The exopolysaccharide alginate protects Pseudomonas aeruginosa biofilm bacteria from IFN-gamma-mediated macrophage killing

YfiBNR mediates cyclic di-GMP dependent small colony variant formation and persistence in Pseudomonas aeruginosa

During long-term cystic fibrosis lung infections, Pseudomonas aeruginosa undergoes genetic adaptation resulting in progressively increased persistence and the generation of adaptive colony morphotypes. This includes small colony variants (SCVs), auto-aggregative, hyper-adherent cells whose appearance correlates with poor lung function and persistence of infection. The SCV morphotype is strongly linked to elevated levels of cyclic-di-GMP, a ubiquitous bacterial second messenger that regulates the transition between motile and sessile, cooperative lifestyles. A genetic screen in PA01 for SCV-related loci identified the yfiBNR operon, encoding a tripartite signaling module that regulates c-di-GMP levels in P. aeruginosa. Subsequent analysis determined that YfiN is a membrane-integral diguanylate cyclase whose activity is tightly controlled by YfiR, a small periplasmic protein, and the OmpA/Pal-like outer-membrane lipoprotein YfiB. Exopolysaccharide synthesis was identified as the principal downstream target for YfiBNR, with increased production of Pel and Psl exopolysaccharides responsible for many characteristic SCV behaviors. An yfi-dependent SCV was isolated from the sputum of a CF patient. Consequently, the effect of the SCV morphology on persistence of infection was analyzed in vitro and in vivo using the YfiN-mediated SCV as a representative strain. The SCV strain exhibited strong, exopolysaccharide-dependent resistance to nematode scavenging and macrophage phagocytosis. Furthermore, the SCV strain effectively persisted over many weeks in mouse infection models, despite exhibiting a marked fitness disadvantage in vitro. Exposure to sub-inhibitory concentrations of antibiotics significantly decreased both the number of suppressors arising, and the relative fitness disadvantage of the SCV mutant in vitro, suggesting that the SCV persistence phenotype may play a more important role during antimicrobial chemotherapy. This study establishes YfiBNR as an important player in P. aeruginosa persistence, and implicates a central role for c-di-GMP, and by extension the SCV phenotype in chronic infections.

During long-term chronic infections of cystic fibrosis patients, Pseudomonas aeruginosa adapts to the lung environment, generating various different morphotypes including small colony variants (SCVs), small, strongly adherent colonies whose appearance correlates with persistence of infection. The SCV morphology is strongly associated with increased levels of the signaling molecule cyclic di-GMP. In this study we investigated the connection between cyclic di-GMP, SCV and persistence of infection. Following a genetic screen for mutants that displayed SCV morphologies, we identified and characterized the YfiBNR system. YfiN is a membrane-bound cyclic di-GMP producing enzyme, whose activity is tightly controlled by YfiR and YfiB. Cyclic di-GMP produced by YfiN boosts exopolysaccharide synthesis, generating an SCV morphotype upon YfiR-mediated release of YfiN repression. The resulting YfiN-mediated SCV morphotype is highly resistant to macrophage phagocytosis in vitro, suggesting a role for the SCV phenotype in immune system evasion. Consistent with this, YfiN de-repression increased the persistence of P. aeruginosa in long-term infections in a mouse model. The observation that the addition of antibiotics decreased the number of suppressors, and the relative fitness disadvantage of the YfiN-mediated SCV morphotype in liquid culture, suggested that SCV-mediated persistence might be favored during antimicrobial chemotherapy.

Effects of Trp- and Arg-containing antimicrobial-peptide structure on inhibition of Escherichia coli planktonic growth and biofilm formation

Biofilms are sessile microbial communities that cause serious chronic infections with high morbidity and mortality. In order to develop more effective approaches for biofilm control, a series of linear cationic antimicrobial peptides (AMPs) with various arginine (Arg or R) and tryptophan (Trp or W) repeats [(RW)(n)-NH(2), where n = 2, 3, or 4] were rigorously compared to correlate their structures with antimicrobial activities affecting the planktonic growth and biofilm formation of Escherichia coli. The chain length of AMPs appears to be important for inhibition of bacterial planktonic growth, since the hexameric and octameric peptides significantly inhibited E. coli growth, while tetrameric peptide did not cause noticeable inhibition. In addition, all AMPs except the tetrameric peptide significantly reduced E. coli biofilm surface coverage and the viability of biofilm cells, when added at inoculation. In addition to inhibition of biofilm formation, significant killing of biofilm cells was observed after a 3-hour treatment of preformed biofilms with hexameric peptide. Interestingly, treatment with the octameric peptide caused significant biofilm dispersion without apparent killing of biofilm cells that remained on the surface; e.g., the surface coverage was reduced by 91.5 + or - 3.5% by 200 microM octameric peptide. The detached biofilm cells, however, were effectively killed by this peptide. Overall, these results suggest that hexameric and octameric peptides are potent inhibitors of both bacterial planktonic growth and biofilm formation, while the octameric peptide can also disperse existing biofilms and kill the detached cells. These results are helpful for designing novel biofilm inhibitors and developing more effective therapeutic methods.

