Effect of polysaccharide interactions on antibiotic susceptibility of Pseudomonas aeruginosa

Staphylococcus aureus persisters are associated with reduced clearance in a catheter-associated biofilm infection

Background

Staphylococcus aureus causes a wide variety of infections, many of which are chronic or relapsing in nature. Antibiotic therapy is often ineffective against S. aureus biofilm-mediated infections. Biofilms are difficult to treat partly due to their tolerance to antibiotics, however the underlying mechanism responsible for this remains unknown. One possible explanation is the presence of persister cells-dormant-like cells that exhibit tolerance to antibiotics. Recent studies have shown a connection between a fumC (fumarase C, a gene in the tricarboxylic acid cycle) knockout strain and increased survival to antibiotics, antimicrobial peptides, and in a Drosophila melanogaster model.

Objective

It remained unclear whether a S. aureus high persister strain would have a survival advantage in the presence of innate and adaptive immunity. To further investigate this, a fumC knockout and wild type strains were examined in a murine catheter-associated biofilm model.

Results

Interestingly, mice struggled to clear both S. aureus wild type and the fumC knockout strains. We reasoned both biofilm-mediated infections predominantly consisted of persister cells. To determine the persister cell population within biofilms, expression of a persister cell marker (Pcap5A::dsRED) in a biofilm was examined. Cell sorting of biofilms challenged with antibiotics revealed cells with intermediate and high expression of cap5A had 5.9-and 4.5-fold higher percent survival compared to cells with low cap5A expression. Based on previous findings that persisters are associated with reduced membrane potential, flow cytometry analysis was used to examine the metabolic state of cells within a biofilm. We confirmed cells within biofilms had reduced membrane potential compared to both stationary phase cultures (2.5-fold) and exponential phase cultures (22.4-fold). Supporting these findings, cells within a biofilm still exhibited tolerance to antibiotic challenge following dispersal of the matrix through proteinase K.

Conclusion

Collectively, these data show that biofilms are largely comprised of persister cells, and this may explain why biofilm infections are often chronic and/or relapsing in clinical settings.

Resistance evolution can disrupt antibiotic exposure protection through competitive exclusion of the protective species

Antibiotic degrading bacteria can reduce the efficacy of drug treatments by providing antibiotic exposure protection to pathogens. While this has been demonstrated at the ecological timescale, it is unclear how exposure protection might alter and be affected by pathogen antibiotic resistance evolution. Here, we utilised a two-species model cystic fibrosis (CF) community where we evolved the bacterial pathogen Pseudomonas aeruginosa in a range of imipenem concentrations in the absence or presence of Stenotrophomonas maltophilia, which can detoxify the environment by hydrolysing β-lactam antibiotics. We found that P. aeruginosa quickly evolved resistance to imipenem via parallel loss of function mutations in the oprD porin gene. While the level of resistance did not differ between mono- and co-culture treatments, the presence of S. maltophilia increased the rate of imipenem resistance evolution in the four μg/ml imipenem concentration. Unexpectedly, imipenem resistance evolution coincided with the extinction of S. maltophilia due to increased production of pyocyanin, which was cytotoxic to S. maltophilia. Together, our results show that pathogen resistance evolution can disrupt antibiotic exposure protection due to competitive exclusion of the protective species. Such eco-evolutionary feedbacks may help explain changes in the relative abundance of bacterial species within CF communities despite intrinsic resistance to anti-pseudomonal drugs.

Depth distributions of signaling molecules in Pseudomonas aeruginosa biofilms mapped by confocal Raman microscopy

Pseudomonas aeruginosa is an opportunistic human pathogen implicated in both acute and chronic diseases, which resists antibiotic treatment, in part by forming physical and chemical barriers such as biofilms. Here, we explore the use of confocal Raman imaging to characterize the three-dimensional (3D) spatial distribution of alkyl quinolones (AQs) in P. aeruginosa biofilms by reconstructing depth profiles from hyperspectral Raman data. AQs are important to quorum sensing (QS), virulence, and other actions of P. aeruginosa. Three-dimensional distributions of three different AQs (PQS, HQNO, and HHQ) were observed to have a significant depth, suggesting 3D anisotropic shapes-sheet-like rectangular solids for HQNO and extended cylinders for PQS. Similar to observations from 2D imaging studies, spectral features characteristic of AQs (HQNO or PQS) and the amide I vibration from peptide-containing species were found to correlate with the PQS cylinders typically located at the tips of the HQNO rectangular solids. In the QS-deficient mutant lasIrhlI, a small globular component was observed, whose highly localized nature and similarity in size to a P. aeruginosa cell suggest that the feature arises from HHQ localized in the vicinity of the cell from which it was secreted. The difference in the shapes and sizes of the aggregates of the three AQs in wild-type and mutant P. aeruginosa is likely related to the difference in the cellular response to growth conditions, environmental stress, metabolic levels, or other structural and biochemical variations inside biofilms. This study provides a new route to characterizing the 3D structure of biofilms and shows the potential of confocal Raman imaging to elucidate the nature of heterogeneous biofilms in all three spatial dimensions. These capabilities should be applicable as a tool in studies of infectious diseases.

