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Welcome to YeastBytes |
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| Like a five year old who has just been refused those must have black chunky shoes, here we are kicking and screaming with YeastBytes no.4. Despite the unrelenting pressure, we have stuck to our task to deliver a late summer bumper edition with some more finely honed pieces in our bimonthly homage to all things Saccharomyces. |
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| The lead article in this issue is something of a party political broadcast on behalf of the 'Yeast Supply' party, often seen as failing to punch its weight and grabbing too few votes. This piece focuses on the benefits of yeast supply or management through the eyes of a 'party worker'. Unable to resist the pun, 'freeze dying' hits home on the perils of using dried yeast from commercial collections for yeast supply. For the first time in YeastBytes' young life we take a deep breath and consider the thorny subject of the 'generation game' and the pros and cons of culling and replacing yeast. Next up Cara new boy, Pieter Swanepoel introduces ('passport control') some of the amazing developments in molecular methods that have made strain fingerprinting a quick and cost-effective monitoring tool. YeastBytes regular Chris Giles mulls over the joys of the QA of microbiological media. Finally, in the bizarre corner that is 'did you know?', YeastBytes catches up on the spread of Australian yeast extract in art! |
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how does your media grow? |
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| The routine quality control of culture media is one of those necessities of microbiological laboratory life which should be done alongside calibrating pH meters, checking incubator temperatures and all the other minutia of Good Laboratory Practice. For some (!) it might not be the most interesting of topics and can be time consuming, but without carrying out these basic tests then how are you going to know if your media does the job for which it is intended? Is it fit for purpose, does it at least grow the bacteria or yeast you expect and inhibit the growth of those you don?t want? |
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| Challenging media with microorganisms that should or shouldn?t grow in it is a key tool in assuring performance. These ?control cultures? should be used to validate every batch of media that is prepared. As with all cultures, good microbiological practice is required in their storage and use. Typically control organisms are sourced from appropriate commercial collections thereby enabling traceability and annual restocking. |
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| One of the simplest techniques is the Econometric Technique. ET (as we call it) involves streaking a known culture to extinction on agar plates according to a set pattern. The results can be compared to those of previous batches of media; any significant changes between them will indicate that an error has occurred during the preparation of the media. For the more sophisticated Microbiologist, this method can be quantified via the Absolute Growth Index or, inevitably, ?AGI?. |
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| Another widely used technique, the ?Productivity Ratio?, is based on the Miles & Misra ?drop count? technique. The media is compared to a nutrient control media, and the number of colonies which grow on the test media is compared to those on the control media. This technique is not as easy to carry out at the econometric technique, and it takes longer to set up. It has one important advantage over the econometric method in that it can be used with broth media - in this configuration, the opticial densities of control and test cultures are compared, rather than colony counts. |
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| Additional tests are necessary to assure media quality. The pH value of plates and broths should be checked. Low gel strength, abnormal colour, poor recovery of microorganisms and poor selective properties can all be a consequence of excessive heating or poor preparation of media. |
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| Finally, the assurance of sterility is of obvious importance! Agar plates or broths are incubated so as ensure that the sterilisation step and handling has not introduced contamination, thus giving rise to false positive results in tests. While sterilisation efficiacy can be assured to an extent via integral data logging or, more simply, through the use of Bowie Dick tape which, through the appearance of dark brown stripes confirms effective autoclaving, test like this are indirect and no substitute for direct evaluation of the media and media performance. Good microbiological practice of course is also essential to minimise the threat of problems arising from media pouring practices. |
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| So, the QC of media, love it or hate it, we should all be doing it!
For more information on the techniques described here (as well as some that are not) take a look here and here
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| Chris Giles |
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 ... printable version » |
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