Burns (1985) 12,91-96 Printe d in Great Britain 91 Antimicrobial cream susceptibility testing J. M. Conly, S. Byrne, J. McLeod, S. Hoban, G. Robertson and A. R. Ronald Deoartments of Medicine, Surgery, Medical Microbiology and the Health Sciences B&n Unit, University of Manitoba, Canada Summary (Bridges et al, 197Y) and sulphadiazine resistant Gram-negative bacilli (Bridges and Lowbury. 1977; Gayle et al., 1978; Hendry and Stewart. 1979) have appeared. These reports have promp- ted the use of in vitro methods to determine topical agent sensitivity. Nathan et al. (1978) noted a reduction or elimination of bacteria in wounds where treatment was based on in vitro sensitivities utilizing an agar cup diffusion method. However, this technique is time con- suming and in a busy general microbiology laboratory may not lend itself to being adopted as a routine procedure. Rodeheaver et al. (lY80) described a simple inexpensive needle extrusion method for performing in vitro cream susceptibil- ity testing and found excellent correlation with the agar cup diffusion method. These preliminary reports are encouraging and we report here the results of our evaluation of antimicrobial cream sensitivities. Our study was initiated with the following objectives: (1) to assess the correlation of zone sizes produced by the needle extrusion method with the minimal inhibitory concentra- tion of the antimicrobial agent in the cream; (2) to report the sensitivity patterns of major pathogen groups in our burn unit to specific burn creams; (3) to assess the clinical utilization of the test when introduced as routine and whether a reduction in bacteria occurred in wounds treated on the basis of in vitro cream sensitivity testing. Utilizing a recently described rapid needle extrusion method for determining the sensitivities of burn wound isolates, we have compiled data on the resistance pat- terns of over 250 isolates from our burn unit. Major isolate groups were Staphy lococcus aureus, l?w_domo- r1a.s auruginma. various Enterobacteriaceae and Enter- cocci. Excellent correlation was exhibited between the inhibitory zone sizes and the minimal inhibitory con- centration for 120 organisms tested. Utilizing the nee- dle extrusion sensitivities to facilitate the selection of the hurn creams, a significant reduction in the micro- biologic flora of the burn wound was noted. The utiliza- tion of this technique in the selection of burn creams deserves controlled trials to assess whether changes in topical therapy might alter clinical outcome. INTRODUCTION DCIKINC; the past two decades, topical antimicro- bial therapy of burns has become standard prac- tice. In the early 1960s. gentamicin, 0.5 per cent silver nitrate and mafenide acetate were intro- duced and in lY68 silver sulphadiazine became available (Lindberg et al., 1965; Moyer et al., lY65: Fox, 1968; Stone, 1971). Silver sulpha- diazine has been reported to have few side effects and is utilized exclusively in many burn units. Carr et al. (1973) determined the in vitro anti- bactericidal activity of silver sulphadiazine to burn wound isolates and found all strains tested to be inhibited by levels of the drug achieved topically. However, several reports on the emergence of silver resistant Pseudomonas Dr J. Conly is supported by a Fellowship from the Medical Research Council of Canada. Presented, in part, at the X&h Annual Meeting of the American Society for Microbiology Meeting, March l%G, St. Louis, Missouri. MATERIALS AND METHODS Bacteria tested The correlation of the needle extrusion method with the standard agar dilution method was de- termined utilizing I17 blood culture isolates. Additional isolates were obtained from patients with burns in our unit.