Pseudomonas aeruginosa alginate promotes Burkholderia cenocepacia persistence in cystic fibrosis transmembrane conductance regulator knockout mice

Pseudomonas aeruginosa, a major respiratory pathogen in cystic fibrosis (CF) patients, facilitates infection by other opportunistic pathogens. Burkholderia cenocepacia, which normally infects adolescent patients, encounters alginate elaborated by mucoid P. aeruginosa. To determine whether P. aeruginosa alginate facilitates B. cenocepacia infection in mice, cystic fibrosis transmembrane conductance regulator knockout mice were infected with B. cenocepacia strain BC7 suspended in either phosphate-buffered saline (BC7/PBS) or P. aeruginosa alginate (BC7/alginate), and the pulmonary bacterial load and inflammation were monitored. Mice infected with BC7/PBS cleared all of the bacteria within 3 days, and inflammation was resolved by day 5. In contrast, mice infected with BC7/alginate showed persistence of bacteria and increased cytokine levels for up to 7 days. Histological examination of the lungs indicated that there was moderate to severe inflammation and pneumonic consolidation in isolated areas at 5 and 7 days postinfection in the BC7/alginate group. Further, alginate decreased phagocytosis of B. cenocepacia by professional phagocytes both in vivo and in vitro. P. aeruginosa alginate also reduced the proinflammatory responses of CF airway epithelial cells and alveolar macrophages to B. cenocepacia infection. The observed effects are specific to P. aeruginosa alginate, because enzymatically degraded alginate or other polyuronic acids did not facilitate bacterial persistence. These observations suggest that P. aeruginosa alginate may facilitate B. cenocepacia infection by interfering with host innate defense mechanisms.

Growth in glucose-based medium and exposure to subinhibitory concentrations of imipenem induce biofilm formation in a multidrug-resistant clinical isolate of Acinetobacter baumannii

Background

Acinetobacter baumannii is emerging as an important nosocomial pathogen. Multidrug resistance, as well as ability to withstand environmental stresses, makes eradication of A. baumannii difficult, particularly from hospital settings.

Results

Over a six-year period, 73 isolates of A. baumannii were collected from infected patients in two hospitals in Italy. While 69 out of the 73 isolates displayed identical multidrug antibiotic resistance pattern, they were susceptible to carbapenems. Genetic profiles of these 69 isolates, determined by Pulsed Field Gel Electrophoresis (PFGE), indicated that they were genetically related and could be clustered in a specific clone, called SMAL. We tested the ability of the SMAL clone to form biofilm, an important determinant for bacterial colonization of the human host and for persistence in the hospital environment. Biofilm formation by A. baumannii SMAL, measured as surface adhesion to polystyrene, is strongly affected by growth conditions, being impaired in rich growth media such as LB, while being favoured in glucose-based medium. Surface adhesion in glucose-based media is inhibited by treatment with cellulase, suggesting that it depends on production of cellulose or of a chemically related extracellular polysaccharide. Exposure of A. baumannii SMAL to subinhibitory concentrations of imipenem resulted in biofilm stimulation and increased production of iron uptake proteins. Growth in iron-supplemented medium also stimulated surface adhesion, thus suggesting that increased intracellular iron concentrations might act as an environmental signal for biofilm formation in A. baumannii SMAL.

Conclusions

Our results indicate that exposure to subinhibitory concentrations of imipenem can stimulate biofilm formation and induce iron uptake in a pathogenic strain of A. baumannii, with potential implications on antibiotic susceptibility and ability to persist in the human host.

In Vivo Effects of Citric Acid/Zwitterionic Surfactant Cleansing Solution on Rabbit Sinus Mucosa

Background

Chronic rhinosinusitis that is refractory to medical or surgical intervention may involve a particularly resistant form of infection known as a bacterial biofilm that is recalcitrant to antibiotics secondary to physical barrier characteristics. Recently, a novel sinus cleansing solution, citric acid/zwitterionic surfactant (CAZS) was shown to be extremely effective in disrupting biofilms in vitro. The purpose of this study was to determine the effects of CAZS on sinonasal epithelium in vivo compared with normal saline.

