Candida albicans ethanol stimulates Pseudomonas aeruginosa WspR-controlled biofilm formation as part of a cyclic relationship involving phenazines

Iron acquisition in the cystic fibrosis lung and potential for novel therapeutic strategies

Iron acquisition is vital to microbial survival and is implicated in the virulence of many of the pathogens that reside in the cystic fibrosis (CF) lung. The multifaceted nature of iron acquisition by both bacterial and fungal pathogens encompasses a range of conserved and species-specific mechanisms, including secretion of iron-binding siderophores, utilization of siderophores from other species, release of iron from host iron-binding proteins and haemoproteins, and ferrous iron uptake. Pathogens adapt and deploy specific systems depending on iron availability, bioavailability of the iron pool, stage of infection and presence of competing pathogens. Understanding the dynamics of pathogen iron acquisition has the potential to unveil new avenues for therapeutic intervention to treat both acute and chronic CF infections. Here, we examine the range of strategies utilized by the primary CF pathogens to acquire iron and discuss the different approaches to targeting iron acquisition systems as an antimicrobial strategy.

Respiratory microbiota resistance and resilience to pulmonary exacerbation and subsequent antimicrobial intervention

Pulmonary symptoms in cystic fibrosis (CF) begin in early life with chronic lung infections and concomitant airway inflammation leading to progressive loss of lung function. Gradual pulmonary function decline is interspersed with periods of acute worsening of respiratory symptoms known as CF pulmonary exacerbations (CFPEs). Cumulatively, CFPEs are associated with more rapid disease progression. In this study multiple sputum samples were collected from adult CF patients over the course of CFPEs to better understand how changes in microbiota are associated with CFPE onset and management. Data were divided into five clinical periods: pre-CFPE baseline, CFPE, antibiotic treatment, recovery, and post-CFPE baseline. Samples were treated with propidium monoazide prior to DNA extraction, to remove the impact of bacterial cell death artefacts following antibiotic treatment, and then characterised by 16S rRNA gene-targeted high-throughput sequencing. Partitioning CF microbiota into core and rare groups revealed compositional resistance to CFPE and resilience to antibiotics interventions. Mixed effects modelling of core microbiota members revealed no significant negative impact on the relative abundance of Pseudomonas aeruginosa across the exacerbation cycle. Our findings have implications for current CFPE management strategies, supporting reassessment of existing antimicrobial treatment regimens, as antimicrobial resistance by pathogens and other members of the microbiota may be significant contributing factors.

Co-occurence of filamentation defects and impaired biofilms in Candida albicans protein kinase mutants

Pathogenicity of Candida albicans is linked with its developmental stages, notably the capacity switch from yeast-like to hyphal growth, and to form biofilms on surfaces. To better understand the cellular processes involved in C. albicans development, a collection of 63 C. albicans protein kinase mutants was screened for biofilm formation in a microtitre plate assay. Thirty-eight mutants displayed some degree of biofilm impairment, with 20 categorised as poor biofilm formers. All the poor biofilm formers were also defective in the switch from yeast to hyphae, establishing it as a primary defect. Five genes, VPS15, IME2, PKH3, PGA43 and CEX1, encode proteins not previously reported to influence hyphal development or biofilm formation. Network analysis established that individual components of some processes, most interestingly MAP kinase pathways, are not required for biofilm formation, most likely indicating functional redundancy. Mutants were also screened for their response to bacterial supernatants and it was found that Pseudomonas aeruginosa supernatants inhibited biofilm formation in all mutants, regardless of the presence of homoserine lactones (HSLs). In contrast, Candida morphology was only affected by supernatant containing HSLs. This confirms the distinct HSL-dependent inhibition of filamentation and the HSL-independent impairment of biofilm development by P. aeruginosa.

Fungal-bacterial interactions and their relevance in health

Cross-kingdom interactions between bacteria and fungi are a common occurrence in the environment. Recent studies have identified various types of interactions that either can take the form of a synergistic relationship or can result in an antagonistic interplay with the subsequent destruction or inhibition of growth of bacteria, fungi or both. This cross-kingdom communication is of particular significance in human health and disease, as bacteria and fungi commonly colonize various human surfaces and their interactions can at times alter the outcome of invasive infections. Moreover, mixed infections from both bacteria and fungi are relatively common among critically ill patients and individuals with weak immune responses. The purpose of this review is to summarize our knowledge on the type of interactions between bacteria and fungi and their relevance in human infections.

