The importance of understanding the infectious microenvironment

Standard doses of antibiotics do not efficiently treat chronic infections of the soft tissue and bone. In this Personal View, we advocate for improving treatment of these infections by taking the infectious microenvironment into account. The infectious microenvironment can cause sensitive bacteria to lose their susceptibility to antibiotics that are effective in standard laboratory susceptibility testing. We propose that bacteria behave substantially different in standard laboratory conditions than they do in actual infections. The infectious microenvironment could impose changes in growth and metabolic activity that result in increased protection against antibiotics. Therefore, we advocate that improved antibiotic treatment of chronic infection is achievable when antibiotics are recommended on the basis of susceptibility testing in relevant in vitro conditions that resemble actual infectious microenvironments. We recommend establishing knowledge of the relevant conditions of the chemical and physical composition of the infectious microenvironment. Recent advances in RNA sequencing, metabolomics, and microscopy have made it possible for the characterisation of the microenvironment of infections and to validate the clinical relevance of in vitro conditions to actual infections.

Delayed neutrophil recruitment allows nascent Staphylococcus aureus biofilm formation and immune evasion

Biofilms that form on implanted medical devices cause recalcitrant infections. The early events enabling contaminating bacteria to evade immune clearance, before a mature biofilm is established, are poorly understood. Live imaging in vitro demonstrated that Staphylococcus aureus sparsely inoculated on an abiotic surface can go undiscovered by human neutrophils, grow, and form aggregates. Small (~50 μm²) aggregates of attached bacteria resisted killing by human neutrophils, resulting in neutrophil lysis and bacterial persistence. In vivo, neutrophil recruitment to a peritoneal implant was spatially heterogenous, with some bacterial aggregates remaining undiscovered by neutrophils after 24 h. Intravital imaging in mouse skin revealed that attached S. aureus aggregates grew and remained undiscovered by neutrophils for up to 3 h. These results suggest a model in which delayed recruitment of neutrophils to an abiotic implant presents a critical window in which bacteria establish a nascent biofilm and acquire tolerance to neutrophil killing.

Oxygen Restriction Generates Difficult-to-Culture P. aeruginosa

Induction of a non-culturable state has been demonstrated for many bacteria, e.g., Escherichia coli and various Vibrio spp. In a clinical perspective, the lack of growth due to these non-culturable bacteria can have major consequences for the treatment of patients. Here, we show how anoxic conditioning (restriction of molecular oxygen, O₂) generates difficult-to-culture (DTC) bacteria during biofilm growth. A significant subpopulation of Pseudomonas aeruginosa entered a DTC state after anoxic conditioning, ranging from 5 to 90% of the total culturable population, in both planktonic and biofilm models. Anoxic conditioning also generated DTC subpopulations of Staphylococcus aureus and Staphylococcus epidermidis (89 and 42% of the total culturable population, respectively). Growth of the DTC populations were achieved by substituting O₂ with 10 mM NO₃^– as an alternative electron acceptor for anaerobic respiration or, in the case of P. aeruginosa, by adding sodium pyruvate or catalase as scavengers against reactive oxygen species (ROS) during aerobic respiration. An increase in normoxic plating due to addition of catalase suggests the molecule hydrogen peroxide as a possible mechanism for induction of DTC P. aeruginosa. Anoxic conditioning also generated a true viable but non-culturable (VBNC) population of P. aeruginosa that was not resurrected by substituting O₂ with NO₃^– during anaerobic respiration. These results demonstrate that habituation to an anoxic micro-environment could complicate diagnostic culturing of bacteria, especially in the case of chronic infections where oxygen is restricted due to the host immune response.

Adjunctive dabigatran therapy improves outcome of experimental left-sided Staphylococcus aureus endocarditis

Background

Staphylococcus aureus is the most frequent and fatal cause of left-sided infective endocarditis (IE). New treatment strategies are needed to improve the outcome. S. aureus coagulase promotes clot and fibrin formation. We hypothesized that dabigatran, could reduce valve vegetations and inflammation in S. aureus IE.

Methods

We used a rat model of severe aortic valve S. aureus IE. All infected animals were randomized to receive adjunctive dabigatran (10 mg/kg b.i.d., n = 12) or saline (controls, n = 11) in combination with gentamicin. Valve vegetation size, bacterial load, cytokine, cell integrins expression and peripheral platelets and neutrophils were assessed 3 days post-infection.

Results

Adjunctive dabigatran treatment significantly reduced valve vegetation size compared to controls (p< 0.0001). A significant reduction of the bacterial load in aortic valves was seen in dabigatran group compared to controls (p = 0.02), as well as expression of key pro-inflammatory markers keratinocyte-derived chemokine, IL-6, ICAM-1, TIMP-1, L-selectin (p< 0.04). Moreover, the dabigatran group had a 2.5-fold increase of circulating platelets compared to controls and a higher expression of functional and activated platelets (CD62p⁺) unbound to neutrophils.

Conclusion

Adjunctive dabigatran reduced the vegetation size, bacterial load, and inflammation in experimental S. aureus IE.

