Plant pathogenic bacteria are typically motile, single-celled organisms, for which a range of light and electron microscope techniques are available to investigate aspects of structure and morphology (Sigee, 1989). Determination of the chemical and structural organisation of plant pathogenic bacteria is important to an understanding of cell function and host—pathogen interactions, and is also an important factor in bacterial taxonomy and pathogen identification. Phytopathogenic bacteria can be examined either during growth in sterile medium (in vitro culture, Fig. 2.1) or in association with higher plants, where they may be present on the plant surface or within infected tissue (growth in planta, see Fig. 2.3c).

Characteristic morphology and fine structure

The small size (0.5–2.0 μm diameter) of plant pathogenic bacteria places them close to the limits of resolution of the light microscope, and the examination of bacterial preparations with this instrument normally involves the use of an oil immersion objective to obtain maximum detail. These organisms are also quite difficult to see in terms of their optical contrast, and light microscope examination normally involves the use of stained preparations or phase-contrast microscopy (see Fig. 6.17a). Under optimal conditions of light microscopy, general features of morphology such as size, shape, and the presence of flagella and a capsule may be resolved, but little further detail can be determined.