Chemotherapy antibiotics

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  • Antibiotics (anti-against, bios-life) are the substances able to inhibit microorganisms.

  • Antibacterial agents are any compounds natural, synthetic, or semi-synthetic that are clinically useful in the treatment of bacterial infections.


  • Bacteriostatic agents (sulfonamides, chloramphenicol) inhibit bacterial growth

  • Bacteriocidal agents (penicillin, streptomycin) significantly reduce the number of viable bacteria in the culture. Bactericidal agents generally kill only growing organisms


  • Narrow spectrum antibacterial agents are preferentially active against other Gram + or Gram- bacteria

  • Broad-spectrum antibacterial agents are active against Gr+ and Gr- bacteria


  • The activity of antibiotics is expressed in international unit (IU). IU of penicillin (oxford unit) is the smallest amount of preparation inhibiting the growth of a standard S. aureus strain.

  • One unit of activity (A.U.) corresponds to the activity of 0.6 micrograms (mg) of the chemically pure crystalline sodium salt of benzylpenicillin.

Classification of antibacterial agents

  • 1. According to origin

  • Antibiotics produced by:

    • Fungi - Penicillins
    • Actinomycetes – Streptomycin, Erythromycin, Tetracycline
    • Bacteria - Gramicidins
  • Semisynthetic antibiotics: Ampicillin, Oxicillin

  • Sulfonamides

  • Derivatives of imidazoles: Metronidazole

  • Quinolones: Norfloxacin, Abactal, Ofloxacin

  • Derivative of pyrimidine: Trimethoprim

  • Derivatives of Arsenic (As), Bismuth (Bi), Mercury (Ar), Acridine.

Classification of antibacterial agents

  • 2. On the of chemical composition

  • Beta-lactams: Penicillins, Cephalosporins

  • Derivatives of dioxiaminophenilpropan: Levomycetin (Chloramphenicol)

  • Tetracyclines

  • Aminoglycosides: Gentamicin

  • Macrolides: Erythromycin

  • Acyclic antibiotics: Nistatin

  • Lincosamides: Lincomycin

Classification of antibacterial agents

  • A. Inhibitors cell wall synthesis

  • B-lactams (Penicillins, Cephalosporins)

  • Other inhibitors of bacterial cell wall synthesis: Cycloserine, Vancomycin, Bacitracin

  • B. Inhibitors nucleotide synthesis

  • Sulfonamides: Trimethoprim, Sulfa-methoxazole-trimethoprim.

Classification of antibacterial agents

  • C. Inhibitors nucleic acid synthesis

  • DNA synthesis inhibitors: Novobiocin, Quinolones, Nitroimidazoles (Metronidazole)

  • RNA synthesis inhibitors: Rifampin

  • D. Inhibitors protein synthesis

  • Inhibitors of the 30 s ribosomal unit: Aminoglycosides, Tetracyclines

  • Inhibitors of the 50 s ribosomal unit: Chloramphenicol, Macrolide (Erythromycin, Clarithromycin), Lincomycin

General principles of effective antibacterial therapy

  • The choice of antibacterial agent should be based on susceptibility

  • When the infection is life-threatening or when early treatment is extremely important, treatment is initiated with broad-spectrum antibiotics without detection susceptibility

General principles of effective antibacterial therapy

  • 3. Early treatment usually involves short-term therapy, which has the advantage of reducing the possibility of superinfection.

  • 4. Infections of poorly vascularized tissues (e.g. Endocarditis, Osteomyelitis) should be treated with bactericidal antibiotics.

Antibiotic susceptibility testing

  • Antibiotic susceptibility testing allows the choice of the best antibiotic with the narrowest spectrum and highest effectiveness against the isolated bacteria

    • Tube dilution test
    • The agar diffusion test (Bauer-Kirby test)
    • E-test method
    • Automated tests

Definition of MIC by Tube Dilution Test

  • Minimal inhibitory concentration (MIC) is the lowest concentration of an antibiotic that will inhibit the growth of a microorganism.

Interpretation of Tube Dilution Test

  • A bacterium is considered clinically susceptible to a given antibiotic if a blood level 2-4 times the MIC can be attained with the usual dosages and no appreciable side effects

Kirby - Bauer technique

  • Broth culture of an isolated bacterium is spread into an agar plate and paper disks impregnated with known concentration of different antibio-tics are dropped on the surface of the seeded plate

Antibiotic susceptibility testing plate

  • You should measure the diameter of zones of inhibition in millimeters. The zone seen here measures 20 mm in diameter

Interpretation of Disc Diffusion Test

E-test method

  • Applying the plastic strip impregnated with different concentrations of antibiotic onto the surface of the agar plate

  • Incubating plates. Plates should be read as early as possible after 24 hours incubation and results recorded in the susceptibility book.

  • Measure MIC of antibiotic as shown in Figure.

E-test method

Automated tests

  • These tests measure the inhibitory effect of the antimicrobial agents in a liquid medium by using light scattering to determine growth of the test organism. Results can be obtained within a few hours.

Bacterial Resistance

  • The two basic mechanisms of bacterial resistance

  • Mutation

  • Transfer of resistance genes through plasmids


  • Intrinsic resistance

  • Alteration in the transport system, cell wall, or cell membrane

  • Enzymatic inactivation of the drug

  • Target alteration

Why has antibiotic resistance occurred?

  • Pressure on doctors, by patients, to prescribe antibiotics even when they are not needed.

  • Patients being prescribed antibiotics without the doctor knowing the cause of the infection.

  • Use of antibiotics in animals for growth promotion and prophylaxis, which allows them to enter the human food chain.






  • 1. Organotoxic action: neurotoxic (Streptomycin); toxic action on the haematopoietic system (Tetracyclines).

  • 2. Allergic reactions: angioneurotic oedema, anaphylactic shock, allergic asthma, contact dermatitis (Penicillin).


  • 3. Development of resistant strains of microorganisms which cause various complications

  • 4. Dysbacteriosis - disorders of normal flora of intestine and appears of infections.

  • 5. Immunosupression action.


  • Interferons - are natural substances that have anti-viral properties. Interferons are synthesized by cells infected an virus.

  • Type I

  • -Interferon-α (IFN-α) has maximal antiviral activity

  • -Interferon-β (IFN-β) has intermediate antiviral activity.

  • Mechanism of action IFN-α and IFN-β:

  • 1. Inhibit viral protein synthesis.

  • 2. May block other stages of viral replication, including budding.


  • Type II

  • -Interferon-α (IFN-α) is more active as a lymphokyne than as an antiviral agents.

  • Type III

  • Mechanism of action Interferon-γ (IFN γ)

  • 1. Inhibits viral replication by increasing intracellular nitric oxide levels;

  • 2. Activates cytotoxic T cells, monocytes, macrophages, and natural killer cells, which are able to kill virus-infected cells.

  • IFN-α is used in the treatment of Hepatitis B, C Herpes simplex virus, cytomegalovirus, AIDS - associated Kaposi’s sarcoma.

  • IFN-γ is used as an immunostimulant in the treatment of oncologic disorders and some immunodeficiency diseases.

Synthetic Antiviral Agents

Synthetic Antiviral Agents

Synthetic Antiviral Agents

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