Antimicrobial Treatment Provides a Competitive Advantage to Mycobacterium abscessus in a Dual-Species Biofilm with Pseudomonas aeruginosa

The physiological factors that contribute to Mycobacterium abscessus lung infections remain unclear. We determined whether antibiotic treatment targeting a major cystic fibrosis pathogen (i.e., Pseudomonas aeruginosa) could provide the ideal conditions for the establishment of M. abscessus infection. Our data showed that P. aeruginosa inhibited M. abscessus biofilm formation under control conditions and that antimicrobial therapy selectively targeting P. aeruginosa diminished this competitive interaction, thereby increasing M. abscessus survival.

Influence of the lung microbiome on antibiotic susceptibility of cystic fibrosis pathogens

The lungs of patients with cystic fibrosis (CF) are colonised by a microbial community comprised of pathogenic species, such as Pseudomonas aeruginosa and Staphylococcus aureus, and microorganisms that are typically not associated with worse clinical outcomes (considered as commensals). Antibiotics directed at CF pathogens are often not effective and a discrepancy is observed between activity of these agents in vitro and in the patient. This review describes how interspecies interactions within the lung microbiome might influence the outcome of antibiotic treatment targeted at common CF pathogens. Protective mechanisms by members of the microbiome such as antibiotic degradation (indirect pathogenicity), alterations of the cell wall, production of matrix components decreasing antibiotic penetration, and changes in metabolism are discussed. Interspecies interactions that increase bacterial susceptibility are also addressed. Furthermore, we discuss how experimental conditions, such as culture media, oxygen levels, incorporation of host–pathogen interactions, and microbial community composition may influence the outcome of microbial interaction studies related to antibiotic activity. Hereby, the importance to create in vitro conditions reflective of the CF lung microenvironment is highlighted. Understanding the role of the CF lung microbiome in antibiotic efficacy may help find novel therapeutic and diagnostic approaches to better tackle chronic lung infections in this patient population.

Interspecies interactions in the lung microbiome may influence the outcome of antibiotic treatment targeted at cystic fibrosis pathogenshttp://bit.ly/2WQp0iQ

Surface-Growing Communities of Pseudomonas aeruginosa Exhibit Distinct Alkyl Quinolone Signatures

A cascade of events leads to the development of microbial biofilm communities that are thought to be responsible for over 80% of infections in humans. However, not all surface-growing bacteria reside in a stationary biofilm state. Here, we have employed confocal Raman microscopy to analyze and compare variations in the alkyl quinolone (AQ) family of molecules during the transition between surface-attached motile-swarming and stationary biofilm communities. The AQs have been established previously as important to Pseudomonas aeruginosa biofilms, interspecies competition, and virulence. The AQ Pseudomonas quinolone signal (PQS) is also a known quorum-sensing signal. We detail spatial identification of AQ, PQS, and 2-alkyl-4-hydroxyquinoline N-oxide (AQNO) metabolites in both swarm and biofilm communities. We find that AQNO metabolites are abundant signatures in active swarming communities.

Interaction of gentamicin sulfate with alginate and consequences on the physico-chemical properties of alginate-containing biofilms

BACKGROUND

Alginate is one of the main extracellular polymeric substances (EPS) in biofilms of Cystic Fibrosis (CF) patients suffering from pulmonary infections. Gentamicin sulfate (GS) can strongly bind to alginate resulting in loss of pharmacological activity; however neither the mechanism nor its repercussion is fully understood. In this study, we investigated how GS modifies the alginate macromolecular network and its microenvironment.

MATERIAL AND METHODS

Alginate gels of two different compositions (either enriched in guluronate units (G) or enriched in mannuronate units (M)) were crosslinked with Ca²⁺ and exposed to GS at varying times and concentrations. The complexes formed were characterized via turbidimetry, mechanical tests, swelling assay, calorimetry techniques, nuclear magnetic resonance, Ca²⁺ displacement, macromolecular probe diffusion and pH alteration.

RESULTS

In presence of GS, the alginate network and its environment undergo a tremendous reorganization in terms of gel density, stiffness, diffusion property, presence and state of the water molecules. We noted that the intensity of those alterations is directly dependent on the polysaccharide motif composition (ratio M/G).

CONCLUSION

Our results underline the importance of alginate as biofilm component, its pernicious role during antibiotherapy and could represent a potential macromolecular target to improve anti-infectious therapies.

