- automatic pipettes
- E. coli
- environmental chamber
- Escherichia coli
- growth curves
- heated chamber
- M9 minimal medium
- mini centrifuges
- time-lapse microscopy
The "national shortage" of M9 medium
31 January 2020
A little while back we switched all work in minimal medium over to M9 because of some precipitation issue with the formerly used 56/2 minimal medium (described in Willetts et al., 1969). To save the hassle of having to make the medium ourselves (56/2 is made of 6 main components, some in very small amounts, which makes the generation a little troublesome) I looked into buying it as a powder. For years we have been buying our main media components such as tryptone, yeast extract and also agar from BD Difco. The products are expensive, but we had a very high consistency in growth parameters. They do offer M9 minimal medium as a powder, and initially this worked very well. However, at some point I tried to order more via my favourite supplier, SLS. After quite a few weeks I was informed that SLS was still waiting for BD to provide them with the item. I agreed with SLS to cancel the order and to order the powder from Sigma instead. However, this produced the same result. The SLS sales representative told me that she had been informed that essentially there was no M9 medium as powder available in the UK. Neither by BD, nor by Sigma. However, Sigma was able to provide a 5 × concentrated solution of M9, which we ordered and which worked fine, with the caveat that keeping a solution sterile requires more care than making medium fresh from a powder.
After almost a year I enquired again whether the powder was available, and BD confirmed that they could indeed deliver. However, when this powder arrived and we prepared plates with it, we noticed that cell growth was much slower than on plates generated with the solution provided by Sigma (obviously all components added in addition, such as magnesium sulphate, calcium chloride and the carbon source, were the same). While on M9 minimal medium supplemented with 0.2% glucose we normally see robust colonies after 48 h at 37°C (and this was the case with the solution from Sigma), we had to grow plates for at least 72 h and the colony sizes were still not comparable. We had bought 2 × 500 g (for almost £90 each) and both showed exactly the same effect.
I reported this back to SLS, and they very kindly tried to liaise with BD to get to the bottom of the problem and also to see whether the money could be reimbursed, as the powder obviously was not usable. BD in the end contacted me directly and I provided them with a completed product questionnaire, including some pictures of the plates we had generated. However, in the end BD simply stated that QC at their end had confirmed that the shipped powder was fine and simply refused to do anything else. They never really acknowledged any sort of problem and they certainly did not offer any compensation in any form for the £180 flushed down the drain.
Taking the "national shortage" of M9 powder into consideration, as well as the fact that M9 is less complex and always was available as a solution, I believe the issue is rather obvious. If BD is correct that QC at their end did not find an issue, then the issue must have arisen on the way to me as a customer. The most likely scenario is a different particle size of the various components caused a demixing effect as the bottles are shaken in transit. If then powder is taken from the top, the composition simply is not right, leading to the observed growth delay (rather than a complete absence of growth).
Of course the "national shortage" is a silly excuse; M9 is easily made from standard chemicals. It is very likely that there was a quality issue with the mix that either BD or their supplier needed to address. However, if true then of course they knew about some quality issues that have arisen in the past – it seems likely that the problem is the same, rather than a different quality issue cropping up.
Regardless of what really has been going on, I am very disappointed by the overall response by BD (or the lack of a response!). They contemplated whether some of the powder should be sent back for them to have a look, but that never happened, at least as far as I know. They simply stated that their QC was fine, that they do not know what caused the growth difficulties at my end – case closed for them. At least a gesture towards an unhappy customer would have gone a long way. But if there really was a quality issue and BD knew about it, then the entire issue becomes even more difficult and they definitely should have taken more time in their response. Given that BD is expensive and similar components can be purchased much cheaper from other companies, this problem has led us to abandon BD alltogether and we use other brands now. Good for us, I guess: reproducibility with cheaper products is pretty much unchanged.