Rickettsia are usually considered as a separate group of bacteria. These microorganisms which multiply by binary fission inside their host cells are a group of obligately intracellular gram-negative mircoorganisms, having cell walls that look exactly as that of a typical bacteria cell wall. They possess no flagella. They are carried as parasites such as lice and tcks, and also cause diseases such as typhus.
**Rickettsiae** resemble small bacteria, but like viruses they can only multiply by invading the cells of another life form.
Explanation:
Rickettsiae are obligate intracellular organisms resembling gram negative bacteria because they stain slightly pink on gram stain.
Like viruses they can only multiply by invading the cells of another life form because they are obligate intracellular and cannot form their own ATP alongwith NADH and Co-Ach. As these are the necessary components for an organism / cell to grow and replicate , they use he machinery of another living organism , spread through bloodstream and survive and replicate in the cells of organism.
Thus they resemble bacteria and viruses but actually they are parasites.
