The Yin and Yang of SagS: Distinct Residues in the HmsP Domain of SagS Independently Regulate Biofilm Formation and Biofilm Drug Tolerance

The role of cyclic nucleotides in bacterial antimicrobial resistance and tolerance

Nucleotide signalling molecules - mainly cyclic 3',5'-adenosine phosphate (cAMP), bis-(3',5')-cyclic diguanosine monophosphate (c-di-GMP), and bis-(3',5')-cyclic diadenosine monophosphate (c-di-AMP) - contribute to the regulation of cellular pathways. Numerous recent works have focused on the involvement of these cyclic nucleotide phosphates (cNPs) in bacterial resistance and tolerance to antimicrobial treatment. Indeed, the rise of antimicrobial resistance (AMR) is a rising global threat to human health, while the rise of antimicrobial tolerance underlies the development of AMR and long-term infections, placing an additional burden on this problem. Here, we summarise the current understanding of cNP signalling in bacterial physiology with a focus on our understanding of how cNP signalling affects AMR and antimicrobial tolerance in different bacterial species. We also discuss additional cNP-related drug targets in bacterial pathogens that may have therapeutic potential.

Binding of GTP to BifA is required for the production of Pel-dependent biofilms in Pseudomonas aeruginosa

The Pel exopolysaccharide is one of the most mechanistically conserved and phylogenetically diverse bacterial biofilm matrix determinants. Pel is a major contributor to the structural integrity of Pseudomonas aeruginosa biofilms, and its biosynthesis is regulated by the binding of cyclic-3',5'-dimeric guanosine monophosphate (c-di-GMP) to the PelD receptor. c-di-GMP is synthesized from two molecules of guanosine triphosphate (GTP) by diguanylate cyclases with GGDEF domains and degraded by phosphodiesterases with EAL or HD-GYP domains. As the P. aeruginosa genome encodes 43 c-di-GMP metabolic enzymes, one way signaling specificity can be achieved is through direct interaction between specific enzyme-receptor pairs. Here, we show that the inner membrane hybrid GGDEF-EAL enzyme, BifA, directly interacts with PelD via its cytoplasmic HAMP, GGDEF, and EAL domains. Despite having no catalytic function, the degenerate active site motif of the BifA GGDEF domain (GGDQF) has retained the ability to bind GTP with micromolar affinity. Mutations that abolish GTP binding result in increased biofilm formation but stable global c-di-GMP levels. Our data suggest that BifA forms a dimer in solution and that GTP binding induces conformational changes in dimeric BifA that enhance the BifA-PelD interaction and stimulate its phosphodiesterase activity, thus reducing c-di-GMP levels and downregulating Pel biosynthesis. Structural comparisons between the dimeric AlphaFold2 model of BifA and the structures of other hybrid GGDEF-EAL proteins suggest that the regulation of BifA by GTP may occur through a novel mechanism.IMPORTANCEc-di-GMP is the most common cyclic dinucleotide used by bacteria to regulate phenotypes such as motility, biofilm formation, virulence factor production, cell cycle progression, and cell differentiation. While the identification and initial characterization of c-di-GMP metabolic enzymes are well established, our understanding of how these enzymes are regulated to provide signaling specificity remains understudied. Here we demonstrate that the inactive GGDEF domain of BifA binds GTP and regulates the adjacent phosphodiesterase EAL domain, ultimately downregulating Pel-dependent P. aeruginosa biofilm formation through an interaction with PelD. This discovery adds to the growing body of literature regarding how hybrid GGDEF-EAL enzymes are regulated and provides additional precedence for studying how direct interactions between c-di-GMP metabolic enzymes and effectors result in signaling specificity.

