Medical device-associated infections account for a large proportion of hospital-acquired infections. A variety of opportunistic pathogens can cause implant infections, depending on the type of the implant and on the anatomical site of implantation. The success of these versatile pathogens depends on rapid adhesion to virtually all biomaterial surfaces and survival in the hostile host environment. Biofilm formation on implant surfaces shelters the bacteria and encourages persistence of infection. Furthermore, implant-infecting bacteria can elude innate and adaptive host defences as well as biocides and antibiotic chemotherapies. In this Review, we explore the fundamental pathogenic mechanisms underlying implant infections, highlighting orthopaedic implants and Staphylococcus aureus as a prime example, and discuss innovative targets for preventive and therapeutic strategies.

Implant infections: adhesion, biofilm formation and immune evasion

Success in nature depends upon an ability to perceive and adapt to the surrounding environment. Bacteria are not an exception; they recognize and constantly adjust to changing situations by sensing environmental and self-produced signals, altering gene expression accordingly. Autoinducer-2 (AI-2) is a signal molecule produced by LuxS, an enzyme found in many bacterial species and thus proposed to enable interspecies communication. Two classes of AI-2 receptors and many layers and interactions involved in downstream signalling have been identified so far. Although AI-2 has been implicated in the regulation of numerous niche-specific behaviours across the bacterial kingdom, interpretation of these results is complicated by the dual role of LuxS in signalling and the activated methyl cycle, a crucial central metabolic pathway. In this article, we present a comprehensive review of the discovery and early characterization of AI-2, current developments in signal detection, transduction and regulation, and the major studies investigating the phenotypes regulated by this molecule. The development of novel tools should help to resolve many of the remaining questions in the field; we highlight how these advances might be exploited in AI-2 quorum quenching, treatment of diseases, and the manipulation of beneficial behaviours caused by polyspecies communities.

AI‐2‐mediated signalling in bacteria

https://digital.csic.es/bitstream/10261/97192/5/Biofilm%20dispersion_Solano.pdf

Biofilm dispersion and quorum sensing

Staphylococci are frequent human commensals and some species can cause disease. Staphylococcus aureus in particular is a dangerous human pathogen. In staphylococci, the ability to sense the bacterial cell density, or quorum, and to respond with genetic adaptations is due to one main system, which is called accessory gene regulator (Agr). The extracellular signal of Agr is a post-translationally modified peptide containing a thiolactone structure. Under conditions of high cell density, Agr is responsible for the increased expression of many toxins and degradative exoenzymes, and decreased expression of several colonization factors. This regulation is important for the timing of virulence factor expression during infection and the development of acute disease, while low activity of Agr is associated with chronic staphylococcal infections, such as those involving biofilm formation. Accordingly, drugs inhibiting Agr are being evaluated for their capacity to control acute forms of S. aureus infection.

/pdf/quorum-sensing-regulation-in-staphylococci-an-overview-192dbsdaac.pdf

Quorum-sensing regulation in staphylococci—an overview

The staphylococci comprise a diverse genus of Gram-positive, nonmotile commensal organisms that inhabit the skin and mucous membranes of humans and other mammals. In general, staphylococci are benign members of the natural flora, but many species have the capacity to be opportunistic pathogens, mainly infecting individuals who have medical device implants or are otherwise immunocompromised. Staphylococcus aureus and Staphylococcus epidermidis are major sources of hospital-acquired infections and are the most common causes of surgical site infections and medical device-associated bloodstream infections. The ability of staphylococci to form biofilms in vivo makes them highly resistant to chemotherapeutics and leads to chronic diseases. These biofilm infections include osteomyelitis, endocarditis, medical device infections, and persistence in the cystic fibrosis lung. Here, we provide a comprehensive analysis of our current understanding of staphylococcal biofilm formation, with an emphasis on adhesins and regulation, while also addressing how staphylococcal biofilms interact with the immune system. On the whole, this review will provide a thorough picture of biofilm formation of the staphylococcus genus and how this mode of growth impacts the host.

The Staphylococcal Biofilm: Adhesins, Regulation, and Host Response.

