Microbiology and Spoilage Trail in Nile Perch (Lates niloticus), Lake ...

hours. After incubation a loopfull of RV medium was streaked on to prepared plates of
xylose lysine desoxycholate (XLD) agar (Oxoid CM0469B) and Bismith sulphite agar
(BSA), (Oxoid CM0201B) media. The plates were incubated at 37 0 C for 24 hours and for
48 hours for negative plates. Using a sterile loop, colonies from positive XLD/BSA plates
were streaked for purification on nutrient agar (NA) plates and incubated at 37 0 C for 24
hours. After purification, using a sterile needle (straight loop), colonies were ioculated
into Triple Sugar Iron (TSI) agar (Oxoid CM0277B) – butt and slant respectively. TSI
tubes were incubated at 37 0 C for 24 hours. Positive TSI slants which produced gas with
black butt and red slant were reported as presumptive Salmonella in 25ml of water
samples.
• Pathogenic Vibrio species detection: 25g (equivalent to 25ml) of water was weighed
and added to 225ml of alkaline peptone water (APW) (Oxoid CM1028B) for enrichment
at 37 0 C for 24 hours. After incubation a loopfull of APW enrichment was streaked onto
prepared plates of selective Cholera TCBS medium (Oxoid CM0333B). The TCBS plates
were incubated at 37 0 C for 24 hours. After incubation suspicious colonies were flat-
yellow for the Vibrio cholerae (3-5mm-diameter) or blue- green for either Vibrio
parahaemolytics (3-5mm -diameter) or Vibrio vulnificus (2-3mm-diameter). Suspected
colonies were sub-cultured by streaking on TSA agar plates for purification and incubated
at 37 0 C for 24 hours. After incubation, pure colonies were subjected to biochemical tests
in Hugh and Leifson (oxidation/fermentation (O/F)) test and TSI agar slants. After
incubation at 37 0 C for 24 hours presumptive results were read on O/F test tubes which
produced a yellow colour with oxidation and or fermentation; and on TSI slants that
developed a yellow colour without gas as presumptive Vibrio cholera in 25ml of water
samples.
3.3.2.1.2 Phase I- trial 2: Sediments
Twenty grams (20g) of sediment samples was weighed into a stomacher bag and mixed
with 180ml of diluent. The mixture was stomached by 400 stomacher for 30 seconds to
make 1/10 dilution. Subsequent dilutions up to 1/1000 were made. The above (3.3.2.1.1)
described test procedures used for analysis of water were all followed.
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