To the advantage of these bacteria, they are able to detect both attractant and repellant chemicals (6). When an increasing concentration of an attractant (sugar) or decreasing concentration of a repellant (fatty acid) is detected, the bacteria will decrease the frequency at which it tumbles and will thus swim for longer periods (5). This gives the bacteria more of an opportunity to swim into an area where the concentration of a needed chemical is high and away from areas which contain toxic chemicals. Conversely, detection of decreasing concentrations of attractants or increasing concentrations of repellants causes bacteria to increase their tumbling frequencies (5). While the tumbling remains random, increasing the amount of tumbling allows bacteria to change direction more often, hopefully oriented away from a repellant and toward an attractant gradient. What must be understood here is that the movement of these bacteria is still random just as it would be in a solution where there was no detection of chemical gradients; however, the bacteria modify their movement patterns by increasing and decreasing the frequency at which they change from swimming to tumbling.
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