I am cultivating Mycobacterium tuberculosis from clinical samples on 7H9 and 7H10 media. The literature describes 'persister-like bacilli' that grow in liquid but not solid media however I cannot find an adequate explanation in the literature as to why Mtb often grows only on solid but liquid media. I was wondering if growth on solid agar is not inhibited by inhibitors in the clinical sample but cannot find literature to back up this assertion. I would be grateful for some help! Many thanks
Dear Andrew,
In the littérature most of data stress the importance of using both liquid and solid media for Mycobacteria primary isolation. The receipe of your media 9H9 is quite similar to 7H10 except for malachite green that can inhibit other bacteria and mycètes growth.
Did you use the same ADC supplement in your liquid 7H9? this is the same supplement for 7H10.
or are you using OADC as supplement? if yes you are adding oleic acid (long chain fatty acid) that is quiet important for mycobacteria metabolism. Pay attention to glycerol concentration too.
We have noticed in our lab that few INH resitant strains grow better in solid 7H10, these strains were catalase negative (deletion of katG gene), the main explanation I found is the Catalase distribution in the solid media that is more homogeneous. And may be the negative catalase strains are using the catalase from the solid media as scavenger.
I would recommend to use different liquid media that is more rich like MGIT or VersaTrek. We use 7H9 only for subculturing.
Hope its helps.
Many thanks for your answer Hafid. My 7H9 is supplemented with 10% OADC as I am growing Mtb from clinical samples. I have used the glycerol and other components are at the concentrations specified by manufacturers / in the published literature. Our clinical laboratory also finds that a substantial minority of isolates only grow on LJ slopes and not in the Bactec MGITs. A lot of papers covering culture and clinical samples mention that they have some isolates that only grow on solid agar but don't go into it further. Given that liquid culture is supposed to be more sensitive and some labs are getting rid of solid culture, it seems that they might be missing some Mtb isolates by using only liquid.
Hi Andrew,
Yes they could miss some mycobacterial isolates.
Liquid media have the advantage to be more rapid. It's why more guidelines recommend using both method (liquid and solid media), Solid media has the advantage of detecting contamination and differenciate between pigmented and unpigmented mycobacteria.
initial isolation on solid media is necessary for the growth of mycobacteria liquid media directly may cause stress of media growth for growth.
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