International Journal of Aquaculture, 2017, Vol.7, No. 3, 15
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22
19
2011). The inhibition of
Vibrio
mimicus
by
Pseudomonas aeruginosa
may be as a result of its production of
antagonistic substances such as pyocyanin and siderophores (Vijayan et al., 2006). Hadi et al. (2013) also reported
strong antibacterial capabilities of four strains of
Pseudomonas
spp. against three shrimp pathogens
V. cholerae
,
V.
parahaemolyticus and V. alginolyticus.
The safety of
Pseudomonas aeruginosa
(Table 1) and the virulence of
Vibrio
mimicus
(Table 2) to
Penaeus
monodon
postlarvae as well as the probiotic potential of
Pseudomonas aeruginosa
(Table 3) were established in
this study. This affirms that
Pseudomonas aeruginosa
strain XyC5 is a beneficial bacterium and has no
pathogenic effects on shrimp. The safety of
Pseudomonas
sp. to fish and shellfish has been reported (Vaschuere et
al., 2000; Chau et al., 2011; Nour and El-Ghiet, 2011; Hadi et al., 2013). The high mortality of the test shrimp
during the four-day pathogenicity test of
Vibrio mimicus
strain XQ could be as a result of the establishment of
vibriosis, a popular disease that affects aquatic animals. This is indicative of high virulence of
Vibrio
species as
has been reported by several authors (Chen et al., 2000; Alday-Sanz et al., 2002; Martin et al., 2004;
Lavilla-Pitogo, 2013; Hadi et al., 2013;). The LC
50
which was the concentration able to kill fifty percent of the test
organisms was found to be 3.2x10
4
cfu/ml revealing
Vibrio mimicus
as a
shrimp pathogen. Vijayan et al
.
(2006)
also reported similar findings on the pathogenicity of
V. parahaemolyticus
a common shrimp pathogen of penaeid
and non-penaeid rearing systems. Furthermore, Alday-Sanz et al. (2002) reported how
P. monodon
larvae suffered
mortalities within 48 hr of immersion challenge with strains of
V. harveyi
and
V. splendidus
.
Mortalities due to vibriosis occur when shrimps are stressed by factors such as: poor water quality, crowding, high
water temperature, low DO (dissolved oxygen) and poor water exchange (Martin et al., 2004). However, the water
quality parameters were within the optimum ranges throughout the culture period (Table 4). The stability of the
shrimp culture water quality may be attributed to the buffering capacity of the bicarbonate incorporated at the
bottom of each tank as culture substrate. A similar result was reported by Suantika et al. (2013).
Information on the health status and pathogenicity of cultural fish and shellfish are usually provided through
histopathological analysis which helps to provide details on their internal organs. The use of hepatopancreas as the
main tissue of target during any infection and an index to check the stress induced conditions and health status of
experimental shrimp is well documented (Pazir et al., 2012). The histopathological result of the experimental
group treated with
Pseudomonas aeruginosa
revealed normal vacuoles and tubule epithelial cells (Figure 4). This
result showed that the shrimp internal organs were not affected by the probiotic candidate. Contrary to this, the
destruction of the hepatopancreas tissue (Figure 5) of the group challenged with
Vibrio mimicus
may be as a result
of the established disease which affected the shrimp and led to their mortalities. This is related to a work done by
Pazir et al
.
(2012) on both
P. monodon and P.vannamei
of which they found out Early Mortality Syndrome (EMS)
caused by
Vibrio parahaemolyticus
. Another histopathological result comparable to this was that reported by
Prachumwat et al. (2012). They found out that epithelial cells of the HP tubules were disfunctioned. This
destructive pathology strongly implies a lethal/toxic etiology and also suggests that the pattern of disease spread
was related to the infectious agent. These results indicate that the indigenous non-pathogenic and antagonistic
Pseudomonas aeruginosa
strain XyC5 could be beneficial as biological control agent for management of vibriosis
in shrimp aquaculture.
3 Materials and Methods
3.1 Collection and acclimatization of shrimp (
Penaeus monodon
) postlarvae
Healthy live shrimp (
Penaeus monodon
) were collected from Sombriero River in Tombia, Rivers State of Nigeria,
with the assistance of local fishermen. They were carefully transported to the laboratory in aerated tanks
containing habitat water. The shrimp were acclimatized at room temperature for two days in aerated tanks
containing habitat water. The average length and weight of the shrimp used were 5.1 ±0.1 cm and 1.5 ±0.1 g
respectively.
3.2 Molecular characterization of
Pseudomonas
sp. Ps2
and Vibrio
sp. V1
Chromosomal DNA extraction was performed with Zymo Research Bacterial DNA MiniPrep Kit. The D
NA samples were quantified using NanoDrop ND-2000 Spectrophotometer. The 16S rRNA region of the
