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12 p.-4 fig.-4 tab., Bacterial surfaces are decorated with carbohydrate structures that may serve as ligands for host receptors. Based on their ability to recognize specific sugar epitopes, plant lectins are extensively used for bacteria typing. We previously observed that the galactose-specific agglutinins from Ricinus communis (RCA) and Viscum album (VAA) exhibited differential binding to nontypeable Haemophilus influenzae (NTHi) clinical isolates, their binding being distinctly affected by truncation of the lipooligosaccharide (LOS). Here, we examined their binding to the structurally similar LOS molecules isolated from strains NTHi375 and RdKW20, using microarray binding assays, saturation transfer difference NMR, and molecular dynamics simulations. RCA bound the LOSRdKW20 glycoform displaying terminal Gal beta(1,4) Glc beta, whereas VAA recognized the Gala(1,4) Gal beta(1,4) Glc beta epitope in LOSNTHi375 but not in LOSRdKW20, unveiling a different presentation. Binding assays to whole bacterial cells were consistent with LOSNTHi375 serving as ligand for VAA, and also suggested recognition of the glycoprotein HMW1. Regarding RCA, comparable binding to NTHi375 and RdKW20 cells was observed. Interestingly, an increase in LOSNTHi375 abundance or expression of HMW1 in RdKW20 impaired RCA binding. Overall, the results revealed that, besides the LOS, other carbohydrate structures on the bacterial surface serve as lectin ligands, and highlighted the impact of the specific display of cell surface components on lectin binding., We gratefully acknowledge financial support from the Spanish Ministry of Economy and Competitiveness(Grants BFU2015-70052-R, CTQ2015-64597-C2-2-P and SAF2015-66520-R), the Department of Health of the Navarra Government (ref 03/2016), the CIBER of Respiratory Diseases (CIBERES), an initiative from the Spanish Institute of Health Carlos III (ISCIII), and the Marie Curie Initial Training Networks DYNANO(Grant PITN-GA-2011-289033), GLYCOPHARM (Grant PITN-GA-2012-317297), and WntsApp (Grant PITN-GA-2013-608180).

Bacterial surfaces are decorated with a diversity of carbohydrate structures that play important roles in the bacteria–host relationships. They may offer protection against host defense mechanisms, elicit strong antigenic responses, or serve as ligands for host receptors, including lectins of the innate immune system. Binding by these lectins may trigger defense responses or, alternatively, promote attachment, thereby enhancing infection. The outcome will depend on the particular bacterial surface landscape, which may substantially differ among species and strains. In this chapter, we describe two novel methods for exploring interactions directly on the bacterial surface, based on the generation of bacterial microarrays and quartz crystal microbalance (QCM) sensor chips. Bacterial microarrays enable profiling of accessible carbohydrate structures and screening of their recognition by host receptors, also providing information on binding avidity, while the QCM approach allows determination of binding affinity and kinetics. In both cases, the chief element is the use of entire bacterial cells, so that recognition of the bacterial glycan epitopes is explored in their natural environment., We gratefully acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (Grants BFU2012-36825, BFU2015-70052-R, SAF2012-31166, and SAF2015-66520-R), the Department of Health of the Navarra Government (ref. 359/2012), the CIBER of Respiratory Diseases (CIBERES), an initiative from the Spanish Institute of Health Carlos III (ISCIII), and the Marie Curie Initial Training Networks DYNANO (Grant PITN-GA-2011-289033), GLYCOPHARM (Grant PITN-GA-2012- 317297), and WntsApp (GA-No. 608180, FP7-PEOPLE-2013). I.K. and D.P. were beneficiaries of Marie Curie contracts from the European Commission.