NAJRAN UNIVERSITY College of Medicine

1. NAJRAN UNIVERSITY College of Medicine Microbiology &Immunology Course Lecture No. 7 By Dr. Ahmed MoradAsaad Associate Professor of Microbiology

2. ANTIMICROBIALAGENTS DEFINITIONS : Antibiotic:An antimicrobial agent produced by a living organism and cankill or inhibit the growth of other organisms. Chemotherapeutic agents:Synthetic substances (drugs) with similar antimicrobial activities to that of antibiotics. Bactericidal agents:Substances (drugs), which kill and destroy bacteria. Bacteriostaticagents:Substances (drugs), which inhibit the growth andmultiplication of bacteria.

3. Properties of an ideal antimicrobial agent: 1. Selective toxicity: The antimicrobial agent must be harmful to bacteria but relatively harmless (non‑toxic) to the host cells. 2. Broad spectrum: it should affect a variety of bacteria (both Gram positive and Gram negative). 3. Bactericidal agents (which kill and destroy bacteria) are better than bacteriostatic agents (which inhibit the growth and multiplication of bacteria). 4. It should be diffusible into distant body areas with slow rate of excretion. 5. Bacteria (target cells) do not develop resistance to it.

4. Mechanism of action of antimicrobial agents: • 1- Inhibition of cell wall synthesis; e.g. penicillin and cephalosporins. • 2- Inhibition of cell membrane function; e.g. polymyxins, amphotericin‑B, nystatin and colistin. • 3- Inhibition of protein synthesis leading to inhibition of cell growth., e.gchloramphenicol, streptomycin, tetracyclines, griseofulvin and aminoglycosides(Amikin, Gentamicin). • 4. Inhibition of nucleic acid synthesis: • a. RNA inhibition; by inhibiting RNA polymerase of bacteria, (Rifampicin). • b. DNA inhibition: nalidixic acid, novobiocin,metronidazole and quinolones. • 5- Competitive inhibition; e.g. sulphonamides compete with para‑ amino­benzoic acid (PABA) for the active site of the enzyme involved in folic acid synthesis.

6. Choice of Antimicrobial Agent 1) In‑vitro tests of bacterial sensitivity to antibiotics: a. Disc diffusion method:A culture medium is inoculated with the organism. Discs impregnated with different antibiotics are placed and the plate is incubated at 37ºC for 24 hrs. The degree of inhibition by the antibiotic is measured by the diameter of the zone of inhibition (the more the zone of inhibition, the more the sensitivity). b. Dilution method: Serial dilutions of the antibiotic are inoculated with the organism to determine the minimal inhibitory concentration (MIC), which is the lowest concentration of the antibiotic that inhibits growth of the organism.

7. 2) Empirical use of antibiotics • Using antibiotics without in‑vitro tests is indicated in the following conditions : • 1- In closed lesions with no available sample (e.g. brain abscess). • 2- While waiting for the result of in‑vitro sensitivity test.

8. Antimicrobial Drug Combinations This is indicated in the following conditions: 1. In severe infections of unknown (undiagnosed) organism. 2. For complete eradication of the organism to avoid occurrence of complications. 3 In severe mixed infections with organisms that have different drug sensitivities. 4. To avoid development of resistant strains especially with prolonged course of treatment, e.g. in tuberculous infections. 5. Synergism: The combined effect of a pair of antimicrobial agents is greater than the sum of their independent activities, E.g., use of penicillin and genatamicin in treatment of enterococcalendocarditis

9. Complications of antimicrobial chemotherapy 1. Toxicity : a. Streptomycin is toxic to the 8 thcranial nerve. b. Chloramphenicol may cause bone marrow depression (aplasia). c. Aminoglycosides (gentamicin, amikacin, ..) are nephrotoxic. d. Tetracyclines may cause teeth deformity and liver damage. 2. Hypersensitivity :Some antimicrobial agents act as haptens. When they bind to host macromolecules they become antigenic resulting in one of the hypersensitivity reactions

10. 3. Superinfection: During treatment with a broad ‑spectrum antibiotic, susceptible members of normal flora at different sites of the body are killed. This will result in a biological imbalance. Bacteria resistant to the antibiotic used may cause a superinfection (i.e. an infection on top of antibiotic administration), e.g. 1- Candida may cause oral thrush or vaginitis. 2- Pseudomonas and Staphylococci may cause wound infections in hospitalized patients. 3- Gram negative bacilli (Proteus, Pseudomonas, E. coli) may cause genital or respiratory tract infections. 4- Clostridium defficileand Staphylococci may cause enterocolitis.

11. 4. Masking the diagnosis: Early administration of antibiotics may decrease the antigenic mass needed to stimulate antibody formation. This will interfere with serological diagnosis. e.g. typhoid fever. 5. Development of resistant strains: Resistant strains may emerge due to: - Selection of bacteria which are naturally resistant to the antibiotic given. -Genetic variations (e.g. mutation or gene transfer by conjugation, transformation, transduction or lysogenic conversion).

12. The organism may acquire resistance due to one of the following mechanisms: • ‑ Production of specific enzymes to inactivate the antibiotic, e.g. Staphylococci produce P‑lactamase enzyme to inactivate penicillin and cephalosporins. • -Decreased permeability to antibiotics to prevent it s entrance. e.g. Streptococci are resistant to aminoglycosides by this mechanism. • -Alteration of target sites or receptors for the drug action, e.g. organisms resistant to erythromycin and chloramphenicol have an altered receptor site in the 50S component of their ribosomes. • - Development of alternative metabolic pathway to bypass the pathway which is inhibited by the drug, e.g. organisms resistant to sulphonamide utilize the folic acid of the surrounding media (sparing ) with no need for PABA.

13. Limitation of Drug Resistance Emergence of drug resistance in infections may be minimized in the following ways: 1. Maintain high levels of the drug in the tissue to inhibit both the original population and first step mutants. 2. Simultaneously administer two drugs, each of which delays the emergence of mutants resistant to the other drug. 3. Avoid exposure of micro‑organisms to a particularly valuable drug by restricting its use

14. Chemoprophylaxis It is the administration of antimicrobial drugs to prevent infection i.e. before the development of a disease. Examples: 1- The injection of penicillin G, once every 3‑4 week to prevent reinfection with group A β-hemolytic streptococci in rheumatic fever. 2- Prevention of meningitis by eradicating the meningococci with rifampin. 3- Prevention of syphilis by the injection of penicillin G. 4- Prevention of cholera by oral administration of tetracyclines. 5- Prevention of recurrent urinary tract infection by oral intake of nitrofurantion. 6- In major surgical procedure, the administration of a broad spectrum bactericidal drug from just before until after the procedure, has been found effective.