Hi, Stefan,

> Also I read in the newspaper, that beneficial bacteria of the species
> E-Coli are normally harmless and needed in the intestines, but can
> become dangerous for people with weakened immune system, especially
> AIDS patients.

The problem with the species E. coli is that there are "beneficial"
strains which are part of the normal intestinal flora and others
which have aquired a whole lot of virulence factors. The latter ones
can cause severe intestinal and/or extraintestinal diseases. Perhaps
you have heard or read somewhere of
ETEC = enterotoxigenic E. coli
EPEC = enteropathogenic E. coli
EIEC = enteroinvasive E. coli
STEC = Shiga Toxin-producing E. coli
EHEC = enterohemorrhagic E. coli
These are NOT benevolent strains and you don't necessarily need to have a
weakened immune system to get infected with them.

Best regards,

Sylvia Franke