2.2.1. Microbiological analysis
Frozen samples were thawed over night at 4 °C in refrigerator before analysis. The thawed shrimp muscles without shells were cut to very small pieces using sterile knife and forceps; then, 10 g was homogenized using 90 ml sterile physiological saline (0.85%) solution, and serial decimal dilutions of each homogenate were prepared using the same dilutent for the respective microbiological analysis (Downes and Ito, 2001). Appropriate dilutions (101–109) were used for enumeration using standard microbiological pour plate technique and recommended culture media of Oxoid (2002). Plate count agar medium was used for enumerating the Total Viable Count (TVC) after incubating at 35–37 °C for 48 h. Violet red bile agar with methyl umbelliferyl glucuronide (VRB-MUG) selective media was used to isolate coliform, gram negative enteric bacteria and rapid detection of E. coli. The prepared dilution of frozen shrimp homogenate was inoculated in sterile petri dishes; then, medium was poured and plates were incubated at 37 °C for 18–24 h. Colonies of lactose negative Enterobacteriaceae are colourless and those of lactose positive are red and often surround by a forbid zone due to precipitation of bile acids; meanwhile, light blue fluorescent colonies under UV-Lamp (336 nm) are denoted as E. coli.
The recommended Difco Baird Parker agar medium by ICMSF (1978) was used to detect coagulase active Staphylococcus aureus after incubating the plates at 35–37 °C for 48 h. The black shiny colonies with narrow white margin and surrounded by clear zones were counted as Staphylococcus aureus.