FAQs

I left my Overnighters on the desk and the purple colour faded. Is this normal?

Yes, it is normal with prolonged exposure to UV. The label recommends storing out of light. Temperature is not so critical, in fact farmers can store them in their pockets for easy access during milking. Long term we may have a solution to the fading, but no promises on that yet...

Can the Overnighter give false positives?

If excess milk is left above the meniscus line, it can ferment its own lactose and slowly cause a non specific false positive reaction. Ensuring excess milk is tapped or flicked out will minimise this. Cases where a lab has not grown a bacteria but the Overnighter has raise questions about the limits of detection of the laboratory. Remember most labs use about 1/5 - 1/10th of the milk volume in an overnighter.

What if my Overnighter doesn't react?

Remember it may well be a "No Growth", which may be common in certain scenarios. People sometimes get disappointed when they don't see a colour change, but in fact this is a meaningful result. If no reaction in 12 hours, hold until 24. Slow staphs should be on the turn by then, with changes starting at the bottom of the tube. After 48 hours though, its all over. Don't incubate beyond then or you increase the risk of non specific fermentation and false positives.

Is is worthwhile testing subclinical cows?

Yes, as knowing whether you are dealing with a strep completely changes prognosis and lactational treatment options. Test only strong RMT positive quarters. The growth may be a lot slower than clinicals, especially if there are low numbers. If strongly suspicious of a hidden staph, freezing and thawing the sample may increase recovery rates.

Can I subculture staphs for sensitivities? How?

Richard has indicated on some visits that this can be done, however we have experienced some difficulties, so if you are interested in this please take a frozen milk and check with us before proceeding. Work in progress.

Can I get a "contaminated" result with the Overnighter? How strict does asepsis need to be?

To our knowledge, this is not a problem. The inhibitors deck all overgrowth. This means that asepsis is not as critical as with normal samples, however if companion milks are being taken for freezing, we don't want to lower the standard, therefore the label does not mention this.

How narrow is the temperature tolerance? If it goes cold during incubation is that a problem?

The temperature range appears to be quite robust. Feedback on incubating them in desk drawers, below oil column heaters, in pockets where clothes have been taken off and on chiller heat exchange units (highly variable!) indicate they are quite robust. The growth is slowed, and may take 48 hours or longer, but is still fairly consistent. On top of water heaters seems to be the best bet, even though they are a little above body temp.

I incubated my Overnighter and the red tube turned an orange colour. What does that mean?

Sometimes the red tube will turn an orange shade as the milk sample ferments. This colour change does not indicate presence of Staph Aureus or Strep uberis bacteria which create distinctive yellow and red/black colour changes, respectively.

Should I store my unused Overnighters in the fridge?

While the product is relatively robust to environmental temperatures, it should be stored in a cool place and protected from the light. A refrigerator would therefore be a good place to store them, but is not essential.

Sometimes when I get a strong positive for Staph, I also get some yellowish (not black) colour change in the purple tube. Is this a mixed infection?

During 24-48 hour incubation of our QA samples we have observed that a proportion of Staph samples will cause some yellowing in the base of the purple tubes, often no more than half way up in the media. The top of the media retains its purple colouring. We believe that this is due to non-specific fermentation of the milk sample during this time, and these should tubes should be read as negative.

Sometimes I get mixed colour changes. What does that mean?

Where there is mixed infection in the sample, you can expect to get mixed colour reactions. A change to yellow in the red tube is definitive for Staph aureus and will mask the red/black change due to Strep uberis. Normally the change in the two tubes allows you to distinguish Strep uberis from Strep dysgalactiae, so if there is mixed staph and strep infection, changes in the purple tube that differentiate the to Strep infections may be difficult to differentiate.