I. Symbioses between Microorganisms II. Plants as Microbial Habitats 2. The Legume–Root Nodule Symbiosis 3. Agrobacterium and Crown Gall Disease 4. Mycorrhizae III. Animals as Microbial Habitats 5. The Mammalian Gut 6. The Rumen and Ruminant Animals 7. The Human Microbiome 8. Termites