Methods

Indwelling catheters were placed into the right maxillary sinus of New Zealand white rabbits. CAZS solution or normal saline (10 mL) was instilled at a rate of 20 mL/minute into the sinus followed by aspiration. Rabbits were killed 1, 3, and 6 days after treatment. Mucosa from both maxillary sinuses was harvested and evaluated for physiological activity (ciliary beating) as well as morphological integrity of the epithelium by scanning electron microscopy.

Results

One day after treatment, beating cilia was evident with morphological analysis shown intact epithelium with 80–85% denudation of cilia compared with saline. Three days after treatment, ciliary activity was again noted with morphological evidence of persistent denuded cilia. By day 6 after treatment, the epithelium had regenerated cilia over the apical surface. Throughout the recovery period beating cilia was evident in CAZS-treated sinuses.

Conclusion

This study shows that although CAZS acutely denudes respiratory cilia, the remaining cilia are active. Additionally, the epithelial barrier appears intact with active ciliogenesis, and reciliation of the mucosal surface occurring 6 days after treatment.

Flagellum-mediated biofilm defense mechanisms of Pseudomonas aeruginosa against host-derived lactoferrin

Chronic infection with the gram-negative organism Pseudomonas aeruginosa is a leading cause of morbidity and mortality in human patients, despite high doses of antibiotics used to treat the various diseases this organism causes. These infections are chronic because P. aeruginosa readily forms biofilms, which are inherently resistant to antibiotics as well as the host's immune system. Our laboratory has been investigating specific mutations in P. aeruginosa that regulate biofilm bacterial susceptibility to the host. To continue our investigation of the role of genetics in bacterial biofilm host resistance, we examined P. aeruginosa biofilms that lack the flgK gene. This mutant lacks flagella, which results in defects in early biofilm development (up to 36 h). For these experiments, the flgK-disrupted strain and the parental strain (PA14) were used in a modified version of the 96-well plate microtiter assay. Biofilms were challenged with freshly isolated human leukocytes for 4 to 6 h and viable bacteria enumerated by CFU. Subsequent to the challenge, both mononuclear cells (monocytes and lymphocytes) and neutrophils, along with tumor necrosis factor alpha (TNF-alpha), were required for optimal killing of the flgK biofilm bacteria. We identified a cytokine cross talk network between mononuclear cells and neutrophils that was essential to the production of lactoferrin and bacterial killing. Our data suggest that TNF-alpha is secreted from mononuclear cells, causing neutrophil activation, resulting in the secretion of bactericidal concentrations of lactoferrin. These results extend previous studies of the importance of lactoferrin in the innate immune defense against bacterial biofilms.

Regular debridement is the main tool for maintaining a healthy wound bed in most chronic wounds

Sharp debridement is the most clinically and cost-effective way of physically removing and suppressing a biofilm. Continued debridement, as part of a multifaceted treatment strategy, will keep the biofilm in a weakened state.

Detection and identification of specific bacteria in wound biofilms using peptide nucleic acid fluorescent in situ hybridization (PNA FISH)

Biofilms provide a reservoir of potentially infectious micro-organisms that are resistant to antimicrobial agents, and their importance in the failure of medical devices and chronic inflammatory conditions is increasingly being recognized. Particular research interest exists in the association of biofilms with wound infection and non-healing, i.e. chronic wounds. In this study, fluorescent in situ hybridization (FISH) was used in combination with confocal laser scanning microscopy (CLSM) to detect and characterize the spatial distribution of biofilm-forming bacteria which predominate within human chronic skin wounds (Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus sp. and Micrococcus sp.). In vitro biofilms were prepared using a constant-depth film fermenter and a reconstituted human epidermis model. In vivo biofilms were also studied using biopsy samples from non-infected chronic venous leg ulcers. The specificity of peptide nucleic acid (PNA) probes for the target organisms was confirmed using mixed preparations of planktonic bacteria and multiplex PNA probing. Identification and location of individual bacterial species within multi-species biofilms demonstrated that P. aeruginosa was predominant. CLSM revealed clustering of individual species within mixed-species biofilms. FISH analysis of archive chronic wound biopsy sections showed bacterial presence and allowed bacterial load to be determined. The application of this standardized procedure makes available an assay for identification of single- or multi-species bacterial populations in tissue biopsies. The technique provides a reliable tool to study bacterial biofilm formation and offers an approach to assess targeted biofilm disruption strategies in vivo.

