The study of microorganisms

Morphology

Morphology refers to the size, shape, and arrangement of cells. The observation of microbial cells requires not only the use of microscopes but also the preparation of the cells in a manner appropriate for the particular kind of microscopy. During the first decades of the 20th century, the compound light microscope was the instrument commonly used in microbiology. Light microscopes have a usual magnification factor of 1000 × and a maximum useful magnification of approximately 2000 ×. Specimens can be observed either after they have been stained by one of several techniques to highlight some morphological characteristics or in living, unstained preparations as a “wet mount.”

Electron microscopy

The development of the electron microscope and complimentary techniques vastly increased the resolving power beyond that attainable with light microscopy. This increase is possible because the wavelengths of the electron beams are so much shorter than the wavelengths of light. Objects as small as 0.02 nm are resolvable by electron microscopy, compared with 0.25 μm—allowing, for instance, the observation of virions and viral structures. Specimens are observed by either transmission electron microscopy or scanning electron microscopy. In TEM the electron beam passes through the specimen and registers on a screen forming the image; in SEM the electron beam moves back and forth over the surface of microorganisms coated with a thin film of metal and registers a three-dimensional picture on the screen.

Advances in microscopes and microscopic techniques continue to be introduced to study cells, molecules, and even atoms. Among these are confocal microscopy, the atomic force microscope, the scanning tunneling microscope, and immunoelectron microscopy. These are particularly significant for studies of microorganisms at the molecular level.

Nutritional and physiological characteristics

Microorganisms as a group exhibit great diversity in their nutritional requirements and in the environmental conditions that will support their growth. No other group of living organisms comes close to matching the versatility and diversity of microbes in this respect. Some species will grow in a solution composed only of inorganic salts (one of the salts must be a compound of nitrogen) and a source of carbon dioxide (CO₂); these are called autotrophs. Many, but not all, of these microbes are autotrophic via photosynthesis. Organisms requiring any other carbon source are called heterotrophs. These microbes commonly make use of carbohydrates, lipids, and proteins, although many microbes can metabolize other organic compounds such as hydrocarbons. Others, particularly the fungi, are decomposers. Many species of bacteria also require specific additional nutrients such as minerals, amino acids, and vitamins. Various protozoans, fungi, and bacteria are parasites, either exclusively (obligate parasites) or with the ability to live independently (facultative parasites).

If the nutritional requirements of a microorganism are known, a chemically defined medium containing only those chemicals can be prepared. More complex media are also routinely used; these generally consist of peptone (a partially digested protein), meat extract, and sometimes yeast extract. When a solid medium is desired, agar is added to the above ingredients. Agar is a complex polysaccharide extracted from marine algae. It has several properties that make it an ideal solidifying substance for microbiological media, particularly its resistance to microbial degradation.

Microorganisms vary widely in terms of the physical conditions required for growth. For example, some are aerobes (require oxygen), some are anaerobes (grow only in the absence of oxygen), and some are facultative (they grow in either condition). Eukaryotic microbes are generally aerobic. Microorganisms that grow at temperatures below 20 °C (68 °F) are called psychrophiles; those that grow best at 20–40 °C (68–104 °F) are called mesophiles; a third group, the thermophiles, require temperatures above 40 °C. Those organisms which grow under optimally under one or more physical or chemical extremes, such as temperature, pressure, pH, or salinity, are referred to as extremophiles. Bacteria exhibit the widest range of temperature requirements. Whereas bacterial (and fungal) growth is commonly observed in food that has been refrigerated for a long period, some isolated archaea (e.g., Pyrodictium occultum and Pyrococcus woesei) grow at temperatures above 100 °C (212 °F).

Other physical conditions that affect the growth of microorganisms are acidity or basicity (pH), osmotic pressure, and hydrostatic pressure. The optimal pH for most bacteria associated with the human environment is in the neutral range near pH 7, though other species grow under extremely basic or acidic conditions. Most fungi are favoured by a slightly lower pH (5–6); protozoa require a range of pH 6.7–7.7; algae are similar to bacteria in their requirements except for the fact that they are photosynthetic.