Antimicrobial Drugs

• Antimicrobial drugs treat infectious diseases by disrupting microorganism growth.
• These drugs should selectively target pathogens while minimizing harm to the host.
• Selective toxicity means a drug can attack specific microbial cells while leaving host cells relatively unharmed.
• Antimicrobials include antibiotics/antibacterials, antivirals, antiparasitic agents, and antifungals.

Definitions

• Antibiotics inhibit bacterial growth.
• Microorganisms commonly produce antibiotic substances that inhibit the growth of other microorganisms.
• Synthetic drugs can also be called antibiotics.
• Bacteriostatic agents stop bacterial replication, allowing the host immune system to act.
• Bactericidal agents directly kill bacteria.

Spectrum of Activity

• The range of microbes affected by an antimicrobial drug defines its spectrum of activity.
• Narrow-spectrum drugs are effective against a limited variety of bacteria.
• Broad-spectrum drugs can inhibit both Gram-positive and Gram-negative bacteria.
• Broad-spectrum drugs advantages include treating unknown infections or infections caused by diverse organisms like bacterial meningitis.
• Broad-spectrum drugs disadvantages include destroying normal flora, leading to superinfections like C. difficile diarrhea.
• Semi-synthetic antibiotics are chemically altered versions of naturally occurring ones for increased effectiveness.
• Synthetic antimicrobials are completely synthesized in a lab.

Gram Negative Outer Membrane Limitations

• The "lipid bilayer" with fatty acid tail hydrocarbons prevents the passage of polar molecules, which most drugs have.
• Small, water-filled "pores" only allow entry by compounds that are water-soluble.
• If a drug cannot reach its target, it is useless.
• Small, hydrophilic drugs can generally enter Gram-negative bacteria.

Inhibition of Cell Wall Synthesis

• The cell wall structure is a major difference between prokaryotic and eukaryotic cells.
• Prokaryotic cells have peptidoglycan.
• Interfering with cell wall synthesis affects actively growing cells, leading to a weakened cell wall, exposed plasma membrane, and lysis.

Penicillin

• Includes a family of over 50 chemically-related antibiotics with a common core β-lactam ring structure.
• Penicillins are also called "β-lactam antibiotics".
• Penicillin V served as the prototype and was the 1st penicillin discovered.
• Penicillin binds to penicillin-binding proteins (PBPs) and enzymes responsible for peptidoglycan synthesis, disrupting cell wall formation.

β-Lactam Ring

• Some bacteria produce β-lactamase, which breaks the β-lactam ring and inactivates penicillin.
• β-lactamase is also called "penicillinase”.
• β-lactamase is a common form of penicillin resistance.

Penicillin Types

• Natural penicillins are extracted directly from Penicillium cultures, and are narrow spectrum (G+).
• Natural penicillins are useful against most Staphylococci, Streptococci, and spirochetes and are susceptible to β-lactamases.
• Pen G and Pen V are the most common natural penicillins.
• Semi-synthetic penicillins feature a β-lactam core made by Penicillium with an R-group added in the lab.
• Semi-synthetic penicillins are engineered for specific characteristics, such as β-lactamase resistance (methicillin) or broader specificity (ampicillin, amoxicillin).

Antibiotic Resistance

• MRSA is methicillin-resistant Staphylococcus aureus.
• Penicillins typically interact with bacterial cell walls via penicillin-binding proteins in the peptidoglycan layer.
• MRSA has a genetic mutation preventing penicillin from binding.
• MRSA produces beta-lactamase and must be isolated.
• MRSA can usually be treated with vancomycin.

Vancomycin

• Named after the word "vanquish".
• Vancomycin is a glycopeptide antibiotic with a completely different structure than penicillin.
• Vancomycin is naturally produced by species of Streptomyces.
• Vancomycin inhibits cross-linkage of peptidoglycan layers.
• Vancomycin is very narrow spectrum and mostly treats MRSA.
• Recently some strains of S. aureus and certain Enterococci are resistant to vancomycin.
• Toxicity used to be a problem, but improved manufacturing procedures have corrected this.