Structure based virtual screening and molecular dynamics of natural anti-biofilm compounds against SagS response regulator/sensor kinase in Pseudomonas aeruginosa

SagS sensor regulator plays a vital role in biofilm development of Pseudomonas aeruginosa which subsequently makes the cells more tolerant to various antimicrobials. The multidrug resistance (MDR) issue has risen substantially in recent years and is considered a global threat. Therefore, alternative compounds should be unearthed immediately to address the issues related to P. aeruginosa drug resistance for which SagS could be a candidate. The present study is an attempt to screen natural anti-biofilm compounds as the potent inhibitors of SagS. Twenty natural anti-biofilm/quorum sensing inhibiting compounds were retrieved from various literatures with significant inhibitory effects against P. aeruginosa biofilm from in-vitro experiments which were screened using various pharmacokinetic parameters. The screened and three standard drugs were docked against SagS-HisKA using AutoDock 4.2 tool, which were further analysed by MD simulations to understand the binding mode of compounds and dynamic behaviour of the complexes. Two potential anti-biofilm natural compounds, pinocembrin with binding affinity (-7.19 kcal/mol), vestitol (-7.18 kcal/mol) and the standard drug ceftazidime (-8.89 kcal/mol) were selected based on filtered parameters and better binding affinity. The trajectory analysis of MD simulations reflected Pinocembrin in stabilizing the system compared to ceftazidime. The existing reports state that the natural products represent promising source of therapy with least or almost nil adverse effect compared to synthetic drugs which is well collated with our in-silico findings. This investigation can save both time and cost required for in-vitro and in-vivo analysis for designing of a novel anti-biofilm agent against P. aeruginosa biofilm-associated infections.Communicated by Ramaswamy H. Sarma.

Manganese Acts as an Environmental Inhibitor of Pseudomonas aeruginosa Biofilm Development by Inducing Dispersion and Modulating c-di-GMP and Exopolysaccharide Production via RbdA

The opportunistic human pathogen Pseudomonas aeruginosa causes chronic infections that involve multicellular aggregates called biofilms. Biofilm formation is modulated by the host environment and the presence of cues and/or signals, likely affecting the pool of the bacterial second messenger cyclic diguanylate monophosphate (c-di-GMP). The manganese ion Mn2+ is a divalent metal cation that is essential for pathogenic bacterial survival and replication during the infection in a host organism. In this study, we investigated how Mn2+ alters P. aeruginosa biofilm formation via the regulation of c-di-GMP levels. Exposure to Mn2+ was found to temporally enhance attachment but impair subsequent biofilm development, apparent by reduced biofilm biomass accumulation and lack of microcolony formation due to the induction of dispersion. Moreover, exposure to Mn2+ coincided with reduced production of the exopolysaccharides Psl and Pel, decreased transcriptional abundance of pel and psl, and decreased levels of c-di-GMP. To determine whether the effect of Mn2+ was linked to the activation of phosphodiesterases (PDEs), we screened several PDE mutants for Mn2+-dependent phenotypes (attachment and polysaccharide production) as well as PDE activity. The screen revealed that the PDE RbdA is activated by Mn2+ and is responsible for Mn2+-dependent attachment, inhibition of Psl production, and dispersion. Taken together, our findings suggest Mn2+ is an environmental inhibitor of P. aeruginosa biofilm development that acts through the PDE RbdA to modulate c-di-GMP levels, thereby impeding polysaccharide production and biofilm formation but enhancing dispersion. IMPORTANCE While diverse environmental conditions such as the availability of metal ions have been shown to affect biofilm development, little is known about the mechanism. Here, we demonstrate that Mn2+ affects Pseudomonas aeruginosa biofilm development by stimulating phosphodiesterase RbdA activity to reduce the signaling molecule c-di-GMP levels, thereby hindering polysaccharide production and biofilm formation but enhancing dispersion. Our findings demonstrate that Mn2+ acts as an environmental inhibitor of P. aeruginosa biofilms, further suggesting manganese to be a promising new antibiofilm factor.

MoaB1 Homologs Contribute to Biofilm Formation and Motility by Pseudomonas aeruginosa and Escherichia coli

