334 DIFCO MANUAL 



for clinical purposes. Since in vivo sensitivities cannot be accurately gauged from 

 in vitro levels of sensitivity, mathematical expressions of these might even be 

 misleading. 



There have been some interpretations in the literature to indicate that the tube 

 dilution method of determining sensitivity is the only accurate procedure and 

 that the Disk method does not present the desired information. Continued wide- 

 spread use of the Disk Technique and proper interpretation of results have dem- 

 onstrated the clinical accuracy, efficiency and reliability of this procedure. The 

 tube dilution method is applicable in research procedures and in cases where 

 quantitative determinations are required and where proper consideration can be 

 made of resistant variants and other factors. The tube dilution method because 

 of the time and materials involved is not suitable for routine diagnostic work, 

 and has not offered any advantages over the Disk method in clinical procedures. 



The results obtained with Bacto-Sensitivity Disks, as with other sensitivity tests 

 similarily conducted, are of a qualitative rather than a quantitative nature. The 

 many variables that enter into a test of this type such as the size of the inoculum, 

 type of growth of the organism, amount of inoculum on the plate, thickness of 

 the medium, incubation conditions, diffusion rate of the antibiotic, rate of dete- 

 rioration of the antibiotic during incubation, preclude its quantitative nature. 

 Sensitivities are to be determined by the presence of and not entirely by the 

 diameter of the zone of inhibition around the disks. Organisms showing a zone 

 of inhibition around the disk with the smallest concentration of the antibiotic 

 would be considered very sensitive to that antibiotic; an organism showing no 

 zone around the lowest, but a zone around the intermediate disk would be classed 

 as sensitive or moderately sensitive, while a zone around only the highest concen- 

 tration disk would indicate that the organism is but slightly sensitive to the 

 antibiotic. Resistant organisms produce no zone of inhibition even around the 

 disk containing the highest concentration. As was pointed out by Jawetz in a 

 personal communication, the information obtained by the use of Sensitivity Disks 

 is not only important as an aid in determining which antibiotic may be effective 

 against a specific infection, but also which antibiotics are ineffective. He stressed 

 that knowledge of the resistance and sensitivity of an organism is equally im- 

 portant in saving time and expense in treatment. 



Information of this type has proven to be invaluable to the clinician under 

 practical conditions in selecting the most appropriate antibiotic for treating in- 

 fections. It is stressed that the differences in the diameters of the zones of inhibi- 

 tion obtained with like concentrations of the various antibiotics do not necessarily 

 indicate their relative therapeutic efficiency. The rate of diffusion of the various 

 therapeutic agents from the disks and through the medium influence the diameter 

 of the zone of inhibition. Howe^s in a study of sensitivity testing using paper disks 

 considered only the presence or absence of bacterial growth and gave no attention 

 to the diameter of the zone of inhibition. 



The importance of culture media in sensitivity testing has been shown by 

 many investigators. Repeated comparative tests in our laboratory have shown 

 that Bacto-Heart Infusion Agar, with or without added blood, is well suited for 

 the plating medium, producing excellent growth of a large variety of pathogenic 

 microorganisms. Hemolytic reactions on this medium containing blood are typi- 

 cal, being an added advantage in diagnostic work. Tryptic soy media have been 

 employed in sensitivity testing but we have found no advantage in their use. 

 Waisbren, Carr and Dunnett-* reported inhibition of activity with certain anti- 

 biotics using media of this type. Bacto-Brain Heart Infusion is recommended as 

 a liquid medium for obtaining a rapid profuse growth of the test organism to be 

 used for inoculating the Heart Infusion Agar employed in the disk plate tech- 

 nique of sensitivity testing. This medium also was employed in a turbidimetric 



