Several clinical and laboratory isolates of Candida albicans have a natural blue surface fluorescence when cultured and observed with sensitive optics. The localization and color of the fluorescence are similar to those of the natural fluorescence of sporulated Saccharomyces cerevisiae which is caused by the generation and surface deposition of the cross-linking
amino acid dityrosine. In S. cerevisiae,
dityrosine production results from the direct action of at least two genes and is responsible for resistance of the ascospores to lytic
enzymes and physicochemical
trauma. Among the criteria for the identification of
dityrosine is pH sensitivity of the fluorescence intensity and a highly characteristic shift of the fluorescence excitation maximum with a change in pH. Video microscopy of whole Candida organisms revealed the characteristic
dityrosine intensity maximum at pH approximately 10 and the intensity minimum at pH approximately 2. Separation of an
acid hydrolysate of Candida cell walls by reverse-phase high-performance liquid chromatography revealed a fluorescence peak that coelutes with the
reagent dityrosine. At pH approximately 10, this peak has a fluorescence excitation maximum of 320 to 325 nm, while at pH approximately 2, the excitation maximum is 285 to 290 nm. This excitation maximum shift and the observed emission maximum of approximately 410 nm are characteristic of
dityrosine. Two separate strains of C. albicans were injected intraperitoneally into mice and harvested at 24 h. Blue surface fluorescence was observed, suggesting that
dityrosine generation occurs in vivo as well as in vitro. This is the first report of the presence of
dityrosine in a human fungal pathogen.