Antimicrobial Drugs

History of Chemotherapy and Antimicrobial Discovery

• Definitions and Concepts:

  • Chemotherapy: The administration of drugs that target living cells and tissues, including pathogenic microorganisms.
  • Antimicrobial Drugs: Chemotherapeutic agents that specifically target micro-organisms to control or eliminate infections.

• Ancient Approaches to Antimicrobial Therapy:

  • Ancestral civilizations utilized antimicrobial substances as early as $350\text{ AD}$, though they possessed no mechanistic understanding of how these treatments functioned.
  • These ancient antimicrobial treatments were primarily acquired through the consumption of specific plants and fermented products, such as traditional beers containing tetracycline residues.

• Key Historical Figures and Discoveries in Modern Chemotherapy:
  • Paul Ehrlich (Early 1900s):
  • Screened more than $600$ arsenic-containing chemical compounds to identify an agent capable of curing syphilis without causing fatal toxicity to the host.
  • Identified compound $606$ (Salvarsan), which specifically targeted Treponema pallidum.
  • Established the foundational systematic chemical screening methodology for modern antimicrobial drug discovery.

• Josef Klarer, Fritz Mietzsch, and Gerhard Domagk (1930s):

  • Evaluated synthetic red dyes and discovered the antimicrobial properties of Prontosil.
  • Prontosil effectively treated Streptococcus infections in vivo.
  • Represented the first synthetic antimicrobial agent developed by science, opening the doorway for the development of all sulfonamide ("sulfa") drugs and subsequent synthetic therapeutics.

• Alexander Fleming (1928):

  • Accidentally discovered the first natural antibiotic, penicillin.
  • Observed a contaminant mold colony, Penicillium notatum, creating a clear zone of bacterial growth inhibition on an agar plate inoculated with Staphylococcus species.
  • Penicillin was subsequently isolated, purified, tested, and mass-produced in the $1940s$.

• Dorothy Hodgkin (1940s):
  • Employed X-ray crystallography to analyze and elucidate the complex three-dimensional chemical structure of penicillin.
  • Determination of this structure enabled scientists to chemically modify natural penicillin, giving rise to semisynthetic antimicrobials.

• Selman Waksman (1940s):
  • Conducted comprehensive screening of soil microorganisms and discovered several key natural antimicrobials, including streptomycin.
  • Soil bacteria belonging to the group Actinomycetes represent the natural biological source for $\mathbf{>50\%}$ of all natural antibiotics.

Fundamentals of Antimicrobial Chemotherapy

• Classifications of Antibacterial Activity:
  • Bacteriostatic vs. Bactericidal Activity:
  • Bacteriostatic: Drugs that reversibly inhibit bacterial growth and multiplication, allowing the host immune system to clear the pathogen.
  • Bactericidal: Drugs that directly kill target bacteria.
  • Spectrum of Activity:
  • Narrow-Spectrum Antimicrobials: Target only a specific subset or group of bacterial pathogens (e.g., Gram-positive bacteria only).
  • Broad-Spectrum Antimicrobials: Target a wide variety of bacterial pathogens (e.g., both Gram-positive and Gram-negative bacteria).
  • Superinfection Mechanism: The clinical administration of broad-spectrum antimicrobials kills non-resistant normal microbiota alongside target pathogens. This depletion of protective normal flora removes microbial competition, allowing drug-resistant secondary pathogens or opportunistic organisms to proliferate rapidly and cause a superinfection.

• Optimization of Antimicrobial Dosing and Administration:
  • Dosage Parameters:
  • Defined as the total concentration of drug administered within a specific time period.
  • Dosing in pediatric patients is calculated explicitly based on body mass ($\text{mg/kg}$).
  • Standardized adult dosing is generally applied to individuals aged $12\text{ years}$ and older regardless of mass.
  • The optimal dosage achieves high clinical drug efficacy while minimizing adverse side effects or host toxicity.
  • Routes of Administration:
  • Choice of route dictates plasma concentration profiles over time.
  • Oral Administration: Slower absorption resulting in lower peak plasma concentration relative to systemic routes; preferred for convenience and patient compliance.
  • Intramuscular (IM) Administration: Achieves higher plasma drug levels than oral routes, with sustained concentration decay over time.
  • Intravenous (IV) Administration: Provides immediate, rapid elevation to peak plasma concentration, achieving significantly higher drug levels in plasma than oral or IM routes.

