This is indeed one of my area of maximum expertise...
I always assume that you keep the starters in a sealed container (either a jar or a Pyrex glass container with a plastic lid, depending in which form you keep your starter) and therefore I do not see problems with the refrigeration storing. The contamination issue is at room temperature, in your kitchen environment, were spores may build up. You can minimize the risk by keeping an absolute clean and disinfected environment and tools. The risk do still exist, and therefore it is also important to keep the cultures at a good level of acidity.
The other issues is that it become more difficult when the cultures keep increasing in numbers. For me two are already too much to look after. As well as Peter quoted, it is extremely difficult that an established starter get overtaken by local microflora. Just to be precise and by no means it affect performance and reliability, the Camaldoli one is not really old, just few years.
Wazza McG,
It is quite normal for a culture (specially if kept in a liquid form) to become to acetic and loose it's characteristics. Do not panic, yo do not need to go back to the dry form (in fact each time you reactivate from a dry state you increase the contamination issue), it just need proper wash procedures, and in the booklet you find such examples. I personally use a technique that I have developed together to microbiology researcher at the University of Naples. It works great for me, but I believe Ed advice are also bullet prof.
Just for example, when I came back from my three weeks trip in America, my Ischia starter was completely unbalanced and lost all the leavening power. Thanks to my "washing" technique, it was back to the status I found him many years ago, within 36 hours... Ciao
