Antimicrobial Chemotherapy

Chemotherapeutic Agents

Definition: Chemical agents used to treat disease, particularly infections caused by pathogenic microbes
they function by inhibiting microbial growth or destruction of the pathogens.
Types:
- The majority of chemotherapeutic agents are classified as antibiotics.
- Antibiotics: These can be categorized into microbial products or their derivatives that kill or inhibit the growth of susceptible microbes. The spectrum of activity can vary greatly.
  - Examples include:
  - Ampicillin and Amoxicillin - Semisynthetic antibiotics effective against a wide range of bacteria.
  - Penicillin G and Penicillin V - Naturally produced antibiotics used primarily for Gram-positive bacterial infections.

Microbial Sources of Antibiotics

Table 34.2: Microbial Sources of Some Antibiotics
- Bacteria:
  - Streptomyces spp.: A significant source of many antibiotics including:
    - Amphotericin B (an antifungal)
    - Chloramphenicol (also synthetic)
    - Kanamycin, Neomycin, Nystatin, Rifampin, Streptomycin, Tetracyclines, Vancomycin, Gentamicin.
  - Micromonospora spp., responsible for producing Streptomycin.
  - Bacillus spp., which include various antibiotics, such as Bacitracin and Polymyxins.
- Fungi:
  - Penicillium spp.: The original source of Penicillin.
  - Cephalosporium spp.: Another group producing various antibiotics, including Bacitracin and Cephalosporins.

General Characteristics of Antimicrobial Drugs

Selective Toxicity: The capacity of the drug to kill or inhibit the pathogen while inducing minimal harm to the host cells.
Therapeutic Use: Represents the effective drug level necessary for clinical treatment to alleviate infections.
Toxic Dose: The drug concentration at which toxic side effects manifest.
Therapeutic Index: Defined as the ratio of the toxic dose to the therapeutic dose and calculated by:
$ext{Therapeutic Index} = \frac{\text{Minimum Toxic Dose to Host}}{\text{Minimum Effective Microbial Lethal Dose}}$
Side Effects: These are undesired effects that may occur during treatment, affecting various bodily functions.
Spectrum of Activity:
- Narrow-Spectrum: Effective against a specific group of pathogens.
- Broad-Spectrum: Effective against a wide variety of pathogens, including both Gram-positive and Gram-negative bacteria.
Effect Mechanisms:
- Bacteriocidal: Agents that kill bacterial cells.
- Bacteriostatic: Agents that inhibit bacterial growth without killing the cells.

Effectiveness of Antimicrobial Agents

Efficacy can differ based on the pathogen involved. For example:
- Penicillins are effective against a broad range of bacteria, including many Gram-positive and some Gram-negative strains.
- Mycobacteria, Gram-negative, and Gram-positive bacteria often necessitate more specific antibiotics like Sulfonamides, Cephalosporins, and Quinolones.
Effectiveness is often quantified through:
- Minimal Inhibitory Concentration (MIC): The smallest concentration of an antibiotic that inhibits bacteria growth.
- Minimal Lethal Concentration (MLC): The lowest concentration of an antibiotic that results in the death of the pathogen.

Properties of an Ideal Antibiotic

Broad Spectrum: Should effectively target a wide array of microbe types.
Stable: Retains its effectiveness over a long shelf-life without degradation.
Soluble: Must be capable of dissolving in bodily fluids to facilitate distribution throughout the body.
Stable Toxicity: Side effects and toxicity must be predictable and manageable.
Nonallergenic: Should have a low incidence of allergic reactions among patients.
Cost-effective: Needs to be reasonably priced for extensive access by patients.
Selectively Toxic: More harmful to pathogens than to human cells.
Resistant Prevention: Should have a low propensity to encourage the emergence of drug-resistant pathogen strains.

Properties of Some Common Antibacterial Drugs

Table 9.1:
- Cell Wall Synthesis Inhibitors:
  - These affect various bacterial types differently;
  - they primarily target Gram-positive bacteria,
  - but some newer agents can impact Gram-negative bacteria.
  - Common side effects include tendonitis and headaches.
- Cell Membrane Disruption:
  - Polymyxin B: Disrupts bacterial cell membrane structure
  - can cause significant kidney damage.
- Antimetabolites:
  - Target microbial folic acid synthesis, including drugs such as Sulfonamides and Trimethoprim;
  - often exhibit broad-spectrum action but accompanied by various side effects.
Key Examples:
- Sulfonamides: Inhibit the utilization of P-aminobenzoic acid (PABA) crucial for the synthesis of folic acid;
- associated with side effects such as nausea and vomiting.
- Isoniazid: Primarily used in the treatment of mycobacterial infections,
- can lead to peripheral neuropathy in susceptible individuals.

