2.2.3 Growth factors
Some organotrophs such as Escherichia coli can grow in simple media containing glucose and. mineral salts, while others, like lactic acid bacteria, require complex media containing various vitamins, amino acids and nucleic acid bases. This is because the latter organisms carmot synthesize certain essential cellular mateiials from only glucose and mineral salts. These required compounds should therefore be supplied in the growth media: such compounds are known as growth factors. Growth factor requirements differ between organisms with Vitamins being the most commonly required growth factors (Table 2.3).
2.3 l Structure of microbial cells Microorganisms are grouped into either prokaryotes or eukaryotes according to their cellular structure. With only a few exceptions, prokaryotic cells do not have subcellular organelles separated from the cytoplasm by phospholipid membranes such as the nuclear and mitochondrial membranes. Organelles like the nucleus, mitochon- dria and endoplasmic reticulum are only found in eukaryotic cells. The detailed structure of prokaryotic cells is described below. 2.3. I Flagella and pili
Motile prokaiyotic cells have an appendage called a flagellum (plural, flagella) involved in motility, and a similar but smaller structure, the fimbria (plural, fimbriae). Fimbriae are not involved in motility and are composed of proteins.
The bacterial flagellum consists of three parts. These are a basal body, a hook and a filament (Figure 2.1). The basal body is embedded in the cytoplasmic membrane and cell surface structure and