Motility is an inheritable phenotype and is a useful criterion for identification and classification of bacteria. Microscopic examination of live bacteria in wet mounts reveals whether the bacteria are motile or nonmotile. Because unstained transparent cells are examined, more examination time is usually needed to visualize and locate the cells than for stained preparations. This is particularly true because 400X rather than 1,000X magnification is used to see bacterial cells in this type of preparation, and therefore examination is critical. Due to these limitations, special techniques are used in order to prevent the wet mount from drying during the time required for microscopic examination. The hanging drop technique is a method in which a drop of bacterial suspension, preferably in mid-logarithmic phase, is enclosed in an air-tight chamber prepared in a special depression slide (having a concave depression in the center) or assembled from modelling clay (plasticine) which is a soft, malleable, and nonhardening material available at toy or hobby stores.
Upon inoculating a batch of culture medium, bacterial cells go through successive phases starting with a "lag" adaptation phase the length of which varies with type of organism and nature of the environment. During this phase cells may experience shock due to the change in environment. In addition, if cells were transferred from an old culture or from the refrigerator, their flagella may have deteriorated; therefore at this stage of growth examination for bacterial motility is not recommended. Bacteria then start dividing at the maximum rate, entering a "log' phase in which numbers increase logarithmically (exponentially). Young actively dividing cells demonstrate the best motility at the log phase. Further incubation, over 8 hours, leads bacteria to enter a "stationary" phase of growth during which the number of bacterial cells that die is almost equivalent to the number of newly formed cells. Also, at this stage it is not recommended to examine cells as motility may be weak or considerably lost. Extended incubation in batch cultures leads eventually to the death and possibly lysis of bacterial cells, a phase usually described as the "decline" phase.
This original Flash animation (made with Macromedia Flash 5) illustrates two techniques used to carry out microscopic motility testing in wet mounts and gives details of the two procedures and hints about some observations made during microscopy.
This animation could serve as a useful tutorial aid in the microbiology laboratory for undergraduates studying microbiology as well as for high school students.