moaB homologs, encoding the molybdopterin biosynthetic protein B1, have been reported to be expressed under anoxic conditions and during biofilm growth in various microorganisms; however, little is known about MoaB's function. Here, we demonstrate that in Pseudomonas aeruginosa, MoaB1 (PA3915) contributes to biofilm-related phenotypes. Specifically, moaB1 expression is induced in biofilms, and insertional inactivation of moaB1 reduced biofilm biomass accumulation and pyocyanin production while enhancing swarming motility, and pyoverdine abundance without affecting attachment, swimming motility, or c-di-GMP levels. Inactivation of the highly conserved E. coli homolog of moaB1, moaBEc, likewise coincided with reduced biofilm biomass accumulation. In turn, heterologous expression of moaBEc restored biofilm formation and swarming motility by the P. aeruginosa moaB1 mutant to wild-type levels. Moreover, MoaB1 was found to interact with other conserved biofilm-associated proteins, PA2184 and PA2146, as well as the sensor-kinase SagS. However, despite the interaction, MoaB1 failed to restore SagS-dependent expression of brlR encoding the transcriptional regulator BrlR, and inactivation of moaB1 or moaBEc had no effect on the antibiotic susceptibility phenotype of biofilms formed by P. aeruginosa and E. coli, respectively. While our findings did not establish a link between MoaB1 and molybdenum cofactor biosynthesis, they suggest that MoaB1 homologs contribute to biofilm-associated phenotypes across species boundaries, possibly hinting at the existence of a previously undescribed conserved biofilm pathway. IMPORTANCE Proteins contributing to the biogenesis of molybdenum cofactors have been characterized; however, the role of the molybdopterin biosynthetic protein B1 (MoaB1) has remained elusive, and solid evidence to support its role in biosynthesis of molybdenum cofactor is lacking. Here, we demonstrate that, in Pseudomonas aeruginosa, MoaB1 (PA3915) contributes to biofilm-related phenotypes in a manner that does not support a role of MoaB1 in the biosynthesis of molybdenum cofactors.

Bacterial Stress Responses as Potential Targets in Overcoming Antibiotic Resistance

Bacteria can be adapted to adverse and detrimental conditions that induce general and specific responses to DNA damage as well as acid, heat, cold, starvation, oxidative, envelope, and osmotic stresses. The stress-triggered regulatory systems are involved in bacterial survival processes, such as adaptation, physiological changes, virulence potential, and antibiotic resistance. Antibiotic susceptibility to several antibiotics is reduced due to the activation of stress responses in cellular physiology by the stimulation of resistance mechanisms, the promotion of a resistant lifestyle (biofilm or persistence), and/or the induction of resistance mutations. Hence, the activation of bacterial stress responses poses a serious threat to the efficacy and clinical success of antibiotic therapy. Bacterial stress responses can be potential targets for therapeutic alternatives to antibiotics. An understanding of the regulation of stress response in association with antibiotic resistance provides useful information for the discovery of novel antimicrobial adjuvants and the development of effective therapeutic strategies to control antibiotic resistance in bacteria. Therefore, this review discusses bacterial stress responses linked to antibiotic resistance in Gram-negative bacteria and also provides information on novel therapies targeting bacterial stress responses that have been identified as potential candidates for the effective control of Gram-negative antibiotic-resistant bacteria.

A previously uncharacterized gene, PA2146, contributes to biofilm formation and drug tolerance across the ɣ-Proteobacteria

Transcriptomic studies have revealed a large number of uncharacterized genes that are differentially expressed in biofilms, which may be important in regulating biofilm phenotypes such as resistance to antimicrobial agents. To identify biofilm genes of unknown function in P. aeruginosa, we made use of RNA-seq and selected 27 uncharacterized genes that were induced upon biofilm growth. Biofilms by respective mutants were subsequently analyzed for two biofilm characteristics, the biofilm architecture and drug susceptibility. The screen revealed 12 out of 27 genes to contribute to biofilm formation and 13 drug susceptibility, with 8 genes affecting both biofilm phenotypes. Amongst the genes affecting both biofilm phenotypes was PA2146, encoding a small hypothetical protein that exhibited some of the most substantial increases in transcript abundance during biofilm growth by P. aeruginosa PAO1 and clinical isolates. PA2146 is highly conserved in ɣ-proteobacteria. Inactivation of PA2146 affected both biofilm phenotypes in P. aeruginosa PAO1, with inactivation of homologs in Klebsiella pneumoniae and Escherichia coli having similar effects. Heterologous expression of PA2146 homologs complemented the P. aeruginosa ∆PA2146, suggesting that PA2146 homologs substitute for and play a similar role as PA2146 in P. aeruginosa.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

Combination of Sanguisorbigenin and Conventional Antibiotic Therapy for Methicillin-Resistant Staphylococcus aureus: Inhibition of Biofilm Formation and Alteration of Cell Membrane Permeability

