Antibiotic treatment of Pseudomonas aeruginosa biofilms stimulates expression of the magnesium transporter gene mgtE

Novel Mg 2+ binding sites in the cytoplasmic domain of the MgtE Mg 2+ channels revealed by X-ray crystal structures

MgtE is a Mg ²⁺-selective channel regulated by the intracellular Mg ²⁺ concentration. MgtE family proteins are highly conserved in all domains of life and contribute to cellular Mg ²⁺ homeostasis. In humans, mutations in the SLC41 proteins, the eukaryotic counterparts of the bacterial MgtE, are known to be associated with various diseases. The first MgtE structure from a thermophilic bacterium, Thermus thermophilus, revealed that MgtE forms a homodimer consisting of transmembrane and cytoplasmic domains with a plug helix connecting the two and that the cytoplasmic domain possesses multiple Mg ²⁺ binding sites. Structural and electrophysiological analyses revealed that the dissociation of Mg ²⁺ ions from the cytoplasmic domain induces structural changes in the cytoplasmic domain, leading to channel opening. Thus, previous works showed the importance of MgtE cytoplasmic Mg ²⁺ binding sites. Nevertheless, due to the limited structural information on MgtE from different species, the conservation and diversity of the cytoplasmic Mg ²⁺ binding site in MgtE family proteins remain unclear. Here, we report crystal structures of the Mg ²⁺-bound MgtE cytoplasmic domains from two different bacterial species, Chryseobacterium hispalense and Clostridiales bacterium, and identify multiple Mg ²⁺ binding sites, including ones that were not observed in the previous MgtE structure. These structures reveal the conservation and diversity of the cytoplasmic Mg ²⁺ binding site in the MgtE family proteins.

The Effect of Benzyl Isothiocyanate on the Expression of Genes Encoding NADH Oxidase and Fibronectin-Binding Protein in Oral Streptococcal Biofilms

Recent studies have shown that antimicrobial treatment results in up- or down regulation of several virulence-associated genes in bacterial biofilms. The genes encoding NADH oxidase (nox) and fibronectin-binding protein (fbp) are known to play important roles in biofilm growth of some oral bacterial species. The objective was to study the effect of benzyl isothiocyanate (BITC), an antimicrobial agent from Miswak plant, on the expression of nox and fbp genes in some oral streptococci. The biofilms were treated with BITC and mRNA expression of nox and fbp genes was measured by comparative ΔΔCt method. The highest amount of biofilm mass was produced by A. defectiva, followed by S. gordonii, S. mutans, G. elegans and G. adiacens. Upon treatment with BITC, S. gordonii biofilms showed highest folds change in mRNA expression for both fbp and nox genes followed by S. mutans, A. defectiva, and G. adiacens. G. elegans mRNA levels for nox were extremely low. In conclusion, BITC treatment of the biofilms caused an upregulation of biofilm-associated genes fbp and nox genes in most of the tested species suggesting the significance of these genes in biofilm lifestyle of these oral bacteria and needs further investigation to understand if it contributes to antimicrobial resistance.

DNA alternate polymerase PolB mediates inhibition of type III secretion in Pseudomonas aeruginosa

Opportunistic pathogen Pseudomonas aeruginosa uses a variety of virulence factors to cause acute and chronic infections. We previously found that alternate DNA polymerase gene polB inhibits P. aeruginosa pyocyanin production. We investigated whether polB also affects T3SS expression. polB overexpression significantly reduced T3SS transcription and repressed translation of the master T3SS regulator ExsA, while not affecting exsA mRNA transcript abundance. Further, polB does not act through previously described genetic pathways that post-transcriptionally regulate ExsA. Our results show a novel T3SS regulatory component which may lead to development of future drugs to target this mechanism.

Sub-minimum inhibitory concentrations of biocides induced biofilm formation in Pseudomonas aeruginosa

It is clear that biofilm formation causes many serious health-care problems. Interestingly, sub minimum inhibitory concentrations (sub-MICs) of some biocides can induce biofilm formation in bacteria. We investigated whether sub-MICs of Savlon, chlorhexidine and deconex®, as biocidal products, can induce biofilm formation in clinical isolates of Pseudomonas aeruginosa. To determine MICs and biofilm formation, we performed microtitre plate assays. All three biocides induced biofilm formation at sub-MICs; Savlon was the most successful antiseptic agent to induce biofilm formation among P. aeruginosa isolates. Deconex had the best inhibition effect on planktonic cultures of P. aeruginosa isolates. We concluded that sub-MICs of Savlon and deconex could significantly induce biofilm formation.

