in vitro culture and isoenzyme analysis of giardia lamblia

3.4 DISCUSSION
Since our source of material for culture initiation was faecally-derived cysts,
progress of this work was largely dependent on successful excystation of
trophozoites. Several excystment procedures were therefore explored in search of
optimal results. It has been acknowledged that none of the available excystment
methods work all the time (Nash, 1988). Furthermore, Schaefer (1990) reported
large variations in excystation results (2-80%) in his review of available techniques
and he concluded that, " Giardia duodenalis do not excyst routinely at high levels
with available methods". There are also other investigators of the same opinion
(Gordts et al., 1985, Mayrhofer et al., 1992). Similarly, in the current study, varying
results were obtained using different excystment methods.
In the classic in vitro excystation work of Bingham and Meyer (1979), it was shown
that hydrochloric acid alone was capable of inducing excystation and that salts
and digestive enzymes did not significantly alter the level of excystation. In our
hands, however, 68 replicated attempts at inducing excystation of cysts obtained
from both symptomatic and asymptomatic donors by use of Hel only and
subsequently incubating in the culture medium (TYI-S-33) as described by AI
Tukhi et al. (1991) were unsuccessful. To exclude the possibility of variation in
ability of cysts to excyst in vitro, the method was repeated (with no success) on
seven samples which had been previously excysted with success using the
method of Hamilton and Jackson, (1990). Therefore failure to successfully repeat
the method described by AI-Tukhi et al. (1991) could not be explained in terms of
an inherent inability of cysts to excyst in vitro.
When salts and trypsin were incorporated into the induction medium (modified
80