Aromatic Esters of Bicyclic Amines as Antimicrobials against Streptococcus pneumoniae

Drug Repositioning as a Therapeutic Strategy against Streptococcus pneumoniae: Cell Membrane as Potential Target

A collection of repurposing drugs (Prestwick Chemical Library) containing 1200 compounds was screened to investigate the drugs' antimicrobial effects against planktonic cultures of the respiratory pathogen Streptococcus pneumoniae. After four discrimination rounds, a set of seven compounds was finally selected, namely (i) clofilium tosylate; (ii) vanoxerine; (iii) mitoxantrone dihydrochloride; (iv) amiodarone hydrochloride; (v) tamoxifen citrate; (vi) terfenadine; and (vii) clomiphene citrate (Z, E). These molecules arrested pneumococcal growth in a liquid medium and induced a decrease in bacterial viability between 90.0% and 99.9% at 25 µM concentration, with minimal inhibitory concentrations (MICs) also in the micromolar range. Moreover, all compounds but mitoxantrone caused a remarkable increase in the permeability of the bacterial membrane and share a common, minimal chemical structure consisting of an aliphatic amine linked to a phenyl moiety via a short carbon/oxygen linker. These results open new possibilities to tackle pneumococcal disease through drug repositioning and provide clues for the design of novel membrane-targeted antimicrobials with a related chemical structure.

Choline-Functionalized Supramolecular Copolymers: Toward Antimicrobial Activity against Streptococcus pneumoniae

Dynamic binding events are key to arrive at functionality in nature, and these events are often governed by electrostatic or hydrophobic interactions. Synthetic supramolecular polymers are promising candidates to obtain biomaterials that mimic this dynamicity. Here, we created four new functional monomers based on the benzene-1,3,5-tricarboxamide (BTA) motif. Choline or atropine groups were introduced to obtain functional monomers capable of competing with the cell wall of Streptococcus pneumoniae for binding of essential choline-binding proteins (CBPs). Atropine-functionalized monomers BTA-Atr and BTA-Atr₃ were too hydrophobic to form homogeneous assemblies, while choline-functionalized monomers BTA-Chol and BTA-Chol₃ were unable to form fibers due to charge repulsion. However, copolymerization of BTA-Chol₃ with non-functionalized BTA-(OH)₃ yielded dynamic fibers, similar to BTA-(OH)₃. These copolymers showed an increased affinity toward CBPs compared to free choline due to multivalent effects. BTA-based supramolecular copolymers are therefore a versatile platform to design bioactive and dynamic supramolecular polymers with novel biotechnological properties.

The search for novel treatment strategies for Streptococcus pneumoniae infections

This review provides an overview of the most important novel treatment strategies against Streptococcus pneumoniae infections published over the past 10 years. The pneumococcus causes the majority of community-acquired bacterial pneumonia cases, and it is one of the prime pathogens in bacterial meningitis. Over the last 10 years, extensive research has been conducted to prevent severe pneumococcal infections, with a major focus on (i) boosting the host immune system and (ii) discovering novel antibacterials. Boosting the immune system can be done in two ways, either by actively modulating host immunity, mostly through administration of selective antibodies, or by interfering with pneumococcal virulence factors, thereby supporting the host immune system to effectively overcome an infection. While several of such experimental therapies are promising, few have evolved to clinical trials. The discovery of novel antibacterials is hampered by the high research and development costs versus the relatively low revenues for the pharmaceutical industry. Nevertheless, novel enzymatic assays and target-based drug design, allow the identification of targets and the development of novel molecules to effectively treat this life-threatening pathogen.

Searching for Antipneumococcal Targets: Choline-Binding Modules as Phagocytosis Enhancers

