Zinc-dependent cytoadherence of Legionella pneumophila to human alveolar epithelial cells in vitro

The Legionella collagen-like protein employs a distinct binding mechanism for the recognition of host glycosaminoglycans

Bacterial adhesion is a fundamental process which enables colonisation of niche environments and is key for infection. However, in Legionella pneumophila, the causative agent of Legionnaires' disease, these processes are not well understood. The Legionella collagen-like protein (Lcl) is an extracellular peripheral membrane protein that recognises sulphated glycosaminoglycans on the surface of eukaryotic cells, but also stimulates bacterial aggregation in response to divalent cations. Here we report the crystal structure of the Lcl C-terminal domain (Lcl-CTD) and present a model for intact Lcl. Our data reveal that Lcl-CTD forms an unusual trimer arrangement with a positively charged external surface and negatively charged solvent exposed internal cavity. Through molecular dynamics simulations, we show how the glycosaminoglycan chondroitin-4-sulphate associates with the Lcl-CTD surface via distinct binding modes. Our findings show that Lcl homologs are present across both the Pseudomonadota and Fibrobacterota-Chlorobiota-Bacteroidota phyla and suggest that Lcl may represent a versatile carbohydrate-binding mechanism.

The Legionella collagen-like protein employs a unique binding mechanism for the recognition of host glycosaminoglycans

Bacterial adhesion is a fundamental process which enables colonisation of niche environments and is key for infection. However, in Legionella pneumophila, the causative agent of Legionnaires' disease, these processes are not well understood. The Legionella collagen-like protein (Lcl) is an extracellular peripheral membrane protein that recognises sulphated glycosaminoglycans (GAGs) on the surface of eukaryotic cells, but also stimulates bacterial aggregation in response to divalent cations. Here we report the crystal structure of the Lcl C-terminal domain (Lcl-CTD) and present a model for intact Lcl. Our data reveal that Lcl-CTD forms an unusual dynamic trimer arrangement with a positively charged external surface and a negatively charged solvent exposed internal cavity. Through Molecular Dynamics (MD) simulations, we show how the GAG chondroitin-4-sulphate associates with the Lcl-CTD surface via unique binding modes. Our findings show that Lcl homologs are present across both the Pseudomonadota and Fibrobacterota-Chlorobiota-Bacteroidota phyla and suggest that Lcl may represent a versatile carbohydrate binding mechanism.

Legionella and Legionnaires' disease: Time to explore in India

Legionella pneumophila was first recognised as a fatal cause of pneumonia more than four decades ago, during the 1976-American Legion convention in Philadelphia, USA. Legionella spp. continue to cause disease outbreaks of public health significance, and at present, Legionnaires' disease (LD) has emerged as an important cause of community and hospital-acquired pneumonia. Parallel to this, the understanding of LD has also increased exponentially. However, the disease is likely to be underreported in many countries because of the dearth of common definitions, diagnostic tests and active surveillance systems. In this review, we outline the basic concepts of Legionella including clinical presentations, epidemiology, laboratory diagnosis and the status of LD in India. This article also summarises the progress of research related to Legionella in this country, identifying the research gaps and discussing priorities to explore this unexplored pathogen in India.

The Legionella pneumophila collagen-like protein mediates sedimentation, autoaggregation, and pathogen-phagocyte interactions

Although only partially understood, multicellular behavior is relatively common in bacterial pathogens. Bacterial aggregates can resist various host defenses and colonize their environment more efficiently than planktonic cells. For the waterborne pathogen Legionella pneumophila, little is known about the roles of autoaggregation or the parameters which allow cell-cell interactions to occur. Here, we determined the endogenous and exogenous factors sufficient to allow autoaggregation to take place in L. pneumophila. We show that isolates from Legionella species which do not produce the Legionella collagen-like protein (Lcl) are deficient in autoaggregation. Targeted deletion of the Lcl-encoding gene (lpg2644) and the addition of Lcl ligands impair the autoaggregation of L. pneumophila. In addition, Lcl-induced autoaggregation requires divalent cations. Escherichia coli producing surface-exposed Lcl is able to autoaggregate and shows increased biofilm production. We also demonstrate that L. pneumophila infection of Acanthamoeba castellanii and Hartmanella vermiformis is potentiated under conditions which promote Lcl dependent autoaggregation. Overall, this study shows that L. pneumophila is capable of autoaggregating in a process that is mediated by Lcl in a divalent-cation-dependent manner. It also reveals that Lcl potentiates the ability of L. pneumophila to come in contact, attach, and infect amoebae.

