(1 in 1000) dilution tubes, using a new sterile pipette for each dilution.
Incubation Incubate the LST tubes at 37±10C for 24 and 48 hours
Reading of enrichment tubes (presumptive test) Record tubes showing gas production after 24hours, and re-incubate negative tubes for further 24 hours, and then record tubes showing gas production.
Confirmed test for coliforms
Transfer a loopful from each gas- positive tube of LST to a separate tube of BGLB broth.
Incubate the BGLB tubes at 35± 10C for 48hours.
The formation of gas confirms the presence of coliform bacteria. Recored the number of positive tubes that were confirmed positive for coliforms.
Calculation (MPN) Note the MPN appropriate to the number of positive tubes from the following table for example: 3 in 1:10, 1 in 1:100 and 0 in 1:1000 the table shows that MPN=43 coliforms/g or ml