Collective antibiotic tolerance: mechanisms, dynamics and intervention

Bacteria have developed resistance against every antibiotic at a rate that is alarming considering the timescale at which new antibiotics are developed. Thus, there is a critical need to use antibiotics more effectively, extend the shelf life of existing antibiotics and minimize their side effects. This requires understanding the mechanisms underlying bacterial drug responses. Past studies have focused on survival in the presence of antibiotics by individual cells, as genetic mutants or persisters. Also important, however, is the fact that a population of bacterial cells can collectively survive antibiotic treatments lethal to individual cells. This tolerance can arise by diverse mechanisms, including resistance-conferring enzyme production, titration-mediated bistable growth inhibition, swarming and interpopulation interactions. These strategies can enable rapid population recovery after antibiotic treatment and provide a time window during which otherwise susceptible bacteria can acquire inheritable genetic resistance. Here, we emphasize the potential for targeting collective antibiotic tolerance behaviors as an antibacterial treatment strategy.

Biofilm-associated persistence of food-borne pathogens

Microbial life abounds on surfaces in both natural and industrial environments, one of which is the food industry. A solid substrate, water and some nutrients are sufficient to allow the construction of a microbial fortress, a so-called biofilm. Survival strategies developed by these surface-associated ecosystems are beginning to be deciphered in the context of rudimentary laboratory biofilms. Gelatinous organic matrices consisting of complex mixtures of self-produced biopolymers ensure the cohesion of these biological structures and contribute to their resistance and persistence. Moreover, far from being just simple three-dimensional assemblies of identical cells, biofilms are composed of heterogeneous sub-populations with distinctive behaviours that contribute to their global ecological success. In the clinical field, biofilm-associated infections (BAI) are known to trigger chronic infections that require dedicated therapies. A similar belief emerging in the food industry, where biofilm tolerance to environmental stresses, including cleaning and disinfection/sanitation, can result in the persistence of bacterial pathogens and the recurrent cross-contamination of food products. The present review focuses on the principal mechanisms involved in the formation of biofilms of food-borne pathogens, where biofilm behaviour is driven by its three-dimensional heterogeneity and by species interactions within these biostructures, and we look at some emergent control strategies.

Development and Control of Bacterial Biofilms on Dairy Processing Membranes

Membrane fouling is a major operational problem that leads to reduced membrane performance and premature replacement of membranes. Bacterial biofilms developed on reverse osmosis membranes can cause severe flux declines during whey processing. Various types of biological, physical, and chemical factors regulate the formation of biofilms. Extracellular polymeric substances produced by constitutive microflora provide an effective barrier for the embedded cells. Cultural and microscopic techniques also revealed the presence of biofilms with attached bacterial cells on membrane surfaces. Presence of biofilms, despite regular cleaning processes, reflects ineffectiveness of cleaning agents. Cleaning efficiency depends upon factors such as pH of the cleaning agent, temperature, pressure, cleaning agent dose, optimum cleaning time, and cross-flow velocity during cleaning. Among different cleaning agents, surfactants help to prevent bacterial attachment to surfaces by reducing the surface tension of water and interfacial tension between the layers. Enzymes mixed with surfactants and chelating agents can be used to penetrate the biofilm matrix formed by microbes. Recent studies have shown the role of quorum-sensing-based cell-to-cell signaling, which provides communication within bacterial cells to form a mature biofilm, and also the role of applying quorum inhibitors to prevent biofilm formation. Major cleaning applications are also summarized in Table .

Interaction between extracellular lipase LipA and the polysaccharide alginate of Pseudomonas aeruginosa

Background

As an opportunistic human pathogen Pseudomonas aeruginosa is able to cause acute and chronic infections. The biofilm mode of life significantly contributes to the growth and persistence of P. aeruginosa during an infection process and mediates the pathogenicity of the bacterium. Within a biofilm mucoid strains of P. aeruginosa simultaneously produce and secrete several hydrolytic enzymes and the extracellular polysaccharide alginate. The focus of the current study was the interaction between extracellular lipase LipA and alginate, which may be physiologically relevant in biofilms of mucoid P. aeruginosa.

Results

Fluorescence microscopy of mucoid P. aeruginosa biofilms were performed using fluorogenic lipase substrates. It showed a localization of the extracellular enzyme near the cells. A microtiter plate-based binding assay revealed that the polyanion alginate is able to bind LipA. A molecular modeling approach showed that this binding is structurally based on electrostatic interactions between negatively charged residues of alginate and positively charged amino acids of the protein localized opposite of the catalytic centre. Moreover, we showed that the presence of alginate protected the lipase activity by protection from heat inactivation and from degradation by the endogenous, extracellular protease elastase LasB. This effect was influenced by the chemical properties of the alginate molecules and was enhanced by the presence of O-acetyl groups in the alginate chain.

