From Gene Expression to Behavior: A Study in Bacteria Cells Gene expression plays an important role in cell metabolism, motility, and replications. In a search for a bistable gene expression, I measured the distribution of maltoporin LamB using a fluorescence protein reporter gene venus. Though it is still not conclusive about the existence of two subpopulations in the protein concentration, the study estimates the burst-like LamB expression to have a mean bursting rate of 9.4 bursts per cell-cycle and a mean bursting number of 82 copies per burst. In another study on bacterial growth curves, I unexpectedly discovered that depletion of 4 amino acids (arginine, methionine, isoleucine, and glycine) can cause diauxie-like growth arrests, which last for 5 to 30 minutes depending on the amino acids used. As a quantitative measure of response times of gene regulations upon starvation, this technique could potentially provide a simple and robust means to determine the time constants in metabolic networks of a variety of bacteria. Proper expression of genes is also significant for robust bacterial swimming and chemotaxis. Unexpectedly, I found that over expression of the chemotactic regulator protein, CheY, in marine bacterium Vibrio alginolyticus not only alters the motor switching frequency but also affects the cell's ability to changing the swimming directions, which is accomplished by sweeping the flagellum about its base at the cell pole. I observed strikingly that very small Vibrio alginolyticus cells are able to reorient by a π -flip (180°) in ~ 60 ms, but this ability is gradually lost when the cell size becomes larger. I also observed that in a cheY mutant of V. alginolyticus, over expression of CheY, or its phosphorylated form CheY-p, significantly diminishes the mutant's ability to change swimming directions. To understand how a flick is accomplished by polar-flagellated bacteria, Prof. Fu came up with a simple model incorporating flagellar off-axial rotation during a flick. I implemented the model with some improvements to explain my experimental observations. Comparing the model prediction with the experiment, I found that all of my observations are consistent with the physical picture that V. alginolyticus' flagellum follows some close path Σ on the surface of a unisphere characterized by the polar θ and azimuthal φ angles. Bacteria of different sizes have different contour lengths, which give rise to different flick angles. Fitting using the model also suggests that the flagellum of cheY mutant follows a smaller closed contour compared with that of wild-type cells, suggesting the possibility of motor remodeling and “stiffening” when CheY concentration is increased.