Implications of the "EUCAST trailing" phenomenon in C. tropicalis for the in vivo susceptibility in invertebrate and murine models. [Susceptibility]

Candida tropicalis isolates often display reduced but persistent growth (trailing) over a broad fluconazole concentration range during EUCAST susceptibility testing. Whereas weak trailing (<25% of the positive growth control) is common and found not to impair fluconazole efficacy, we investigated if more pronounced trailing impacted treatment efficacy.

Fluconazole efficacy against two weakly (≤25% growth), two moderately (26-50% growth), one resistant heavily (>70% growth) trailing, and one resistant (100% growth) isolates was investigated in vitro and in vivo (a Galleria mellonella survival model and two non-lethal murine models). CDR1 expression levels and ERG11 sequences were characterised.

Survival in fluconazole-treated G. mellonella was inversely correlated with the degree of trailing (71% to 9% survival in treatment groups). In mice, resistant and heavily trailing isolates responded poorly to fluconazole treatment. The CDR1 expression was significantly higher in trailing and resistant compared to wild-type isolates (1.4-fold to 10-fold higher). All isolates exhibited ERG11 wild-type alleles.

Heavily trailing isolates were less responsive to fluconazole in all in vivo models indicating an impact on fluconazole efficacy. CDR1 upregulation may have contributed to the observed differences. Moderately trailing isolates responded less well to fluconazole in larvae only. This confirms clinical data suggesting fluconazole efficacy against infections with such isolates in less severely ill patients and supports the current 50% growth endpoint for susceptibility testing. However, it is still unclear if the gradual loss of efficacy observed for moderately trailing isolates in the larvae model may be a reason for concern in selected vulnerable patient populations.

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