Methicillin-resistant Staphylococcus aureus (MRSA) infection is challenging to eradicate because of antibiotic resistance and biofilm formation. Novel antimicrobial agents and alternative therapies are urgently needed. This study aimed to evaluate the synergy of sanguisorbigenin (SGB) isolated from Sanguisorba officinalis L. with six conventional antibiotics to achieve broad-spectrum antibacterial action and prevent the development of resistance. A checkerboard dilution test and time-to-kill curve assay were used to determine the synergistic effect of SGB combined with antibiotics against MRSA. SGB showed significant synergy with antibiotics and reduced the minimum inhibitory concentration of antibiotics by 2-16-fold. Biofilm inhibition assay, quantitative RT-PCR, crystal violet absorption, and transmission electron microscopy were performed to evaluate the synergy mechanism. The results indicated that SGB could inhibit biofilm formation and alter cell membrane permeability in MRSA. In addition, SGB was found to exhibit quite low cytotoxicity and hemolysis. The discovery of the superiority of SGB suggests that SGB may be an antibiotic adjuvant for use in combination therapy and as a plant-derived antibacterial agent targeting biofilms.

SagS and its unorthodox contributions to Pseudomonas aeruginosa biofilm development

The Pseudomonas aeruginosa orphan sensor SagS (PA2824) was initially reported as one of three orphan sensor kinases capable of activating HptB, a component of the HptB signaling pathway that intersects with the Gac/Rsm signaling pathway and fine-tunes P. aeruginosa motility and pathogenesis. Since then, this orphan sensor has been reported to be involved in other, unorthodox signaling pathways serving additional functions. The present review is aimed at summarizing the various functions of SagS, with an emphasis on its toggle or dual switch functions, and highlighting the role of SagS as a hub at which the various signaling pathways intersect, to regulate the transition from the planktonic to the sessile mode of growth, as well as the transition of surface-associated cells to a drug tolerant state.

Glucose-6-Phosphate Acts as an Extracellular Signal of SagS To Modulate Pseudomonas aeruginosa c-di-GMP Levels, Attachment, and Biofilm Formation

In Pseudomonas aeruginosa, the orphan two-component sensor SagS contributes both to transition to biofilm formation and to biofilm cells gaining their heightened tolerance to antimicrobials. However, little is known about the identity of the signals or conditions sensed by SagS to induce the switch to the sessile, drug-tolerant mode of growth. Using a modified Biolog phenotype assay to screen for compounds that modulate attachment in a SagS-dependent manner, we identified glucose-6-phosphate to enhance attachment in a manner dependent on the glucose-6-phosphate concentration and SagS. The stimulatory effect was not limited to the attachment since glucose-6-phosphate likewise enhanced biofilm formation and also enhanced the expression of select biofilm marker genes. Moreover, exposure to glucose-6-phosphate coincided with decreased swarming motility but increased cellular cyclic-di-GMP (c-di-GMP) levels in biofilms. No such response was noted for compounds modulating attachment and biofilm formation in a manner independent of SagS. Modulation of c-di-GMP in response to glucose-6-phosphate was due to the diguanylate cyclase NicD, with NicD also being required for enhanced biofilm formation. The latter was independent of the sensory domain of NicD but dependent on NicD activity, SagS, and the interaction between NicD and SagS. Our findings indicate that glucose-6-phosphate likely mimics a signal or conditions sensed by SagS to activate its motile-sessile switch function. In addition, our findings provide new insight into the interfaces between the ligand-mediated two-component system signaling pathway and c-di-GMP levels.IMPORTANCE Pathogens sense and respond to signals and cues present in their environment, including host-derived small molecules to modulate the expression of their virulence repertoire. Here, we demonstrate that the opportunistic pathogen Pseudomonas aeruginosa responds to glucose-6-phosphate. Since glucose-6-phosphate is primarily made available due to cell lysis, it is likely that glucose-6-phosphate represents a cross-kingdom cell-to-cell signal that enables P. aeruginosa to adapt to the (nutrient-poor) host environment by enhancing biofilm formation, cyclic-di-GMP, and the expression of genes linked to biofilm formation in a concentration- and SagS-dependent manner.

Difference in biofilm formation between carbapenem-resistant and carbapenem-sensitive Klebsiella pneumoniae based on analysis of mrkH distribution

OBJECT

To analyze the difference in biofilm formation between carbapenem-resistant and carbapenem-sensitive Klebsiella pneumoniae based on analysis of mrkH distribution and to further explore the function of mrkH for biofilm formation from the perspective of gene regulation.

