I've found a journal reference now.  The abstract is here: http://www.ncbi.nlm.nih.gov/pubmed/11018140?dopt=Abstract 
 I'm relatively new to editing wikipedia and I don't know how to insert a reference.  The above comment is correct - in the words of the abstract "...the G+C content of pathogenicity islands often differs from that of the rest of the genome...".  I added the italics to make my point clear.