Based on the affinity of ligand 1Gal for LecA, it was anticipated that this series of glycoconjugate complexes with varying topologies and stoichiometries might interact with PA with antimicrobial or anti-virulence effects. Initially, bactericidal behaviour was assessed by testing the ability of galactosides incubated with bacteria at a range of concentrations from 1 μM to 10 mM to inhibit growth of PAO1 strain of the bacterium, as compared with bacteria-only control and a positive control (where 1 μg/mL of commercial antibiotic ciprofloxacin was added). For ligand 1Gal, complexes [Eu⋅(1Gal)](CF3SO3)3 and [Eu⋅(1Gal)3](CF3SO3)3, modest toxicity to the bacterium was observed only at high concentrations (10 mM, see Figure ), reducing bacterial growth by ca. 30%, with no significant inhibition below this. 1Lac was also tested and showed a similar level of toxicity at 10 mM, likely due to its terminal galactose unit. The zinc and nickel complexes of 1Gal were both found to be completely innocuous to the bacteria, even at high concentrations ( Since these complexes have low toxicity, potential antivirulence activity was also probed by in vitro assay. Biofilm inhibition assays were carried out for all galactoside compounds by crystal violet assay in six replicates, and percentage inhibition of biofilm measured as optical density (OD) at 590 nm, normalised relative to the mean of a bacteria only control. Ligand 1Gal did not show any biofilm inhibition, even at higher concentrations, nor did the Ni(II) complex, while alternative ligands with glucosyl or lactosyl epitopes were also ineffective at biofilm inhibition. Non-toxic complex [Zn⋅(1Gal)2](OAc)2 showed 45% inhibition at 1mM but only 20% at 10mM, this counterintuitive and non-linear drop in inhibitory effect may be explained by zinc being an endogenous metal for organisms, which upon higher concentration could be abstracted from the complex by the bacteria and used as a micronutrient. Since monoleptic complex [Eu⋅(1Gal)]( CF3SO3)3 was toxic to PA at >10 mM, its biofilm inhibition at lower concentrations is of most note: it showed a remarkable 47% biofilm inhibition at 0.1 mM, and 35% even as dilute as 0.01 mM. Control compounds of the same complex structure with 1Glc or 1Lac did not demonstrate decrease in biofilm formation (Figure S-28, ESI †), suggesting that this activity may be driven by the strong interactions of this galactoside with LecA established by ITC, since LecA is known to play a role in biofilm formation. While the complexes with 1:1 stoichiometry demonstrate strong effects, on the other hand the Eu(III) complex with 1:3 stoichiometry had much more modest effects with 24% biofilm inhibition when bacteria were incubated with 0.1 mM of the complex, despite presenting additional galactoside epitopes. This suggests that Eu(III) plays an important role in the mechanism of inhibition but it is unlikely that the biofilm inhibition is caused simply by chelation between carbohydratebinding active sites by the ligands.