It was somewhere outside Barstow when "Ian_m" wrote:

Soft water stops it, since fitting a water softener the black mould growths
in washing machine and shower silcone have stopped. I assume is living using
the salts in hard water.

This is an interesting observation - I had a long discussion about it
with a local uni researcher who is seeking another IgNobel prize for
some research on kitchen filth and what grows in the sinks of
students.

The mould isn't living _on_ the dissolved salts, but the soap scum
that tends to accumulate with harder water provides a mechanical
substrate that allows the moulds not to be washed away in the initial
stages.


I find Milton works as a de-gunker and de-ponger for washing machines.
Hydrogen peroxide works even better, but it's a bit harsh on rubber
door seals for regular use.