Evolving concepts in biofilm infections

Several pathogens associated with chronic infections, including Pseudomonas aeruginosa in cystic fibrosis pneumonia, Haemophilus influenzae and Streptococcus pneumoniae in chronic otitis media, Staphylococcus aureus in chronic rhinosinusitis and enteropathogenic Escherichia coli in recurrent urinary tract infections, are linked to biofilm formation. Biofilms are usually defined as surface-associated microbial communities, surrounded by an extracellular polymeric substance (EPS) matrix. Biofilm formation has been demonstrated for numerous pathogens and is clearly an important microbial survival strategy. However, outside of dental plaques, fewer reports have investigated biofilm development in clinical samples. Typically biofilms are found in chronic diseases that resist host immune responses and antibiotic treatment and these characteristics are often cited for the ability of bacteria to persist in vivo. This review examines some recent attempts to examine the biofilm phenotype in vivo and discusses the challenges and implications for defining a biofilm phenotype.

Azithromycin increases survival and reduces lung inflammation in cystic fibrosis mice

OBJECTIVE AND DESIGN

Azithromycin (AZM) has been used as an anti-inflammatory agent in the treatment of cystic fibrosis (CF), particularly those with chronic infection with P. aeruginosa (PA). To investigate mechanisms associated with the beneficial effects of AZM in CF, we examined bacterial load, cytokine levels, and clearance of inflammatory cells in CF mice infected with mucoid PA and treated with AZM.

METHODS

Gut-corrected Cftr(tm1Unc)-TgN(FABPCFTR)#Jaw CF mice infected with an alginate-overproducing PA CF-isolate were treated with AZM or saline and examined for survival of animals, lung bacterial load, inflammation, cytokine levels, and apoptotic cells up to 5 days post-infection.

RESULTS

Administration of AZM (20 mg/kg) 24 h after the infection improved 5-day survival to 95% compared with treatment with saline (56%). AZM administration was associated with significant reductions in bacterial load, decreased lung inflammation, and increased levels of IFN-gamma. AZM increased macrophage clearance of apoptotic neutrophils from the lung.

CONCLUSION

Azithromycin enhances bacterial clearance and reduces lung inflammation by improving innate immune defense mechanisms in CF mice.

Molecular characterization of Alg8, a putative glycosyltransferase, involved in alginate polymerisation

The topology of Alg8, the proposed catalytic subunit of the alginate polymerase, was assessed using PhoA and LacZ fusion protein analysis. This analysis suggested that the periplasmic loop comprises only three amino acid residues with the adjacent transmembrane helices at positions 361-387 and 393-416. Accordingly, the extended cytosolic loop could be located at positions 71-361 and was proposed to contain important catalytic residues. Further experimental evidence for this cytosolic domain was obtained by independently demonstrating this protein region as purified soluble protein domain. The soluble protein domain was identified by MALDI-TOF/MS and presumably represents the cytosolic catalytic domain of Alg8. Site-directed mutagenesis of 11 conserved residues in the cytosolic loop showed that D-188/D-190 (DXD motif), D-295/D-296 (acid-base catalysts) and K-297 were each essential for in vivo polymerase activity, whereas D-179/D-181 (DXD motif), C-244, R-263, D-279, and E-282 were not directly involved in the polymerisation reaction. The role of these amino acid residues with respect to the catalysed alginate polymerisation reaction was discussed with the aid of the recently developed structural model of Alg8.

Rhamnolipid-induced shedding of flagellin from Pseudomonas aeruginosa provokes hBD-2 and IL-8 response in human keratinocytes

Several 'pathogen-associated molecular pattern' (PAMP) of the opportunistic pathogen Pseudomonas aeruginosa activate the innate immune system in epithelial cells. Particularly the production of antimicrobial peptides such as the human beta-defensin-2 (hBD-2) and proinflammatory cytokines as the interleukin (IL)-8 is boosted. In the present study culture supernatants of static grown P. aeruginosa were found to be potent hBD-2 and IL-8 inducers, indicating a soluble or shedded PAMP, comparable to that of heat-killed bacterial supernatants. In subsequent analyses this PAMP was identified as flagellin, the major structural protein of the flagella. Flagellin is known to be an immunostimulatory potent factor, but the mechanisms by which P. aeruginosa is able to remove flagellin from the flagella remain unknown. Here we provide evidence for the presence of a factor responsible for release of flagellin from the flagella. Purification of this factor and subsequent mass spectrometry analyses identified rhamnolipids as responsible agents. Our findings indicate that maybe upon adhesion to surfaces P. aeruginosa alters the outer membrane composition in a rhamnolipid-depending manner, thereby shedding flagellin from the flagella. In turn epithelial cells recognize flagellin and cause the synthesis of antimicrobial peptides as well as recruitment of inflammatory cells by induction of proinflammatory cytokines.

