalexin (Keflex) and cephalothin (Keflin).
  • The use of cephalosporins account for 25% of pharmacy costs in North American hospitals.

Carbapenems

• Carbapenems are very broad-spectrum antibiotics used to treat nosocomial infections, and examples includes doripenem.
• Carbapenems are B-lactam antibiotics where a carbon atom is substituted for a sulfur atom and a double bond is added to the penicillin nucleus.
  • Doripenem is especially useful against Pseudomonas aeruginosa infections.

Polypeptide Antibiotics

• Bacitracin is used for treating streptococci & staphylococci infections but is limited to topical use because of kidney toxicity and is used against gram-positive bacteria.
• Vancomycin is a glycopeptide antibiotic that is penicillinase resistant and used to treat gram-positive bacteria, though it is a relatively toxic antibiotic that may cause hearing loss and kidney damage.
  • It is administered intravenously and used as a last resort to treat MRSA (Methicillin-resistant Staphylococcus a.).
  • Widespread use has led to the appearance of vancomycin-resistant enterococci (VRE).

Antimycobacterial Antibiotics

• Isoniazid and ethambutol are effective in the treatment of Mycobacterium infection.
  • Isoniazid inhibits synthesis and ethambutol inhibits incorporation of mycolic acid into the cell wall.

Agents that Cause Injury to the Plasma Membrane

• Polymyxins are narrow-spectrum antibiotics effective against gram-negative bacteria.
  • They are used topically to treat skin infections caused by pseudomonas, but internal usages can cause kidney damage, respiratory arrest, & numbness in the extremities.
• The five polymyxins are labeled from A to E, with B and E being the most commonly used.

Antibacterial Inhibitors of Protein Synthesis - Aminoglycosides

• Aminoglycosides are broad-spectrum antibiotics that were the first used to treat gram-negative infections.
  • Streptomycin was discovered in 1944 and although used to treat gram-negative infections many bacteria are now resistant to it, plus it can have serious toxic effects.
  • Neomycin is a nonprescription antibiotic used in topical applications.
  • Gentamicin & tobramycin can be used to treat bone and joint infections because they efficiently penetrate to these areas.
• Most aminoglycosides have pronounced toxicity and are not given orally.
• They can damage or kill kidney cells & the auditory nerve, and around 5–10% of people treated with aminoglycosides experience some side effect, such as affecting their hearing, sense of balance or kidneys.
  • Damage is relatively minor and reversible once the medication is stopped.
  • They Are administered by intramuscular or intravenous injection, irrigation, topical skin application, or inhalation.
  • If the infection being treated involves the central nervous system, the drug can be injected into the spinal canal.
• Aminoglycosides change the shape of the 30S ribosomal subunit because they cause the mRNA to be read incorrectly.

Tetracyclines

• They prevent tRNA entry into the ribosome.
• They are absorbed well from the stomach and cannot be taken with dairy products because they combine with calcium.
• Oxytetracycline, chlortetracycline, & doxycycline have the widest spectrum of any antibiotics.
• These are suitable for treating rickettsial, chlamydial, and mycoplasmal infections, which are intracellular pathogens with limited antimicrobials effective in destroying them.
• A disadvantage of tetracyclines is that they can destroy the normal microbiota, that they have a wide range of activity and can produce severe gastrointestinal disorders.
• Tetracycline is not advised for children under 5 years of age, and usage discolors teeth by complexing with Ca2+ and can also cause liver damage in pregnant women

Glycylcyclines

• Glycylcyclines are broad-spectrum and bacteriostatic; they bind to the 30S ribosomal subunit, and are a new generation of antibiotics derived from tetracyclines, and useful agains MRSA.
  • These drugs were developed to overcome issues with bacterial resistance to tetracyclines

Chloramphenicol

• It is a synthesized broad-spectrum antibiotic.
• Over prolonged use, it damages bone marrow and results in aplastic anemia and bone marrow failing to produce adequate numbers of blood cells.
• Chloramphenicol inhibits the bacterial enzyme peptidyl transferase on the 50S portion of the 70S ribosome to prevent the growth of the polypeptide chain during protein synthesis.
• Despite the risks, it is used to treat typhoid fever & infections of the central nervous system.

Macrolides

• Macrolides attach to the 50S ribosomal subunit, which then blocks the release of uncharged tRNA from the P site and therefore prevents ribosome movment (aka. translocation).
• Erythromycin is an example macrolide, and is particularly bacteriostatic.
  • It is a broad spectrum antibiotic that is one of the least toxic of the antimicrobial agents.
  • It can be used to treat Legionellosis.
• Azithromycin and telithromycin are semisynthetic examples, with a broader spectrum and better tissue penetration than erythromycin

Streptogramins

• These antibiotics attach to the 50S subunit and work against gram-positive bacteria that are resistant to other antibiotics.

Oxazolidinones

• They bind to the 50S/30S subunit interface and are synthetic and combat MRSA.
  • Eg. Linezolid

Pleuromutilins

• For example, Retapamulin is a topical treatment that is effective agains gram-positive bacteria.

