For Solus One Salmonella presumptive confirmation why sub-culture into RVS as well as streak plates?

Most often there will be sufficient Salmonella on present in the primary broth. These will be detected on plates and there will be no need to take the RVS subculture any further. So why bother with the RVS step?

Because occasionally a presumptive positive result from the single enrichment in supplemented BPW may not show distinct colonies when streaked on an agar plate. In this case having already prepared an RVS subculture can be beneficial. The RVS should preferentially allow the growth of the Salmonella in the secondary media to high enough numbers that when streaked onto plates will give distinct colonies and confirm the presumptive positive result.

What would be the reason that the Salmonella might not grow on the plates but would grow after being subbed into RVS? Simple numbers game. You might only have 1 x 10³ cfu/ml Salmonella in the primary enrichment. Therefore a 10µl loopful could only have a maximum of 10 viable colonies which may not be seen on a plate.

However, if Salmonella are present in the primary enrichment, subbing into RVS would allow them to grow up to say 1 x 10⁶ cfu/ml, which will result in a possible 10,000 colonies in a 10µl loopful. This quantity would be seen on an agar plate.

So by both streaking onto plates and sub-culturing into RVS you ensure that you are able to confirm the positive result.