non-diffusion in agar

[M Des]

We have purified a small (< 1 ka) antibacterial peptide that we tested for 
activity in solid media (routinely, wells cut in TSBYE, 0.75% agar, seeded 
with the indicator bacteria). We also deposed a spot (10 uL) directly on the 
agar. After incubation, absolutely no inhibition zone appeared around the 
wells, but curiously a clear zone was visible just under the spot. A 
positive control (gentamicin) was normal. Tested in liquid media, the 
inoculum was totally inhibited by the peptide.

I was surprised that this antimicrobial doesn't diffuse in a soft solid 
media. We supposed that the proteins agglomerate together and remained 
trapped in the agar matrix, but adding Tween in the fractions did not help. 
Does somebody have a clue to explain the non-diffusion of the peptide ?

Thanks

Michel
kayamicdes at hotmail.com

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