Antibacterial Inhibitors of Nucleic Acid Synthesis

• Rifamycins inhibit the synthesis of mRNA
  • Rifampin is an example, and is used to treat tuberculosis and leprosy.
• Quinolones & Fluoroquinolones selectively inhibit a bacterial enzyme known as gyrase, which is important for the synthesis of bacterial DNA
  • The Quinolone group of antimicrobials includes fluoroquinolones, frequently prescribed because they: have a broad spectrum of activity and are safe for adults, except for pregnant women with good tissue penetration
    • Norfloxacin and ciprofloxacin are the most widely quinolones used and are often prescribed for urinary tract infections

Competitive Inhibitors of Essential Metabolites

• Sulfonamides or sulfa drugs, include a combination of trimethoprim & sulphamethoxazole and is a Broad spectrum drug.
  • It is an example of drug synergism, the combination of which, means that only 10% of the concentration of each drug is needed compared to when each drug is used alone
• Drug synergism occurs when drugs interact in ways that enhance or magnify one or more effects of those drugs
• The combination of trimethoprim & sulfamethoxazole act as competitive inhibitors of the enzymes (PABA) the bacteria use to synthesize folic acid

Antibiotic Resistance

• There are four mechanisms whereby bacteria become resistant to antibiotics:
  1. Destruction or inactivation of the drug.
  2. Preventing the drug from accessing the target site.
  3. Altering the drugs target site.
  4. Pumping the drug out of the cell before it can be effective
• The overuse and misuse of antibiotics, a lack of hospital infection control, agricultural use of antibiotics, drug promotion, and deterioration of public health surveillance are all contributing factors in the rise in antibiotic-resistant bacteria.

Antiviral Drugs

• They are often nucleoside analogs.
• It is difficult to develop antiviral drugs due to simple structures and the fact that they engage in few physiological activities other than replication.
• Entry & fusion inhibitors blocks the receptors on the host cell that bind to the virus and/or the fusion of the virus & cell.
• Uncoating, genome integration, & nucleic acid synthesis inhibitors prevent viral uncoating; inhibits viral DNA integration into the host genome; nucleoside analogs inhibit RNA or DNA synthesis; non-nucleoside inhibitors of RNA synthesis exist.
  • Amantadine & zanamivir prevent influenza viruses from attaching to their host cell membranes.
  • Acyclovir & ganciclovir interfere with herpes virus replication
  • Azidothymidine & ribavirin interfere with HIV replication

Success Stories - Antiviral Drugs

• Acyclovir is one of the most successful antiviral drugs and targets herpes infections: HSV-1&2, and VZV.
  • It is a nucleoside analog that lacks the 3'-OH, and thus blocks viral replication.
• AZT was one of the first anti-HIV drugs, and is an nucleoside analog that targets reverse transcriptase.

Assembly & Exit Inhibitors

• Protease inhibitors block the cleavage of protein precursors.
• Exit inhibitors inhibit neuraminidase, which is required for some viruses to bud from the host cell.

Antifungal Drugs

• They are difficult to develop due to fungi lacking limited selective toxicity because fungi are eukaryotes, so their cells are not that different from those of humans.
• Agents that affect fungal sterols interrupt the synthesis of ergosterol, which makes the membrane excessively permeable.
  • Polyenes are examples of antifungal drugs.
  • Amphotericin B is the drug of choice for serious fungal infections, though it is toxic to most structures in the body.
    • It is produced by Streptomyces and effective for treating serious diseases.
  • Nystatin is often prescribed for infections of the intestine, vagina, or oral cavity and is sold as an ointment cream or suppository.
• Treatment of Topical Imidazoles treats cutaneous mycoses.
• Treatment of Systemic Triazole treats systemic fungal infections.

Allylamines

• Used to treat azole-resistant infections.

Antiprotozoal Drugs

• Antimalarial drugs are examples.
• Quinine, which is synthesized from tree bark, has largely been replaced in favor of chloroquine due its toxicity.
• Metronidazole is one of the most widely used antiprotozoal drugs; it Acts against parasitic protozoa & obligate anaerobic bacteria.
  • This drug interferes with DNA synthesis under anaerobic conditions, and is the drug of choice for vaginitis caused by Trichomonas vaginalis
  • It is also used in treating giardiasis & amebic dysentery
    • giardiasis is an intestinal infection caused by parasite called Giardia.
    • Amebic dysentery is a gastrointestinal illness

Measurement of Antimicrobial Activity

• Minimal inhibitory concentration is the lowest concentration of an antimicrobial compound that prevents growth of a microbe in vitro.
• Minimal cidal concentration is the lowest concentration of antimicrobial compounds that kills a microbe in vitro.
• Antibiotic synergism occurs when the effects of a combination of antibiotics are greater than the sum of the effective of the individual antibiotics
• Antibiotic antagonism occurs when one antibiotic, usually the one with the least effect, interferes with the effectiveness of another antibiotic
• Penicillins are antagonistic to bacteriostatic drugs such as tetracyclines, because penicillins require actively growing cells and static drugs can inhibit growth.
• Aminoglycosides are synergistic when used in combination with penicillins.

Resistance to Antimicrobial Drugs

• Persister cells are microbes with genetic characteristics allowing for their survival when exposed to an antibiotic.
• Superbugs are bacteria that are resistant to large numbers of antibiotics.
• Resistance genes are often spread horizontally among bacteria on plasmids or transposons via conjugation or transduction pathways.
• Mechanisms of resistance include enzymatic destruction or inactivation of the drug, prevention of penetration to the target site within the microbe, alteration of the drug's target site, rapid efflux (ejection) of the antibiotic, and variations of these mechanisms.