Biofilms: the stronghold of Legionella pneumophila

Legionellosis is mostly caused by Legionella pneumophila and is defined as a severe respiratory illness with a case fatality rate ranging from 5% to 80%. L. pneumophila is ubiquitous in natural and anthropogenic water systems. L. pneumophila is transmitted by inhalation of contaminated aerosols produced by a variety of devices. While L. pneumophila replicates within environmental protozoa, colonization and persistence in its natural environment are also mediated by biofilm formation and colonization within multispecies microbial communities. There is now evidence that some legionellosis outbreaks are correlated with the presence of biofilms. Thus, preventing biofilm formation appears as one of the strategies to reduce water system contamination. However, we lack information about the chemical and biophysical conditions, as well as the molecular mechanisms that allow the production of biofilms by L. pneumophila. Here, we discuss the molecular basis of biofilm formation by L. pneumophila and the roles of other microbial species in L. pneumophila biofilm colonization. In addition, we discuss the protective roles of biofilms against current L. pneumophila sanitation strategies along with the initial data available on the regulation of L. pneumophila biofilm formation.

Microbial and physicochemical parameters associated with Legionella contamination in hot water recirculation systems

Hot water recirculation systems (HWRS) in hotels and nursing homes, which are common in countries such as Spain, have been related to outbreaks of legionellosis. To establish the relationships of microbial and physicochemical parameters, especially protozoa, with the occurrence of Legionella in HWRS, 231 samples from hotels and nursing homes were analysed for Legionella, protozoa, heterotrophic plate counts (HPC) at 22 and 37 °C, Pseudomonas, metals, temperature and others. Legionella pneumophila was the dominant species isolated, and 22 % were sg. 1. The sampling method became particularly important in order to define which factors were involved on the occurrence of Legionella. Results showed that the bacteria and the accompanying microbiota were more abundant in the first flush water whose temperature was lower. The bacteria occurred in those samples with high HPC and were inversely correlated with high temperatures. Multivariate regression showed that a concentration above 1 × 10(5) CFU/100 mL of HPC at 37 °C, Fe above 0.095 ppm and the presence of protozoa increased significantly the risk of Legionella colonization, while univariant regression showed that the presence of Cu above 0.76 ppm and temperature above 55 °C diminished it. Therefore, to reduce the risk associated with Legionella occurrence in HWRS these parameters should be taken into consideration.

Lcl of Legionella pneumophila is an immunogenic GAG binding adhesin that promotes interactions with lung epithelial cells and plays a crucial role in biofilm formation

Legionellosis is mostly caused by Legionella pneumophila and is defined by a severe respiratory illness with a case fatality rate ranging from 5 to 80%. In vitro and in vivo, interactions of L. pneumophila with lung epithelial cells are mediated by the sulfated glycosaminoglycans (GAGs) of the host extracellular matrix. In this study, we have identified several Legionella heparin binding proteins. We have shown that one of these proteins, designated Lcl, is a polymorphic adhesin of L. pneumophila that is produced during legionellosis. Homologues of Lcl are ubiquitous in L. pneumophila serogroups but are undetected in other Legionella species. Recombinant Lcl binds to GAGs, and a Δlpg2644 mutant demonstrated reduced binding to GAGs and human lung epithelial cells. Importantly, we showed that the Δlpg2644 strain is dramatically impaired in biofilm formation. These data delineate the role of Lcl in the GAG binding properties of L. pneumophila and provide molecular evidence regarding its role in L. pneumophila adherence and biofilm formation.