Choline-binding proteins (CBPs) from Streptococcus pneumoniae comprise a family of modular polypeptides involved in essential events of this pathogen. They recognize the choline residues present in the teichoic and lipoteichoic acids of the cell wall using the so-called choline-binding modules (CBMs). The importance of CBPs in pneumococcal physiology points to them as novel targets to combat antimicrobial resistances shown by this organism. In this work we have tested the ability of exogenously added CBMs to act as CBP inhibitors by competing with the latter for the binding to the choline molecules in the bacterial surface. First, we carried out a thorough physicochemical characterization of three native CBMs, namely C-LytA, C-Cpl1, and C-CbpD, and assessed their affinity for choline and macromolecular, pneumococcal cell-wall mimics. The interaction with these substrates was evaluated by molecular modeling, analytical ultracentrifugation, surface plasmon resonance, and fluorescence and circular dichroism spectroscopies. Van't Hoff thermal analyses unveiled the existence of one noncanonical choline binding site in each of the C-Cpl1 and C-CbpD proteins, leading in total to 5 ligand-binding sites per dimer and 4 sites per monomer, respectively. Remarkably, the binding affinities of the CBMs do not directly correlate with their native oligomeric state or with the number of choline-binding sites, suggesting that choline recognition by these modules is a complex phenomenon. On the other hand, the exogenous addition of CBMs to pneumococcal planktonic cultures caused extensive cell-chaining probably as a consequence of the inhibition of CBP attachment to the cell wall. This was accompanied by bacterial aggregation and sedimentation, causing an enhancement of bacterial phagocytosis by peritoneal macrophages. In addition, the rational design of an oligomeric variant of a native CBM led to a substantial increase in its antibacterial activity by multivalency effects. These results suggest that CBMs might constitute promising nonlytic antimicrobial candidates based on the natural induction of the host defense system.

Widening the antimicrobial spectrum of esters of bicyclic amines: In vitro effect on gram-positive Streptococcus pneumoniae and gram-negative non-typeable Haemophilus influenzae biofilms

Antibiotic resistance is a global current threat of increasing importance. Moreover, biofilms represent a medical challenge since the inherent antibiotic resistance of their producers demands the use of high doses of antibiotics over prolonged periods. Frequently, these therapeutic measures fail, contributing to bacterial persistence, therefore demanding the development of novel antimicrobials. Esters of bicyclic amines (EBAs), which are strong inhibitors of Streptococcus pneumoniae growth, were initially designed as inhibitors of pneumococcal choline-binding proteins on the basis of their structural analogy to the choline residues in the cell wall. However, instead of mimicking the characteristic cell chaining phenotype caused by exogenously added choline on planktonic cultures of pneumococcal cells, EBAs showed an unexpected lytic activity. In this work we demonstrate that EBAs display a second, and even more important, function as cell membrane destabilizers. We then assayed the inhibitory and disintegrating activity of these molecules on pneumococcal biofilms. The selected compound (EBA 31) produced the highest effect on S. pneumoniae (encapsulated and non-encapsulated) biofilms at very low concentrations. EBA 31 was also effective on mixed biofilms of non-encapsulated S. pneumoniae plus non-typeable Haemophilus influenzae, two pathogens frequently forming a self-produced biofilm in the human nasopharynx. These results support the role of EBAs as a promising alternative for the development of novel, broad-range antimicrobial drugs encompassing both Gram-positive and Gram-negative pathogens.

Choline Binding Proteins from Streptococcus pneumoniae: A Dual Role as Enzybiotics and Targets for the Design of New Antimicrobials

Streptococcus pneumoniae (pneumococcus) is an important pathogen responsible for acute invasive and non-invasive infections such as meningitis, sepsis and otitis media, being the major cause of community-acquired pneumonia. The fight against pneumococcus is currently hampered both by insufficient vaccine coverage and by rising antimicrobial resistances to traditional antibiotics, making necessary the research on novel targets. Choline binding proteins (CBPs) are a family of polypeptides found in pneumococcus and related species, as well as in some of their associated bacteriophages. They are characterized by a structural organization in two modules: a functional module (FM), and a choline-binding module (CBM) that anchors the protein to the choline residues present in the cell wall through non-covalent interactions. Pneumococcal CBPs include cell wall hydrolases, adhesins and other virulence factors, all playing relevant physiological roles for bacterial viability and virulence. Moreover, many pneumococcal phages also make use of hydrolytic CBPs to fulfill their infectivity cycle. Consequently, CBPs may play a dual role for the development of novel antipneumococcal drugs, both as targets for inhibitors of their binding to the cell wall and as active cell lytic agents (enzybiotics). In this article, we review the current state of knowledge about host- and phage-encoded pneumococcal CBPs, with a special focus on structural issues, together with their perspectives for effective anti-infectious treatments.