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Can M. tuberculosis grow on a M. smegmatis plate?


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#1 Serratia Mars

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Posted 20 June 2018 - 06:51 AM

Can M. tuberculosis accidentally grow (as a contaminant) on a 7H9 media plate on which we are growing M. smegmatis?

 

If yes, is there a way to avoid the pathogen from growing? And can we distinguish just based on colony morphology?



#2 bob1

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Posted 20 June 2018 - 07:15 AM

M. tuberculosis will grow in this medium, but I don't know if it will grow on the plate - the 7H9 medium is typically used as a liquid medium. 

 

M tuberculosis typically grows very slowly on plates (if I recall my clinical microbiology correctly), whereas smegmatis would grow faster (but how much??) I'm not sure you could distinguish them morphologically, but you should  be able to do molecular testing and possibly biochemical testing.



#3 OldCloner

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Posted 20 June 2018 - 09:39 AM

What is your source of inoculum for the M. smegmatis? And are you working in a clinical/TB lab?  M. tuberculosis should not just be floating around in a student lab; it has to have an infectious animal/human source of inoculum.

 

Anyway, whatever your contaminant is, you should be able to purify your M. smegmatis by picking a single colony away from the contaminant, and streaking it out on a fresh plate and incubating it again.



#4 Serratia Mars

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Posted 22 June 2018 - 12:12 AM

Hi bob1 and OldCloner. Thank you both for your valuable answers. So M. tuberculosis CAN grow on this media.

 

There is a certain team in my lab who borrowed an M. smegmatis culture from an academic institution without characterization (basically they borrowed it on trust). I asked them to go for ATCC, but that didn't work. Also, these guys are growing it outside at room temperature. My concern was primarily from safety point of view. Ours is a non-clinical, research lab. These plates started showing colonies after 10-11 days. Some literature say M. smeg grows (visible) in 5 days, whereas M. tuberculosis will show up in more than 7 days. The latter was true with these plates; hence I was wondering if we are risking safety by inadvertently allowing M. tuberculosis to grow along side M. smegmatis.



#5 bob1

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Posted 22 June 2018 - 06:12 AM

The only way to be sure is to characterize. 16s rRNA sequencing should work.



#6 Phil Geis

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Posted 24 June 2018 - 06:56 AM

I endorse what Old Cloner said. If there is any risk of mtb, your very irresponsible colleague is placing everyone in the lab at significant risk.  Unless the casual source of culture can assure it is only M. smegmatis, all cultures plates should be sterilized - do not attempt any further work with isolates.  Where is your PI on this matter?

 

Even if assured the isolate is smegmatis, your colleague will be expected to have confirmed identification if publication is anticipated.



#7 Serratia Mars

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Posted 24 June 2018 - 11:56 PM

Thank you Phil Geis & bob1 for your replies. I will talk to my boss regarding this. The problem is; the head of the dept is an engineer by education and dependent upon certain biology PhD guys whom he trust would be doing the right thing as far as biology is concerned. But unfortunately, it is these very guys (the bio guys) who are being careless and casual about the safety aspect. In fact, these guys also work with a "supposedly non-infectious" HIV that has a single-base mutation in the RT enzyme gene; which some publications have already proved to be easily reversed on random. And they treat this HIV like e-coli when working with it; totally trusting it is safe!wacko.png mad.gif



#8 Phil Geis

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Posted 25 June 2018 - 02:57 AM

Perhaps your school has a biological safety officer to whom you can bring your concerns.  Think I'd offer to your PI or dpt chair that you should be safe in fact and perception.  The biology PhD guys should be asked to substantiate the safety of their work to you and everyone who is interested.  If they're so confident, that should not be a difficult task.

 

Your lab should be in compliance to the appropriate CDC lab standard. https://www.cdc.gov/...BL5_sect_IV.pdf



#9 Serratia Mars

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Posted 25 June 2018 - 03:02 AM

Thank you Phil! Yes, I am going to bring this up in our next general meeting.smile.png



#10 OldCloner

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Posted 29 June 2018 - 01:59 PM

All right, if you have an untrustworthy source and you suspect this could be MTb, don't even open it- this should not be worked with outside a glove box and personnel should have respirators! Frankly, you should contact your state health department and ask for help. Don't try to ID it yourself. They can ID the stuff and if you have been exposed you may need testing/prophylactic treatment.  MTb is not to mess with!



#11 Phil Geis

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Posted 06 July 2018 - 01:15 AM

Relevant - https://www.usnews.c...xposure-reports






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