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.-------

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Antibiotic sensitivity test

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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 

     overnight.

 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                         

                            keijihagiwara@gmail.com