Extracellular hydrogen peroxide measurements using a flow injection system in combination with microdialysis probes - Potential and challenges

There is a strong need for techniques that can quantify the important reactive oxygen species hydrogen peroxide (H₂O₂) in complex media and in vivo. We combined chemiluminescence-based H₂O₂ measurements on a commercially available flow injection analysis (FIA) system with sampling of the analyte using microdialysis probes (MDPs), typically used for measurements in tissue. This allows minimally invasive, quantitative measurements of extracellular H₂O₂ concentration and dynamics utilizing the chemiluminescent reaction of H₂O₂ with acridinium ester. By coupling MDPs to the FIA system, measurements are no longer limited to filtered, liquid samples with low viscosity, as sampling via a MDP is based on a dynamic exchange through a permeable membrane with a specific cut-off. This allows continuous monitoring of dynamic changes in H₂O₂ concentrations, alleviates potential pH effects on the measurements, and allows for flexible application in different media and systems. We give a detailed description of the novel experimental setup and its measuring characteristics along with examples of application in different media and organisms to highlight its broad applicability, but also to discuss current limitations and challenges. The combined FIA-MDP approach for H₂O₂ quantification was used in different biological systems ranging from marine biology, using the model organism Exaiptasia pallida (light stress induced H₂O₂ release up to ~ 2.7 µM), over biomedical applications quantifying enzyme dynamics (glucose oxidase in a glucose solution producing up to ~ 60 µM H₂O₂ and the subsequent addition of catalase to monitor the H₂O₂ degradation process) and the ability of bacteria to modify their direct environment by regulating H₂O₂ concentrations in their surrounding media. This was shown by the bacteria Pseudomonas aeruginosa degrading ~ 18 µM background H₂O₂ in LB-broth. We also discuss advantages and current limitations of the FIA-MDP system, including a discussion of potential cross-sensitivity and interfering chemical species.

Implants induce a new niche for microbiomes

Although much work is being done to develop new treatments, research and knowledge regarding factors underlying implant-related microbial colonization leading to infection are less comprehensive. Presence of microorganisms in and around implants clinically characterized as uninfected remains unknown. The objective of this study was to detect and identify bacteria and fungi on implants from various groups of patients with no prior indications of implant related infections. Patient samples (implants and tissue) were collected from five different hospitals in the Capital region of Denmark. By in-depth microbiological detection methods, we examined the prevalence of bacteria and fungi on 106 clinically uninfected implants from four patient groups (aseptic loosening, healed fractures, craniofacial complications and recently deceased). Of 106 clinically uninfected implants and 39 negative controls investigated, 66% were colonized by bacteria and 40% were colonized by fungi (p < 0.0001 compared to negative controls). A large number of microbes were found to colonize the implants, however, the most prevalent microbes present were not common aetiological agents of implant infections. The findings indicate that implants provide a distinct niche for microbial colonization. These data have broad implications for medical implant recipients, as well as for supporting the idea that the presence of foreign objects in the body alters the human microbiome by providing new colonization niches.

The Consequences of Being in an Infectious Biofilm: Microenvironmental Conditions Governing Antibiotic Tolerance

The main driver behind biofilm research is the desire to understand the mechanisms governing the antibiotic tolerance of biofilm-growing bacteria found in chronic bacterial infections. Rather than genetic traits, several physical and chemical traits of the biofilm have been shown to be attributable to antibiotic tolerance. During infection, bacteria in biofilms exhibit slow growth and a low metabolic state due to O₂ limitation imposed by intense O₂ consumption of polymorphonuclear leukocytes or metabolically active bacteria in the biofilm periphery. Due to variable O₂ availability throughout the infection, pathogen growth can involve aerobic, microaerobic and anaerobic metabolism. This has serious implications for the antibiotic treatment of infections (e.g., in chronic wounds or in the chronic lung infection of cystic fibrosis patients), as antibiotics are usually optimized for aerobic, fast-growing bacteria. This review summarizes knowledge about the links between the microenvironment of biofilms in chronic infections and their tolerance against antibiotics.

Pseudomonas aeruginosa biofilm hampers murine central wound healing by suppression of vascular epithelial growth factor

Biofilm-infected wounds are clinically challenging. Vascular endothelial growth factor and host defence S100A8/A9 are crucial for wound healing but may be suppressed by biofilms. The natural course of Pseudomonas aeruginosa biofilm infection was compared in central and peripheral zones of burn-wounded, infection-susceptible BALB/c mice, which display delayed wound closure compared to C3H/HeN mice. Wounds were evaluated histopathologically 4, 7 or 10 days post-infection. Photoplanimetry evaluated necrotic areas. P. aeruginosa biofilm suppressed vascular endothelial growth factor levels centrally in BALB/c wounds but increased peripheral levels 4-7 days post-infection. Central zones of the burn wound displayed lower levels of central vascular endothelial growth factor as observed 4 and 7 days post-infection in BALB/c mice compared to their C3H/HeN counterparts. Biofilm suppressed early, centrally located S100A8/A9 in BALB/c and centrally and peripherally later on in C3H/HeN wounds as compared to uninfected mice. Peripheral polymorphonuclear-dominated inflammation and larger necrosis were observed in BALB/c wounds. In conclusion, P. aeruginosa biofilm modulates wounds by suppressing central, but inducing peripheral, vascular endothelial growth factor levels and reducing host response in wounds of BALB/c mice. This suppression is detrimental to the resolution of biofilm-infected necrosis.