Biofilms and chronic wound inflammation

In contrast to the commonly accepted hypothesis of host-centred pathology, it is possible that surface bacteria, not host dysfunction, cause the chronicity and perpetual inflammation associated with chronic non-healing wounds.

Association between hypermutator phenotype, clinical variables, mucoid phenotype, and antimicrobial resistance in Pseudomonas aeruginosa

The presence of hypermutator Pseudomonas aeruginosa was associated with poorer lung function in patients at the Adult West Midlands CF Unit. Mucoid isolates were more likely to be hypermutators. The presence of resistant mutant subpopulations was associated with hypermutator phenotype but was not good enough to be used as a test for this phenotype.

The relationship of biofilms to chronic rhinosinusitis

PURPOSE OF REVIEW

To provide an update on the state of biofilm research in otolaryngology.

RECENT FINDINGS

Chronic rhinosinusitis is a polymicrobial infection, which includes planktonic and biofilm infections with bacterial and fungal elements. The importance of genetic shift in microbes, when converting into a biofilm state, as well as the multiple phenotypes in each bacterial colony cannot be overemphasized. This creates a very sophisticated community of pathogens, some of which will likely survive a simple chemical treatment. Sinus cultures cannot be expected to provide a complete knowledge of the cause of chronic sinusitis. A new diagnostic method and innovative treatment plans will be necessary to provide a lasting treatment of chronic rhinosinusitis. Surgery combined with postoperative treatment is the most effective mean of controlling the majority of chronic rhinosinusitis infections. The challenges associated with the treatment of chronic rhinosinusitis patients may be met by focusing more on the community of microorganism present in the sinuses.

SUMMARY

The understanding of the implication of chronic biofilm infections is growing rapidly but will require an enormous effort to completely control chronic rhinosinusitis.

Medical biofilms

For more than two decades, Biotechnology and Bioengineering has documented research focused on natural and engineered microbial biofilms within aquatic and subterranean ecosystems, wastewater and waste-gas treatment systems, marine vessels and structures, and industrial bioprocesses. Compared to suspended culture systems, intentionally engineered biofilms are heterogeneous reaction systems that can increase reactor productivity, system stability, and provide inherent cell:product separation. Unwanted biofilms can create enormous increases in fluid frictional resistances, unacceptable reductions in heat transfer efficiency, product contamination, enhanced material deterioration, and accelerated corrosion. Missing from B&B has been an equivalent research dialogue regarding the basic molecular microbiology, immunology, and biotechnological aspects of medical biofilms. Presented here are the current problems related to medical biofilms; current concepts of biofilm formation, persistence, and interactions with the host immune system; and emerging technologies for controlling medical biofilms.

Population-level virulence factors amongst pathogenic bacteria: relation to infection outcome

The study of population-level virulence traits among communal bacteria represents an emerging discipline in the field of bacterial pathogenesis. It has become clear over the past decade-and-a-half that bacteria exhibit many of the hallmarks of multicellular organisms when they are growing as biofilms and communicating among each other using quorum- sensing systems. Each of these population-level behaviors provides for multiple expressions of virulence that individual free-swimming bacteria do not possess. Population-level virulence traits are largely associated with chronic or persistent infections, whereas individual bacterial virulence traits are associated with acute infections. Thus, there is a natural dichotomy between acute and chronic infectious processes, which helps to explain the medical community's success in combating the former, but its utter failure in dealing with the latter. The recent recognition of multicellularity among chronic bacterial pathogens will lead the way towards new multimodality therapies.

Effects of biomaterial-induced inflammation on fibrosis and rejection

Evidence is emerging that biomaterials cause inflammation by ligating innate immune receptors on antigen presenting cells. Although inflammation is usually viewed as detrimental, it has unexpected and potentially beneficial effects on fibrosis and transplant rejection. For example, the magnitude of inflammation due to a biomaterial is not predictive of the extent of fibrosis. Similarly, biomaterials do not always show adjuvancy. Some biomaterials suppressed T cell rejection responses in vivo and in vitro, while others non-specifically stimulated T cell proliferation. Understanding these complex inter-relationships is the key to designing a biomaterial that stimulates regeneration and induces tolerance in tissue engineering applications.

