Organisms are host to their memes, and to the genes of their gut bacteria

Unless the gut bacteria where somehow only transmitted from say mother to children. Then it is basically like mitochondria, a integrated part of that particular organism.

But where does this leave "driving genes" - alleles that help themselves rather than the bearer. It is quite easy to imagine such a gene. To take the metaphor of a computer program:

Gene X: When sexual recombination occurs ALWAYS copy this line.

Clearly this dosen't add to the usability of the program or the fitness of the organism one bit. Another line of our allegorical computer program:

Gene Y:When reading Gene X substitute "this line" with "lines A to Y".

So the line between a driving gene and a "regular" gene depends on its envrionment, that includes other genes. I know from your writing that you are familiar with gene-centered view of evolution.

So in sexually reproducing animals it seems that host may indeed be an appropriate word for the relationship between the organism and its own genes. After all to consider regular usage the mitochondria - rest of cell relationship is often considered a symbiosis. It really comes down to how useful it is to break down host-parasite/smybiont-smybiont systems versus just calling them "organisms" for convenience.