Pseudomonas aeruginosa DesB Promotes Staphylococcus aureus Growth Inhibition in Coculture by Controlling the Synthesis of HAQs

Pseudomonas aeruginosa is a pathogen that can cause serious infections and usually coexists with other pathogens, such as Staphylococcus aureus. Virulence factors are important for maintaining a presence of the organisms in these multispecies environments, and DesB plays an important role in P. aeruginosa virulence. Therefore, we investigated the effect of DesB on S. aureus reduction under competitive situation. Liquid cultures of P. aeruginosa wild type (WT) and its desB mutant were spotted on agar plates containing S. aureus, and the size of the clear zones was compared. In addition, interbacterial competition between P. aeruginosa and S. aureus was observed over time during planktonic coculture. The transcriptional profiles of the WT and desB mutant were compared by qRT-PCR and microarray to determine the role of DesB in S. aureus reduction at the molecular level. As a result, the clear zone was smaller for the desB mutant than for P. aeruginosa PAO1 (WT), and in planktonic coculture, the number of S. aureus cells was reduced in the desB mutant. qRT-PCR and microarray revealed that the expression of MvfR-controlled pqsA-E and phnAB operons was significantly decreased, but the mexEF-oprN operon was highly expressed. The results indicate that intracellular levels of 4-hydroxy-2-heptylquinoline (HHQ), a ligand of MvfR, are reduced due to MexEF-OprN-mediated efflux in desB mutant, resulting in the decrease of MvfR binding to pqsA-E promoter and the reduction of 4-hydroxy-2-alkylquinolines (HAQs) synthesis. Overexpression of mexEF-oprN operon in desB mutant was phenotypically confirmed by observing significantly increased resistance to chloramphenicol. In conclusion, these results suggest that DesB plays a role in the inhibition of S. aureus growth by controlling HAQ synthesis.

Pseudomonas aeruginosa Biofilm Response and Resistance to Cold Atmospheric Pressure Plasma Is Linked to the Redox-Active Molecule Phenazine

Pseudomonas aeruginosa is an important opportunistic pathogen displaying high antibiotic resistance. Its resistance is in part due to its outstanding ability to form biofilms on a range of biotic and abiotic surfaces leading to difficult-to-treat, often long-term infections. Cold atmospheric plasma (CAP) is a new, promising antibacterial treatment to combat antibiotic-resistant bacteria. Plasma is ionized gas that has antibacterial properties through the generation of a mix of reactive oxygen and nitrogen species (RONS), excited molecules, charged particles and UV photons. Our results show the efficient removal of P. aeruginosa biofilms using a plasma jet (kINPen med), with no viable cells detected after 5 min treatment and no attached biofilm cells visible with confocal microscopy after 10 min plasma treatment. Because of its multi-factorial action, it is widely presumed that the development of bacterial resistance to plasma is unlikely. However, our results indicate that a short plasma treatment (3 min) may lead to the emergence of a small number of surviving cells exhibiting enhanced resistance to subsequent plasma exposure. Interestingly, these cells also exhibited a higher degree of resistance to hydrogen peroxide. Whole genome comparison between surviving cells and control cells revealed 10 distinct polymorphic regions, including four belonging to the redox active, antibiotic pigment phenazine. Subsequently, the interaction between phenazine production and CAP resistance was demonstrated in biofilms of transposon mutants disrupted in different phenazine pathway genes which exhibited significantly altered sensitivity to CAP.

Constitutive production of c-di-GMP is associated with mutations in a variant of Pseudomonas aeruginosa with altered membrane composition

Most bacteria can form multicellular communities called biofilms on biotic and abiotic surfaces. This multicellular response to surface contact correlates with an increased resistance to various adverse environmental conditions, including those encountered during infections of the human host and exposure to antimicrobial compounds. Biofilm formation occurs when freely swimming (planktonic) cells encounter a surface, which stimulates the chemosensory-like, surface-sensing system Wsp and leads to generation of the intracellular second messenger 3',5'-cyclic-di-guanosine monophosphate (c-di-GMP). We identified adaptive mutations in a clinical small colony variant (SCV) of Pseudomonas aeruginosa and correlated their presence with self-aggregating growth behavior and an enhanced capacity to form biofilms. We present evidence that a point mutation in the 5' untranslated region of the accBC gene cluster, which encodes components of an enzyme responsible for fatty acid biosynthesis, was responsible for a stabilized mRNA structure that resulted in reduced translational efficiency and an increase in the proportion of short-chain fatty acids in the plasma membrane. We propose a model in which these changes in P. aeruginosa serve as a signal for the Wsp system to constitutively produce increased amounts of c-di-GMP and thus play a role in the regulation of adhesion-stimulated bacterial responses.