Conclusion

We demonstrate that the extracellular lipase LipA from P. aeruginosa interacts with the polysaccharide alginate in the self-produced extracellular biofilm matrix of P. aeruginosa via electrostatic interactions suggesting a role of this interaction for enzyme immobilization and accumulation within biofilms. This represents a physiological advantage for the cells. Especially in the biofilm lifestyle, the enzyme is retained near the cell surface, with the catalytic centre exposed towards the substrate and is protected from denaturation and proteolytic degradation.

Chlorine dioxide disinfection of single and dual species biofilms, detached biofilm and planktonic cells

Disinfection efficacy testing is usually done with planktonic cells or more recently, biofilms. While disinfectants are much less effective against biofilms compared to planktonic cells, questions regarding the disinfection tolerance of detached biofilm clusters remain largely unanswered. Burkholderia cepacia and Pseudomonas aeruginosa were grown in chemostats and biofilm tubing reactors, with the tubing reactor serving as a source of detached biofilm clusters. Chlorine dioxide susceptibility was assessed for B. cepacia and P. aeruginosa in these three sample types as monocultures and binary cultures. Similar doses of chlorine dioxide inactivated samples of chemostat and tubing reactor effluent and no statistically significant difference between the log(10) reductions was found. This contrasts with chlorine, shown previously to be generally less effective against detached biofilm particles. Biofilms were more tolerant and required chlorine dioxide doses ten times higher than chemostat and tubing reactor effluent samples. A second species was advantageous in all sample types and resulted in lower log(10) reductions when compared to the single species cultures, suggesting a beneficial interaction of the species.

Interference of Pseudomonas aeruginosa signalling and biofilm formation for infection control

Pseudomonas aeruginosa is the best described bacterium with regards to quorum sensing (QS), in vitro biofilm formation and the development of antibiotic tolerance. Biofilms composed of P. aeruginosa are thought to be the underlying cause of many chronic infections, including those in wounds and in the lungs of patients with cystic fibrosis. In this review, we provide an overview of the molecular mechanisms involved in QS, QS-enabled virulence, biofilm formation and biofilm-enabled antibiotic tolerance. We now have substantial knowledge of the multicellular behaviour of P. aeruginosa in vitro. A major task for the future is to investigate how such in vitro data correlate with the in vivo behaviour of P. aeruginosa, and how to treat chronic infections of this bacterium in patients.

A colorimetric microtiter plate method for assessment of phage effect on Pseudomonas aeruginosa biofilm

Bacteriophages have a potential in biofilm control. The aim of the study was to develop a method for selection of the most effective Pseudomonas aeruginosa phages for inhibition of biofilm formation and its eradication. The microtiter plate method is based on crystal violet staining and measuring of optical density.

In situ evidence for microdomains in the polymer matrix of bacterial microcolonies

Confocal laser scanning microscopy and fluorescent lectin-binding analyses (FLBA) were used to study the form, arrangement, and composition of exopolymeric substances (EPS) surrounding naturally occurring microcolonies in biofilms. FLBA, using multiple lectin staining and multichannel imaging, indicated that the EPS of many microcolonies exhibit distinct multiple binding regions. A common pattern in the microcolonies is a three zone arrangement with cell-associated, intercellular, and an outer layer of EPS covering the exterior of the colony. Differential binding of lectins suggests that there are differences in the glycoconjugate composition or their arrangement in the EPS of microcolonies. The combination of FLBA with fluorescent in situ hybridization (FISH) indicates that the colonies consist of the major groups, α- and β-Proteobacteria. It is suggested that the EPS arrangement observed provides a physical structuring mechanism that can segregate extracellular activities at the microscale.

Enhanced biofilm formation and increased resistance to antimicrobial agents and bacterial invasion are caused by synergistic interactions in multispecies biofilms

Most biofilms in their natural environments are likely to consist of consortia of species that influence each other in synergistic and antagonistic manners. However, few reports specifically address interactions within multispecies biofilms. In this study, 17 epiphytic bacterial strains, isolated from the surface of the marine alga Ulva australis, were screened for synergistic interactions within biofilms when present together in different combinations. Four isolates, Microbacterium phyllosphaerae, Shewanella japonica, Dokdonia donghaensis, and Acinetobacter lwoffii, were found to interact synergistically in biofilms formed in 96-well microtiter plates: biofilm biomass was observed to increase by >167% in biofilms formed by the four strains compared to biofilms composed of single strains. When exposed to the antibacterial agent hydrogen peroxide or tetracycline, the relative activity (exposed versus nonexposed biofilms) of the four-species biofilm was markedly higher than that in any of the single-species biofilms. Moreover, in biofilms established on glass surfaces in flow cells and subjected to invasion by the antibacterial protein-producing Pseudoalteromonas tunicata, the four-species biofilms resisted invasion to a greater extent than did the biofilms formed by the single species. Replacement of each strain by its cell-free culture supernatant suggested that synergy was dependent both on species-specific physical interactions between cells and on extracellular secreted factors or less specific interactions. In summary, our data strongly indicate that synergistic effects promote biofilm biomass and resistance of the biofilm to antimicrobial agents and bacterial invasion in multispecies biofilms.