• Drug-Drug Interactions:
  • Synergistic Interactions:
  • Occur when two drugs administered simultaneously produce an overall therapeutic effect greater than the sum of their individual effects.
  • Example: Co-administration of Trimethoprim and Sulfamethoxazole (Bactrim) produces a combined bactericidal action where each individual drug is merely bacteriostatic.
  • Antagonistic Interactions:
  • Occur when two drugs interact to decrease efficacy or cause increased adverse host toxicity.
  • Example: Concurrent administration of Rifampin and oral contraceptives leads to enhanced hepatic metabolism of estrogen, markedly decreasing birth control efficacy.

Mechanisms of Antibacterial Drugs

• Selective Toxicity:
  • The core requirement of antimicrobial agents to selectively target, inhibit, or destroy microbial structures or metabolic pathways without causing damage to human host cells.

• Inhibitors of Bacterial Cell Wall Synthesis:
  • $\mathbf{\beta}$-Lactam Antibiotics:
  • Characterized structurally by the presence of a central $\beta$-lactam ring.
  • Structurally resemble the D-alanyl-D-alanine peptide terminus of peptidoglycan subunits.
  • Directly bind Penicillin-Binding Proteins (PBPs), inhibiting transpeptidase activity and blocking cross-linking of peptide side chains during new peptidoglycan synthesis.
  • Penicillins: Natural derivatives obtained from fungi (Penicillium); narrow-spectrum primarily targeting Gram-positive species and a few Gram-negative species.
    • Penicillin G & Penicillin V: Natural penicillins; Penicillin G administered parenterally, Penicillin V orally.
    • Ampicillin & Amoxicillin: Semisynthetic penicillins with enhanced spectrum against Gram-negative bacteria.
    • Methicillin: Semisynthetic penicillin engineered to resist degradation by bacterial penicillinase ($\beta$-lactamase); narrow-spectrum Gram-positive activity.
  • Cephalosporins: Possess a $\beta$-lactam ring fused to a six-membered ring; structurally resistant to $\beta$-lactamase degradation.
    • Cephalosporin C: Natural antibiotic with narrow spectrum similar to penicillin but improved Gram-negative coverage.
    • First-Generation Cephalosporins: Semisynthetic; narrow-spectrum.
    • Second-Generation Cephalosporins: Semisynthetic; increased Gram-negative spectrum compared to first generation.
    • Third- and Fourth-Generation Cephalosporins: Semisynthetic; broad-spectrum against Gram-positive and Gram-negative bacteria, active against some $\beta$-lactamase producers.
    • Fifth-Generation Cephalosporins: Semisynthetic; broad-spectrum activity including Methicillin-Resistant Staphylococcus aureus (MRSA).
  • Monobactams:
    • Aztreonam: Semisynthetic monocyclic $\beta$-lactam drug; narrow-spectrum activity against Gram-negative bacteria only, including some $\beta$-lactamase producers.
  • Carbapenems:
    • Imipenem, Meropenem, Doripenem: Semisynthetic $\beta$-lactam drugs possessing the broadest spectrum of activity among all $\beta$-lactams against Gram-positive and Gram-negative bacteria, including many $\beta$-lactamase-producing pathogens.
  • Glycopeptides:
  • Vancomycin: Natural large glycopeptide molecule that binds directly to the D-Ala-D-Ala peptide chain end of peptidoglycan subunits, sterically blocking both transglycosylation and transpeptidation reactions. Narrow-spectrum against Gram-positive bacteria only, including multidrug-resistant strains.
  • Polypeptide Cell Wall Inhibitors:
  • Bacitracin: Natural antibiotic derived from Bacillus subtilis. Directly blocks the activity of bactoprenol, preventing the active transport of peptidoglycan precursors (NAG-NAM units) across the cytoplasmic membrane to the exterior cell wall. Broad-spectrum activity against Gram-positive and Gram-negative bacteria.