METHODS

40 imipenem-resistant strains and 40 imipenem-sensitive strains were selected to conduct experiments. Carbapenem (imipenem) susceptibility test was performed by the agar-dilution method. bla_KPC resistance gene, type 3 fimbriae-related coding genes (mrkA and mrkD) and regulation gene (mrkH) were screened by PCR. Biofilm formation assay was performed using crystal violet staining method in MHB. The relative expression of genes that critically involved in biofilm formation (mrkA, luxS, pgaA) and carbapenem resistance (ompk35, ompk36, acrB) were measured by quantitative real-time PCR (qRT-PCR). Furthermore, the mrkH cassette was cloned into pGEM-T Easy plasmid to yield pGEM:pmrkH and expressed in Escherichia coli DH5α and K. pneumoniae FK1911, and the biofilm formation assay after transformation was further tested.

RESULTS

The MICs of imipenem were all more than 16 μg/mL in 40 imipenem-resistant strains and ranged from 0.125 μg/mL to 0.5 μg/mL in 40 imipenem-sensitive strains. Moreover, the bla_KPC was identified in the 40 imipenem-resistant K. pneumoniae strains. All 80 K. pneumoniae strains were found to carry mrkA and mrkD genes. Interestingly, the mrkH gene was detected in 43 strains, of which 32 were carbapenem-sensitive strains. The biofilm formation capacity of strains carried mrkH cassette was significantly higher than other 37 strains in MHB media. The relative expression of mrkA in K. pneumoniae carrying mrkH gene was significantly up-regulated. Importantly, the biofilm formation ability of FK1911-pGEM:pmrkH strain was more higher than the strain of FK1911 in MHB medium.

CONCLUSIONS

Our data demonstrated that MrkH played a crucial role in the regulation of biofilm formation by K. pneumoniae. In contrast to carbapenem-sensitive K. pneumoniae, carbapenem-resistant K. pneumoniae was less likely to have strong biofilm-forming capacity because it does not carry the mrkH gene.

Conceptual Model of Biofilm Antibiotic Tolerance That Integrates Phenomena of Diffusion, Metabolism, Gene Expression, and Physiology

Transcriptomic, metabolomic, physiological, and computational modeling approaches were integrated to gain insight into the mechanisms of antibiotic tolerance in an in vitro biofilm system. Pseudomonas aeruginosa biofilms were grown in drip flow reactors on a medium composed to mimic the exudate from a chronic wound. After 4 days, the biofilm was 114 μm thick with 9.45 log₁₀ CFU cm^-2 These biofilms exhibited tolerance, relative to exponential-phase planktonic cells, to subsequent treatment with ciprofloxacin. The specific growth rate of the biofilm was estimated via elemental balances to be approximately 0.37 h^-1 and with a reaction-diffusion model to be 0.32 h^-1, or one-third of the maximum specific growth rate for planktonic cells. Global analysis of gene expression indicated lower transcription of ribosomal genes and genes for other anabolic functions in biofilms than in exponential-phase planktonic cells and revealed the induction of multiple stress responses in biofilm cells, including those associated with growth arrest, zinc limitation, hypoxia, and acyl-homoserine lactone quorum sensing. Metabolic pathways for phenazine biosynthesis and denitrification were transcriptionally activated in biofilms. A customized reaction-diffusion model predicted that steep oxygen concentration gradients will form when these biofilms are thicker than about 40 μm. Mutant strains that were deficient in Psl polysaccharide synthesis, the stringent response, the stationary-phase response, and the membrane stress response exhibited increased ciprofloxacin susceptibility when cultured in biofilms. These results support a sequence of phenomena leading to biofilm antibiotic tolerance, involving oxygen limitation, electron acceptor starvation and growth arrest, induction of associated stress responses, and differentiation into protected cell states.IMPORTANCE Bacteria in biofilms are protected from killing by antibiotics, and this reduced susceptibility contributes to the persistence of infections such as those in the cystic fibrosis lung and chronic wounds. A generalized conceptual model of biofilm antimicrobial tolerance with the following mechanistic steps is proposed: (i) establishment of concentration gradients in metabolic substrates and products; (ii) active biological responses to these changes in the local chemical microenvironment; (iii) entry of biofilm cells into a spectrum of states involving alternative metabolisms, stress responses, slow growth, cessation of growth, or dormancy (all prior to antibiotic treatment); (iv) adaptive responses to antibiotic exposure; and (v) reduced susceptibility of microbial cells to antimicrobial challenges in some of the physiological states accessed through these changes.

