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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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College of Applied Medical Sciences

General Microbiology Course

Lecture No. 10


Dr. Ahmed MoradAsaad

Associate Professor of Microbiology


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


Properties of ideal antimicrobial agent

1- Selective toxicity

2- Broad spectrum

3- Bactericidal agent

4- Diffusable

5- No bacterial resistance


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)


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


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


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


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


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