Why did my Streptococci not grow on my PEA agar?

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Joined: Mon Feb 06, 2012 4:50 am

Why did my Streptococci not grow on my PEA agar?

Post by SweetSarah » Tue Mar 06, 2012 4:49 am

PEA agar stands for phenylethyl alcohol agar and is selective for gram positives. It is pretty much similar to blood agar.
What I don't get is why my gram positive organism, which is Streptococci (probably S. pneumoniae or mitis) barely grew on the PEA agar? If it selects for gram positives shouldn't it have grown more?? Anyways I done this long time ago.
This was how I described the colony morphology, would this be accurate or make sense for Strep pneumoniae or Strep mitis (am still unsure which it could be but know it's one those ones since it was alpha hemolytic on the blood agar)

Colony morphology on PEA: barely any growth; few tiny pinpoint, white, translucent, circular colonies with a smooth apperance.
.....Does this seem accurate?? There was more growth from my gram negative organism on my Mac than there was for my gram positive on PEA. I couldn't find anything online saying Strep not growing on this media so doesn't make sense :|

(*note:We had to use PEA for our unknowns in lab by the way, so that's why I used it and on top of blood agar)

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Re: Why did my Streptococci not grow on my PEA agar?

Post by JorgeLobo » Tue Mar 06, 2012 12:38 pm

Folks overinterpret selective agar - phenylethyl alcohol inhibits all bacteria and Gram negatives are more sensitive (lower MIC) so it's a matter of differential not absolute inhibition. It's used more for the isolation of staph than strep as staph is less inhibited. Though strep pheumo should grow, there are little to no data on mitis and the context in which one finds these bacteria would not be one that typically would have a confounding Gram negative presence that one would need or want to exclude. That your pneumo or mitis isolate grew poorly is not that surprising and colony morphology one this agar should not be a differential criterion.

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