Biofilm development with an emphasis on Bacillus subtilis

Our understanding of the molecular mechanisms involved in biofilm formation has increased tremendously in recent years. From research on diverse bacteria, a general model of bacterial biofilm development has emerged. This model can be adjusted to fit either of two common modes of unicellular existence: nonmotile and motile. Here we provide a detailed review of what is currently known about biofilm formation by the motile bacterium Bacillus subtilis. While the ability of bacteria to form a biofilm appears to be almost universal and overarching themes apply, the combination of molecular events necessary varies widely, and this is reflected in the other chapters of this book.

Role of PAR2 in murine pulmonary pseudomonal infection

Proteinases can influence lung inflammation by various mechanisms, including via cleavage and activation of protease-activated receptors (PAR) such as PAR2. In addition, proteinases such as neutrophil and/or Pseudomonas-derived elastase can disarm PAR2 resulting in loss of PAR2 signaling. Currently, the role of PAR2 in host defense against bacterial infection is not known. Using a murine model of acute Pseudomonas aeruginosa pneumonia, we examined differences in the pulmonary inflammatory response between wild-type and PAR2(-/-) mice. Compared with wild-type mice, PAR2(-/-) mice displayed more severe lung inflammation and injury in response to P. aeruginosa infection as indicated by higher bronchoalveolar lavage fluid neutrophil numbers, protein concentration, and TNF-alpha levels. By contrast, IFN-gamma levels were markedly reduced in PAR2(-/-) compared with wild-type mice. Importantly, clearance of P. aeruginosa was diminished in PAR2(-/-) mice. In vitro testing revealed that PAR2(-/-) neutrophils killed significantly less bacteria than wild-type murine neutrophils. Further, both neutrophils and macrophages from PAR2(-/-) mice displayed significantly reduced phagocytic efficiency compared with wild-type phagocytes. Stimulation of PAR2 on macrophages using a PAR2-activating peptide resulted in enhanced phagocytosis directly implicating PAR2 signaling in the phagocytic process. We conclude that genetic deletion of PAR2 is associated with decreased clearance of P. aeruginosa. Our data suggest that a deficiency in IFN-gamma production and impaired bacterial phagocytosis are two potential mechanisms responsible for this defect.

Role of polysaccharides in Pseudomonas aeruginosa biofilm development

During the past decade, there has been a renewed interest in using Pseudomonas aeruginosa as a model system for biofilm development and pathogenesis. Since the biofilm matrix represents a crucial interface between the bacterium and the host or its environment, considerable effort has been expended to acquire a more complete understanding of the matrix composition. Here, we focus on recent developments regarding the roles of alginate, Psl, and Pel polysaccharides in the biofilm matrix.

Biofilm formation, epiphytic fitness, and canker development in Xanthomonas axonopodis pv. citri

The phytopathogenic bacterium Xanthomonas axonopodis pv. citri is responsible for the canker disease affecting citrus plants throughout the world. Here, we have evaluated the role of bacterial attachment and biofilm formation in leaf colonization during canker development on lemon leaves. Crystal violet staining and confocal laser scanning microscopy analysis of X. axonopodis pv. citri strains expressing the green fluorescent protein were used to evaluate attachment and biofilm formation on abiotic and biotic (leaf) surfaces. Wild-type X. axonopodis pv. citri attached to and formed a complex, structured biofilm on glass in minimal medium containing glucose. Similar attachment and structured biofilm formation also were seen on lemon leaves. An X. axonopodis pv. citri gumB mutant strain, defective in production of the extracellular polysaccharide xanthan, did not form a structured biofilm on either abiotic or biotic surfaces. In addition, the X. axonopodis pv. citri gumB showed reduced growth and survival on leaf surfaces and reduced disease symptoms. These findings suggest an important role for formation of biofilms in the epiphytic survival of X. axonopodis pv. citri prior to development of canker disease.

Pseudomonas aeruginosa forms biofilms in acute infection independent of cell-to-cell signaling

Biofilms are bacterial communities residing within a polysaccharide matrix that are associated with persistence and antibiotic resistance in chronic infections. We show that the opportunistic pathogen Pseudomonas aeruginosa forms biofilms within 8 h of infection in thermally injured mice, demonstrating that biofilms contribute to bacterial colonization in acute infections as well. Using light, electron, and confocal scanning laser microscopy, P. aeruginosa biofilms were visualized within burned tissue surrounding blood vessels and adipose cells. Although quorum sensing (QS), a bacterial signaling mechanism, coordinates differentiation of biofilms in vitro, wild-type and QS-deficient P. aeruginosa strains formed similar biofilms in vivo. Our findings demonstrate that P. aeruginosa forms biofilms on specific host tissues independently of QS.