Monobactams

• Synthetic and semi-synthetic antibiotics offer a potential advantage by not being found in nature.
• The potential advantage provides pathogens with less time to develop resistance.
• Monobactam structures are similar to penicillin but are different enough that they are not sensitive to β-lactamase.
• Monobactams bind to PBPs and enzymes responsible for peptidoglycan synthesis.
• Monobactams are effective against certain Gram-negative bacteria such as E. coli, H. influenzae, P. aeruginosa and treat these infections in Cystic Fibrosis patients.

Cephalosporins

• Cephalosporins have a similar chemical structure to penicillin, featuring a β-lactam ring.
• Examples include cephalothin and cefixime and come from the fungus Cephalosporium.
• Cephalosporins inhibit cell wall synthesis in the same way as penicillin, but tend to be more broad-spectrum than natural penicillin.
• Cephalosporins bind to PBPs and enzymes responsible for peptidoglycan synthesis.
• Cephalosporins are susceptible to a different group of β-lactamases.

Mycobacteria

• M. tuberculosis and M. leprae cause tuberculosis and leprosy, respectively.
• Mycobacterial cell walls contain mycolic acids and a little peptidoglycan.
• Anti-mycobacterial antibiotics interfere with mycolic acid incorporation or synthesis and have minimal effect on other bacteria.
• Isoniazid inhibits mycolic acid synthesis.
• Ethambutol inhibits the incorporation of mycolic acid and is fairly weak on its own, so it is administered as part of a "cocktail" to prevent resistance.
• Dapsone treats Leprosy and inhibits nucleic acid synthesis

Inhibition of Protein Synthesis

• Ribosomes are the site of protein synthesis.
• Eukaryotic and prokaryotic ribosomes differ; eukaryotic: 80S (40S + 60S subunits), prokaryotic: 70S (30S + 50S subunits).
• Targeting 70S ribosomes directs action against bacteria; mitochondria also have 70S ribosomes.
• Some drugs targeting 70S ribosomes may have toxic effects on humans.
• Bacteria are sensitive to lower concentrations of the 70S ribosome drugs than human cells.
• Protein synthesis inhibitors have slightly different modes of action but the same end result.

Mechanisms of Actions

• Chloramphenicol prevents peptide bond formation by irreversibly binding to the peptidyl transferase component of the 50S subunit.
• Aminoglycosides impair translational proofreading and cause misreading of mRNA, leading to premature release of aberrant peptide chains from the 30S ribosome.
• Tetracyclines block the attachment of tRNA to the ribosome, preventing elongation at the 30S ribosome.
• Macrolides prevent continuation of synthesis by inhibiting translocation and peptide elongation at the 50S ribosome.

Chloramphenicol Properties

• Chloramphenicol is broad spectrum, has a simple structure allowing it to diffuse into inaccessible areas, and is inexpensive.
• Chloramphenicol's downside is serious toxicity, including bone marrow suppression, aplastic anemia, and affects blood cell formation.
• Chloramphenicol effects 1 in 40,000 users, compared to 1 in 500,000 normally.
• Physicians advise avoiding Chloramphenicol when alternatives exist, due to the fact it is teratogenic in neonates and causes gray baby syndrome.

Aminoglycosides

• Aminoglycosides were among the first antibiotics against Gram- bacteria but are bactericidal and can be toxic, declining use.
• Aminoglycosides can cause permanent damage to the auditory nerve (Ototoxicity) and kidneys (Nephrotoxic).
• Aminoglycosides may be used to treat cystic fibrosis where lung infections with Pseudomonas aeruginosa, which are G- and difficult to treat, are common.
• Tobramycin is delivered as an aerosol to control Pseudomonas aeruginosa infections.

Tetracyclines

• Effective against Gram+ and Gram- intracellular bacteria, with an ability to penetrate tissues and cells well.
• Semi-synthetic versions like doxycycline and minocycline have longer retention in the body.
• Tetracyclines treat UTIs, Mycoplasma, Chlamydia, and Rickettsia infections, and serve as alternatives for syphilis and gonorrhea instead of penicillins.
• Tetracycline problems include suppressing normal flora, causing GI upsets leading to superinfections like C. albicans, brown teeth discoloration in kids younger than 8, and liver damage in pregnant women.