Biofilm matrix of Candida albicans and Candida tropicalis: chemical composition and role in drug resistance

Matrix material was extracted from biofilms of Candida albicans and Candida tropicalis and analysed chemically. Both preparations contained carbohydrate, protein, hexosamine, phosphorus and uronic acid. However, the major component in C. albicans matrix was glucose (32 %), whereas in C. tropicalis matrix it was hexosamine (27 %). Biofilms of C. albicans were more easily detached from plastic surfaces by treatment with the enzyme lyticase (β-1,3-glucanase) than were those of C. tropicalis. Biofilms of C. albicans were also partially detached by treatment with proteinase K, chitinase, DNase I, or β-N-acetylglucosaminidase, whereas C. tropicalis biofilms were only affected by lipase type VII or chitinase. To investigate a possible role for the matrix in biofilm resistance to antifungal agents, biofilms of C. albicans were grown under conditions of continuous flow in a modified Robbins device (MRD). These biofilms produced more matrix material than those grown statically, and were significantly more resistant to amphotericin B. Biofilms of C. tropicalis synthesized large amounts of matrix material even when grown statically, and such biofilms were completely resistant to both amphotericin B and fluconazole. Mixed-species biofilms of C. albicans and a slime-producing strain of Staphylococcus epidermidis (RP62A), when grown statically or in the MRD, were also completely resistant to amphotericin B and fluconazole. Mixed-species biofilms of C. albicans and a slime-negative mutant of S. epidermidis (M7), on the other hand, were completely drug resistant only when grown under flow conditions. These results demonstrate that the matrix can make a significant contribution to drug resistance in Candida biofilms, especially under conditions similar to those found in catheter infections in vivo, and that the composition of the matrix material is an important determinant in resistance.

Alginate as an auxiliary bacterial membrane: binding of membrane-active peptides by polysaccharides*

The chronicity of Pseudomonas aeruginosa infections in cystic fibrosis (CF) patients is characterized by overproduction of the exopolysaccharide alginate, in which biofilm bacteria are embedded. Alginate apparently contributes to the antibiotic resistance of bacteria in this form by acting as a diffusion barrier to positively charged antimicrobial agents. We have been investigating cationic antimicrobial peptides (CAPs) (prototypic sequence: KKAAAXAAAAAXAAWAAXAAAKKKK-NH(2), where X is any of the 20 commonly occurring amino acids) that were originally designed as transmembrane mimetic peptides. Peptides of this group above a specific hydrophobicity threshold insert spontaneously into membranes and have antibacterial activity at micromolar concentrations. While investigating the molecular basis of biofilm resistance to peptides, we found that the anionic alginate polysaccharide induces conformational changes in the most hydrophobic of these peptides typically associated with insertion of such peptides into membrane environments [Chan et al., J. Biol. Chem. (2004) vol. 279, pp. 38749-38754]. Through a combination of experiments measuring release of the fluorescent dye calcein from phospholipid vesicles, peptide interactions with vesicles in the presence and absence of alginate, and affinity of peptides for alginate as a function of net peptide core hydrophobicity, we show here that alginate offers a microenvironment that provides a protective mechanism for the encased bacteria by both binding and promoting the self-association of the CAPs. The overall results indicate that hydrophilic alginate polymers contain a significant hydrophobic compartment, and behave as an 'auxiliary membrane' for bacteria, thus identifying a unique protective role for biofilm exopolysaccharide matrices.

Penetration of Candida biofilms by antifungal agents

A filter disk assay was used to investigate the penetration of antifungal agents through biofilms containing single and mixed-species biofilms containing Candida. Fluconazole permeated all single-species Candida biofilms more rapidly than flucytosine. The rates of diffusion of either drug through biofilms of three strains of Candida albicans were similar. However, the rates of drug diffusion through biofilms of C. glabrata or C. krusei were faster than those through biofilms of C. parapsilosis or C. tropicalis. In all cases, after 3 to 6 h the drug concentration at the distal edge of the biofilm was very high (many times the MIC). Nevertheless, drug penetration failed to produce complete killing of biofilm cells. These results indicate that poor antifungal penetration is not a major drug resistance mechanism for Candida biofilms. The abilities of flucytosine, fluconazole, amphotericin B, and voriconazole to penetrate mixed-species biofilms containing C. albicans and Staphylococcus epidermidis (a slime-producing wild-type strain, RP62A, and a slime-negative mutant, M7) were also investigated. All four antifungal agents diffused very slowly through these mixed-species biofilms. In most cases, diffusion was slower with biofilms containing S. epidermidis RP62A, but amphotericin B penetrated biofilms containing the M7 mutant more slowly. However, the drug concentrations reaching the distal edges of the biofilms always substantially exceeded the MIC. Thus, although the presence of bacteria and bacterial matrix material undoubtedly retarded the diffusion of the antifungal agents, poor penetration does not account for the drug resistance of Candida biofilm cells, even in these mixed-species biofilms.

