Comparison StudyofBrothMacrodilution andMicrodilution Antifungal Susceptibility Tests

Anevaluation ofbroth dilution antifungal susceptibility tests was performed bydetermining boththemicroandmacrodilution MICsofamphotericin B,flucytosine, fluconazole, ketoconazole, andcilofungin against 38 isolates ofCandida albicans, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Cryptococcus neoformans, andTorulopsis glabrata. Thefollowing preliminary antifungal working grouprecommendations ofthe National Committee forClinical Laboratory Standards forbrothmacrodilution tests withantifungal agents wereused: inocula standardized to1x 104to5x 104CFU/mlwith aspectrophotometer, RPMI1640medium buffered withmorpholinopropanesulfonic acid (pH7.0), incubation at35°Cfor24to48h,andan additive drug dilution procedure. Brothmicrodilution MICswere higher (twoor more dilutions) thanbrothmacrodilution MICsforall isolates tested withamphotericin Bandformostisolates tested withketoconazole, fluconazole, and cilofungin. MICsofflucytosine were thesame bybothtechniques or lowerbythebrothmicrodilution test except intests withC.neoformans. However, theonlystatistically significant differences between thetwotests wereobserved withamphotericin Bagainst allisolates (P= 0.01 to0.07), ketoconazole against C.neoformans (P= 0.01 to0.02), andcilofungin against C.albicans (P= 0.05 to0.14). Tests performed withless denseinocula (1x 103to5x 103) produced similar results. MICsofantifungal drugs canbedetermined byusing broth macro- andmicrodilution andagar dilution techniques. MICsquantitatively measure theinvitro activities ofantifungal compounds against isolates ofeither yeasts ormolds. Infections causedbyfungihavebecomemore frequent because oftheincreased use ofcytotoxic andantibacterial drugsand,more recently, because oftheoccurrence of serious fungal infections inpatients withAIDS.Although there isno needtoperform antifungal susceptibility testsfor everyfungal isolate that causesdisease, there areinstances, listed below, whenthese tests are warranted andmay be extremely useful. Susceptibility testing withthesynthetic antimetabolite flucytosine isdesirable for allyeastisolates, some isolates of Aspergillus spp.,andsome ofthedematiaceous fungi. Most otherfungi, including thedimorphic fungi, areresistant to flucytosine, andhence, these tests arenotneeded. Invitro susceptibility testswithflucytosine arenotonlyimportant foryeasts prior totreatment butalsoforisolates associated withtherapeutic failures orrelapses. Acquired resistance to flucytosine by some yeastshasbeendemonstrated (17). Mostfungal isolates aresusceptible tothepolyene amphotericin B;however, resistance tothis drugbysome isolates ofCandida spp.,Candida lusitaniae inparticular, hasbeen reported. Also, variable results may beobtained withisolates ofAspergillus spp.(MICrange,0.05to8 ,ug/ml), Pseudallescheria boydii (MICrange,1.56to-100,ug/ml), Sporothrix schenckii (MICrange,1.56to12.5,ug/ml), and theZygomycetes (MICrange,0.78to4jig/ml) (17). Susceptibility testing ofthesynthetic imidazoles andtriazoles for yeasts hasyettobeprovenbeneficial. Thesusceptibilities of yeasts tothese drugs arevariable, andselected resistance to themhasbeenreported (16). Thesystemic fungi areusually susceptible totheimidazoles. Forallthesereasons, selected