Microenvironmental characteristics and physiology of biofilms in chronic infections of CF patients are strongly affected by the host immune response

In vitro studies of Pseudomonas aeruginosa and other pathogenic bacteria in biofilm aggregates have yielded detailed insight into their potential growth modes and metabolic flexibility under exposure to gradients of substrate and electron acceptor. However, the growth pattern of P. aeruginosa in chronic lung infections of cystic fibrosis (CF) patients is very different from what is observed in vitro, for example, in biofilms grown in flow chambers. Dense in vitro biofilms of P. aeruginosa exhibit rapid O₂ depletion within <50-100 μm due to their own aerobic metabolism. In contrast, in vivo investigations show that P. aeruginosa persists in the chronically infected CF lung as relatively small cell aggregates that are surrounded by numerous PMNs, where the activity of PMNs is the major cause of O₂ depletion rendering the P. aeruginosa aggregates anoxic. High levels of nitrate and nitrite enable P. aeruginosa to persist fueled by denitrification in the PMN-surrounded biofilm aggregates. This configuration creates a potentially long-term stable ecological niche for P. aeruginosa in the CF lung, which is largely governed by slow growth and anaerobic metabolism and enables persistence and resilience of this pathogen even under the recurring aggressive antimicrobial treatments of CF patients. As similar slow growth of other CF pathogens has recently been observed in endobronchial secretions, there is now a clear need for better in vitro models that simulate such in vivo growth patterns and anoxic microenvironments in order to help unravel the efficiency of existing or new antimicrobials targeting anaerobic metabolism in P. aeruginosa and other CF pathogens. We also advocate that host immune responses such as PMN-driven O₂ depletion play a central role in the formation of anoxic microniches governing bacterial persistence in other chronic infections such as chronic wounds.

Incidence of Propionibacterium acnes in initially culture-negative thioglycollate broths-a prospective cohort study at a Danish University Hospital

The aim of this study was to prospectively investigate the incidence of Propionibacterium acnes in thioglycollate broths reported as culture-negative at the Department of Clinical Microbiology, Rigshospitalet, to evaluate whether 5 days of incubation was enough to find all relevant cases. Five hundred thioglycollate broths reported as culture-negative after 5 days were consecutively collected and incubated for at least a further 9 days (at least 14 days of incubation in total). Only tissue samples from sterile sites of the body (n = 298), bone tissue (n = 197) and foreign material (n = 5) were included in this study. Samples were divided into two groups: infected group and control group. This made it possible to compare findings between groups, thereby making it possible to estimate the level of true-positive findings and contamination. Samples from 296 participants were included in this study. After exclusion criteria were met, P. acnes was cultured from ten out of 151 patients (6.6%) in the infected group and from one out of 138 participants (0.7%) in the control group. This resulted in more findings of P. acnes in the infected group on day 14 than on day 5 (p 0.002). Furthermore, P. acnes was cultured more often from bone tissue and tissue surrounding foreign materials on day 14 than on day 5 (p 0.04). Clinical microbiology laboratories should consider incubating thioglycollate broths for at least 14 days to find all relevant cases of P. acnes, especially when it comes to bone tissue and tissue surrounding foreign materials.

Role of Multicellular Aggregates in Biofilm Formation

In traditional models ofin vitrobiofilm development, individual bacterial cells seed a surface, multiply, and mature into multicellular, three-dimensional structures. Much research has been devoted to elucidating the mechanisms governing the initial attachment of single cells to surfaces. However, in natural environments and during infection, bacterial cells tend to clump as multicellular aggregates, and biofilms can also slough off aggregates as a part of the dispersal process. This makes it likely that biofilms are often seeded by aggregates and single cells, yet how these aggregates impact biofilm initiation and development is not known. Here we use a combination of experimental and computational approaches to determine the relative fitness of single cells and preformed aggregates during early development ofPseudomonas aeruginosabiofilms. We find that the relative fitness of aggregates depends markedly on the density of surrounding single cells, i.e., the level of competition for growth resources. When competition between aggregates and single cells is low, an aggregate has a growth disadvantage because the aggregate interior has poor access to growth resources. However, if competition is high, aggregates exhibit higher fitness, because extending vertically above the surface gives cells at the top of aggregates better access to growth resources. Other advantages of seeding by aggregates, such as earlier switching to a biofilm-like phenotype and enhanced resilience toward antibiotics and immune response, may add to this ecological benefit. Our findings suggest that current models of biofilm formation should be reconsidered to incorporate the role of aggregates in biofilm initiation.

IMPORTANCE

During the past decades, there has been a consensus around the model of development of a biofilm, involving attachment of single planktonic bacterial cells to a surface and the subsequent development of a mature biofilm. This study presents results that call for a modification of this rigorous model. We show how free floating biofilm aggregates can have a profound local effect on biofilm development when attaching to a surface. Our findings show that an aggregate landing on a surface will eventually outcompete the biofilm population arising from single cells attached around the aggregate and dominate the local biofilm development. These results point to a regime where preformed biofilm aggregates may have a fitness advantage over planktonic cells when it comes to accessing nutrients. Our findings add to the increasingly prominent comprehension that biofilm lifestyle is the default for bacteria and that planktonic single cells may be only a transition state at the most.