• Inhibitors of Bacterial Protein Synthesis:
  • Agents Targeting the 30S Ribosomal Subunit:
  • Aminoglycosides:
    • Streptomycin, Gentamicin, Neomycin, Kanamycin: Bind to the 30S ribosomal subunit and impair proofreading activity. Causes codon-anticodon mismatches during translation, leading to faulty proteins that insert into and disrupt the bacterial cytoplasmic membrane. Broad-spectrum, bactericidal.
  • Tetracyclines:
    • Tetracycline, Doxycycline, Tigecycline: Bind to the 30S ribosomal subunit and sterically block the attachment of aminoacyl-tRNA to the mRNA-ribosome complex at the A site. Broad-spectrum, bacteriostatic.
  • Agents Targeting the 50S Ribosomal Subunit:
  • Macrolides:
    • Erythromycin, Azithromycin, Telithromycin: Bind to the 50S subunit and inhibit peptide bond formation by blocking transpeptidation or translocation along mRNA. Broad-spectrum, bacteriostatic.
  • Lincosamides:
    • Lincomycin, Clindamycin: Bind to the 50S subunit and inhibit peptide bond formation. Narrow-spectrum, bacteriostatic; active against streptococcal and staphylococcal species.
  • Chloramphenicol:
    • Binds to the 50S subunit and inhibits peptidyl transferase activity, preventing peptide bond formation. Broad-spectrum, bacteriostatic; clinical usage is severely restricted due to risk of lethal side effects such as aplastic anemia.
  • Oxazolidinones:
    • Linezolid: Binds to the 50S ribosomal subunit and interferes with the formation of the initiation complex between 50S and 30S subunits. Broad-spectrum, bacteriostatic.

• Inhibitors of Bacterial Membrane Function:
  • Polymyxins:
  • Lipophilic polypeptide molecules that act as surfactant detergents.
  • Polymyxin B: Interacts with lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria, disrupting outer and cytoplasmic membranes. Used in topical preparations to treat/prevent wound infections.
  • Polymyxin E (Colistin): Disruption of Gram-negative outer/inner membranes; administered orally for bowel decontamination in immunocompromised patients, or intravenously to treat systemic infections caused by multidrug-resistant Gram-negative pathogens.

• Lipopeptides:

  • Daptomycin: Cyclic lipopeptide that inserts into the cytoplasmic membrane of Gram-positive bacteria, inducing membrane depolarization and cell death. Narrow-spectrum Gram-positive activity, used for complicated skin infections and bacteremia caused by MRSA.

  • Inhibitors of Bacterial Nucleic Acid Synthesis:

• Metronidazole:

  • Interfere with DNA replication by inducing DNA strand breakage; requires intracellular reduction found in anaerobic conditions. Targets anaerobic bacteria and protozoa; broad-spectrum, bactericidal.

• Rifamycins:

  • Rifampin: Directly inhibits bacterial RNA polymerase activity, blocking transcription. Narrow-spectrum against Gram-positive and select Gram-negative organisms; active against Mycobacterium tuberculosis. Used in combination therapy due to rapid resistance emergence; exhibits hepatotoxicity and drug antagonism.

• Fluoroquinolones:

  • Ciprofloxacin, Ofloxacin, Moxifloxacin: Directly inhibit bacterial DNA gyrase (topoisomerase II) and topoisomerase IV, blocking DNA replication fork progression. Broad-spectrum, bactericidal.

  • Inhibitors of Metabolic Pathways (Antimetabolites):

• Folic Acid Synthesis Inhibitors:

  • Sulfonamides (Sulfamethoxazole): Structural analogs of para-aminobenzoic acid (PABA); competitively inhibit the enzyme folate synthase (dihydropteroate synthase), blocking dihydrofolic acid synthesis. Broad-spectrum, bacteriostatic.
  • Sulfones (Dapsone): Inhibits folic acid synthesis pathway; broad-spectrum activity.
  • Trimethoprim: Structural analog of dihydrofolic acid; competitively inhibits dihydrofolate reductase, blocking conversion of dihydrofolic acid to tetrahydrofolic acid. Broad-spectrum, bacteriostatic.
  • Synergistic Combination: Sulfamethoxazole and Trimethoprim administered together provide sequential blockade of folic acid synthesis, resulting in bactericidal action.

• Mycolic Acid Synthesis Inhibitors:

  • Isoniazid: Specifically targets mycobacteria by inhibiting enzymatic synthesis of mycolic acid, an essential lipid component of mycobacterial cell walls. Narrow-spectrum against Mycobacterium species, including M. tuberculosis.

  • Inhibitors of ATP Synthase:

• Diarylquinolines:

  • Inhibits mycobacterial growth by directly binding to the C-subunit of bacterial ATP synthase, disrupting cellular energy generation and reducing intracellular ATP pools.