Macrolides

• Macrolides are a broad-spectrum (G+) alternative to penicillin, but are too big to enter G- cells.
• Macrolides inhibit protein synthesis and are administered orally.
• Orange-flavored suspensions are used to treat streptococcal and staphylococcal infections in children.
• Macrolides are useful for treating people allergic to β-lactams.
• Azithromycin and clarithromycin are macrolides with broader specificity and better tissue penetration.
• Important for intracellular bacteria treatment such as Chlamydia.

Injury to Plasma Membrane

• These drugs change the permeability of the plasma membrane, causing essential metabolites to leave the cell.
• Drugs affecting the plasma membrane are probably not the best choice because the eukaryotic plasma membrane is similar to bacteria.
• An example is polymyxin B, which is most active against gram(-) Pseudomonas.
• Polymyxin B attaches to phospholipids, causing disruption and significant internal toxicity.
• Polymyxin B is used as a topical treatment for superficial infections sold in non-prescription antibiotic ointments like Polysporin.

Nucleic Acid Synthesis Inhibitors

• These drugs interfere with replication or transcription and may harm the human host; however, they are more harmful to bacteria than the host.
• Rifamycins, such as rifampin, inhibit mRNA synthesis and are bactericidal.
• Rifamycins side effect = orange-red urine, feces, tears, sweat, saliva.
• Rifamycins can penetrate tissues and achieve therapeutic levels in CSF and abscesses.
• Rifamycins are useful for TB treatment with isoniazid and ethambutol when tissue penetration is required.

Quinolones

• These are bactericidal with broad or narrow spectrum, which specifically inhibit bacterial DNA replication.
• Quinolones inhibit bacterial DNA topoisomerase type II or type IV.
• Quinolones include Nalidixic acid, which is an early drug (1960's) with limited use.
• Nalidixic acid only application is UTIs.
• Fluoroquinolones are a form of quinolones, and developed in 1980's, norfloxacin and ciprofloxacin (Cipro).
• Fluoroquinolones are safe for adults, but not recommended for children, adolescents, and pregnant women, because they affect cartilage development.
• New synthetic versions with a broader spectrum are being developed but adversely affect some drugs that control heart rhythm.

Drugs inhibiting metabolic pathways

• Blocking essential enzymes within a cell can be done by using specifically-designed drugs.
• Competitive inhibition involves a drug with a structure similar to the normal substrate, which can "block" the active site of an enzyme and prevent its normal function.

Sulfonamides

• Sulfa drugs are among the first synthetic antimicrobials.
• Para-aminobenzoic acid (PABA) is required to make nucleic acids in pathogens but not humans.
• TMP-SMZ (trimethoprim & sulfamethoxazole) is a synergistic combo widely used today.
• Broad-spectrum and used for control of pneumonia caused by Pneumocystis carnii and effective in penetration of brain and CSF.

Metronidazole

• Metronidazole was originally introduced as an oral agent to treat Trichomonas vaginitis.
• It is effective in treating amebiasis, giardiasis, and serious anaerobic bacterial infections.
• Metronidazole has no significant activity against aerobic or facultatively anaerobic bacteria.

Combining Drugs Considerations

• Synergism occurs when combined drugs are more effective together than alone.
  • An example is penicillin (damage cell wall) + streptomycin (inhibit protein synthesis at ribosome).
  • Penicillin allows entry by streptomycin by damaging the cell wall.
• Antagonism occurs when one drug's activity works against another.
  • Example: penicillin (inhibits ACTIVE cell wall synthesis) + tetracycline (stops bacterial growth).
  • Bacteria that are not making a cell wall are not affected by penicillin.

Antifungals

• Fungal infections are increasing in frequency due to opportunistic infections, immunosuppression, and AIDS.
• Toxicity is a major problem since fungi are eukaryotes.
• Azoles: block fungal sterol synthesis with clotrimazole and miconazole and are topical treatments for athlete's foot, and yeast infections.
• Triazoles are less toxic, but still cause some liver damage.
  • Fluconazole and ketoconazole treat systemic mycoses.
• Polyenes kill fungal cells via sterol recognition, such as Amphotericin B.
  • Amphotericin B is commonly used in the treatment for systemic mycoses.
  • Toxicity in the kidney limits the use of Amphotericin B.

