which the microorganism must traverse on its route to systemic distribution
would thus have been avoided. The apparent resistance of
tilapia having less than 20 g body weight would therefore probably
not be absolute and might also be broken by manipulating their environment;
this would lend support to our previous hypothesis that
some handling factors must be involved for tilapia weighing more
than 20 g for the disease to develop, besides the obvious presence
of the microorganism. It must be remembered that tilapia weighing
less than 20 g are grown in semi-intensive conditions at 1000 g/m3
density, whereas individuals over that weight are cultured in floating
cages at greater than 40,000 g/m3 density. It is worth recalling that
the fish live in equilibrium with potentially pathogenic bacteria in
both natural and fish-farm aquatic environments and that stability
lasts until the balance becomes broken (Schulze et al., 2006). Both potentially
pathogenic bacteria and actively beneficial and innocuous
microorganisms participate in establishing such a balance, thereby
determining a self-regulating aquatic environment in all its aspects
through their complex physical, chemical and biological interactions.
A nested-PCR for detecting S. agalactiae in frozen and paraffinwax-
embedded tilapia tissue was thus successfully used for identifying
S. agalactiae in naturally infected adult red tilapia. Tilapia having
less than 20 g bodyweight (larvae and fry) proved negative for the
presence of the S. agalactiae 16S-23S rRNA gene, thereby confirming
our previous observations regarding the absence of S. agalactiae
infection in such animals. The results also supported the hypothesis
that establishing tilapia breeding farms which are free of this
pathogen would be a feasible goal as a first prevention and control
measure against the infection and disease. The nested-PCR used
here is a highly reliable technique for research, diagnosis and epidemiological
studies.