International Journal of Aquaculture, 2017, Vol.7, No.23, 143-158
149
was also recorded as the highest phytase-producing strain (4.71±0.06U) among yeast strains evaluated. Strain
ONF8.1A produced highest xylanolytic activity (1.15±0.09U), which was followed by ONF7.1C (0.92±0.08U).
Considering these facts, the isolates ONF7.1C was finally selected as the most efficient yeast strain.
Table 3 Enzyme activity (unit activity, U) by the primarily selected microbial strains isolated from the GI tracts of tilapia
(
Oreochromis niloticus
). Data are Mean ±Standard error (n=3)
Enzyme activity (U)
Strains
Amylase
1
Protease
2
Lipase
3
Cellulase
4
Phytase
5
Xylanase
6
Bacterial strains
Proximal Intestine
ONF4L
142.28±3.25
f
52.69±2.23
g
4.02±0.14
i
36.51±1.88
d
42.58±1.57
i
3.52±0.08
e
ONF1X
177.62±4.36
h
51.08±2.48
fg
3.95±0.09
i
39.43±1.59
d
37.50±1.41
h
4.29±0.18
f
ONF1P
191.46±4.05
i
64.55±2.39
h
3.99±0.07
i
53.33±2.21
f
51.11±1.63
k
8.69±0.75
h
ONF1T
170.48±4.77
h
47.33±2.86
f
3.78±0.05
h
32.55±1.59
c
46.35±1.54
j
-
Distal Intestine
ONH1Ph
141.05±3.56
ef
38.49±1.84
e
3.84±0.06
hi
20.06±1.12
a
13.67±0.94
d
2.88±0.06
d
ONH1A
101.44±2.55
c
53.69±2.35
g
3.44±0.02
e
35.28±1.61
cd
20.16±1.14
f
-
ONH2A
143.56±3.39
fg
29.61±1.46
c
3.64±0.04
g
33.62±1.68
cd
21.47±1.10
fg
5.06±0.69
f
ONH2Ph
149.63±3.50
g
49.90±2.98
fg
3.55±0.03
f
36.92±1.58
d
17.77±1.08
e
-
ONH1C
188.40±4.05
i
37.45±1.41
e
3.89±0.06
hi
48.73±1.79
e
23.58±1.19
g
-
ONH2C
139.73±3.01
ef
55.88±2.36
g
3.41±0.02
e
45.66±1.80
e
-
6.93±0.74
g
Yeast strains
Proximal Intestine
ONF7.1C
135.13±3.26
e
33.42±1.87
d
3.58±0.05
fg
39.87±2.14
d
4.71±0.06
c
0.92±0.09
b
ONF8.1A
90.28±1.98
b
28.15±1.21
c
3.12±0.08
d
36.42±2.74
cd
-
1.15±0.09
c
ONF14.1C
84.21±1.65
a
23.71±1.12
b
1.96±0.07
a
34.64±2.61
cd
3.97±0.08
b
0.73±0.07
a
ONF21.1B
119.61±2.89
d
18.38±1.16
a
2.84±0.06
c
-
3.78±0.07
a
0.86±0.08
ab
Distal Intestine
ONH15.1B
103.58±2.51
c
24.23±1.09
b
2.35±0.02
b
26.53±1.23
b
-
-
Note: Values with the same superscripts in the same vertical column are not significantly different (
P
<0.05).
1
μg of maltose liberated mg
-1
protein min
-1
;
2
μg of tyrosine liberated mg
-1
protein min
-1
;
3
μg of free fatty acid liberated mg
-1
protein
min
-1
;
4
μg of glucose liberated mg
-1
protein min
-1
;
5
μg of inorganic phosphate liberated mg
-1
protein min
-1
;
6
mg of D-xylose liberated
mg
-1
protein min
-1
The primarily selected 15 extracellular enzyme-producing isolates (10 bacteria and 5 yeasts) were further screened
for antagonism against said eight fish pathogens through ‘cross-streaking’ method and the result has been shown
in Table 4. Out of the 15 isolates, 6 strains (3 bacteria and 3 yeasts) were noticed to inhibit at least one of the
tested fish pathogens.
Bacillus mycoides
can be inhibited by most of the isolates (4 bacteria and 3 yeasts), the
highest inhibition zone being 8 mm by the strain ONF1P.
Pseudomonas
sp. and
Pseudomonas fluorescence
can be
inhibited by 6 bacterial strains, however, no yeast strain was able to inhibit this pathogen.
Aeromonas veronii
was
inhibited by only one bacterial isolate, ONH2A and none of the yeast strains could antagonize this pathogen.
Aeromonas hydrophila
was inhibited by another bacterial isolate, ONF4L. It appeared that the strain ONF1P
could inhibit 5 fish pathogens producing distinct inhibition zones, and the yeast strain ONF7.1C could inhibit two
fish pathogens including
Aeromonas hydrophila
. The strains ONF1P and ONF7.1C were able to produce
considerable amount of exo-enzymes and were also noticed to be antagonistic against 7 tested fish pathogens.
Therefore, these promising strains (ONF1P and ONF7.1C) were finally selected for identification in view of
possible future use.
Co-culture test (
in vitro
) against ten autochthonous fish gut bacteria revealed that the selected yeast strain
(ONF7.1C) could not affect the growth of the autochthonous gut bacteria, however, selected bacterial strain
(ONF1P) couldn’t grow with
Enterobacter hormaechei
(KF559347). Therefore, the selected extracellular
enzyme-producing yeast strain was mostly compatible with the commonly occurring autochthonous fish gut
bacteria. Both ONF1P and ONF7.1C showed tolerance against diluted bile juice. The selected bacterial strains