Mechanisms of Non-Antibacterial Antimicrobial Drugs

• Antifungal Drugs:
  • Targeting Membrane Sterols:
  • Human cell membranes utilize cholesterol, whereas fungal membranes incorporate ergosterol. Selective toxicity relies on targeting ergosterol synthesis or integrity.

* **Imidazoles:** *Miconazole, Ketoconazole, Clotrimazole:* Disrupt ergosterol biosynthesis. Clinical use: fungal skin infections (tinea pedis, tinea cruris, ringworm) and vaginal yeast infections.
* **Triazoles:** *Fluconazole:* Inhibits ergosterol biosynthesis. Administered orally or intravenously for systemic yeast infections, oral thrush, and cryptococcal meningitis.
* **Allylamines:** *Terbinafine (Lamisil):* Inhibits squalene epoxidase, an early enzyme in ergosterol biosynthesis. Clinical use: dermatophytic skin and nail infections.
* **Polyenes:** *Nystatin, Amphotericin B:* Directly bind ergosterol in fungal membranes, forming transmembrane pores that cause ion leakage and cell lysis. Nystatin is used topically; Amphotericin B is reserved for systemic fungal infections.

• Targeting Fungal Cell Wall Synthesis:
  • Echinocandins: Caspofungin: Inhibits synthesis of $\beta(1\rightarrow 3)$ glucan. Clinical use: aspergillosis and systemic yeast infections.
  • Polyoxins and Nikkomycins: Nikkomycin Z: Inhibits chitin synthesis in fungal cell walls. Clinical use: Valley fever (Coccidioidomycosis) and yeast infections.
• Targeting Fungal Microtubules and Cell Division:
  • Griseofulvin: Binds fungal tubulin, disrupting microtubule formation and mitotic spindle assembly during cell division. Used for dermatophytic skin infections.
• Targeting Fungal Nucleic Acid and Protein Synthesis:
  • Flucytosine: Fluorinated pyrimidine analog converted inside fungal cells into an active metabolite that inhibits fungal RNA processing and DNA replication.
• Targeting Fungal Mitochondria:
  • Atovaquone: Antimetabolite targeting fungal mitochondrial cytochrome function.

• Antiprotozoan Drugs:

  • Inhibition of Electron Transport: Atovaquone (Naphthoquinone): Blocks mitochondrial electron transport. Clinical use: malaria, babesiosis, and toxoplasmosis.
  • Inhibition of Folic Acid Synthesis: Proguanil: Inhibits protozoal folic acid synthesis; co-administered with atovaquone. Sulfadiazine and Pyrimethamine target protozoal folate pathways in malaria and toxoplasmosis.
  • Production of Reactive Oxygen Species (ROS): Artemisinin: Generates cytotoxic free radicals in target parasites. Used in combination therapy for malaria.
  • Inhibition of DNA Synthesis: Metronidazole & Tinidazole: Induce lethal DNA strand breakage. Clinical use: Giardia lamblia, Entamoeba histolytica, and Trichomonas vaginalis. Pentamidine: Cleaves kinetoplast DNA and inhibits tRNA binding; used for African sleeping sickness and leishmaniasis.
  • Inhibition of Heme Detoxification: Quinolines (Chloroquine, Mepacrine, Mefloquine): Block the polymerization of toxic heme into non-toxic hemozoin inside malaria-infected erythrocytes.

• Antihelminthic Drugs:

  • Benzimidazoles: Mebendazole & Albendazole: Inhibit helminthic microtubule formation, blocking glucose uptake.
  • Avermectins: Ivermectin: Binds glutamate-gated chloride channels in invertebrate nerve and muscle cells, inducing hyperpolarization, paralysis, and starvation. Clinical use: roundworms, river blindness, strongyloidiasis, and parasitic lice/mites.
  • Niclosamide: Inhibits ATP synthesis under anaerobic conditions. Used for intestinal tapeworm infections.
  • Praziquantel: Induces massive calcium influx into worm tissues, causing intense spastic muscle paralysis. Clinical use: schistosomiasis (blood flukes) and tapeworms.
  • Thioxanthenones: Lucanthone, Hycanthone, Oxamniquine: Inhibit helminthic RNA synthesis; used for schistosomiasis.