Development of a rechargeable optical hydrogen peroxide sensor – sensor design and biological application

Measure and recharge!A reversible sensor concept enables repetitive and quantitative measurement of H₂O₂with high spatial and temporal resolution in environmental and biomedical samples.

Increased bactericidal activity of colistin on Pseudomonas aeruginosa biofilms in anaerobic conditions

Tolerance towards antibiotics of Pseudomonas aeruginosa biofilms is recognized as a major cause of therapeutic failure of chronic lung infection in cystic fibrosis (CF) patients. This lung infection is characterized by antibiotic-tolerant biofilms in mucus with zones of O2 depletion mainly due to polymorphonuclear leukocytic activity. In contrast to the main types of bactericidal antibiotics, it has not been possible to establish an association between the bactericidal effects of colistin and the production of detectable levels of OH ˙ on several strains of planktonic P. aeruginosa. Therefore, we propose that production of OH ˙ may not contribute significantly to the bactericidal activity of colistin on P. aeruginosa biofilm. Thus, we investigated the effect of colistin treatment on biofilm of wild-type PAO1, a catalase-deficient mutant (ΔkatA) and a colistin-resistant CF isolate cultured in microtiter plates in normoxic- or anoxic atmosphere with 1 mM nitrate. The killing of bacteria during colistin treatment was measured by CFU counts, and the OH⋅ formation was measured by 3(')-(p-hydroxylphenyl fluorescein) fluorescein (HPF) fluorescence. Validation of the assay was done by hydrogen peroxide treatment. OH⋅ formation was undetectable in aerobic PAO1 biofilms during 3 h of colistin treatment. Interestingly, we demonstrate increased susceptibility of P. aeruginosa biofilms towards colistin during anaerobic conditions. In fact, the maximum enhancement of killing by anaerobic conditions exceeded 2 logs using 4 mg L(-1) of colistin compared to killing at aerobic conditions.

Low efficacy of tobramycin in experimental Staphylococcus aureus endocarditis

The empiric treatment of infective endocarditis (IE) varies widely and, in some places, a regimen of penicillin in combination with an aminoglycoside is administered. The increasing incidence of Staphylococcus aureus IE, poor tissue penetration by aminoglycosides and low frequency of penicillin-susceptible S. aureus may potentially lead to functional tobramycin monotherapy. Therefore, this study aimed to evaluate tobramycin monotherapy in an experimental S. aureus IE rat model. Catheter-induced IE at the aortic valves were established with S. aureus (NCTC 8325-4) and rats were randomised into untreated (n = 22) or tobramycin-treated (n = 13) groups. The treatment group received tobramycin once-daily. Animals were evaluated at 1 day post infection (DPI), 2 DPI or 3 DPI. Quantitative bacteriology and cytokine expression were measured for valves, myocardium and serum. A decrease of bacterial load was observed in valves and the spleens of the treated (n = 6) compared to the untreated group at 2 DPI (n = 8) (p ≤ 0.02 and p ≤ 0.01, respectively), but not at 3 DPI (n = 7). Quantitative bacteriology in the myocardium was not different between the groups. Keratinocyte-derived chemokine (KC) in the aortic valves was significantly reduced at 2 DPI in the tobramycin-treated group (p ≤ 0.03). However, the expression of interleukin (IL)-1b, IL-6 and granulocyte-colony stimulating factor (G-CSF) in the valves was not different between the two groups. In the myocardium, a significant reduction in IL-1b was observed at 2 DPI (p ≤ 0.001) but not at 3 DPI. Tobramycin as functional monotherapy only reduced bacterial load and inflammation transiently, and was insufficient in most cases of S. aureus IE.

Anti-Pseudomonas aeruginosa IgY Antibodies Induce Specific Bacterial Aggregation and Internalization in Human Polymorphonuclear Neutrophils

Polymorphonuclear neutrophils (PMNs) are essential cellular constituents in the innate host response, and their recruitment to the lungs and subsequent ubiquitous phagocytosis controls primary respiratory infection. Cystic fibrosis pulmonary disease is characterized by progressive pulmonary decline governed by a persistent, exaggerated inflammatory response dominated by PMNs. The principal contributor is chronic Pseudomonas aeruginosa biofilm infection, which attracts and activates PMNs and thereby is responsible for the continuing inflammation. Strategies to prevent initial airway colonization with P. aeruginosa by augmenting the phagocytic competence of PMNs may postpone the deteriorating chronic biofilm infection. Anti-P. aeruginosa IgY antibodies significantly increase the PMN-mediated respiratory burst and subsequent bacterial killing of P. aeruginosa in vitro. The mode of action is attributed to IgY-facilitated formation of immobilized bacteria in aggregates, as visualized by fluorescence microscopy and the induction of increased bacterial hydrophobicity. Thus, the present study demonstrates that avian egg yolk immunoglobulins (IgY) targeting P. aeruginosa modify bacterial fitness, which enhances bacterial killing by PMN-mediated phagocytosis and thereby may facilitate a rapid bacterial clearance in airways of people with cystic fibrosis.

