This goes like any other drug preparation that need to be added to a cell culture. Yours is even worse because you have a good medium/solution that supports bacterial growth. If the solution is what you intend to analyse, you have to filter it through a sterile filter - we commonly use 0.22 micron that does not allow bacteria to pass through. If there is a resistance to push it through (e.g., via a syringe), you can pass it first through a 0.45 micrometer filter and then double filter using 0.22 micrometer. For anything over an hour of incubation, I would not use a non-sterile drug/treatment preparation on a cell-based assay.

Thanks Solomon ...will try that and see

If you want co-culturing of probiotic strain and PBMCs, you need a serial dilution of the probiotic strain in PBMC culture and by comparing MTT results with untreated PBMC culture you cant calculate the cell viability and cytotoxicity. However if treatment of PBMC culture with the bacterial free medium is desired, then you can only apply a sterile supernatant, the MTT results of treated and control PBMC results can then be compared for Cell viability/vitality and cytotoxicity.