Chemotactic Response of Escherichia Coli to Repellents, CoCl2 and NiCl2

Abstract

Negative chemotaxis refers to the motion of microorganisms away from harmful chemicals. Soft agarose gel assay has been traditionally used to characterize the response to various repellents. In this study, we use the “chemical-in-plug” method to quantify the motion of Escherichia coli in the presence of repellents, NiCl2 and CoCl2, over a broad range of concentrations. These experiments were complimented with drift velocity measurements of individual bacteria in controlled gradients using a capillary assay. The latter also revealed the tumbling frequency and steady state clockwise bias for varying concentrations of repellents thereby providing insight into adaptation to repellents. The experimental technique yielded the motion of the bacteria in space and time and further related the motion to the evolving concentration profile of the repellent. Results show that the bacteria exhibit logarithmic sensing to the repellents, i.e., the drift velocity of E.coli is proportional to the logarithmic concentration gradient suggesting Weber law. The predictions of a standard population based model agreed with the observed linear behavior when the binding of the repellent to the receptor was sub-sensitive. This was borne out by a low value of Hill coefficient (n<<1) used to describe the binding characteristics of the receptors. The analysis shows that the binding characteristics for the repellents was sub-sensitive in contrast to an ultra-sensitive response observed for attractants suggesting a negative cooperative behavior of receptors. The above experiments suggest that negative cooperativity allows the cells to respond to harmful chemicals without saturation even at high concentration.