Exhaled breath analysis using electronic nose in cystic fibrosis and primary ciliary dyskinesia patients with chronic pulmonary infections

The current diagnostic work-up and monitoring of pulmonary infections may be perceived as invasive, is time consuming and expensive. In this explorative study, we investigated whether or not a non-invasive exhaled breath analysis using an electronic nose would discriminate between cystic fibrosis (CF) and primary ciliary dyskinesia (PCD) with or without various well characterized chronic pulmonary infections. We recruited 64 patients with CF and 21 with PCD based on known chronic infection status. 21 healthy volunteers served as controls. An electronic nose was employed to analyze exhaled breath samples. Principal component reduction and discriminant analysis were used to construct internally cross-validated receiver operator characteristic (ROC) curves. Breath profiles of CF and PCD patients differed significantly from healthy controls p = 0.001 and p = 0.005, respectively. Profiles of CF patients having a chronic P. aeruginosa infection differed significantly from to non-chronically infected CF patients p = 0.044. We confirmed the previously established discriminative power of exhaled breath analysis in separation between healthy subjects and patients with CF or PCD. Furthermore, this method significantly discriminates CF patients suffering from a chronic pulmonary P. aeruginosa (PA) infection from CF patients without a chronic pulmonary infection. Further studies are needed for verification and to investigate the role of electronic nose technology in the very early diagnostic workup of pulmonary infections before the establishment of a chronic infection.

Physiological levels of nitrate support anoxic growth by denitrification of Pseudomonas aeruginosa at growth rates reported in cystic fibrosis lungs and sputum

Chronic Pseudomonas aeruginosa lung infection is the most severe complication in patients with cystic fibrosis (CF). The infection is characterized by the formation of biofilm surrounded by numerous polymorphonuclear leukocytes (PMNs) and strong O2 depletion in the endobronchial mucus. We have reported that O2 is mainly consumed by the activated PMNs, while O2 consumption by aerobic respiration is diminutive and nitrous oxide (N2O) is produced in infected CF sputum. This suggests that the reported growth rates of P. aeruginosa in lungs and sputum may result from anaerobic respiration using denitrification. The growth rate of P. aeruginosa achieved by denitrification at physiological levels (~400 μM) of nitrate (NO(-) 3) is however, not known. Therefore, we have measured growth rates of anoxic cultures of PAO1 and clinical isolates (n = 12) in LB media supplemented with NO(-) 3 and found a significant increase of growth when supplementing PAO1 and clinical isolates with ≥150 μM NO(-) 3 and 100 μM NO(-) 3, respectively. An essential contribution to growth by denitrification was demonstrated by the inability to establish a significantly increased growth rate by a denitrification deficient ΔnirS-N mutant at <1 mM of NO(-) 3. Activation of denitrification could be achieved by supplementation with as little as 62.5 μM of NO(-) 3 according to the significant production of N2O by the nitrous oxide reductase deficient ΔnosZ mutant. Studies of the promoter activity, gene transcripts, and enzyme activity of the four N-oxide reductases in PAO1 (Nar, Nir, Nor, Nos) further verified the engagement of denitrification, showing a transient increase in activation and expression and rapid consumption of NO(-) 3 followed by a transient increase of NO(-) 2. Growth rates obtained by denitrification in this study were comparable to our reported growth rates in the majority of P. aeruginosa cells in CF lungs and sputum. Thus, we have demonstrated that denitrification is required for P. aeruginosa growth in infected endobronchial CF mucus.

Nitric oxide production by polymorphonuclear leucocytes in infected cystic fibrosis sputum consumes oxygen

Chronic Pseudomonas aeruginosa lung infection in cystic fibrosis (CF) patients is characterized by persisting mucoid biofilms in hypoxic endobronchial mucus. These biofilms are surrounded by numerous polymorphonuclear leucocytes (PMNs), which consume a major part of present molecular oxygen (O(2)) due to production of superoxide (O(2)(-)). In this study, we show that the PMNs also consume O(2) for production of nitric oxide (NO) by the nitric oxide synthases (NOS) in the infected endobronchial mucus. Fresh expectorated sputum samples (n = 28) from chronically infected CF patients (n = 22) were analysed by quantifying and visualizing the NO production. NO production was detected by optode measurements combined with fluorescence microscopy, flow cytometry and spectrophotometry. Inhibition of nitric oxide synthases (NOS) with N(G) -monomethyl-L-arginine (L-NMMA) resulted in reduced O(2) consumption (P < 0·0008, n = 8) and a lower fraction of cells with fluorescence from the NO-indicator 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) (P < 0·002, n = 8). PMNs stained with DAF-FM and the superoxide indicator hydroethidine (HE) and host cells with inducible NOS (iNOS) were identified in the sputum. In addition, the production of the stable end-products of NO in CF sputum was correlated with the concentration of PMNs; NO(3)(-) (P < 0·04, r = 0·66, n = 10) and NO(2)(-) (P< 0·006, r = 0·78, n = 11). The present study suggests that besides consumption of O(2) for production of reactive oxygen species, the PMNs in CF sputum also consume O(2) for production of NO.

