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Examination of bacterial flagellation by dark-field microscopy

A method is described for visualizing unstained bacterial flagella by dark-field light microscopy. Since individual filaments can be seen, a genus such as Salmonella, which is peritrichously flagellated, can readily be distinguished from a polarly flagellated genus such as Pseudomonas. Polarly flagellated bacteria generally swim much faster than peritrichously flagellated bacteria, and turn by abrupt reversals. The differences in flagellation and motility provide diagnostic criteria that may be useful in clinical microbiology.