Antivirals

• In developed countries, the most serious infections are often caused by viruses, but few antiviral drugs exist.
• HIV drugs:
  • Celsentri binds to CCR5 and prevents a GP120 interaction and is referred to as a chemokine receptor antagonist or a CCR5 inhibitor
  • Fuzeon binds to gp41 and interferes with its ability to approximate the two membranes, and is referred to as a fusion inhibitor
• Influenza:
  • Relenza and Tamiflu are used to treat influenza by inhibiting the enzyme neuraminidase, which is required for attachment and detachment of the influenza virus to host cells.

Nucleoside Analogs

• These block transcription of DNA.
• Nucleoside analogs can also affect host cells.
• Virally infected cells are more active than host cells.
• Nucleoside analog drugs have the strongest effect on infected cells.
• Acyclovir and ganciclovir treat Herpes Simplex virus infections.
• Zidovudine or azidothymidine (AZT) treats HIV, and is an analog of thymidine, nucleoside analog reverse transcriptase inhibitor (NRTI) which blocks viral reverse transcriptase.
• Nucleoside analogs mimic natural molecules and prevent virus replication.
• The virus mistakes acyclovir for deoxyguanosine and incorporates it into the DNA chain.
• The 3’-OH group is required for adding the next base in a new DNA chain, which nucleoside analogs miss.
• It impossible to add another base once a nucleoside analog incorporates into a DNA chain, terminating the chain prematurely.

Viral Attachment

• HIV: Celsentri binds to CCR5 to prevent GP120 interaction and is a chemokine receptor antagonist/CCR5 inhibitor, and Fuzeon binds to gp41 to interfere with membrane approximation and is a fusion inhibitor.
• Influenza: Relenza and Tamiflu treat influenza by inhibiting enzyme neuraminidase, which is required for attachment and detachment of the influenza virus to host cells.

Interferons

• Interferon is a cellular protein released during an immune response to viral infection and prevents spread.
• Alpha-interferon treat chronic hepatitis (B and C) infections, but include a lot of side effects.
• Severe flu-like symptoms and bone-marrow suppression are side effects of Alpha-interferon.
• Imiquimod stimulates interferon production and treat genital warts with some side effects.

Antihelminthic Drugs

• Niclosamide treats tapeworm by inhibiting aerobic ATP production and first choice acquired from sushi.
• Mebendazole treats Ascariasis and pinworms and interferes with the worm’s ability to absorb nutrients, with few side effects.

Testing

• Testing determines the appropriate antimicrobial therapy.
• Different species and strains of bacteria are susceptible to different drugs and susceptibility can change over time.
• The best treatment tests for susceptibility before starting and is not always possible.
• Testing is not required if identification is enough and susceptibility is predictable.

Kirby Bauer Test

• Each disk contains a different drug, and after incubation measures inhibition of bacterial growth.
• The diameter measures how sensitive bacteria are to the tested drug.
• Results are compared to a standard table for each drug.
• This test simple and inexpensive.

MIC or Minimum Inhibitory Concentration.

• This is the lowest amount of drug that inhibits growth and reproduction.
• It is a quantitative test for potency of antimicrobial agent.

E-Test

• Combines MIC and disk diffusion to estimate the minimum antibiotic concentration that prevents visible bacterial growth.
• The strip contains a gradient of antibiotic concentrations.
• The MIC is read directly from the scale printed on the strip.

Resistance

• Microorganisms produce enzymes that destroy the active drug.
• Examples Staphylococci produce a B-lactamase destroy penicillin G.
• Microorganisms change their permeability to the drug caused by an outer membrane change decreasing active transport.

Resistance Mechanisms

• Altered structural target for the drug, such as with Penicillin resistance in Streptococcus pneumoniae and enterococci that is attributable to altered PBPs.
• Development of an altered metabolic pathway that bypasses the reaction inhibited by the drug.
• Development of an altered enzyme that can still perform its metabolic function but is much less affected by the drug.

Efflux Pumps and Resistance

• Microorganisms can develop efflux pumps that transport antibiotics out of the cell.
• The development of efflux pumps includes many Gram-positive and Gram-negative organisms, these include tetracyclines, macrolides, fluoroquinolones, and B-lactam agents.