Formation of hydroxyl radicals contributes to the bactericidal activity of ciprofloxacin against Pseudomonas aeruginosa biofilms

Antibiotic-tolerant, biofilm-forming Pseudomonas aeruginosa has long been recognized as a major cause of chronic lung infections of cystic fibrosis patients. The mechanisms involved in the activity of antibiotics on biofilm are not completely clear. We have investigated whether the proposed induction of cytotoxic hydroxyl radicals (OH˙) during antibiotic treatment of planktonically grown cells may contribute to action of the commonly used antibiotic ciprofloxacin on P. aeruginosa biofilms. For this purpose, WT PAO1, a catalase deficient ΔkatA and a ciprofloxacin resistant mutant of PAO1 (gyrA), were grown as biofilms in microtiter plates and treated with ciprofloxacin. Formation of OH˙ and total amount of reactive oxygen species (ROS) was measured and viability was estimated. Formation of OH˙ and total ROS in PAO1 biofilms treated with ciprofloxacin was shown but higher levels were measured in ΔkatA biofilms, and no ROS production was seen in the gyrA biofilms. Treatment with ciprofloxacin decreased the viability of PAO1 and ΔkatA biofilms but not of gyrA biofilms. Addition of thiourea, a OH˙ scavenger, decreased the OH˙ levels and killing of PAO1 biofilm. Our study shows that OH˙ is produced by P. aeruginosa biofilms treated with ciprofloxacin, which may contribute to the killing of biofilm subpopulations.

Bead-size directed distribution of Pseudomonas aeruginosa results in distinct inflammatory response in a mouse model of chronic lung infection

Chronic Pseudomonas aeruginosa lung infection in cystic fibrosis (CF) patients is characterized by biofilms, tolerant to antibiotics and host responses. Instead, immune responses contribute to the tissue damage. However, this may depend on localization of infection in the upper conductive or in the peripheral respiratory zone. To study this we produced two distinct sizes of small alginate beads (SB) and large beads (LB) containing P. aeruginosa. In total, 175 BALB/c mice were infected with either SB or LB. At day 1 the quantitative bacteriology was higher in the SB group compared to the LB group (P < 0·003). For all time-points smaller biofilms were identified by Alcian blue staining in the SB group (P < 0·003). Similarly, the area of the airways in which biofilms were identified were smaller (P < 0·0001). A shift from exclusively endobronchial to both parenchymal and endobronchial localization of inflammation from day 1 to days 2/3 (P < 0·05), as well as a faster resolution of inflammation at days 5/6, was observed in the SB group (P < 0·03). Finally, both the polymorphonuclear neutrophil leucocyte (PMN) mobilizer granulocyte colony-stimulating factor (G-CSF) and chemoattractant macrophage inflammatory protein-2 (MIP-2) were increased at day 1 in the SB group (P < 0·0001). In conclusion, we have established a model enabling studies of host responses in different pulmonary zones. An effective recognition of and a more pronounced host response to infection in the peripheral zones, indicating that increased lung damage was demonstrated. Therefore, treatment of the chronic P. aeruginosa lung infection should be directed primarily at the peripheral lung zone by combined intravenous and inhalation antibiotic treatment.

Poor antioxidant status exacerbates oxidative stress and inflammatory response to Pseudomonas aeruginosa lung infection in guinea pigs

Considerable evidence supports the presence of oxidative stress in cystic fibrosis (CF). The disease has long been associated with both increased production of reactive oxygen species and impaired antioxidant status, in particular during the chronic pulmonary infection with Pseudomonas aeruginosa, which is the main cause of morbidity and mortality in CF. Guinea pigs are unable to synthesize ascorbate (ASC) or vitamin C, a major antioxidant of the lung, and thus like human beings rely on its presence in the diet. On this basis, guinea pigs receiving ASC-deficient diet have been used as a model of oxidative stress. The aim of our study was to investigate the consequences of a 7-day biofilm-grown P. aeruginosa lung infection in 3-month-old guinea pigs receiving either ASC-sufficient or ASC-deficient diet for at least 2 months. The animals receiving ASC-deficient diet showed significantly higher mortality during infection and increased respiratory burst of peripheral polymorphonuclear neutrophils (PMNs) compared with the animals receiving ASC sufficient diet. The inflammatory response at the site of lung infection consisted of PMNs and mononuclear leucocytes (MN), and higher PMN/MN ratios were present in animals on ASC-deficient diet compared with animals on ASC sufficient diet. Measurements of the ASC levels in the lung were significantly decreased in infected compared with non-infected animals. Interestingly, the infection by itself decreased the antioxidant capacity of the plasma (measured as plasma oxidizability) more than the ASC-deficient diet, suggesting a high consumption of the antioxidants during infection. Our data show that poor antioxidant status exacerbates the outcome of biofilm-related infections.

Lactate in cystic fibrosis sputum

BACKGROUND

Antibiotic therapy is thought to improve lung function in patients with cystic fibrosis (CF) by decreasing neutrophil-derived inflammation. We investigated the origin and clinical significance of lactate in the chronically inflamed CF lung.

METHODS

Lactate was measured in sputa of 18 exacerbated and 25 stable CF patients via spectrophotometry and gaschromatography. Lung function was assessed via spirometry. Seven patients with chronic obstructive pulmonary disease (COPD) and three patients with acute lung inflammation served as control groups. Neutrophil and bacterial lactate production was assessed under aerobic and anaerobic conditions.