Pseudomonas aeruginosa biofilms exposed to imipenem exhibit changes in global gene expression and beta-lactamase and alginate production

The lungs of cystic fibrosis (CF) patients are commonly colonized with Pseudomonas aeruginosa biofilms. Chronic endobronchial P. aeruginosa infections are impossible to eradicate with antibiotics, but intensive suppressive antibiotic therapy is essential to maintain the lung function of CF patients. The treatment often includes beta-lactam antibiotics. How these antibiotics influence gene expression in the surviving biofilm population of P. aeruginosa is not clear. Thus, we used the microarray technology to study the effects of subinhibitory concentrations of a beta-lactam antibiotic, imipenem, on gene expression in biofilm populations. Many genes showed small but statistically significant differential expression in response to imipenem. We identified 34 genes that were induced or repressed in biofilms exposed to imipenem more than fivefold compared to the levels of induction or repression for the controls. As expected, the most strongly induced gene was ampC, which codes for chromosomal beta-lactamase. We also found that genes coding for alginate biosynthesis were induced by exposure to imipenem. Alginate production is correlated to the development of impaired lung function, and P. aeruginosa strains isolated from chronically colonized lungs of CF patients are nearly always mucoid due to the overproduction of alginate. Exposure to subinhibitory concentrations of imipenem caused structural changes in the biofilm, e.g., an increased biofilm volume. Increased levels of alginate production may be an unintended adverse consequence of imipenem treatment in CF patients.

The biofilm matrix

The extracellular matrix is a complex and extremely important component of all biofilms, providing architectural structure and mechanical stability to the attached population. The matrix is composed of cells, water and secreted/released extracellular macromolecules. In addition, a range of enzymic and regulatory activities can be found within the matrix. Together, these different components and activities are likely to interact and in so doing create a series of local environments within the matrix which co-exist as a functional consortium. The matrix architecture is also subject to a number of extrinsic factors, including fluctuations in nutrient and gaseous levels and fluid shear. Together, these intrinsic and extrinsic factors combine to produce a dynamic, heterogeneous microenvironment for the attached and enveloped cells.

A simple alfalfa seedling infection model for Pseudomonas aeruginosa strains associated with cystic fibrosis shows AlgT (sigma-22) and RhlR contribute to pathogenesis

A sensitive plant infection model was developed to identify virulence factors in nontypeable, alginate overproducing (mucoid) Pseudomonas aeruginosa strains isolated from cystic fibrosis (CF) patients with chronic pulmonary disease. Nontypeable strains with defects in lipopolysaccharide O-side chains are common to CF and often exhibit low virulence in animal models of infection. However, 1,000 such bacteria were enough to show disease symptoms in the alfalfa infection. A typical mucoid CF isolate, FRD1, and its isogenic mutants were tested for alfalfa seedling infection. Although defects in the global regulators Vfr, RpoS, PvdS, or LasR had no discernable effect on virulence, a defect in RhlR reduced the infection frequency by >50%. A defect in alginate biosynthesis resulted in plant disease with >3-fold more bacteria per plant, suggesting that alginate overproduction attenuated bacterial growth in planta. FRD1 derivatives lacking AlgT, a sigma factor required for alginate production, were reduced >50% in the frequency of infection. Thus, AlgT apparently regulates factors in FRD1, besides alginate, important for pathogenesis. In contrast, in a non-CF strain, PAO1, an algT mutation did not affect its virulence on alfalfa. Conversely, PAO1 virulence was reduced in a mucA mutant that overproduced alginate. These observations suggested that mucoid conversion in CF may be driven by a selection for organisms with attenuated virulence or growth in the lung, which promotes a chronic infection. These studies also demonstrated that the wounded alfalfa seedling infection model is a useful tool to identify factors contributing to the persistence of P. aeruginosa in CF.

Alginate lyase promotes diffusion of aminoglycosides through the extracellular polysaccharide of mucoid Pseudomonas aeruginosa

We demonstrated that a 2% suspension of Pseudomonas aeruginosa alginate completely blocked the diffusion of gentamicin and tobramycin, but not that of carbenicillin, illustrating how alginate production can help protect P. aeruginosa growing within alginate microcolonies in patients with cystic fibrosis (CF) from the effects of aminoglycosides. This aminoglycoside diffusion barrier was degraded with a semipurified preparation of P. aeruginosa alginate lyase, suggesting that this enzyme deserves consideration as an adjunctive agent for CF patients colonized by mucoid strains of P. aeruginosa.