Potentially pathogenic amoeba-associated microorganisms in cooling towers and their control

Cooling towers provide a favorable environment for the proliferation of microorganisms. Cooling towers generate a biofilm and often aerosolize contaminated water, thereby increasing the risk of microorganism dissemination by human inhalation. This pathogen dissemination was first revealed by the epidemics of Legionnaires’ disease that were directly related to the presence of cooling towers, and since then, the ecology of Legionella pneumophila has been well studied. Each country has specific standards regarding the acceptable amount of microorganisms in cooling tower systems. However, those standards typically only concern L. pneumophila, even though many other microorganisms can also be isolated from cooling towers, including protozoa, bacteria and viruses. Microbiological control of the cooling tower system can be principally achieved by chemical treatments and also by improving the system’s construction. Several new treatments are being studied to improve the efficiency of disinfection. However, as most of these treatments continue to focus solely on L. pneumophila, reports of other types of pathogens continue to increase. Therefore, how their dissemination affects the human populous health should be addressed now.

Purification of Legiobactin and importance of this siderophore in lung infection by Legionella pneumophila

When cultured in a low-iron medium, Legionella pneumophila secretes a siderophore (legiobactin) that is both reactive in the chrome azurol S (CAS) assay and capable of stimulating the growth of iron-starved legionellae. Using anion-exchange high-pressure liquid chromatography (HPLC), we purified legiobactin from culture supernatants of a virulent strain of L. pneumophila. In the process, we detected the ferrated form of legiobactin as well as other CAS-reactive substances. Purified legiobactin had a yellow-gold color and absorbed primarily from 220 nm and below. In accordance, nuclear magnetic resonance spectroscopy revealed that legiobactin lacks aromatic carbons, and among the 13 aliphatics present, there were 3 carbonyls. When examined by HPLC, supernatants from L. pneumophila mutants inactivated for lbtA and lbtB completely lacked legiobactin, indicating that the LbtA and LbtB proteins are absolutely required for siderophore activity. Independently derived lbtA mutants, but not a complemented derivative, displayed a reduced ability to infect the lungs of A/J mice after intratracheal inoculation, indicating that legiobactin is required for optimal intrapulmonary survival by L. pneumophila. This defect, however, was not evident when the lbtA mutant and its parental strain were coinoculated into the lung, indicating that legiobactin secreted by the wild type can promote growth of the mutant in trans. Legiobactin mutants grew normally in murine lung macrophages and alveolar epithelial cells, suggesting that legiobactin promotes something other than intracellular infection of resident lung cells. Overall, these data represent the first documentation of a role for siderophore expression in the virulence of L. pneumophila.

In vivo effect of adhesion inhibitor heparin on Legionella pneumophila pathogenesis in a murine pneumonia model

OBJECTIVE

To examine the effect of intratracheal heparin instillation on Legionella pneumophila-related acute lung injury (ALI) and systemic dissemination.

DESIGN

Prospective, controlled experimental study.

SETTING

University research laboratory.

INTERVENTIONS

A/J mice received 5 microg of sulfated heparin intratracheally co-instilled with 10(6) or 10(8) colony-forming units (CFU) of a virulent isolate of L. pneumophila.

MEASUREMENTS AND RESULTS

ALI was assessed in control groups (PBS and PBS-heparin) and on days 1, 2 and 3 post-infection, in terms of the lung wet-to-dry (W/D) weight ratios and of lung endothelial permeability to radio-labeled albumin (Perm-I(125)). Lung bacterial loads were measured and systemic spread was assessed by blood and target organ culture. The alveolar inflammatory response was evaluated by measuring the cytokine levels (TNF-alpha, IFN-gamma, IL-6 and IL-12p70) in bronchoalveolar lavage fluids (BALF). Co-instilled heparin improved mouse survival after the 10(8) CFU challenge (p < 0.01). On day 2, heparin co-instillation significantly reduced the W/D ratio and Perm-I(125) (p < 0.01 and p < 0.001 respectively), improved lung bacterial clearance (p < 0.001), prevented systemic dissemination (blood, liver, spleen, kidneys and brain cultures, all p < 0.05) and significantly increased IFN-gamma and IL-12p70 levels in BALF (p < 0.05).

CONCLUSIONS

Heparin co-instillation during intratracheal L. pneumophila challenge has a protective effect on the alveolar-capillary barrier and prevents bacterial dissemination. These results tend to confirm the competitive inhibition by heparin of L. pneumophila attachment to lung epithelium in vivo, and point to the possible involvement of a heparan-sulfate adhesin in L. pneumophila binding to pneumocytes.