Pseudomonas aeruginosa, Chromobacterium violaceum, Burkholderia cenocepacia and Ralstonia solanacearum are saprophytic bacteria found in various environments, including soil, water and vegetation. The first three bacteria occasionally become aggressive opportunistic animal and human pathogens which attack mainly immunocompromised patients, e.g. patients suffering from cystic fibrosis. The fourth bacterium is economically important plant pathogen. All these bacteria produce lectins. Lectins are proteins which are capable of binding diverse carbohydrates with varying specificity. This fact is very important especially in host-pathogen interactions, where lectins help pathogen in adhesion to sugar moieties presented on host cells. Foregoing bacteria produce lectins belonging to a so-called “PA-IIL family”, namely PA-IIL (P. aeruginosa), CV-IIL (C. violaceum), BC2L-A (B. cenocepacia) and RS-IIL (R. solanacearum). These lectins are orthologues with significant sequential and structural similarities. Although they bind similar saccharides, they differ in preferences towards these saccharides. RS-IIL and BC2L-A prefer D-mannose, while PA-IIL and CV-IIL prefer L-fucose. The aim of this work was preparation of mutated variants of these lectins with the amino acid substitution corresponding to the position Thr98 (in PA-IIL), which participates on sugar binding. Changes in affinities/ specificities of the mutants were measured by sothermal titration calorimetry (ITC) and surface plasmon resonance (SPR).