Two things to try.
(1) Increase the agar content from 1.5% to 2.0% or maybe 2.5%. This
makes it a bit of a pig to pour but reduces the aailability of the
(2) Try incorporating finely divided activated charcoal. Spreading
(excess motility is often a result of the bug attempting to "swim away"
from metabolic products. The charcaol adsorbs the product. Used to work
for Proteus - might work for you.
Centre for Articulated Contextualisation,
The University of Reading,