Almost daily some Web developer asks some e-mail list whether meta keyword tags should contain commas and spaces. There's no good answer to this, which is why they keep asking. Each search engine uses meta tags in its own way. Some don't use them at all.
People spend whole careers trying to interpret what search engines are doing.
But often their work confuses more than it clarifies simply because each search engine behaves differently. In the Members section of Search Engine Watch , Danny Sullivan confirms this. Some people leave out one or the other to cram in keywords for search engines that limit keywords by character count rather than word count. But using commas to define specific search phrases can also be helpful. So, he advises, use the strategy that seems to work for you.