• Antiviral Drugs:

  • Nucleoside/Nucleotide Analogs:
  • Acyclovir: Guanosine analog. Monophosphorylated specifically by viral thymidine kinase, then converted to acyclovir triphosphate by host cellular kinases. Competitively inhibits viral DNA polymerase and causes chain termination during replication. Used for Herpes simplex virus and Varicella-zoster virus.

* *Azidothymidine/Zidovudine (AZT):* Nucleoside reverse transcriptase inhibitor (NRTI) for HIV.
* *Ribavirin:* Used for Hepatitis C virus and Respiratory Syncytial Virus (RSV).
* *Sofosbuvir:* Nucleoside analog for Hepatitis C.

• Inhibitors of Viral Uncoating / Endosomal Escape:
  • Amantadine & Rimantadine: Bind to the M2 ion channel protein of Influenza A, preventing endosomal acidification, viral uncoating, and genomic RNA release.

* *Pleconaril:* Blocks viral uncoating in serious enterovirus infections.

• Neuraminidase Inhibitors:
  • Oseltamivir (Tamiflu), Zanamivir, Peramivir: Inhibit viral neuraminidase (NA), preventing enzymatic cleavage of host cell sialic acid residues and blocking the release of progeny influenza virus particles.
• Antiretroviral Therapy (ART) for HIV:
  • HIV therapeutic challenges: targets CD4+ white blood cells, integrates into the host cell genome as a provirus, and rapidly develops drug resistance mutations.
  • Treatment utilizes a "cocktail" of multiple antiviral drug classes simultaneously:
    • Reverse Transcriptase Inhibitors (AZT, Etravirine): Block viral RNA conversion to DNA.
    • Protease Inhibitors (Ritonavir): Block post-translational processing of viral precursor polyproteins into functional proteins.
    • Integrase Inhibitors (Raltegravir): Block integration of viral cDNA into the host cell chromosome.
    • Fusion / Entry Inhibitors (Enfuvirtide): Block GP120/GP41 structural interaction with CD4/CCR5 receptors, preventing envelope fusion.

Antimicrobial Drug Resistance

• Evolution and Development of Resistance:

  • Arises through natural selection under heightened selective pressure.
  • Factors accelerating resistance development:
  • Misuse and inappropriate prescribing of antimicrobials.
  • Administration of subtherapeutic dosages.
  • Patient noncompliance with full treatment schedules.
  • Resistance traits are acquired via vertical gene transfer (spontaneous mutation and progeny inheritance) or horizontal gene transfer (transformation, transduction, conjugation).

• Biochemical Mechanisms of Antimicrobial Resistance:

  • Enzymatic Inactivation / Modification: Microbes produce enzymes that hydrolyze or alter the drug (e.g., $\beta$-lactamases hydrolyzing $\beta$-lactam rings).
  • Target Modification: Structural alterations in drug binding targets prevent binding (e.g., mutations altering rRNA, RNA polymerase, or PBP active sites).
  • Target Overproduction: Microbe synthesizes excessive quantities of the target enzyme to overwhelm drug concentration.
  • Enzymatic Bypass: Microbe bypasses the targeted enzymatic step by utilizing an alternative metabolic route.
  • Target Mimicry: Microbe synthesizes decoy proteins that bind and sequester the antimicrobial agent away from its functional cellular target (e.g., M. tuberculosis producing pentapeptides that mimic DNA to sequester fluoroquinolones).
  • Prevention of Penetration / Accumulation: Downregulation of outer membrane porins to block drug entry, or upregulation of active efflux pumps to pump drugs out of the cell.