RESULTS

In sputum specimens of patients with respiratory exacerbations lactate concentrations decreased significantly (p<0.005) from 3.4±2.3mmol/L to 1.4±1.4mmol/L after 2-3 weeks of intravenous antibiotics. Successful treatment was reflected in 16 patients (88.9%) by FVC increase associated with lactate decrease (p<0.05). In every single sputum lactate was detectable (3.0±3.1mmol/L, range 0.2-14.1mmol/L). Lactate was lower (1.6±0.8mmol/L) in sputa from seven COPD patients, and it was below the detection limit in three patients with acute lung inflammation. Neutrophil lactate production accumulated up to 10.5mmol/L after 4 days, whereas bacterial lactate production did not appear to contribute substantially to sputum lactate concentrations.

CONCLUSIONS

Successful antibiotic therapy is reflected by a decrease in lactate concentrations. Neutrophils are the most likely source for lactate in sputum of CF patients. Therefore lactate may be used to monitor responses to antibiotic therapy as an adjunct to lung function measurements.

Thermal injury induces impaired function in polymorphonuclear neutrophil granulocytes and reduced control of burn wound infection

Severe thermal injury induces immunosuppression, involving all parts of the immune system, especially when large fractions of the total body surface area are affected. An animal model was established to characterize the burn-induced immunosuppression. In our novel mouse model a 6% third-degree burn injury was induced in mice with a hot-air blower. The third-degree burn was confirmed histologically. The mice were allocated into five groups: control, shave, burn, infection and burn infection group. At 48 h, a decline in the concentration of peripheral blood leucocytes was observed in the group of mice with burn wound. The reduction was ascribed to the decline in concentration of polymorphonuclear neutrophil leucocytes and monocytes. When infecting the skin with Pseudomonas aeruginosa, a dissemination of bacteria was observed only in the burn wound group. Histological characterization of the skin showed a more polymorphonuclear neutrophil granulocytes (PMNs)-dominated inflammation in the group of mice with infected burn wound compared with the with burn wound group. In contrast, a higher degree of inflammation was observed in the burn wound group compared with the group of mice with infected burn wound. Furthermore, the oxidative burst and the phagocytic capacity of the PMNs were reduced in the group of mice with burn wound. Using this novel mouse model of thermal injury a decline of peripheral leucocytes was observed, whereas the increased local inflammatory response at the site of infection showed reduced capacity to contain and eliminate the infection.

Flow cytometric analysis of RNA synthesis by detection of bromouridine incorporation

RNA synthesis has traditionally been investigated by a laborious and time-consuming radiographic method involving incorporation of tritiated uridine. Now a faster non-radioactive alternative has emerged, based on immunocytochemical detection. This method utilizes the brominated RNA precursor bromouridine, which is taken into a cell, phosphorylated, and incorporated into nascent RNA. The BrU-substituted RNA is detected by permeabilizing the cells and staining with certain anti-BrdU antibodies. This dynamic approach yields information complementing that provided by cellular RNA content analysis at a given time and may be of value in studies of cellular activation and gene expression.

Rapid necrotic killing of polymorphonuclear leukocytes is caused by quorum-sensing-controlled production of rhamnolipid by Pseudomonas aeruginosa

Quorum sensing (QS) denotes a density-dependent mode of inter-bacterial communication based on signal transmitter molecules. Active QS is present during chronic infections with the opportunistic pathogen Pseudomonas aeruginosa in immunocompromised patients. The authors have previously demonstrated a QS-regulated tolerance of biofilm bacteria to the antimicrobial properties of polymorphonuclear leukocytes (PMNs). The precise QS-regulated effect on the PMNs is, however, unknown. Incubation of human PMNs with supernatants from dense P. aeruginosa cultures showed that the QS-competent P. aeruginosa induced rapid necrosis of the PMNs. This mechanism was also observed in mouse lungs infected with P. aeruginosa, and in sputum obtained from P.-aeruginosa-infected patients with cystic fibrosis. Evidence is presented that the necrotic effect was caused by rhamnolipids, production of which is QS controlled. The results demonstrate the potential of the QS system to facilitate infections with P. aeruginosa by disabling the PMNs, which are a major first line of defence of the host. Furthermore, the study emphasizes the inhibition of QS as a target for the treatment of infections with P. aeruginosa.

[Foreign body infections--biofilms and quorum sensing]

Biofilms are structured consortia of bacteria embedded in self-produced polymer matrix. Biofilms are resistant to antibiotics, disinfectives and phagocytosis. The persistence of foreign body infections is due to biofilms. Chronic P. aeruginosa lung infection in cystic fibrosis patients is a biofilm. Bacteria in biofilms communicate by means of quorum sensing which activates genes for virulence factors. Biofilms can be prevented by antibiotic prophylaxis or early therapy or by quorum sensing inhibitors which make them susceptible to antibiotics and phagocytosis.