Bacterial Biofilm Eradication Agents: A Current Review

Most free-living bacteria can attach to surfaces and aggregate to grow into multicellular communities encased in extracellular polymeric substances called biofilms. Biofilms are recalcitrant to antibiotic therapy and a major cause of persistent and recurrent infections by clinically important pathogens worldwide (e.g., Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus). Currently, most biofilm remediation strategies involve the development of biofilm-inhibition agents, aimed at preventing the early stages of biofilm formation, or biofilm-dispersal agents, aimed at disrupting the biofilm cell community. While both strategies offer some clinical promise, neither represents a direct treatment and eradication strategy for established biofilms. Consequently, the discovery and development of biofilm eradication agents as comprehensive, stand-alone biofilm treatment options has become a fundamental area of research. Here we review our current understanding of biofilm antibiotic tolerance mechanisms and provide an overview of biofilm remediation strategies, focusing primarily on the most promising biofilm eradication agents and approaches. Many of these offer exciting prospects for the future of biofilm therapeutics for a large number of infections that are currently refractory to conventional antibiotics.

RNA-Dependent Regulation of Virulence in Pathogenic Bacteria

During infection, bacterial pathogens successfully sense, respond and adapt to a myriad of harsh environments presented by the mammalian host. This exquisite level of adaptation requires a robust modulation of their physiological and metabolic features. Additionally, virulence determinants, which include host invasion, colonization and survival despite the host's immune responses and antimicrobial therapy, must be optimally orchestrated by the pathogen at all times during infection. This can only be achieved by tight coordination of gene expression. A large body of evidence implicate the prolific roles played by bacterial regulatory RNAs in mediating gene expression both at the transcriptional and post-transcriptional levels. This review describes mechanistic and regulatory aspects of bacterial regulatory RNAs and highlights how these molecules increase virulence efficiency in human pathogens. As illustrative examples, Staphylococcus aureus, Listeria monocytogenes, the uropathogenic strain of Escherichia coli, Helicobacter pylori, and Pseudomonas aeruginosa have been selected.

Phosphoglycerate mutase affects Stenotrophomonas maltophilia attachment to biotic and abiotic surfaces

Stenotrophomonas maltophilia biofilm formation is of increasing medical concern, particularly for lung infections. However, the molecular mechanisms facilitating the biofilm lifestyle in S. maltophilia are poorly understood. We generated and screened a transposon mutant library for mutations that lead to altered biofilm formation compared to wild type. One of these mutations, in the gene for glycolytic enzyme phosphoglycerate mutase (gpmA), resulted in impaired attachment on abiotic and biotic surfaces. As adherence to a surface is the initial step in biofilm developmental processes, our results reveal a unique factor that could affect S. maltophilia biofilm initiation and, possibly, subsequent development.

Yersinia pseudotuberculosis BarA-UvrY Two-Component Regulatory System Represses Biofilms via CsrB

The formation of biofilms by Yersinia pseudotuberculosis (Yptb) and Y. pestis requires the hmsHFRS genes, which direct production of a polysaccharide extracellular matrix (Hms-ECM). Despite possessing identical hmsHFRS sequences, Yptb produces much less Hms-ECM than Y. pestis. The regulatory influences that control Yptb Hms-ECM production and biofilm formation are not fully understood. In this study, negative regulators of biofilm production in Yptb were identified. Inactivation of the BarA/UvrY two-component system or the CsrB regulatory RNA increased binding of Congo Red dye, which correlates with extracellular polysaccharide production. These mutants also produced biofilms that were substantially more cohesive than the wild type strain. Disruption of uvrY was not sufficient for Yptb to cause proventricular blockage during infection of Xenopsylla cheopis fleas. However, this strain was less acutely toxic toward fleas than wild type Yptb. Flow cytometry measurements of lectin binding indicated that Yptb BarA/UvrY/CsrB mutants may produce higher levels of other carbohydrates in addition to poly-GlcNAc Hms-ECM. In an effort to characterize the relevant downstream targets of the BarA/UvrY system, we conducted a proteomic analysis to identify proteins with lower abundance in the csrB::Tn5 mutant strain. Urease subunit proteins were less abundant and urease enzymatic activity was lower, which likely reduced toxicity toward fleas. Loss of CsrB impacted expression of several potential regulatory proteins that may influence biofilms, including the RcsB regulator. Overexpression of CsrB did not alter the Congo-red binding phenotype of an rcsB::Tn5 mutant, suggesting that the effect of CsrB on biofilms may require RcsB. These results underscore the regulatory and compositional differences between Yptb and Y. pestis biofilms. By activating CsrB expression, the Yptb BarA/UvrY two-component system has pleiotropic effects that impact biofilm production and stability.

