DRUG RESISTANCE OF FURUNCULOSIS IN TROUT 



561 



the zones and abundance and rapidity' of frrowtli, 

 was in tlie medium witii pH S.O. 



As a result of this preliminary test to determine 

 thcoptimumpH range, another experiment wascar- 

 riedout with fi-tippedconmiereial mult i])le disks and 

 single disks prepareil in the laboratory. On stand- 

 ard furunculosis medium, zones of partial growth 

 inhibition appeared around disks witlil milligram 

 of sodium sulfisoxazole, 1.0 and 0.5 milligram oi 

 sodium sulfadiazine, and 0.1 milligram of thio- 

 sulfil.^ No zones appeared around disks containing 

 0.1 milligram of sulfisoxazole, sulfailiazine, elkosin, 

 sulfamerazine, or triple sulfa.'' The same test 

 was repeated with (1) Case Laboratory Sensitivity 

 Test medium "C," as prepared by the manu- 

 facturer, and (2) the same medium bulTered and 

 alkalized to pH 8.0. Residts are presented in 

 table 7. 



Growth of A. salmonicida was strikingly less 

 abundant on nonbuffered "Case" medium, prob- 

 ably because of the presence of dextrose. To test 

 the effect of dextrose, a standard furunculosis 

 medium buffered and adjusted to pH 8.0 was en- 

 riched with 0.5 percent of this sugar. On such a 

 medium and with multiple disks containing as 

 much as 0.5 milligram of sulfonamides per tip, no 

 clear zones appeared with sensitive cultures. 



Table 7. — Sensitiiity of sulfa-sensitive strains of A. sal- 

 niouicida to sulfonamides cultured on Sensitii'ity Test 

 7nediuni "C" {Case) 

 lL = di.'iks prepared at Leetown: PZ = zones of partial growth inhibition] 



A similar test was performed with commercial 

 Mueller Hinton and Trypticase Soy agar media 

 and 7-tipped Multidisks containing 0.1 or 0.5 

 milligram of sulfonamides per disk. The width of 

 the zones could not be recorded because the clear 

 zones were so wide that practically no growth 

 appeared on tlie 10-cm. petri plates. Therefore 



' Ayerst Latx>ratories. 



' A mixture of sulfamerazine, sulfamethazine, and sulfadiazine. 



the test was repeated with single disks, prepared 

 at the Leetown laboratory, which contained 0.1, 

 0.25, and 0.5 milligram of sodium sulfadiazine. 



The results presented in table 8 show that the 

 lack of growth on 10-cm. petri i)lates using multiple 

 ilisks was due to tiie great width of zones produced 

 by disks containing more than 0.1 milligram of sul- 

 fonamide. Therefore use of multiple disks with 

 liigher concentrations of sulfonamides and with 

 Mueller Hinton and Trypticase Soy agar media 

 in 10-cm. petri plates is not advisable. 



Table 8. — Determining sulfonamide sensitivity of A. 

 salmoiiicida using two media and single disks 



Quantity of sodium sulfadiazine per disk 



Width of clear zone 

 (mm.) on— 



O.I milligram. 

 0.25 milligram 

 0.5 milligram. 



Mueller 

 Hinton 

 medium 

 (Dlfco) 



Mm. 

 12 

 13-15 

 15-16 



Trypticase 



Soy agar 



(BBL) 



A/m. 



7-8 

 10 

 12-13 



The sulfonamide-sensitivity test in which filter 

 paper disks containing drugs were employed could 

 be much more conveniently performed under field 

 conditions if petri plates were replaced by test 

 tubes. Therefore, the buffered and alkalized 

 standard furunculosis medium, Mueller Hinton 

 medium (Difco), and Trypticase Soy agar (BBL) 

 were used as agar slants. Media were inoculated 

 by making longitudinal streaks with a loop con- 

 taining a suspension of .4. salmunicida, and disks 

 were deposited in the center of the slant. Excel- 

 lent clear zones appeared (fig. 2) with sulfa- 

 sensitive strains. 



Another approach to the performance of the 

 sulfonamide-sensitivity test under simulated field 

 conditions was made with agar media containing 

 sodium sulfadiazine added to the bufferetl and 

 alkalized standard furunculosis medium. These 

 solutions were sterilized by filtration or were 

 incorporatetl in the medium before sterilization 

 in the autoclave. Li all cases, clear differences 

 were noticed in the growth of sulfonamide- 

 sensitive and sulfonamide-resistant strains of A. 

 salmonicida. The optimum concentrations of 

 sodium sulfadiazine in the medium were 1:500 

 and 1:1,000. 



It is expected that extensive field trials based 

 on the results presented here will permit selection 



