Classification and pathogenicity of microbes

This chapter discusses the classification and pathogenicity of microbes. The microbial causes of human disease include viruses, chlamydiae, rickettsiae, mycoplasmas, bacteria, fungi, and protozoa. Viruses differ greatly from all the other microbes as they consist essentially of only nucleic acid surrounded by a protein coat and contain only one instead of two types of nucleic acid. Once inside human cells, the viruses remove the normal nuclear control of the cells to take over cellular metabolism for the synthesis of new virions. Chlamydiae and rickettsiae are also obligate intracellular parasites, have both DNA and RNA, and multiply by binary fission. Mycoplasmas, bacteria, and fungi can be cultured in cell-free media. Bacterial causes of disease are mainly lower bacteria that are unicellular. Multiplication is predominantly by asexual binary fission, although biological variation is facilitated in some species by sex, especially with Gram-negative species such as Escherichia coli. Only a few higher bacteria cause disease in man, such as Actinomycetes israelit, which are filamentous Gram-positive bacilli.