YSIRK-G/S-directed translocation is required for Streptococcus suis to deliver diverse cell wall anchoring effectors contributing to bacterial pathogenicity

Type 1 fimbriae-mediated collective protection against type 6 secretion system attacks

Bacterial competition may rely on secretion systems such as the type 6 secretion system (T6SS), which punctures and releases toxic molecules into neighboring cells. To subsist, bacterial targets must counteract the threats posed by T6SS-positive competitors. In this study, we used a comprehensive genome-wide high-throughput screening approach to investigate the dynamics of interbacterial competition. Our primary goal was to identify deletion mutants within the well-characterized E. coli K-12 single-gene deletion library, the Keio collection, that demonstrated resistance to T6SS-mediated killing by the enteropathogenic bacterium Cronobacter malonaticus. We identified 49 potential mutants conferring resistance to T6SS and focused our interest on a deletion mutant (∆fimE) exhibiting enhanced expression of type 1 fimbriae. We demonstrated that the presence of type 1 fimbriae leads to the formation of microcolonies and thus protects against T6SS-mediated assaults. Collectively, our study demonstrated that adhesive structures such as type 1 fimbriae confer collective protective behavior against T6SS attacks.IMPORTANCEType 6 secretion systems (T6SS) are molecular weapons employed by gram-negative bacteria to eliminate neighboring microbes. T6SS plays a pivotal role as a virulence factor, enabling pathogenic gram-negative bacteria to compete with the established communities to colonize hosts and induce infections. Gaining a deeper understanding of bacterial interactions will allow the development of strategies to control the action of systems such as the T6SS that can manipulate bacterial communities. In this context, we demonstrate that bacteria targeted by T6SS attacks from the enteric pathogen Cronobacter malonaticus, which poses a significant threat to infants, can develop a collective protective mechanism centered on the production of type I fimbriae. These adhesive structures promote the aggregation of bacterial preys and the formation of microcolonies, which protect the cells from T6SS attacks.

Host PTX3 Protein and Bacterial Capsule Coordinately Regulate the Inflammatory Response during Streptococcus suis Infection

Streptococcus suis serotype 2 (SS2) is a noteworthy zoonotic pathogen that has been responsible for large economic losses in pig production and a great threat to human health. Pentraxin 3 (PTX3) is an essential regulator of the innate immune response to bacterial pathogens; however, its role during SS2 infection is not fully understood. In this study, we found that the SS2 strain HA9801 induced a significant inflammatory response in the mouse air pouch model; this response was amplified by the treatment of exogenous PTX3 simultaneously in terms of the results of inflammatory cell recruitment and proinflammatory cytokine IL-6 production. In addition, PTX3 facilitated the phagocytosis of macrophage Ana-1 against SS2 strain HA9801. The supplementation of exogenous PTX3 significantly reduced the bacterial loads in a dose-dependent manner in lungs, livers and bloods of SS2-infected mice compared to the samples with HA9801 infection alone; this finding indicated that PTX3 may facilitate the bacterial clearance through enhancing the host inflammatory response during SS2 infection. Both PTX3 and SS2 capsular polysaccharide (CPS2) were required for the robust inflammatory response, implying that the host PTX3 protein and SS2 surface CPS2 modulate the host innate immune response in concert. All of these results suggested that PTX3 is a potential novel biological agent for the SS2 infection; however, the recommended dose of PTX3 must be evaluated strictly to avoid inducing an excessive inflammatory response that can cause serious tissue injury and animal death.

GPApred: The first computational predictor for identifying proteins with LPXTG-like motif using sequence-based optimal features

The cell surface proteins of gram-positive bacteria are involved in many important biological functions, including the infection of host cells. Owing to their virulent nature, these proteins are also considered strong candidates for potential drug or vaccine targets. Among the various cell surface proteins of gram-positive bacteria, LPXTG-like proteins form a major class. These proteins have a highly conserved C-terminal cell wall sorting signal, which consists of an LPXTG sequence motif, a hydrophobic domain, and a positively charged tail. These surface proteins are targeted to the cell envelope by a sortase enzyme via transpeptidation. A variety of LPXTG-like proteins have been experimentally characterized; however, their number in public databases has increased owing to extensive bacterial genome sequencing without proper annotation. In the absence of experimental characterization, identifying and annotating these sequences is extremely challenging. Therefore, in this study, we developed the first machine learning-based predictor called GPApred, which can identify LPXTG-like proteins from their primary sequences. Using a newly constructed benchmark dataset, we explored different classifiers and five feature encodings and their hybrids. Optimal features were derived using the recursive feature elimination method, and these features were then trained using a support vector machine algorithm. The performance of different models was evaluated using independent datasets, and a final model (GPApred) was selected based on consistency during cross-validation and independent assessment. GPApred can be an effective tool for predicting LPXTG-like sequences and can be further employed for functional characterization or drug targeting. Availability: https://procarb.org/gpapred/.