• Clinically Significant Resistant Pathogens ("Superbugs"):
  • Multidrug-Resistant Microbes (MDRs): Pathogens resistant to multiple antimicrobial classes; cause over $2\text{ million}$ infections and $23,000$ deaths annually in the United States.
  • Cross-Resistance: A single resistance mechanism (e.g., efflux pump upregulation) confers resistance to multiple distinct antibiotic classes simultaneously.
  • ESKAPE Pathogens: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species; primary drivers of severe nosocomial infections.
  • Vancomycin Resistance:
  • Vancomycin-Resistant Enterococci (VRE): Alteration of peptidoglycan subunit terminal peptides from D-Ala-D-Ala to D-Ala-D-Lac, preventing vancomycin binding.
  • Vancomycin-Resistant Staphylococcus aureus (VRSA): Acquired resistance transposons from VRE via horizontal gene transfer.
  • Vancomycin-Intermediate Staphylococcus aureus (VISA): Cell wall thickening with excessive un-crosslinked peptide chains that bind and trap vancomycin in outer cell wall layers.
  • Methicillin-Resistant Staphylococcus aureus (MRSA): Acquired the mecA gene encoding a novel Penicillin-Binding Protein (PBP2a) with low affinity for all $\beta$-lactam antibiotics.
  • Extended-Spectrum $\mathbf{\beta}$-Lactamases (ESBLs): Plasmid-encoded enzymes that hydrolyze all penicillins, cephalosporins, monobactams, and $\beta$-lactamase inhibitors, but remain susceptible to carbapenems.
  • Carbapenem-Resistant Enterobacteriaceae (CRE): Produce carbapenemase enzymes that hydrolyze all $\beta$-lactams including carbapenems; can exhibit pan-resistance.
  • Resistant Tuberculosis:
  • MDR-TB: Resistant to both Isoniazid and Rifampin.
  • XDR-TB: Resistant to Isoniazid, Rifampin, any fluoroquinolone, and at least one second-line injectable drug (capreomycin, kanamycin, amikacin).

* Global Epidemiology ($2017$ data): $10\text{ million}$ new cases worldwide, $1.6\text{ million}$ deaths, $558,000$ new MDR-TB cases. High burden countries: India ($2.7\text{ million}$), China ($889,000$), Indonesia ($842,000$), Philippines ($581,000$), Pakistan ($525,000$), Nigeria ($418,000$), Bangladesh ($364,000$), South Africa ($322,000$).

Testing the Effectiveness of Antimicrobials

• Kirby-Bauer Disk Diffusion Assay:
  • Standardized agar diffusion test used to evaluate bacterial susceptibility to antimicrobial agents.
  • Filter paper disks impregnated with known concentrations of drug are placed on an agar plate inoculated with a bacterial lawn.
  • Measurement of the clear Zone of Inhibition around each disk reflects antibacterial activity, and comparison against standard reference charts classifies strains as Resistant, Intermediate, or Susceptible.

• Quantitative Dilution Testing Methods:
  • Minimal Inhibitory Concentration (MIC): The lowest drug concentration that visibly inhibits growth of the target bacterial strain in culture.
  • Minimal Bactericidal Concentration (MBC): The lowest drug concentration that kills $\mathbf{\ge 99.9\%}$ of the original bacterial inoculum.
  • Broth Dilution Test: Serial dilution of antimicrobials in broth tubes to quantify MIC.

• Etest (Epsilometer Test): Combination of disk diffusion and dilution techniques. A plastic strip containing a predefined exponential concentration gradient of an antimicrobial is placed on an inoculated agar surface. The MIC value is read directly where the elliptical zone of inhibition intersects the printed strip scale.

• Microdilution Tray Testing: Automated broth dilution assays performed in $96$-well microtiter plates to evaluate multiple antimicrobial MICs concurrently.

Current Strategies for Antimicrobial Discovery

• Modern Discovery Approaches:

  • Development of modified semisynthetic derivatives to overcome known resistance mechanisms.
  • High-throughput automated screening of vast microbial and soil product libraries.
  • Cultivating previously unculturable soil bacteria in situ using specialized platforms such as the iChip, which led to the discovery of Teixobactin (a cell wall inhibitor targeting peptidoglycan and teichoic acid synthesis).
  • Bioprospecting marine organisms and environments for novel antimicrobial metabolites.
  • Application of combinatorial chemistry to generate extensive chemical structural variants.
  • Developing targeted resistance mechanism inhibitors (adjuvants) to restore the clinical efficacy of legacy drugs (e.g., $\beta$-lactamase inhibitors like clavulanic acid).
  • Developing anti-virulence agents that disrupt pathogenic virulence factors without exerting direct selective pressure on bacterial growth.

• FDA Antimicrobial Approval Trends ($1983$–$2012$):

  • The number of new antimicrobial drugs approved by the FDA declined continuously over a 30-year period:
  • $1983$–$1987$: $16$ approvals
  • $1988$–$1992$: $14$ approvals
  • $1993$–$1997$: $10$ approvals
  • $1998$–$2002$: $7$ approvals
  • $2003$–$2007$: $5$ approvals
  • $2008$–$2012$: $2$ approvals