Questions & Answers

No, you do not need to add a frit to all samples. Only samples that have debris in the tube after heat-treatment need a frit. It is advisable to check all tubes after heat-treatment to establish if frits are required.

Use a frit in any sample that has significant debris in the sample tube after heat-treatment.

High fat and high protein foods may develop debris as they are heated and cooled. It is therefore very important to allow the sample to cool, to allow fats to solidify, before adding a frit to the tube. If a frit is added prior to fats solidifying then the frit will have no filtering effect and clogging may still occur during liquid transfer into the assay plate.

A frit is a small round filter disk. It fits snuggly into the polypropylene tubes that Solus supply for heat treating sample aliquots. It can be used to prevent food debris from blocking pipette tips during transfer from the sample tubes to the Solus assay plate. This may be particularly useful for single enrichment protocols.

For more information on frits, why not watch our short video?

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.

It depends on what sample size you are working with.

For standard 25g samples there’s no need to pre-warm the supplemented media. Although it wouldn’t cause you a problem if you did.

For 100g, 375g and environmental sponges or swabs you do need to pre-warm your supplemented media to 37°C.

How long do I need to pre-warm for? That will very much depend on the volume of media that you are working with. Obviously a 5L bottle would take longer to reach 37°C than a 1L bottle. During internal validation procedures, each lab should determine the most appropriate way to heat the supplemented BPW depending on volumes and incubator capacity. The following can be used as a guide.

For media batch volumes greater than 1 litre, media can be placed in walk-in incubators for a minimum of 16 hours (i.e. overnight). If walk-in incubators aren’t available, existing fan circulated incubators can be used that are capable of heating batches greater than 1 litre for a minimum of 16 hours. This will ensure the media is adequately heated to achieve maximum recovery of Salmonella from the sample.

The Solus One Salmonella assay uses a wash buffer activator. The contents of an activator sachet is dissolved in deionised (DI) water prior to the addition of
the concentrated wash buffer reagent.

The addition of the activator enhances the efficiency of the wash buffer and helps eliminate unwanted non-specific interactions.

Learn more about the Solus One Salmonella assay

Solus One Salmonella Supplement, when added to BPW, inhibits the growth of competing flora. However unlike other supplement formulations, Salmonella are not suppressed by the supplement. This is a common problem of supplements containing brilliant green for example.

Therefore Salmonella will grow to levels that are easily detected by the immunoassay, when this supplement is used.

Dedicated media

Solus One Salmonella Supplement is part of our dedicated media range. The media in our range has been shown over time to perform consistently and reliably with our pathogen ELISA tests.

Good, consistent quality is important for the sensitive detection of target organisms and also the control of false positive results due to overgrowth of any background flora.

Solus dedicated media is available for our entire pathogen testing range in a number of convenient formats.

Learn more about Solus dedicated media

Solus One products provide a faster time to result than our original ELISA kits. Negative and presumptive positive results can be obtained within 24 hours from a single highly selective enrichment step.

A single enrichment is sufficient because the immunoassay is more sensitive than our standard ELISA. Capture antibody binding is maximised by use of a proprietary indirect coating system. This leads to excellent sensitivity and low background signals.

Conjugate binding to the pathogen is also improved by the use of a novel buffer formulation.

Learn more about Solus One

To achieve the best results from Solus One Listeria, the preparation of the single enrichment media, SOLO+, must be carried out correctly. The media can be prepared in batches and stored at 2-8°C for up to 2 weeks. However, it must be warmed up to 30°C immediately prior to use in the enrichment of environmental swabs and sponges. During internal validation procedures, each lab should determine the most appropriate way to heat the SOLO+ depending on volumes and incubator capacity. The following can be used as a guide.

The kit instructions state that the SOLO+ media must be heated for 2 hours at 30°C. This refers to media batch volumes up to 1 litre, as internally validated, and can be achieved by placing the media in an incubator with fan circulation or a walk-in incubator.

For media batch volumes greater than 1 litre, media can be placed in walk-in incubators for a minimum of 16 hours (i.e. overnight). If walk-in incubators aren’t available, existing fan circulated incubators can be used that are capable of heating batches greater than 1 litre for a minimum of 16 hours. This will ensure the media is adequately heated to achieve maximum recovery of Listeria if present in swabs.

It should be noted that unopened, autoclaved media can be kept at 30°C for up to 72 hours with no detrimental effect on performance. This allows technicians to put SOLO+ media in to warm on a Friday evening for use on Monday morning should they wish.