SxsA, a novel surface protein mediating cell aggregation and adhesive biofilm formation of Staphylococcus xylosus

Biofilm formation of staphylococci has been an emerging field of research for many years. However, the underlying molecular mechanisms are still not fully understood, and vary widely between species and strains. The aim of this study was to identify new effectors impacting biofilm formation of two Staphylococcus (S.) xylosus strains. We identified a novel surface protein conferring cell aggregation, adherence to abiotic surfaces and biofilm formation. The S. xylosus surface protein A (SxsA) is a large protein occurring in variable sizes. It lacks sequence similarity to other staphylococcal surface proteins but shows similar structural domain organization and functional features. Upon deletion of sxsA, adherence of S. xylosus strain TMW 2.1523 to abiotic surfaces was completely abolished, and significantly reduced in TMW 2.1023. Macro- and microscopic aggregation assays further showed that TMW 2.1523 sxsA mutants exhibit reduced cell aggregation compared to the wildtype. Comparative genomic analysis revealed that sxsA is part of the core genome of S. xylosus, S. paraxylosus and S. nepalensis and additionally encoded in a small group of S. cohnii and S. saprophyticus strains. This study provides insights into protein-mediated biofilm formation of S. xylosus and identifies a new cell wall-associated protein influencing cell aggregation and biofilm formation.

Bap-Independent Biofilm Formation in Staphylococcus xylosus

The biofilm associated protein (Bap) is recognised as the essential component for biofilm formation in Staphylococcus aureus V329 and has been predicted as important for other species as well. Although Bap orthologs are also present in most S. xylosus strains, their contribution to biofilm formation has not yet been demonstrated. In this study, different experimental approaches were used to elucidate the effect of Bap on biofilm formation in S. xylosus and the motif structure of two biofilm-forming S. xylosus strains TMW 2.1023 and TMW 2.1523 was compared to Bap of S. aureus V329. We found that despite an identical structural arrangement into four regions, Bap from S. xylosus differs in key factors to Bap of S. aureus, i.e., isoelectric point of aggregation prone Region B, protein homology and type of repeats. Disruption of bap had no effect on aggregation behavior of selected S. xylosus strains and biofilm formation was unaffected (TMW 2.1023) or at best slightly reduced under neutral conditions (TMW 2.1523). Further, we could not observe any typical characteristics of a S. aureus Bap-positive phenotype such as functional impairment by calcium addition and rough colony morphology on congo red agar (CRA). A dominating role of Bap in cell aggregation and biofilm formation as reported mainly for S. aureus V329 was not observed. In contrast, this work demonstrates that functions of S. aureus Bap cannot easily be extrapolated to S. xylosus Bap, which appears as non-essential for biofilm formation in this species. We therefore suggest that biofilm formation in S. xylosus follows different and multifactorial mechanisms.

Spatial regulation of protein A in Staphylococcus aureus

Surface proteins of Staphylococcus aureus play vital roles in bacterial physiology and pathogenesis. Recent work suggests that surface proteins are spatially regulated by a YSIRK/GXXS signal peptide that promotes cross-wall targeting at the mid-cell, though the mechanisms remain unclear. We previously showed that protein A (SpA), a YSIRK/GXXS protein and key staphylococcal virulence factor, mis-localizes in a ltaS mutant deficient in lipoteichoic acid (LTA) production. Here, we identified that SpA contains another cross-wall targeting signal, the LysM domain, which, in addition to the YSIRK/GXXS signal peptide, significantly enhances SpA cross-wall targeting. We show that LTA synthesis, but not LtaS, is required for SpA septal anchoring and cross-wall deposition. Interestingly, LTA is predominantly found at the peripheral cell membrane and is diminished at the septum of dividing staphylococcal cells, suggesting a restriction mechanism for SpA septal localization. Finally, we show that D-alanylation of LTA abolishes SpA cross-wall deposition by disrupting SpA distribution in the peptidoglycan layer without altering SpA septal anchoring. Our study reveals that multiple factors contribute to the spatial regulation and cross-wall targeting of SpA via different mechanisms, which coordinately ensures efficient incorporation of surface proteins into the growing peptidoglycan during the cell cycle.