1 Microbes as parasites
The varieties of microbes
Prokaryotes and eukaryotes
Bacteria are prokaryotes, all other organisms are eukaryotes
There are many differences between the two major divisions: prokaryotes and eukaryotes, of cellular organisms (Fig. 1.1). These include the following.
• A distinct nucleus is absent.
• DNA is in the form of a single circular chromosome. Additional ‘extrachromosomal’ DNA is carried in plasmids.
• Transcription and translation can be carried out simultaneously.
• DNA is carried on several chromosomes within a nucleus.
• The nucleus is bounded by a nuclear membrane.
• Transcription requires formation of messenger RNA (mRNA) and movement of mRNA out of the nucleus into the cytoplasm
• Translation takes place on ribosomes.
• The cytoplasm is rich in membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes) which are absent in prokaryotes.
Living inside or outside cells
Pathogens within cells are protected from many of the host’s defence mechanisms
As will be discussed in greater detail in Chapter 13, the intracellular pathogens pose problems for the host that are quite different from those posed by extracellular organisms. Pathogens that live within cells are largely protected against many of the host’s defence mechanisms while they remain there, particularly against the action of specific antibodies. Control of these infections depends therefore on the activities of intracellular killing mechanisms, short-range mediators or cytotoxic agents, although the latter may destroy both the pathogen and the host cell, leading to tissue damage. This problem, of targeting activity against the pathogen when it lives within a vulnerable cell, also arises when using drugs or antibiotics, as it is difficult to achieve selective action against the pathogen while leaving the host cell intact. Even more problematic is the fact that many intracellular pathogens live inside the very cells responsible for the host’s immune and inflammatory mechanisms and therefore depress the host’s defensive abilities. For example, a variety of viral, bacterial and protozoal pathogens live inside macrophages, and several viruses (including HIV) are specific for lymphocytes.