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