Pseudomonas aeruginosa Magnesium Transporter MgtE Inhibits Type III Secretion System Gene Expression by Stimulating rsmYZ Transcription

Pseudomonas aeruginosa causes numerous acute and chronic opportunistic infections in humans. One of its most formidable weapons is a type III secretion system (T3SS), which injects powerful toxins directly into host cells. The toxins lead to cell dysfunction and, ultimately, cell death. Identification of regulatory pathways that control T3SS gene expression may lead to the discovery of novel therapeutics to treat P. aeruginosa infections. In a previous study, we found that expression of the magnesium transporter gene mgtE inhibits T3SS gene transcription. MgtE-dependent inhibition appeared to interfere with the synthesis or function of the master T3SS transcriptional activator ExsA, although the exact mechanism was unclear. We now demonstrate that mgtE expression acts through the GacAS two-component system to activate rsmY and rsmZ transcription. This event ultimately leads to inhibition of exsA translation. This inhibitory effect is specific to exsA as translation of other genes in the exsCEBA operon is not inhibited by mgtE Moreover, our data reveal that MgtE acts solely through this pathway to regulate T3SS gene transcription. Our study reveals an important mechanism that may allow P. aeruginosa to fine-tune T3SS activity in response to certain environmental stimuli.IMPORTANCE The type III secretion system (T3SS) is a critical virulence factor utilized by numerous Gram-negative bacteria, including Pseudomonas aeruginosa, to intoxicate and kill host cells. Elucidating T3SS regulatory mechanisms may uncover targets for novel anti-P. aeruginosa therapeutics and provide deeper understanding of bacterial pathogenesis. We previously found that the magnesium transporter MgtE inhibits T3SS gene transcription in P. aeruginosa In this study, we describe the mechanism of MgtE-dependent inhibition of the T3SS. Our report also illustrates how MgtE might respond to environmental cues, such as magnesium levels, to fine-tune T3SS gene expression.

Acidosis potentiates the host proinflammatory interleukin-1β response to Pseudomonas aeruginosa infection

Infection by Pseudomonas aeruginosa, and bacteria in general, frequently promotes acidification of the local microenvironment, and this is reinforced by pulmonary exertion and exacerbation. However, the consequence of an acidic environment on the host inflammatory response to P. aeruginosa infection is poorly understood. Here we report that the pivotal cellular and host proinflammatory interleukin-1β (IL-1β) response, which enables host clearance of the infection but can produce collateral inflammatory damage, is increased in response to P. aeruginosa infection within an acidic environment. Synergistic mechanisms that promote increased IL-1β release in response to P. aeruginosa infection in an acidic environment are increased pro-IL-1β induction and increased caspase-1 activity, the latter being dependent upon a functional type III secretion system of the bacteria and the NLRC4 inflammasome of the host. Using an in vivo peritonitis model, we have validated that the IL-1β inflammatory response is increased in mice in response to P. aeruginosa infection within an acidic microenvironment. These data reveal novel insights into the regulation and exacerbation of inflammatory responses to P. aeruginosa.

Involvement of stress-related genes polB and PA14_46880 in biofilm formation of Pseudomonas aeruginosa

Chronic infections of Pseudomonas aeruginosa are generally established through production of biofilm. During biofilm formation, production of an extracellular matrix and establishment of a distinct bacterial phenotype make these infections difficult to eradicate. However, biofilm studies have been hampered by the fact that most assays utilize nonliving surfaces as biofilm attachment substrates. In an attempt to better understand the mechanisms behind P. aeruginosa biofilm formation, we performed a genetic screen to identify novel factors involved in biofilm formation on biotic and abiotic surfaces. We found that deletion of genes polB and PA14_46880 reduced biofilm formation significantly compared to that in the wild-type strain PA14 in an abiotic biofilm system. In a biotic biofilm model, wherein biofilms form on cultured airway cells, the ΔpolB and ΔPA14_46880 strains showed increased cytotoxic killing of the airway cells independent of the total number of bacteria bound. Notably, deletion mutant strains were more resistant to ciprofloxacin treatment. This phenotype was linked to decreased expression of algR, an alginate transcriptional regulatory gene, under ciprofloxacin pressure. Moreover, we found that pyocyanin production was increased in planktonic cells of mutant strains. These results indicate that inactivation of polB and PA14_46880 may inhibit transition of P. aeruginosa from a more acute infection lifestyle to the biofilm phenotype. Future investigation of these genes may lead to a better understanding of P. aeruginosa biofilm formation and chronic biofilm infections.