Antimicrobial Tests

Dilution Susceptibility Tests: Involves inoculating media with different concentrations of drugs to assess microbial susceptibility;
allows for quantification of resistance levels.
Disk Diffusion Tests:
- Utilizes disks embedded with antibiotics on agar plates where bacteria have been inoculated;
- relies on the diffusion of the antibiotic which creates clear zones indicating areas free of bacterial growth.
The Etest:
- A method that employs a gradient of antibiotics along a strip;
- the MIC can be derived from the intersection point of the zone of inhibition
- the strip on the agar plate.

Classification of Antimicrobial Drugs by Mode of Action

Inhibitors of Cell Wall Synthesis: These drugs disrupt the integrity of the bacterial cell wall, causing cell lysis.
Protein Synthesis Inhibitors: Impact bacterial ribosomes, preventing the derivation of proteins critical for bacterial survival and replication.
Metabolic Antagonists: Block essential microbial metabolic pathways.
Nucleic Acid Synthesis Inhibition: Interfere with the replication and transcription of bacterial DNA or RNA.
Others:
- Incorporates antibacterial, antifungal, antiviral, anti-protozoan drugs, and vaccines, each targeting specific pathogens or pathological processes.
Inhibitors of Cell Wall Synthesis: These drugs disrupt the integrity of the bacterial cell wall, causing cell lysis.
- Penicillins: including Penicillin G, Penicillin V, Ampicillin, Methicillin, Ticarcillin
- Cephalosporins: including Cephalothin, Cefoxitin, Cefoperazone, and Ceftriaxone
- Other Agents: include Vancomycin and Teicoplanin
  - A glycopeptide antibiotic used for the treatment of serious Gram-positive bacterial infections, often reserved for patients who are allergic to penicillin or when resistance to other antibiotics is a concern.
Protein Synthesis Inhibitors: Impact bacterial ribosomes, preventing the derivation of proteins critical for bacterial survival and replication.
- Aminoglycosides:
- Streptomycin
- Gentamicin
- Tetracyclines:
- Chlortetracycline
- Doxycycline
- Macrolides:
- Erythromycin
- Chloramphenicol
Metabolic Antagonists: Block essential microbial metabolic pathways.
- Sulfonamides/Sulfa Drugs:
- Sulfamethoxazole
- Sulfanilamide
- Folic Acid
- Trimethoprim
Nucleic Acid Synthesis Inhibition: These agents interfere with the replication and transcription of bacterial DNA or RNA.
Key Examples:
- Quinolones: These are synthetic antibiotics that inhibit bacterial DNA gyrase, leading to impaired DNA replication.
- Nalidixic acid is an example of this class, primarily used to treat urinary tract infections.
- Rifampin: This antibiotic inhibits RNA synthesis by binding to bacterial RNA polymerase, thereby blocking transcription. It is often used in combination therapies, particularly for treating tuberculosis and other serious infections.

Factors Influencing Antimicrobial Drug Effectiveness

Ability of the drug to penetrate and reach the site of infection effectively.
Susceptibility of the specific microbial pathogen to the particular drug.
The concentration of drug necessary to maintain levels above the MIC must be achievable in bodily fluids.
Presence and mechanisms of drug resistance exhibited by pathogens.

Antimicrobial Drug Resistance

Timeline of Antibiotic Discovery and Resistance: Documents how resistance has been identified over time against various antibiotic classes, highlighting critical dates in antibiotic development.
- Antibiotic Classes:
  - Penicillins: Resistance emerged in 1940.
  - Tetracyclines: Resistance noted in 1953.
  - Macrolides: Resistance identified in 1985.
  - Carbapenems: Resistance observed in 1993.
  - Fluoroquinolones: Resistance arose subsequent to their introduction.
- There is a significant concern over the lack of new antibiotic classes introduced during the past three decades since the initial discoveries in the 1940s.

Spread of Antibiotic Resistance

Mechanisms of Resistance Spread:
- Resistance can develop in animals treated with antibiotics, which may propagate through contaminated meat products during cooking and handling.
- Contaminated water that is used for irrigation or fertilizers can harbor resistant bacteria leading to human infections.
- Human infections due to antibiotic misuse can lead to the emergence and spread of resistant strains within communities and healthcare environments.

The Antibiotic Access vs. Excess Dilemma

Access Issues:
- Remote communities often face logistical challenges that delay access to diagnostics and treatment,
- leading to increased morbidity and mortality from treatable infections.
Excess Issues:
- Urban areas frequently experience overdiagnosis and inappropriate use of antibiotics,
- contributing significantly to the growing problem of antimicrobial resistance.

Genetic Mechanisms of Resistance Acquisition

Conjugative Plasmids: Most resistance genes are transferred among bacteria via plasmids, encompassing processes like:
1. The transfer of the F plasmid from donor to recipient cells.
2. Integration of chromosomal DNA from donor cells into the recipient.
3. Clonal replication and retention of transferred resistance genes in newly formed bacterial populations through division.