Introduction

1 THE MICROBIAL CELL

Microbiology can be defined as the study of organisms that are too small to be clearly visible to the naked eye. This will include all organisms with a diameter of less than approximately 1 mm.

Micro-organisms are widely distributed between different taxonomic groups and include bacteria, protozoa, fungi, and algae. Viruses, although frequently considered in microbiology text books, are not cells.

The discovery of micro-organisms caused numerous problems relating to the placing of these species into the traditional plant and animal kingdoms, and in the mid-19th century a third kingdom, the Protista was suggested to include the protozoans, fungi, algae, and bacteria.

The development of electron microscopy showed that there was a major dichotomy between the various groups relating to the internal structure and organisation of cells. Two very different types of cells were discovered, the small, relatively simple procaryotic cell, and the more complex eucaryotic cell which is usually considerably larger.

1.1 Procaryotic Cells

The bacteria are procaryotic cells. This group also includes the organisms which used to be known as the 'Blue–green Algae' or Cyanophyceae, and are now known as the Cyanobacteria.

The vast majority of procaryotic organisms are unicellular, although multi-cellular bacteria forming filaments are also found. A few types are more complex structurally and may produce fruiting bodies, appendages, or stalks, although these would not generally be found in a typical analytical laboratory. Bacteria forming filaments frequently show a marked tendency to break up into individual cells. Figure 1.1 shows a typical procaryotic cell.

Procaryotic cells are surrounded by a cell membrane, a thin flexible sheet composed of protein and lipid. This has an important function in regulating the molecules which can pass into and out of the cell.

In the great majority of cases, there is a rigid cell wall outside the membrane. This cell wall has a chemistry which is unique to the procaryotic cell, and contains a number of compounds which are not found in the eucaryotes. These include peptidoglycan (murein), which is a repeating disaccharide containing the amino sugar, N-acetylmuramic acid, linked to a peptide containing D-amino acids, and diamino acids such as diaminopimelic acid. This cell wall chemistry is an important linking factor between the true bacteria and the blue–green algae. A few groups such as Mycoplasma do not possess a cell wall.

In the procaryotes, there is no discrete nucleus surrounded by a nuclear membrane, and the deoxyribonucleic acid (DNA) is not associated with the basic proteins known as histones. The DNA consists of covalently closed circles.

There are no organelles such as mitochondria present within the cell. Flagella are present in the case of motile cells; the position of these varies from species to species and may be used diagnostically. Certain species may contain spores which are highly resistant to heat and a variety of chemicals. A number of species are capable of photosynthesis, that is they can convert solar energy into chemical energy. Those that do carry out photosynthesis have chromatophores which are extensions of the cell membrane, and which are morphologically distinct from the chloroplast of the eucaryotic higher plant (see later).

There is also a considerable difference in the ribosome structure of procaryotic and eucaryotic cells. Ribosomes are the structures upon which proteins are synthesised. Procaryotic cells possess ribosomes which are 70S in size, whereas eucaryotes have 80 S ribosomes.

1.2 Eucaryotic Cells

Organisms containing eucaryotic cells include animals, plants, fungi, algae, and protozoa. Figure 1.2 shows typical animal and plant eucaryotic cells.

A cell membrane consisting of a protein and lipid mixture is present, but the eucaryotic cell membrane contains significant quantities of steroids, which are not found in most procaryotic cells. There may or may not be a cell wall outside the membrane depending on the type of cell.

Eucaryotic cells possess a nucleus, surrounded by a nuclear membrane which delineates the nucleus from the rest of the cell. This nucleus contains the majority of the cell DNA and is associated with histones. The nuclear DNA is linear. The nucleus is also the centre for the synthesis of ribonucleic acid (RNA).

All material inside the cell membrane, but outside the nuclear membrane, is known as the cytoplasm. Other organelles are found in the cytoplasm of the eucaryotic cell. These include the mitochondria, which are the powerhouse of the eucaryotic cell producing much of the energy in the form of adenosine triphosphate (ATP). Chloroplasts are found in photosynthetic species such as plants and algae. Neither of these structures is found in procaryotes. Various other organelles such as a nucleolus, centrioles, and a Golgi apparatus are also found. There may be one or more cilia or flagella in the case of motile cells, but these differ considerably from the flagella of bacterial cells.

Eucaryotic micro-organisms of importance to the analytical chemist include the fungi, protozoa and algae.

1.3 Nomenclature and Taxonomy

Micro-organisms are named using a binomial system. Every species has two latinised names, the first being the generic name and the second the specific name which identifies the organism within the genus. The first letter of the generic name is in capitals and the whole name is either italicised or underlined. In situations where no ambiguity can arise only the first letter or a shortened version of the generic name may be used, although the full name should always be used the first time it arises.

Nomenclature may cause problems. Occasionally, newly isolated organisms may not be recognised for what they are, and are given a new name. This can cause considerable confusion when the organism is finally correctly identified.

Taxonomy is the science of classification and has several functions. The first is to describe the species which is the basic taxonomic unit. The second is to catalogue these species into some arrangement enabling the relationships between species to be recognised. A third practical aspect is identification, that is, the matching of an unknown organism with a known species.

The division between species at the plant and animal levels is generally relatively easy on the basis of their morphology (appearance), although problems can arise in distinguishing between the simpler algae and protozoa. This however is not true with bacteria as the range of distinguishable shapes and sizes is too small.

Bacteria are therefore classified and identified into strains on the basis of a number...