332 DIFCO MANUAL 



static or bactericidal materials has been described by numerous investigators. 

 Likewise, many investigators have used this technique to determine the resistance 

 or susceptibility of microorganisms to antibiotics or other inhibitory substances. 

 Morley^ dried sulfathiazole and penicillin on paper disks for determining the 

 sensitivity of wound bacteria to these materials. O'Toole^ reported that this 

 method proved to be simple and economical. She suggested modifications making 

 the test suitable for determining the sensitivity of various species of micro- 

 organisms in the routine laboratory and gave tables showing the stability of the 

 dried disks, and also the size of zones produced by various concentrations of 

 penicillin, streptomycin and sulfathiazole. Bondi, Spaulding, Smith and Dietz^ 

 described a method for the rapid determination of the susceptibility of micro- 

 organisms to penicillin and other antibiotics stressing that this method can be 

 used at the time of primary isolation, thereby eliminating the time required for 

 pure culture isolation and identification prior to selecting the antibiotic for treat- 

 ment. They saturated sterile disks with solutions of the antibiotic and placed 

 them on the surface of Blood Agar plates immediately after inoculation with 

 clinical specimens. Kolmer, Spaulding and Robinson* described the paper disk 

 technique of determining susceptibility of bacteria to antibiotics. They pointed 

 out that the method is a practical, simple and a rapid means of estimating sensi- 

 tivity. Many modifications of these and similar tests are in current use to deter- 

 mine the resistance or sensitivity of microorganisms to antibiotics and other 

 therapeutic agents. 



The use of antibiotics in the treatment of many infectious diseases is an estab- 

 lished procedure. The sensitivity of groups of many pathogenic bacteria to the 

 various antibiotics and other therapeutic agents is known, and infections caused 

 by these microorganisms usually respond to treatment with the designated anti- 

 biotic. However, the clinician frequently finds that an infection is not responding 

 as expected to a specific treatment. In such cases he must accurately and rapidly 

 determine the most effective antibiotic to use to overcome the infection. If treat- 

 ment is not effective against the organism, there is danger of the possible sensitiza- 

 tion of a patient to an antibiotic or other therapeutic agent which may be re- 

 quired at some future date. It is also possible to increase the resistance of micro- 

 organisms to these materials if ineffective dosages are used in treatment. 



Determination of susceptibility of microorganisms to various antibiotics has 

 shown a wide variation in individual strains. The publications of Finland, Frank 

 and Wilcox^ '^''^•^ clearly show a wide range of sensitivity between different 

 species or groups of organisms. Barber and Rozwadowska-Dowzenko^ reported 

 that the proportion of staphylococci isolated from hospital patients resistant to 

 penicillin increased from 14 per cent in 1946 to about 60 per cent in 1948. Davi- 

 son^o suggested that penicillin blood levels in 1949 had to be 15 times higher than 

 in 1946 in order to inhibit the growth of 80 per cent of the Gram-positive cocci 

 found in nasal exudates. The more sensitive organisms are killed by inadequate 

 dosage of penicillin while the more resistant strains survive, becoming progres- 

 sively more resistant. Miller and Sherlock-Hubbard^^ reported that over the 

 seven year period, 1945-1951, the resistance of pathogenic staphylococci to peni- 

 cillin had increased markedly. In addition during the last two years comparative 

 resistance to the broad spectrum antibiotics on the part of the usually sensitive 

 organisms has increased. Increased resistance of the tubercle bacillus to strepto- 

 mycin was also shown. The clinician must rely on the laboratory for the deter- 

 mination of the sensitivity of microorganisms in cases where prompt response is 

 not obtained with the usual antibiotic therapy. The use of Bacto-Sensitivity Disks 

 supplies this information for all the more commonly employed antibiotics with 

 a minimum of laboratory time. 



Sweeney, Davis and Barnes^^ ^^^^ Bacto-Sensitivity Disks to determine the 