Biofilm susceptibility to antimicrobials

Microbial biofilms, where organisms are intimately associated with each other and a solid substratum through binding and inclusion within an exopolymer matrix, are widely distributed in nature and disease. In the mouth, multispecies biofilms are associated not only with dental plaque and tooth decay but also with soft tissues of the buccal cavity and with most forms of periodontal disease. Organization of micro-organisms within biofilms confers, on the component species, properties which are not evident with the individual species grown independently or as planktonic populations in liquid media. While many of these properties relate to the establishment of functional, mixed-species consortia within the exopolymeric matrices, others relate to the establishment of physico-chemical gradients, within the biofilm, that modify the metabolism of the component cells. A consequence of biofilm growth that has profound implications for their control in the environment and in medicine is a markedly enhanced resistance to chemical antimicrobial agents and antibiotics. Mechanisms associated with such resistance in biofilms will form the substance of the present review. While some aspects of biofilm resistance are yet only poorly understood, the dominant mechanisms are thought to be related to: (i) modified nutrient environments and suppression of growth rate within the biofilm; (ii) direct interactions between the exopolymer matrices, and their constituents, and antimicrobials, affecting diffusion and availability; and (iii) the development of biofilm/attachment-specific phenotypes.

Theoretical aspects of antibiotic diffusion into microbial biofilms

Antibiotic penetration into microbial biofilm was investigated theoretically by the solution of mathematical equations describing various combinations of the processes of diffusion, sorption, and reaction. Unsteady material balances on the antibiotic and on a reactive or sorptive biomass constituent, along with associated boundary and initial conditions, constitute the mathematical formulations. Five cases were examined: diffusion of a noninteracting solute; diffusion of a reversibly sorbing, nonreacting solute; diffusion of an irreversibly sorbing, nonreacting solute; diffusion of a stoichiometrically reacting solute; and diffusion of a catalytically reacting solute. A noninteracting solute was predicted to penetrate biofilms of up to 1 mm in thickness relatively quickly, within a matter of seconds or minutes. In the case of a solute that does not sorb or react in the biofilm, therefore, the diffusion barrier is not nearly large enough to account for the reduced susceptibility of biofilms to antibiotics. Reversible and irreversible sorption retards antibiotic penetration. On the basis of data available in the literature at this point, the extent of retardation of antibiotic diffusion due to sorption does not appear to be sufficient to account for reduced biofilm susceptibility. A catalytic (e.g., enzymatic) reaction, provided it is sufficiently rapid, can lead to severe antibiotic penetration failure. For example, calculation of beta-lactam penetration indicated that the reaction-diffusion mechanism may be a viable explanation for failure of certain of these agents to control biofilm infections. The theory presented in this study provides a framework for the design and analysis of experiments to test these mechanisms of reduced biofilm susceptibility to antibiotics.

Pseudomonas aeruginosa clinical isolates: serotypes, resistance phenotypes and plasmid profiles

78 Pseudomonas aeruginosa strains were isolated from the respiratory tract of 56 patients, 15 of which were affected by cystic fibrosis (CF). The epidemiological typing scheme was based on serotyping, antibiotic resistance pattern and plasmid DNA profile. All strains (except 2 mucoid strains) were typed using a rapid slide O-agglutination technique. Most common serotypes in both group were 0:1, 0:10 and 0:6. Moreover we observed a correlation among 0:12 serotype and CF patients. Plasmid DNA analysis showed that 45.2% (on average) of strains isolated from patients with and w/o CF harboured 1-3 plasmids ranging in size from 1 to 15 Md. Plasmid prevalence was higher in strains isolated from CF patients in specimens collected after antibiotic therapy. A correlation was found between 1 and 1.9 Md plasmids and resistance to aminoglycosides. Our results indicate that the analysis of antibiotic resistance phenotypes combined with plasmid analysis may be useful, in association to serotyping, to characterize the circulation of P. aeruginosa strains and the spread of resistance in these bacteria.

Antibiotic resistance of biofilms

Microbial biofilms are notably recalcitrant towards treatment with antibiotics, biocides or disinfectants that would adequately control the same organisms growing in planktonic mode. Much of this resistance has been attributed to an organisation of the biofilm cells within exopolymer matrices. Whilst such exopolymers are unlikely to hinder the diffusion and access of antimicrobial agents to the underlying cells, they will chemically quench reactive biocides such as chlorine and peroxygens, and bind highly charged antibiotics, such as tobramycin and gentamycin, thereby providing some protection to the more deep lying cells. Extracellular enzymes, bound within the glycocalyx and able to degrade the treatment agents, will further reduce the access of susceptible compounds. Diffusion limitation however, is unlikely to be the sole moderator of the resistance properties of microbial biofilms. In addition, gradients of oxygen and nutrients established across the biofilm community will cause growth rates to be much reduced at points remoted from the accessible nutrient. Slow growth rates, and the associated induction of stringent responses further contribute towards this resistance. Finally, there have been recent demonstrations that attachment of microorganisms to surfaces promotes the expression of genes that are not normally expressed in planktonic culture. Whether or not the expression of such genes alters the phenotype in a manner which alters the response of the cells to antimicrobial agents remains to be demonstrated.

