Bacteria

BACTERIA

Introduction

The smallest free-living microorganisms are the prokaryotes, comprising bacteria and archaea. Prokaryote is a term used to define cells that lack a true nuclear membrane; they contrast with eukaryotic cells (e.g. plants, animals and fungi) that possess a nuclear membrane and internal compartmentalization. Indeed, a major feature of eukaryotic cells, absent from prokaryotic cells, is the presence in the cytoplasm of membrane-enclosed organelles.

Bacteria and archaea share many traits and it was not until the early 1980s that differences first became evident from analyses of gene sequences. One major difference is the composition of cell walls. A more striking contrast is in the structure of the lipids that make up their cytoplasmic membranes. Differences also exist in their respective patterns of metabolism: most archaea are anaerobes, and are often found inhabiting extreme environments. It is possible that their unusual membrane structure gives archaeal cells greater stability under extreme conditions. Of notable interest is the observation that no disease causing archaea have yet been identified; the vast majority of prokaryotes of medical and pharmaceutical significance are bacteria.

Bacteria represent a large and diverse group of microorganisms that can exist as single cells or as cell clusters. Moreover, they are generally able to carry out their life processes of growth, energy generation and reproduction independently of other cells. In these respects they are very different from the cells of animals and plants, which are unable to live alone in nature and can exist only as part of a multicellular organism. They are capable of growing in a range of different environments and can not only cause contamination and spoilage of many pharmaceutical products but also a range of different diseases. For this reason, only bacteria are referred to throughout the remainder of this chapter.

Bacterial diversity and ubiquity

Bacterial diversity can be seen in terms of variation in cell size and shape (morphology), adaptation to environmental extremes, survival strategies and metabolic capabilities. Such diversity allows bacteria to grow in a multiplicity of environments ranging from hot sulphur springs (65 °C) to deep freezers ( −20°C), from high (pH 1) to low (pH

acidity and high (0.7 m) to low osmolarity (water). In addition, they can grow in both nutritionally rich (compost) and nutritionally poor (distilled water) situations. Hence, although each organism is uniquely suited to its own particular environmental niche and rarely grows out of it, the presence of bacteria may be considered ubiquitous. Indeed, there is no natural environment that is free from bacteria. This ubiquity is often demonstrated by terms used to describe organisms that grow and/or survive in particular environments. An example of such descriptive terminology is shown in Table 3.1.