Signal Sensing and Transduction Are Conserved between the Periplasmic Sensory Domains of BifA and SagS

The hybrid sensor kinase SagS of Pseudomonas aeruginosa plays a key role in the transition from the planktonic to the biofilm mode of growth. Recently, we have shown that distinct sets of residues in its periplasmic HmsP sensory domain are involved in the regulation of biofilm formation or antibiotic tolerance. Interestingly, the HmsP domain of the phosphodiesterase BifA shows great predicted structural similarity to that of SagS, despite moderate sequence conservation and only a number of residues involved in SagS signaling being conserved between both proteins. Based on this observation, we hypothesized that BifA and SagS may use similar mechanisms to sense and transduce signals perceived at their periplasmic HmsP domains and, therefore, may be interchangeable. To test this hypothesis, we constructed SagS hybrids in which the HmsP domain of SagS was replaced by that of BifA (and vice versa) or by the DISMED2 sensory domain of NicD. The SagS-BifA hybrid restored attachment and biofilm formation by the ΔbifA mutant. Likewise, while the NicD-SagS hybrid was nonfunctional, the BifA-SagS hybrid partially restored pathways leading to biofilm formation and antibiotic tolerance in a ΔsagS mutant background. Furthermore, alanine substitution of key residues previously associated with the biofilm formation and antibiotic tolerance pathways of SagS impaired signal transduction by the BifA-SagS hybrid in a similar way to SagS. In conclusion, our data indicate that the nature of the sensory domain is important for proper functionality of the cytoplasmic effector domains and that signal sensing and transduction are likely conserved in SagS and BifA.IMPORTANCE Biofilms have been associated with more than 60% of all recalcitrant and chronic infections and can render bacterial cells up to a thousand times more resistant to antibiotics than planktonic cells. Although it is known that the transition from the planktonic to the biofilm mode of growth involves two-component regulatory systems, increased c-di-GMP levels, and quorum sensing systems among others, the exact signaling events that lead to biofilm formation remain unknown. In the opportunistic pathogen Pseudomonas aeruginosa, the hybrid sensor kinase SagS regulates biofilm formation and antibiotic tolerance through two independent pathways via distinct residues in its periplasmic sensory domain. Interestingly, the sensory domains of SagS and BifA show great predicted structural similarity despite moderate sequence conservation. Here we show that the sensory domains of BifA and SagS are functionally interchangeable and that they use a similar mechanism of signal sensing and transduction, which broadens our understanding of how bacteria perceive and transduce signals when transitioning to the biofilm mode of growth.

Roles of two-component regulatory systems in antibiotic resistance

Two-component regulatory systems (TCSs) are a major mechanism by which bacteria sense and respond to changes in their environment. TCSs typically consist of two proteins that bring about major regulation of the cell genome through coordinated action mediated by phosphorylation. Environmental conditions that activate TCSs are numerous and diverse and include exposure to antibiotics as well as conditions inside a host. The resulting regulatory action often involves activation of antibiotic defenses and changes to cell physiology that increase antibiotic resistance. Examples of resistance mechanisms enacted by TCSs contained in this review span those found in both Gram-negative and Gram-positive species and include cell surface modifications, changes in cell permeability, increased biofilm formation, and upregulation of antibiotic-degrading enzymes.

