When testing growth inhibition of S. mutans in BHI with different concentrations of silver ions, I have observed some weird reaction resulting on different color of the medium. Does anyone have a clue which component of BHI medium could be interacting with silver and why?
Hey Lillian, a little more info would help... What color does the medium turns to? Does the medium is exposed to light? Are all batches of medium reacting independently of the concentration you use? Have you tried spiking other type of medium to see if the effect is reporducible or is just oberved in BHI media? The phenomenon you we are observing is undoubtely caused by the reduction potential of silver ions.
It might be reduction of silver ions that are changing absorbance values, and it is concentration dependent. I have tried more appropriate media and this effect is not reproducible. But the question is: What in the BHI media is possibly causing this reduction? Are bacteria also responsible for this process?
Hey Lilian, it is hard to say. Silver ions can react with nucleophilic amino acid residues in proteins, attached to sulfhydryl, amino, imidazole, phosphate and carbonyl groups of membrane or enzyme proteins. My bet would be in thiol groups, this effect may not be evident in other media as this BHI is fortified for enhanced growth of fastidious organisms. One may guess the same effect may be seen in for example Fluid Thioglycollate medium. On the other hand, usually BHI medium contains relatively elevated concetrations of sodium chloride (up to 6.5%) which is used to differentiate the enterococci from nonenterococcal group D streptococci. Precipitated silver chloride still is photosensitive and may generate purple coloration. In my opinion is a definite probablilty that bacterial interaction (cell wall, DNA, enzymes, membrane proteins) may be playing a role however if this is the case one may expect to be seen in other types of media. May I suggest a small experiment using a fixed silver ion concentration in TSA media with similar i. salt concentration as BHI, ii. dextrose concentration as BHI iii. TSA enriched with cysteine and iv. BHI inoculated with other microorganism than S. mutans and v. with no MO inoculation at all to see if any of these give similar results as the ones you are observing. Hope this helps.
Thank you, Fabio! It helped to clarify some points about what might be happening.
- Badges
- Science topic
- Similar topics
- Medicine
- Disease
- Infectious Diseases