Analysis of Virulence Factors in Streptococcus iniae
S.iniae was first isolated in 1976 from
skin lesions in a captive dolphin. Since then it has been the
cause of significant mortality in farmed fish particularly tilapia.
Since the beginning of 1996, approximately 12 cases of cellulitis
in humans, associated with handling of tilapia, have been reported
in Toronto. This is the first account of human disease caused
by S.iniae. We have found that there is a particular virulent
strain associated with disease both in fish and in humans. Having
successfully identified sagA by Tn916 mutagenesis we used a similar
approach to inactivate the hemolysin gene(s) from S. iniae.
Tn916 was mobilized from Enterococcus faecalis CG110 to
a hemolytic virulent S. iniae strain. A hemolysin-deficient
mutant containing a single insertion was identified and was used
in a murine model to assess its effect on virulence. A recent
experiment in mice has shown that when injected subcutaneously,
S.iniae does not cause a necrotic lesion as is observed
with GAS, but rather causes bacteremia. Organisms are recovered
from the blood, liver, spleen, and also the brain. The fact that
S.iniae appears to readily cross the blood-brain barrier
in mice is interesting considering that it causes meningoencephalitis
in fish. A preliminary pilot experiment has shown that the hemolysin-deficient
mutant is non-bacteremic. If this observation is reproducible,
the putative hemolysin gene will be cloned following the strategy
previously used for cloning the sagA gene from GAS. CAMP factor
genes from S.uberis and S.agalactiae have already
been cloned and gene sequences are available in the Genbank database.
Using primers based on the conserved regions between these genes,
we have amplified a homologue from S.iniae. This CAMP-factor
like homologue is currently being cloned and will also be inactivated
by insertional mutagenesis in order to assess its role in virulence.
