06 October, 2008

Antibiotic sensitivity testing

Photo: Staphylococcus aureus were seeded in agar plate.
       Left: Aztreonam disc(30μg/ml).
        Right: transparent hallow(no growth of bacteria) is 
                    seen inVancomycin disc(30μg/ml).
Photo: The antibiotic sensitivity testing 
             (Disc diffusion method).
    The minimal inhibitory concentration(MIC) of VAM and AZT
      against to this staphylococcus strain were >32μg/ml and <=2μg/ml,
      respectively. MIC is the lowest concentration of an antibiotics that
      inhibit the visible growth of a bacteria after overnight incubation.
      ----- Mr. Mizuno, Department of Clinical Laboratory, Yamaguchi
             University School of Medicine.-------


In 1940, Heatley devised the "cylinder plate assay" to test the 
strength of penicillin.

The culture plates were made in the ordinary way, and sown
evenly with test organism usually the 'standard staphylococcus'.
Then placed short lengths of glass tubing on the surface of the
agar, having bevelled the lower edges to sharpen them.
These little cylinders, usually six or eight to each plate, were
then filled the test solution, the cover put on, and the plate
incubated while the solution in the cylinders diffused into the
agar in a widening circle. If it contained penicillin, the organisms
failed to grow within a certain distance of the cylinder, and the
diameter of this circle of inhibition was an index of the strength
of the penicillin solution.

Basically, the same method are using for the antibiotic sensitivity 
testing in clinical laboratory.

  i) Tested bacteria, which isolated from clinical specimen, 
      for example, Staphylococcus aureus was seeded the agar plate.
 ii) After seeding, small wafers(discs), which contain Vancomycin 
     or Aztreonam, were placed on agar plate and incubated 
 iii) Large clear hallow(photo, right) which indicate no growth of 
     bacterias, was seen in Vancomycin disc(VAM,30μg/ml, right) 
     and not in Aztreonam disc(AZT,30μg/ml, left).

Any questions:  write to Keiji Hagiwara, MD