I am testing for the presence of staph aureus on the raw materials for our new products. The agar that I use is Vogel and Johnson Agar. At the same time, I am also running another test to obtain the total bacterial count using TSA. The problem is that some raw materials show positive staph aureus growth on VJA (black colonies with yellow zones) but no growth on TSA. Is there any explanation on this? I tested the same samples but still got the same result.
I assume you mean the colony on VJ agar is picked to TSA ? (ie not the original raw material [which is?])
Does the TSA have sufficient salt ? (although it should work OK [some salt-loving species may need a boost, eg Vibrio], eg -
S.aureus growth on TSA,MS.pdf 1.47MB
does TSA work OK with a reference sample of S.aureus COP
Do the colonies on VJ agar show coag.+ result ?
Never used VJ agar myself, don't recall possible false positives.
Edited by Charles.C, 01 December 2015 - 02:41 AM.