Biology Reference
In-Depth Information
Antagonistic Bacteria
The most common method to select potential probiotic bacteria has
been by testing the inhibition potential (
via
production of antagonistic
compounds) of candidate strains against known pathogens, mostly by
growing both on an artifi cial medium (Gram and Hjelm, 2002; Gullian
et al., 2004; Hai et al., 2007; Spanggaard et al., 2001) and seldom in an
in
vivo
challenge (Makridis
et al
.
, 2005). As mentioned by Gram and Hjelm
(2002), “a word of caution should be included since no studies have
documented, (i) that inhibitory substances produced
in vitro
are actually
effective
in vivo
or, (ii) that
in vitro
antagonism is a predictor of
in vivo
effect”. Considering this, we have also to acknowledge that identifying
antagonistic strains can be a hard process. Of 1018 isolates tested for their
antagonistic properties in an agar diffusion assay, only 45 (4.4%) showed
inhibitory properties against
Listonella anguillarum
(although commonly
used, the synonym
Vibrio anguillarum
is not a valid name), a common fi sh
pathogen (Spanggaard
et al
.
, 2001). And even so, only six of the 45 strains
tested, showed any improvement in infected trout; that is 0.59% of the
total strains tested. A similar result was obtained for strains isolated from
a Chilean scallop (
Argopecten purpuratus
) culture system; only 2.2% of the
506 strains analyzed showed any inhibitory effect on a
V. anguillarum
-
related pathogen (Riquelme
et al
.
, 1997). Prado and colleagues (2009) also
showed inhibitory action in 52 of 523 strains tested (9.9% ) to three
Vibrio
pathogens, but only 11 (2.1%) inhibited all three. One of these (PP-154)
identifi ed as
Phaeobacter
sp., was able to show antagonistic activity with
more than 25
Vibrio
strains as well as other pathogens, such as
Aeromonas
hydrophila, A. salmonicida, Pseudomonas anguilliseptica, Tenacibaculum
maritimum,
and
Streptococcus parauberis.
Methods to Measure Antagonism
Many methods have been devised or adapted to evaluate the antagonism
between potential probiotic bacterial strains and pathogenic strains. The
most common, perhaps because of its simplicity is to culture both strains
in a solid medium crossing each other (cross-streaking) and observing if
an inhibition halo in the pathogenic strains is observed at the crossing
point. This method has the disadvantage that it is very diffi cult to observe
an inhibition.
Another common method is the evaluation of bacteriocin-like
inhibitory substances (BLIS) (Gibson
et al
.
, 1998) or its modifi cations (Hai
et al
.
, 2007). This method has the same principle of the cross-streaking
method but the tested strain is streaked, incubated and the bacterial
growth removed mechanically and any remaining viable cells killed with
chloroform vapors. The pathogenic strain is then streaked at a right angle