Autoinducer 2 (AI-2) is widely recognized as a signal molecule for intra- and interspecies communication in Gram-negative bacteria, but its signaling function in Gram-positive bacteria, especially in Staphylococcus aureus, remains obscure. Here we reveal the role of LuxS in the regulation of capsular polysaccharide synthesis in S. aureus NCTC8325 and show that AI-2 can regulate gene expression and is involved in some physiological activities in S. aureus as a signaling molecule. Inactivation of luxS in S. aureus NCTC8325 resulted in higher levels of transcription of capsular polysaccharide synthesis genes. The survival rate of the luxS mutant was higher than that of the wild type in both human blood and U937 macrophages. In comparison to the luxS mutant, a culture supplemented with chemically synthesized 4,5-dihydroxy-2,3-pentanedione (DPD), the AI-2 precursor molecule, restored all the parental phenotypes, suggesting that AI-2 has a signaling function in S. aureus. Furthermore, we demonstrated that the LuxS/AI-2 signaling system regulates capsular polysaccharide production via a two-component system, KdpDE, whose function has not yet been clarified in S. aureus. This regulation occurred via the phosphorylation of KdpE binding to the cap promoter. Quorum sensing (QS) is a cell-cell communication mechanism in which bacteria secrete and sense small diffusible molecules called autoinducers (AIs) to coordinate social activities, such as bioluminescence, biofilm formation, swarming behavior, antibiotic production, and virulence factor secretion (7, 23, 38, 59). Many QS mechanisms have evolved among bacteria. In general, Gram-negative bacteria use acylated homoserine lactones (AHLs) as AIs, and Gram-positive bacteria use oligopeptide AIs, which act through two-component phosphorelay cascades. Studies have shown that one QS mechanism is shared by both Gram-positive and Gram-negative bacteria and involves the production of autoinducer 2 (AI-2) (4, 38, 59, 60), which is synthesized by the LuxS enzyme in a metabolic pathway known as the activated methyl cycle (50, 57, 61). AI-2 is not a single compound but a family of interconverting compounds derived from 4,5-dihydroxy-2,3-pentanedione (DPD), which cyclizes spontaneously to form two epimeric furanoses, (2R,4S)- and (2S,4S)-2,4-dihydroxy-2-methyldihydrofuran-3one (R- and S-DHMF), respectively. Hydration of R- and SDHMF produces (2R ,4 S)- and (2S ,4 S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R- and S-THMF), respectively (40). In contrast to the other autoinducers that are usually involved in intraspecies communication, AI-2 is widely present in bacteria, leading to the suggestion that it is a universal language for interspecies communication (50, 64).

https://iai.asm.org/content/iai/78/8/3506.full.pdf

Staphylococcus aureus AI-2 Quorum Sensing Associates with the KdpDE Two-Component System To Regulate Capsular Polysaccharide Synthesis and Virulence

Staphylococcus aureus is an important pathogen that causes biofilm-associated infection in humans. Autoinducer 2 (AI-2), a quorum-sensing (QS) signal for interspecies communication, has a wide range of regulatory functions in both Gram-positive and Gram-negative bacteria, but its exact role in biofilm formation in S. aureus remains unclear. Here we demonstrate that mutation of the AI-2 synthase gene luxS in S. aureus RN6390B results in increased biofilm formation compared with the wild-type (WT) strain under static, flowing and anaerobic conditions and in a mouse model. Addition of the chemically synthesized AI-2 precursor in the luxS mutation strain (ΔluxS) restored the WT phenotype. Real-time RT-PCR analysis showed that AI-2 activated the transcription of icaR, a repressor of the ica operon, and subsequently a decreased level of icaA transcription, which was presumably the main reason why luxS mutation influences biofilm formation. Furthermore, we compared the roles of the agr-mediated QS system and the LuxS/AI-2 QS system in the regulation of biofilm formation using the ΔluxS strain, RN6911 and the Δagr ΔluxS strain. Our data indicate a cumulative effect of the two QS systems on the regulation of biofilm formation in S. aureus. These findings demonstrate that AI-2 can decrease biofilm formation in S. aureus via an icaR-activation pathway. This study may provide clues for therapy in S. aureus biofilm-associated infection.

/pdf/staphylococcus-aureus-autoinducer-2-quorum-sensing-decreases-20oun9l5g0.pdf

Staphylococcus aureus autoinducer-2 quorum sensing decreases biofilm formation in an icaR-dependent manner

In quorum sensing (QS) process, bacteria regulate gene expression by utilizing small signaling molecules called autoinducers in response to a variety of environmental cues. Autoinducer 2 (AI-2), a QS signaling molecule proposed to be involved in interspecies communication, is produced by many species of gram-negative and gram-positive bacteria. In Escherichia coli and Salmonella typhimurium, the extracellular AI-2 is imported into the cell by a transporter encoded by the lsr operon. Upstream of the lsr operon, there is a divergently transcribed gene encoding LsrR, which was reported previously to repress the transcription of the lsr operon and itself. Here, we have demonstrated for the first time that LsrR represses the transcription of the lsr operon and itself by directly binding to their promoters using gel shift and DNase I footprinting assays. The β-galactosidase reporter assays further suggest that two motifs in both the lsrR and lsrA promoter regions are crucial for the LsrR binding. Furthermore, in agreement with the conclusion that phosphorylated AI-2 can relieve the repression of LsrR in previous studies, our data show that phospho-AI-2 renders LsrR unable to bind to its own promoter in vitro.

/pdf/lsrr-binding-site-recognition-and-regulatory-characteristics-3p2dlxuvg7.pdf

LsrR-binding site recognition and regulatory characteristics in Escherichia coli AI-2 quorum sensing.

http://sunlab.ustc.edu.cn/pdf/ijaa_2012.pdf

LuxS/AI-2 system is involved in antibiotic susceptibility and autolysis in Staphylococcus aureus NCTC 8325.

http://sunlab.ustc.edu.cn/pdf/ijmm_2014.pdf

Liping Zhao

Papers

Staphylococcus aureus AI-2 Quorum Sensing Associates with the KdpDE Two-Component System To Regulate Capsular Polysaccharide Synthesis and Virulence

Staphylococcus aureus autoinducer-2 quorum sensing decreases biofilm formation in an icaR-dependent manner

LsrR-binding site recognition and regulatory characteristics in Escherichia coli AI-2 quorum sensing.

LuxS/AI-2 system is involved in antibiotic susceptibility and autolysis in Staphylococcus aureus NCTC 8325.

Staphylococcus aureus glucose-induced biofilm accessory proteins, GbaAB, influence biofilm formation in a PIA-dependent manner.