Warthin Starry is a good stain for Cat-scratch. It is recommended that you 
use a cat-scratch control, but these can be difficult to find.

The organisms in the water tend to grow in the lines and the holding tanks 
after the filtration steps. This is usually mitigated by having a system 
that continuously re-circulates (organisms don't tend to accumulate in 
water that doesn't stand still). Millipore makes convenient little culture 
paddles to test the system for bacteria growth. If you specify water 
quality for procedures (and this is required by CAP) then you must test for 
organisms periodically.

Bert