Impact of Pseudomonas aeruginosa quorum sensing on biofilm persistence in an in vivo intraperitoneal foreign-body infection model

Pseudomonas aeruginosa is an opportunistic human pathogen that causes chronic biofilm-based infections in host organisms. P. aeruginosa employs quorum sensing (QS) to control expression of its virulence, and to establish and maintain chronic infections. Under such conditions, the biofilm mode of growth contributes significantly to P. aeruginosa tolerance to the action of the innate and adaptive defence system and numerous antibiotics. In the present study, an in vivo foreign-body infection model was established in the peritoneal cavity of mice. Experimental data showed that QS-deficient P. aeruginosa are cleared more rapidly from silicone implants as compared to their wild-type counterparts. Concurrently, treatment with the QS inhibitor furanone C-30 of mice harbouring implants colonized with the wild-type P. aeruginosa resulted in a significantly faster clearing of the implants as compared to the placebo-treated group. These results were obtained with both an inbred (BALB/c) and an outbred (NMRI) mouse strain. The present results support a model by which functional QS systems play a pivotal role in the ability of bacteria to resist clearing by the innate immune system and strongly suggest that the efficiency of the mouse innate defence against biofilm-forming P. aeruginosa is improved when the bacteria are treated with QS drugs that induce QS deficiency.

Serum concentrations of GM-CSF and G-CSF correlate with the Th1/Th2 cytokine response in cystic fibrosis patients with chronic Pseudomonas aeruginosa lung infection

The inflammation in cystic fibrosis (CF) patients with chronic Pseudomonas aeruginosa lung infection is dominated by polymorphonuclear neutrophils (PMNs). There seems to be a relationship between the PMN-dominated inflammation, pronounced antibody production and a Th2-dominated response. Apart from mobilizing monocytes and PMNs from the bone marrow, GM-CSF, G-CSF and IL-3 select subsets of dendritic cells, which subsequently induce distinct Th responses. Therefore, the present study examines the correlation between the mobilizing cytokines in serum and the Th responses. The IFN-gamma and IL-4 production by peripheral blood mononuclear cells, and the concentrations of GM-CSF and G-CSF in serum as well as lung function, were determined in 37 CF patients with and 6 CF patients without chronic P. aeruginosa lung infection. The GM-CSF/G-CSF ratio correlated both with the IFN-gamma production and good lung function. In addition, an inverse correlation between IL-3 and IFN-gamma was observed. The results indicate involvement of endogenous GM-CSF, G-CSF and IL-3 in the skewed Th response in CF, and change to a Th1-dominated response might be achieved with GM-CSF treatment.

Faster activation of polymorphonuclear neutrophils in resistant mice during early innate response to Pseudomonas aeruginosa lung infection

Polymorphonuclear neutrophils (PMNs) are crucial for the outcome of Pseudomonas aeruginosa lung infection in patients with cystic fibrosis. We compared PMNs and inflammatory cytokines in the lungs and blood from susceptible BALB/c and resistant C3H/HeN mice 1 and 2 days after intratracheal challenge with alginate embedded P. aeruginosa. These parameters were correlated with the quantitative bacteriology and histopathology of the lungs. After challenge, the content of granulocyte colony-stimulating factor (G-CSF) and macrophage inflammatory protein-2 (MIP-2) was increased in the lungs and the sera and the percentage of PMNs was increased in the blood. However, 2 days after challenge the concentration of G-CSF and MIP-2 was higher in the lungs and sera of BALB/c mice. CD11b expression was higher on the PMNs of the C3H/HeN mice. The expression of CD62L on PMNs of both strains of mice was decreased 1 day after bacterial challenge, whereas the expression was increased after 2 days of challenge on PMNs of C3H/HeN mice only. These changes were accompanied by a more severe lung inflammation in BALB/c mice and faster clearance of the bacteria in C3H/HeN mice. In conclusion, the rapid early bacterial clearance in the lungs of C3H/HeN mice could be explained by faster activation of the PMNs, as indicated by the higher up-regulation of CD11b. The severe lung inflammation in BALB/c mice may be caused by the early higher content of G-CSF in the sera mobilizing PMNs from the bone marrow and the persistent chemotactic gradient provided by MIP-2 in the lungs.

A highly selective CCR2 chemokine agonist encoded by human herpesvirus 6

The chemokine-like, secreted protein product of the U83 gene from human herpesvirus 6, here named vCCL4, was chemically synthesized to be characterized in a complete library of the 18 known human chemokine receptors expressed individually in stably transfected cell lines. vCCL4 was found to cause calcium mobilization as efficiently as the endogenous chemokine ligand CCL2 through the CCR2 receptor, whereas the virally encoded chemokine did not affect any of the other 17 human chemokine receptors tested. Mutual cross-desensitization between CCL2 and vCCL4 was demonstrated in the CCR2-transfected cells. The affinity of vCCL4 for the CCR2 receptor was 79 nm as determined in competition binding against radioactively labeled CCL2. In the murine pre-B lymphocyte cell line L1.2 stably transfected with the CCR2 receptor, vCCL4 acted as a relatively low potency but highly efficacious chemoattractant being equally or more efficacious in causing cell migration than CCL2 and CCL7 and considerably more efficacious than CCL8 and CCL13. It is concluded that human herpesvirus 6 encodes a highly selective and efficacious CCR2 agonist, which will attract CCR2 expressing cells, for example macrophages and monocytes, conceivably for the virus to infect and to establish latency in. It is suggested that vCCL4 during reactivation of the virus in for example monocyte-derived microglia could perhaps be involved in the pathogenesis of the CCR2-dependent disease, multiple sclerosis.