Controlling chronic Pseudomonas aeruginosa infections by strategically interfering with the sensory function of SagS

The hybrid sensor SagS plays a central role in the formation of Pseudomonas aeruginosa biofilms, by enabling the switch from the planktonic to the biofilm mode of growth and by facilitating the transition of biofilm cells to a highly tolerant state. In this study, we examined the importance of the SagS key amino acid residues associated with biofilm formation (L154) and antibiotic tolerance (D105) in P. aeruginosa virulence. Recombinant P. aeruginosa ΔsagS and ΔsagS chromosomally expressing wild-type sagS, or its two variants D105A and L154A, were tested for their potential to form biofilms and cause virulence in plants and mouse models of acute and chronic pneumonia. Although mutation of sagS did not alter P. aeruginosa virulence during acute infections, a significant difference in pathogenicity of sagS mutants was observed during chronic infections, with the L154A variant showing reduced bacterial loads in the chronic pneumonia model, while interference with the D105 residue enhanced the susceptibility of P. aeruginosa biofilms during tobramycin treatment. Our findings suggest that interference with the biofilm or tolerance regulatory circuits of SagS affects P. aeruginosa pathogenicity in chronic but not acute infections, and reveal SagS to be a promising new target to treat P. aeruginosa biofilm infections.

Two Component Regulatory Systems and Antibiotic Resistance in Gram-Negative Pathogens

Gram-negative pathogens such as Klebsiella pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa are the leading cause of nosocomial infections throughout the world. One commonality shared among these pathogens is their ubiquitous presence, robust host-colonization and most importantly, resistance to antibiotics. A significant number of two-component systems (TCSs) exist in these pathogens, which are involved in regulation of gene expression in response to environmental signals such as antibiotic exposure. While the development of antimicrobial resistance is a complex phenomenon, it has been shown that TCSs are involved in sensing antibiotics and regulating genes associated with antibiotic resistance. In this review, we aim to interpret current knowledge about the signaling mechanisms of TCSs in these three pathogenic bacteria. We further attempt to answer questions about the role of TCSs in antimicrobial resistance. We will also briefly discuss how specific two-component systems present in K. pneumoniae, A. baumannii, and P. aeruginosa may serve as potential therapeutic targets.

Pyruvate-depleting conditions induce biofilm dispersion and enhance the efficacy of antibiotics in killing biofilms in vitro and in vivo

The formation of biofilms is a developmental process initiated by planktonic cells transitioning to the surface, which comes full circle when cells disperse from the biofilm and transition to the planktonic mode of growth. Considering that pyruvate has been previously demonstrated to be required for the formation of P. aeruginosa biofilms, we asked whether pyruvate likewise contributes to the maintenance of the biofilm structure, with depletion of pyruvate resulting in dispersion. Here, we demonstrate that the enzymatic depletion of pyruvate coincided with the dispersion of established biofilms by S. aureus and laboratory and clinical P. aeruginosa isolates. The dispersion response was dependent on pyruvate fermentation pathway components but independent of proteins previously described to contribute to P. aeruginosa biofilm dispersion. Using porcine second-degree burn wounds infected with P. aeruginosa biofilm cells, we furthermore demonstrated that pyruvate depletion resulted in a reduction of biofilm biomass in vivo. Pyruvate-depleting conditions enhanced the efficacy of tobramycin killing of the resident wound biofilms by up to 5-logs. Our findings strongly suggest the management of pyruvate availability to be a promising strategy to combat biofilm-related infections by two principal pathogens associated with wound and cystic fibrosis lung infections.

Modulation of S. aureus and P. aeruginosa biofilm: an in vitro study with new coumarin derivatives

Coumarin is an important heterocyclic molecular framework of bioactive molecules against broad spectrum pathological manifestations. In the present study 18 new coumarin derivatives (CDs) were synthesized and characterized for antibiofilm activity against two model bacteria such as Staphylococcus aureus and Pseudomonas aeruginosa. It was observed that all the CDs executed significant effect in moderating activities against both planktonic and biofilm forms of these selected bacteria. Hence, to interpret the underlying probable reason of such antibiofilm effect, in-silico binding study of CDs with biofilm and motility associated proteins of these organisms were performed. All CDs have shown their propensity for occupying the native substrate binding pocket of each protein with moderate to strong binding affinities. One of the CDs such as CAMN1 showed highest binding affinity with these proteins. Interestingly, the findings of in-silico studies coincides the experimental results of antibiofilm and motility affect of CDs against both S. aureus and P. aeruginosa. Moreover, in-silico studies suggested that the antibiofilm activity of test CDs may be due to the interference of biofilm and motility associated proteins of the selected model organisms (PilT from P. aeruginosa and TarK, TarO from S. aureus). The detailed synthesis, characterization, methodology and results of biological screening along with computational studies have been reported. This study could be of greater interest in the context of the development of new anti-bacterial agent in the future.