Some protozoa, e.g., American and African trypanosomes causing Chagas disease (Schenkman et al., 1991)
and sleeping sickness (Engstler et al., 1995), are
sialylated on their outer surface at various stages during
the course of their life cycle. They are unable to
synthesize Sia by themselves, but acquire these sugars
by a special enzyme, trans-sialidase, from host material,
either from the blood meal of the insect vector or during
their life in the blood stream. Pathogenic fungi can be
surface-sialylated, too, although the mechanism of
acquisition of Sia is unknown (Alviano et al., 1999).
Recently, however, a sialyltransferase was discovered in
Cryptococcus neoformans(Rodrigues et al., 2003).