4.4.2 Morphology is affected by the medium on which the organism is grown and by the temperatureof incubation. Organisms are typical and in their most natural state in young cultures; in wet, unstained preparations, they are best observed by phase-contrast or dark-ground microscopy. Such examination will show not only the shape(s) of organisms but,when prepared from suitable material (see Section 3.3.5), will show motility if it is present, and whether the cells remain rigid (as in most bacteria), flex(spirochaetes), or glide (cytophagas). The distinction between spheres (cocci) and rods (bacilli) is not always clear-cut, and genera such as Acinetobacterand Moraxella cannot be placed categorically in one morphological group. Although it is usual to describe organisms of both these genera as either coccobacilli or short rods, electron micrographs clearly show the coccal nature of some acinetobacter strains (Thornley, 1967). Baumann, Doudoroff & Stanier (1968b) found that the differences in morphology ofthese genera corresponded with the growth phase:plump rods in the logarithmic phase and coccoidforms in the stationary phase. Brzin (1965) described what she called a sphaeroplasting effect, in that prolonged incubation of 'Acinetobacter anitratus (Acinetobacter calcoaceticus) strains at 37 °C pro-duced polymorphism.Bizarre-shaped cells may suggest particular genera and the presence of clubs or dumb-bell forms will call for staining by methods such as those of Neisseror Albert or with Loeffler's methylene blue, capable of showing metachromatic granules.Electron microscopy is still not yet available in all diagnostic laboratories and without it the site of insertion of bacterial flagella cannot be determined accurately. Fortunately such information is not often needed for the identification of motile bacteria, andwe use it only for the genus Campylobacter for which the simple flagella stain of Kodaka et al. (1982) is recommended. Scanning electron microscopy seems to have special advantages in the identification of the actinomycetes (Williams & Davies, 1967).
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