Bacteria are prokaryotes. In addition to bacteria, like mycoplasma, chlamydia, rickettsia, spirochetes and actinomycetes, they are all prokaryotes. The major difference with eukaryotes is the nucleus, which is not wrapped in a nuclear membrane, and the DNA material within the nucleus, which is not combined with proteins to form chromatin. Bacteria are also distinguished from other pathogenic microorganisms by the absence of any organelles other than ribosomes within the cytoplasm. The structure of bacteria also includes the cytoplasm, cell membrane, and cell wall, which are common to all bacteria. Some bacteria also have special structures, such as budding spores, flagella, bacteriophages, and pods, and there are substances that are associated with bacterial pathogenicity.