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NAJRAN UNIVERSITY College of Applied Medical Sciences. General Microbiology Course Lecture No. 10. By. Dr. Ahmed Morad Asaad Associate Professor of Microbiology. Antimicrobial agents

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Najran university college of applied medical sciences

NAJRAN UNIVERSITY

College of Applied Medical Sciences

General Microbiology Course

Lecture No. 10

By

Dr. Ahmed MoradAsaad

Associate Professor of Microbiology


Najran university college of applied medical sciences

Antimicrobial agents

Antibiotic: An antimicrobial agent produced by a living organism and can kill or inhibit the growth of other oragnisms

Chemotherapeutic agent: synthetic substance s (drugs) with similar antimicrobial activities to that of antibiotics

Bactericidal agents: substances which kill and destroy bacteria

Bacteriostatic agents: substances which inhibit the growth and multiplication of bacteria


Najran university college of applied medical sciences

Properties of ideal antimicrobial agent

1- Selective toxicity

2- Broad spectrum

3- Bactericidal agent

4- Diffusable

5- No bacterial resistance


Najran university college of applied medical sciences

Mechanism of action of antimicrobial agents

1- Inhibition of cell wall synthesis (Penicillin and cephalosporins)

2- Inhibition of cell membrane function (polymyxin, amphotericin B and colistin)

3- Inhibition of protein synthesis (chloramphenicol, tetracycline, streptomycin and aminoglycosides: amikacin, gentamycin)


Najran university college of applied medical sciences

4- Inhibition of nucleic acid synthesis:

RNA synthesis: by inhibiting RNA polymerase of bacteria (rifampicin)

DNA inhibition (nalidixic acid, novobiocin and quinolones)

5- Competitive Inhibition: sulphonamides compete with para-amino-benzoic acid (PABA) for the active site of the enzyme involved in folic acid synthesis


Najran university college of applied medical sciences

Choice of antimicrobial agent

1- In vitro tests of bacterial sensitivity to antibiotics

2- Emprical use of antibiotics

1- In vitro tests

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

- The degree of inhibition by antibiotic is measured by the diameter of the inhibition zone


Najran university college of applied medical sciences

  • B- Dilution method

  • Serial dilutions of the antibiotics are inoculated with the organism to determine:

  • Minimum inhibitory concentration (MIC): The lowest concentration of the antibiotic that inhibit the growth of the organism

  • Minimum bactericidal concentration (MBC): The lowest concentration of the antibiotic that kill the organism

  • C- E tsest

  • Strips with antibiotic gradients

  • D- Automated systems


Najran university college of applied medical sciences

Antibiograms:

periodic reports that indicate the susceptibility of clinically isolated organisms to the antibiotics in current local use.


Najran university college of applied medical sciences

Empirical use of antibiotics:

Use of antibiotics without in vitro tests is indicated in:

1- In closed lesions with no available samples (brain abscess)

2- While waiting for the results of in vitro sensitivity tests