Phenotypic and functional characterization of Bacillus anthracis biofilms

Biofilms, communities of micro-organisms attached to a surface, are responsible for many chronic diseases and are often associated with environmental reservoirs or lifestyles. Bacillus anthracis is a Gram-positive, endospore-forming bacterium and is the aetiological agent of pulmonary, gastrointestinal and cutaneous anthrax. Anthrax infections are part of the natural lifecycle of many ruminants in North America, including cattle and bison, and B. anthracis is thought to be a central part of this ecosystem. However, in endemic areas in which humans and livestock interact, chronic cases of cutaneous anthrax are commonly reported. This suggests that biofilms of B. anthracis exist in the environment and are part of the ecology associated with its lifecycle. Currently, there are few data that account for the importance of the biofilm mode of life in B. anthracis, yet biofilms have been characterized in other pathogenic and non-pathogenic Bacillus species, including Bacillus cereus and Bacillus subtilis, respectively. This study investigated the phenotypic and functional role of biofilms in B. anthracis. The results demonstrate that B. anthracis readily forms biofilms which are inherently resistant to commonly prescribed antibiotics, and that antibiotic resistance is not solely the function of sporulation.

The immunity of splenic and peritoneal F4/80(+) resident macrophages in mouse mixed allogeneic chimeras

Mixed allogeneic chimeras are emerging as a prospective approach to induce immune tolerance in clinics. However, the immunological function of macrophages in mixed chimeras has not been evaluated. Using a B6-->BALB/c mixed chimera model, we investigated the phenotype and function of F4/80(+) resident peritoneal exudate macrophage (PEMs) and splenic macrophages (SPMs) in vitro and in vivo. Recipient F4/80(+)PEMs and SPMs in mixed chimeras expressed significantly lower levels of MHC-II, CD54, and CD23 than those in non-chimeric mice before lipopolysaccharide stimulation. Recipient F4/80(+)PEMs and SPMs in mixed chimeras induced normal cell proliferation and delayed-type hypersensitivity of allo-T cells, but they induced more IFN-gamma and IL-2 products and less IL-10 and TGF-beta products of allo-T cells compared with those of non-chimeras. Furthermore, recipient F4/80(+)PEMs and SPMs had significantly higher phagocytotic capacity against chicken red blood cells or allo-T cells than those of controls while they had normal phagocytosis to Escherichia coli. Although some slight but significant alterations of recipient macrophages have been detected, these results provide direct evidences for the efficient immunity of recipient macrophages in mixed allogeneic chimeras. The present study also, for the first time, offered basic information for macrophages maturing in heterogeneous environments.

Interaction of Candida albicans with adherent human peripheral blood mononuclear cells increases C. albicans biofilm formation and results in differential expression of pro- and anti-inflammatory cytokines

Monocytes and macrophages are the cell types most commonly associated with the innate immune response against Candida albicans infection. Interactions between the host immune system and Candida organisms have been investigated for planktonic Candida cells, but no studies have addressed these interactions in a biofilm environment. In this study, for the first time, we evaluated the ability of C. albicans to form biofilms in the presence or absence of adherent peripheral blood mononuclear cells (PBMCs; enriched for monocytes and macrophages by adherence). Our analyses using scanning electron and confocal scanning laser microscopy showed that the presence of PBMCs enhanced the ability of C. albicans to form biofilms and that the majority of PBMCs were localized to the basal and middle layers of the biofilm. In contrast to the interactions of PBMCs with planktonic C. albicans, where PBMCs phagocytose fungal cells, PBMCs did not appear to phagocytose fungal cells in biofilms. Furthermore, time-lapse laser microscopy revealed dynamic interactions between C. albicans and PBMCs in a biofilm. Additionally, we found that (i) only viable PBMCs influence Candida biofilm formation, (ii) cell surface components of PBMCs did not contribute to the enhancement of C. albicans biofilm, (iii) the biofilm-enhancing effect of PBMCs is mediated by a soluble factor released into the coculture medium of PBMCs with C. albicans, and (iv) supernatant collected from this coculture contained differential levels of pro- and anti-inflammatory cytokines. Our studies provide new insight into the interaction between Candida biofilm and host immune cells and demonstrate that immunocytes may influence the ability of C. albicans to form biofilms.