Modification by surface association of antimicrobial susceptibility of bacterial populations

In the majority of natural situations in which bacteria are found, they are associated with and attached to surfaces. In the presence of moisture and nutrients, they grow to form extensive bacterial films which are often enveloped within copius exopolymeric matrices. Biofilms are ubiquitous to many different situations in industry, the environment and medicine. Their presence can be either beneficial or more commonly detrimental to such systems. In this respect, biofilm populations possess physiological properties distinct from those of unattached, planktonic bacteria. Moreover, it is generally accepted that bacteria growing within a biofilm are more resistant to antimicrobial agents than their planktonic counterparts. However, although the consequences of attachment to antimicrobial resistance have been known for many years, the mechanistic bases for such effects have still to be fully elucidated. In this article the nature of different resistance mechanisms, including those of the exopolymeric matrix, environmental modulation, attachment-specific physiologies and quorum sensing are reviewed.

Non-canonical mechanisms of antibiotic resistance

Although the current in vitro methods used for detection and analysis of the phenotypes of antibiotic resistance in the laboratory are well established, other resistance mechanisms of resistance exist which may escape detection using the standard approach. The present article reviews some of these mechanisms which are grouped under the term 'non-canonical mechanisms' of antibiotic resistance. Such mechanisms include gene dosage, heterologous induction or selection, populational resistance and synergism between mechanisms of low resistance. The role of these mechanisms in the failure of therapy is discussed.

A sandwich cup method for the penetration assay of antimicrobial agents through Pseudomonas exopolysaccharides

We developed new sandwich cup method to assay the penetration of various antimicrobial agents through Pseudomonas exopolysaccharides. Using alginate extracted from mucoid-type Pseudomonas aeruginosa and gellan gum from Pseudomonas elodea, the role of exopolysaccharides as a barrier against drug penetration was examined. The penetration of positively charged hydrophilic drugs such as aminoglycosides and polypeptides was markedly inhibited by the gels tested, but that of beta-lactams, quinolones, and macrolides was not inhibited. The penetration of gentamicin was strongly influenced by the gel concentration, the solution to be used, and the presence of Ca2+. These results suggest that the microenvironment at the infection site could greatly influence drug penetration through biofilms in vivo.

Continuous culture studies on the synthesis of capsular polysaccharide by Klebsiella pneumoniae K1

The synthesis of capsular polysaccharide by Klebsiella pneumoniae K1 was investigated in a minimal salts medium by continuous culture. The organism produced larger amounts of polysaccharide under nitrogen-limited conditions than under carbon-limited conditions. The synthesis of polysaccharide was dependent not only on the availability of excess carbon, but also on growth rate. The rate of polysaccharide synthesis was greatest at low dilutions, low temperature (30 degrees C) and at neutral pH. Prolonged growth in nitrogen-limited culture resulted in the development of non-mucoid variants, possibly due to a selective growth advantage over mucoid cells. The non-mucoid isolate was more susceptible to some bacteriophages, possibly due to the reduction nor absence of capsular polysaccharide.

Effect of sub-MIC antibiotics on the cell surface and extracellular virulence determinants of Pseudomonas cepacia

The effects of sub-MICs of ciprofloxacin and tobramycin on the cell surface characteristics and extracellular virulence factors of Pseudomonas cepacia were evaluated. Cells were grown in batch culture under iron-deficient and iron-replete conditions. At sub-MIC levels that did not affect bacterial growth cell surface hydrophobicity decreased under both iron-replete and iron-depleted conditions with ciprofloxacin, but increased with tobramycin under iron-sufficient conditions. Exopolysaccharide synthesis, lipase production and siderophore production were all significantly increased by the presence of ciprofloxacin under both growth conditions. Outer membrane protein and lipopolysaccharide profiles were not affected by exposure to the two antibiotics.

Polysaccharide production in Pseudomonas cepacia

The effects of glucose, osmolarity, temperature and mode of growth on exopolysaccharide production in Pseudomonas cepacia was studied in batch culture using a chemically defined growth medium. Polymer production was maximal under conditions of a 2% (w/v) glucose supplement, 0.4 M NaCl and an incubation temperature of 35 degrees C. In addition, polysaccharide composition and molecular weight varied with mode of growth. On agar culture there was a decrease in pyruvate and rhamnose content yet an increase in the amount of acetate compared to the polymer isolated from broth culture equivalents. The clinical implications of these results are discussed in relation to the potential pathogenicity of P. cepacia in cystic fibrosis patients.