Hi,
A friend of mine suggested that the reaction that makes Salmonella colonies appear black on XLD or TSI agar might show an increased rate at a slightly lower pH. I was thinking of adding a bit of baking soda to our culture media to make it more basic, but without pH strips or a pH meter in our lab, I'm afraid of adding too much and inhibiting bacterial growth. Does anyone have any ideas of how much I should start with? Or does this sound like a terrible idea?
Thanks in advance.
Hi there,
If you want to lower the pH you need to add an acid to make it more acidic, not a base. My advice: get something to measure the pH.
I suggest that you might dissolve the medium components in a buffer at the pH you want.
Some bacteria utilize thiosulfate anion as a terminal electron acceptor, reducing it to sulfide. If this occurs, the newly formed hydrogen sulfide (H 2S) reacts with ferrous sulfate in the medium to form ferrous sulfide, which is visible as a black precipitate. Examples of sulfide-producing bacteria include Salmonella, Proteus, Citrobacter and Edwardsiella species. The blackening of the medium is almost always observed in the butt (bottom) of the medium.
and it is independent produce acid by bacteria.
i am wondering about your questions!!!!!!
Hi, to make culture media more basic you could try a few drops of Sodium Hydroxide (NaOH) this is a strong base, but a few drops should bring up the pH of the media.....you do need some method for checking pH though in your lab......
Shocking news!
USDA does not have money to provide to their researchers / student apprentices at least pH-strips or litmus paper (if they can't provide modern technology regarding pH-meters, irrespective of being these floor models or pocket versions) for measuring pH at least in an approximation?
Guessing about the real task you want to perform.... basic - acid milieu
Alireza Mordadi pointed to facts regarding blackening and rightly spoke out: "i am wondering about your questions!!!!!!"
Apologize if that statement 'hits you' personally - we do not know about your current situation / position while affiliated to US-DA....
Hi Victoria
you stated in the question that you might want to slightly lower pH but then ask about adding base ingredients. I presume you wanted to make the media more acidic ? In that case you need something to measure the pH ! a quick search on the internet here finds the price of pH paper to be around 5 euro's. Anyway , you can adjust the pH of your media to more acidic conditions using phosphate buffer pH 5.7-8.0 . Calculate how much you need for your volume etc from this site http://www.ochemonline.com/Buffer_solutions and then add after sterilization; sterilize the buffer by filter sterilisation. Both the media you suggest are selective media and contain phenol red, a pH indicator (and weak bacterial inhibitor). If you lower the pH too much, the media will change from red to yellow !
Hi
At least you should use pH strips otherwise you will face the problem to reproduce your results.
Good Luck !!!
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