Biofilms and their relevance to veterinary medicine

Bacteria are renowned for their ability to tolerate and adapt to a wide range of adverse environmental conditions. The primary mechanism that facilitates these adaptations is thought to be the capacity to form and maintain biofilms. Within a biofilm, bacteria become attached to a surface where they exist in complex communities which are able to interact with each other through intracellular communication and thus rapidly adapt to changing environments. The organisms within biofilms are notorious for their resistance towards the host immune response and antibacterial agents compared to their free-living planktonic counterparts. Consequently, biofilms are of significant importance to both clinical and veterinary science. However, although bacterial infections are widely reported in animals their association with biofilms is rarely discussed. The aim of this review is to look at the characteristics of biofilm infections in humans and to relate this knowledge to veterinary science in order to assess their relevance in this area.

The ColRS two-component system regulates membrane functions and protects Pseudomonas putida against phenol

As reported, the two-component system ColRS is involved in two completely different processes. It facilitates the root colonization ability of Pseudomonas fluorescens and is necessary for the Tn4652 transposition-dependent accumulation of phenol-utilizing mutants in Pseudomonas putida. To determine the role of the ColRS system in P. putida, we searched for target genes of response regulator ColR by use of a promoter library. Promoter screening was performed on phenol plates to mimic the conditions under which the effect of ColR on transposition was detected. The library screen revealed the porin-encoding gene oprQ and the alginate biosynthesis gene algD occurring under negative control of ColR. Binding of ColR to the promoter regions of oprQ and algD in vitro confirmed its direct involvement in regulation of these genes. Additionally, the porin-encoding gene ompA(PP0773) and the type I pilus gene csuB were also identified in the promoter screen. However, it turned out that ompA(PP0773) and csuB were actually affected by phenol and that the influence of ColR on these promoters was indirect. Namely, our results show that ColR is involved in phenol tolerance of P. putida. Phenol MIC measurement demonstrated that a colR mutant strain did not tolerate elevated phenol concentrations. Our data suggest that increased phenol susceptibility is also the reason for inhibition of transposition of Tn4652 in phenol-starving colR mutant bacteria. Thus, the current study revealed the role of the ColRS two-component system in regulation of membrane functionality, particularly in phenol tolerance of P. putida.

Comparative antibody-mediated phagocytosis of Staphylococcus epidermidis cells grown in a biofilm or in the planktonic state

Staphylococcus epidermidis is an important cause of nosocomial infections. Virulence is attributable to elaboration of biofilms on medical surfaces that protect the organisms from immune system clearance. Even though leukocytes can penetrate biofilms, they fail to phagocytose and kill bacteria. The properties that make biofilm bacteria resistant to the immune system are not well characterized. In order to better understand the mechanisms of resistance of bacteria in biofilms to the immune system, we evaluated antibody penetration throughout the biofilm and antibody-mediated phagocytic killing of planktonic versus biofilm cells of S. epidermidis by using a rabbit antibody to poly-N-acetylglucosamine (PNAG). These antibodies are opsonic and protect against infection with planktonic cells of PNAG-positive Staphylococcus aureus and S. epidermidis. Antibody to PNAG readily penetrated the biofilm and bound to the same areas in the biofilm as did wheat germ agglutinin, a lectin known to bind to components of staphylococcal biofilms. However, biofilm cells were more resistant to opsonic killing than their planktonic counterparts in spite of producing more PNAG per cell than planktonic cells. Biofilm extracts inhibited opsonic killing mediated by antibody to PNAG, suggesting that the PNAG antigen within the biofilm matrix prevents antibody binding close to the bacterial cell surface, which is needed for efficient opsonic killing. Increased resistance of biofilm cells to opsonic killing mediated by an otherwise protective antibody was due not to a biofilm-specific phenotype but rather to high levels of antigen within the biofilm that prevented bacterial opsonization by the antibody.

Bacterial alginates: from biosynthesis to applications

Alginate is a polysaccharide belonging to the family of linear (unbranched), non-repeating copolymers, consisting of variable amounts of beta-D-mannuronic acid and its C5-epimer alpha- L-guluronic acid linked via beta-1,4-glycosidic bonds. Like DNA, alginate is a negatively charged polymer, imparting material properties ranging from viscous solutions to gel-like structures in the presence of divalent cations. Bacterial alginates are synthesized by only two bacterial genera, Pseudomonas and Azotobacter, and have been extensively studied over the last 40 years. While primarily synthesized in form of polymannuronic acid, alginate undergoes chemical modifications comprising acetylation and epimerization, which occurs during periplasmic transfer and before final export through the outer membrane. Alginate with its unique material properties and characteristics has been increasingly considered as biomaterial for medical applications. The genetic modification of alginate producing microorganisms could enable biotechnological production of new alginates with unique, tailor-made properties, suitable for medical and industrial applications.