Lactose permease gene (lacY) was overexpressed in the septuple knock-out mutant of Escherichia coli, previously engineered for hydrogen production from glucose. It was expected that raising the lactose transporter activity would elevate the intracellular lactose concentration, inactivate the lactose repressor, induce the lactose operon, and in result, stimulate the overall lactose consumption and conversion. However, overexpression of the lactose transporter caused a considerable growth delay of the recombinant strain on lactose, resembling to some extent the 'lactose killing' phenomenon, described by Dykhuizen and Hartl [1]. Therefore, the recombinant strain was subjected to selection on lactose-containing media. Selection on plates with 3% lactose yielded a strain with a decreased content of the recombinant plasmid, yet with an improved ability to grow and produce hydrogen on lactose. Macromolecular analysis of its biomass by means of FT-IR spectroscopy demonstrated that increase of the cellular polysaccharide content might contribute to the adaptation of E. coli to lactose stress. This article is protected by copyright. All rights reserved.