International Journal of Aquaculture, 2017, Vol.7, No.23, 143-158
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promising exo-enzyme producing strains ONF1P and ONF7.1C were capable of producing all of the six studied
enzymes in considerable amount, which was determined by quantitative assay. ONF1P was identified as
B.
sonorensis
(Accession No. KT362744) based on the 16S rRNA partial gene sequence analysis. Previous report by
Mary et al. (1975) recorded that
Bacillus
and
Corynebacterium
were principal groups of microorganisms present
in tilapia to tolerate the adverse effects of digestive enzymes. Moreover, previous studies have recorded
B.
sonorensis
from the gut of Indian major carps,
Cirrhinus mrigala
(Dutta and Ghosh, 2015) and
Catla catla
(Dutta
et al., 2015), although, occurrence of
B. sonorensis
depicted in the present study has not been reported previously.
In the present investigation, considerable populations of amylolytic and proteolytic bacteria were detected, which
was in agreement with Mondal et al. (2008) who reported amylolytic and proteolytic bacteria in the
gastrointestinal tract of Nile tilapia correlating to its omnivorous feeding aptitude. Bairagi et al. (2002) isolated an
efficient protease-producing strain, TP3A from the gut of
Oreochromis mossambica
. However, none of them
made an attempt to identify the efficient enzyme-producing strains. Large number of cellulolytic bacteria was
detected in this study. This result might indicate that cellulolytic bacteria exist in the GI tract of Nile tilapia and
support the hypothesis that bacteria contribute to the exogenous production of cellulase in fish. Endogenous
phytase activity has been reported in hybrid tilapia (
O. niloticus
×
O. aureus
) (LaVorgna, 1998). However, to the
authors’ knowledge, occurrence of phytase and xylanase-producing microorganisms in Nile tilapia has not been
reported previously.
Tilapia have a high level of immunity compared to other fresh water fishes that guard against the infection
(Thillaimaharani et al., 2012).
In vivo
studies involving scanning electron microscopy indicated that yeast adhere
firmly to the gut mucosal epithelia, which may resolve the colonization capacity of yeasts within the fish GI tract
(Ofek and Beachey, 1980; Traore et al., 1994). In the present study, one promising extracellular
enzyme-producing autochthonous yeast (ONF7.1C) was identified as
Pichia kudriavzevi
(Accession No.
KT582009) based on 18S rRNA partial gene sequence analysis.
Pichia
was described as good exo-enzyme
producer in
Cirrhinus mrigala
(Banerjee and Ghosh, 2014). Tannase producing
P. kudriavzevi
in fish gut has also
been documented (Mandal and Ghosh, 2013a, b). Probiotic yeasts with phytase activity have been demonstrated in
the GI tract of sea cucumbers (Hirimuthugoda et al., 2007). Present study also support considerable amount of
phytase production by isolated yeast strains from
Oreochromis niloticus
. The presence of cellulase and xylanase
producing yeasts in the GI tracts of this fish species might indicate their probable role in degradation of dietary
non starch polysaccharides.
Demonstration of antagonistic properties against some fish pathogens was considered as the second criteria for the
selection of candidate probionts. Apart from nutritional benefits, the enzyme-producing gut bacteria in fish have
been assumed to compete continuously with pathogens through the competitive exclusion or the production of
antimicrobial compounds (Ray et al., 2012). Although several strains of Bacilli were demonstrated as probiotics
for fish, antagonism of pathogens by the Bacilli has been seldom indicated (Lalloo et al., 2010; Geraylou et al.,
2014; Mukherjee and Ghosh, 2016). In the present study, most of the promising exo-enzyme producing bacteria
and yeast strains displayed antagonism against at least two of the tested fish pathogens. To the best of the authors’
knowledge, exoenzyme-producing ability along with pathogen inhibitory potential of gut microflora from Nile
tilapia depicted in the present study has not been reported previously.
It is essential for every probiotic microorganism to be capable of withstanding fish GI conditions. The gut
microorganisms should be able to adhere to mucus of the GI tract and capable of tolerating bile juice in order to
demonstrate its beneficial attributes. Both the selected microorganisms, ONF1P and ONF7.1C, grew well in fish
mucus, although, minor differences were noted in growth rate of bacteria and yeast, which might be due to
difference in specific nutritional requirements of prokaryotes and eukaryotes (Geraylou et al., 2014). In addition,
as evident from the log viable count of bacteria and yeast inoculums in fish bile juice
B. sonorensis
ONF1P and
P.
kudriavzevii
ONF7.1C were found to tolerate 12.5% and 7.5% of the diluted bile juice, respectively. In this
context, it can be stated that the physiological concentration of bile in the fish GI tract was estimated to be
approximately 0.4-1.3% (Balcázar et al., 2008). Therefore, it can be concluded that both the microorganisms were
