Bacterial infections and infestations of man can be caused by both microbes and non-microbes. Microbes include bacteria, viruses, fungi, and protozoa. Non-microbes include worms, insects, and arachnids. This chapter concentrates on the basic biology of bacteria. A pathogen is an organism that is able to cause disease in its host and the pathogenicity of any organism is its ability to produce disease. Microbes express their pathogenicity by means of their virulence. The virulence of any pathogen is determined by any of its structural, biochemical, or genetic features that enable it to cause disease in the host. The relationship between a host and a potential pathogen is non- static; the likelihood of any pathogen causing disease in its host depends both on the virulence of the pathogen and the degree of resistance or susceptibility of the host, due mainly to the effectiveness of the host’s defence mechanisms. Two of the main factors influencing a bacteria’s pathogenicity are its ability to invade and it ability to produce toxins—either exotoxins or endotoxins. Bacteria are unicellular prokaryotic micro-organisms, unlike human cells, which are eukaryotic. Fungi, protozoa, helminths, and arthropods are also eukaryotic. Prokaryotic organisms contain both DNA and RNA, but their genetic material exists unbound in the cytoplasm of the cell as, unlike eukaryotic cells, they have no nuclear membrane. Sometimes bacteria contain additional smaller circular DNA molecules, called plasmids. The main features of a bacterium are the cell wall, cytoplasm, and cell membrane. However, some bacteria have additional features such as spores, capsules, fimbriae (pili), and flagellae. The construction of the cell wall is different in different bacteria, but all cell walls contain peptidoglycan. The structure of the cell wall determines the staining characteristics when stained using the Gram stain. Although its first use was over a hundred and fifty years ago, is still the standard method for primary classification of bacteria. Occasionally, bacteria do not have a cell wall. Gram staining of a fixed smear of bacteria is used to separate bacteria into Gram positive or Gram negative, and also to demonstrate their shape. Bacteria with a thick peptidoglycan layer but with no outer membrane stain purple and are called Gram positive.
Histochemistry of the Gram-staining reaction for micro-organisms