How to adjust the pH of a bacterial culture medium using a buffer? - (Dec/08/2016 )

Hi 

I am trying to adjust the pH of a bacterial culture medium containing Peptone, 10g, Beef extract, 10 g, NaCl, 5g, pH 7.3 ± 0.1  in order to test the pH range my bacterial isolates can tolerate. I am going to use sodium acetate/acetic acid, Tris/HCl and  glycine/sodium hydroxide buffers to adjust the pH of the medium to below 6, 6-9, above 9, respectively. Could anyone help me with the whole procedure. which buffering capacity for each buffer I should go with? Should I add the buffer after autoclaving or before? 

Thank you

Does it need to be buffered? Currently your system is not buffered (assuming those components are the only ones). Generally you can adjust the pH of non-buffered systems using HCl or NaOH. Acetate would work too for low pH, but you should consider the bio-availability of the free acetate and the role this might play in metabolism - simple molecules can have big differences on bacteria.

I decided to buffer the system because the pH of the medium would drop after autoclaving. Maybe I should have adjusted the pH after autoclaving. for that matter, is it neccessary to sterilize NaOH or HCl (by autoclaving or filter sterilizing)?

The pH change after autoclaving is normal and most pre-made complex media indicate this in the SDS/label/manufacturer's website. If you need such an strict pH try to account for the pH change and adjust the pH to a slightly higher value before autoclaving. That;s the safest bet.

Adjusting pH in sterile conditions is a pain and introducing a buffer will change the culturing conditions... and introducing different buffers for different pH values may create very different conditions. e.g. you are adding potential C and/or N sources (acetate and glycine)