The Effect of Fructose, Galactose, and Glucose on the Induction of β-Galactosidase in Escherichia coli

Carbon catabolite repression (CCR) refers to the ability of bacteria to preferentially utilize one carbon source over another. Phosphoenolpyruvate-dependent carbohydrate: phosphotransferase system (PTS) is the mechanism by which CCR is exerted. In the absence of preferred PTS-carbon sources, cAMP-CAP complexes are abundant and thus, the enzymes encoded in the lac operon can be expressed maximally. In our experiment, IPTG was used to induce expression of the lac operon in E. coli cells grown in 0.2% glycerol and then transferred to 0.1% of fructose, galactose, and glucose. β-galactosidase levels of each culture were measured by absorption spectroscopy of nitrophenol produced from the ONPG substrate. Results showed that the addition of galactose exhibited a decrease in βgalactosidase induction. However, both glucose and fructose displayed lower, but similar enzyme induction patterns to galactose. By the end of the experiment, enzyme induction in all three conditions (glucose, fructose, and galactose) was approximately 33% that of the control (glycerol). Initially, according to our “composition theory,” we hypothesized that induction levels of galactose would follow that of lactose and induction levels with fructose would follow that of sucrose. However according to our results, galactose may exhibit feedback inhibition when there are higher levels, while there seems to be no preferential usage between glucose and fructose; thus, monosaccharides derived from dissacchrides are not necessarily utilized by bacteria in the same way.