Antimicrobial substances and effects on sessile bacteria

Biofilm antimicrobial susceptibility testing: where are we and where could we be going?

Our knowledge about the fundamental aspects of biofilm biology, including the mechanisms behind the reduced antimicrobial susceptibility of biofilms, has increased drastically over the last decades. However, this knowledge has so far not been translated into major changes in clinical practice. While the biofilm concept is increasingly on the radar of clinical microbiologists, physicians, and healthcare professionals in general, the standardized tools to study biofilms in the clinical microbiology laboratory are still lacking; one area in which this is particularly obvious is that of antimicrobial susceptibility testing (AST). It is generally accepted that the biofilm lifestyle has a tremendous impact on antibiotic susceptibility, yet AST is typically still carried out with planktonic cells. On top of that, the microenvironment at the site of infection is an important driver for microbial physiology and hence susceptibility; but this is poorly reflected in current AST methods. The goal of this review is to provide an overview of the state of the art concerning biofilm AST and highlight the knowledge gaps in this area. Subsequently, potential ways to improve biofilm-based AST will be discussed. Finally, bottlenecks currently preventing the use of biofilm AST in clinical practice, as well as the steps needed to get past these bottlenecks, will be discussed.

Pharmacodynamics of ciprofloxacin against Pseudomonas aeruginosa planktonic and biofilm-derived cells

The influence of growth phase and state on the survival and recovery of Pseudomonas aeruginosa exposed to ciprofloxacin was investigated using batch culture grown planktonic cells and disaggregated biofilm populations. Biofilms were either nonantibiotic exposed or previously exposed to ciprofloxacin before disaggregation and subsequent challenge with ciprofloxacin. Viable counts showed that late stationary phase cells were tolerant to ciprofloxacin over 24 h exposure, while all other populations presented a biphasic killing pattern. In contrast, the metabolic activity of planktonic and biofilm-derived cells remained similar to controls during the initial 6 h of ciprofloxacin exposure, despite a significant reduction in viable cell numbers. A similar effect was observed when assessing the postantibiotic effect of 1 h ciprofloxacin exposure. Thus, although cell reduction occurred, the metabolic status of the cells remained unchanged. The recovery of disaggregated biofilm cells previously exposed to ciprofloxacin was significantly quicker than naïve biofilm cells, and this latter population's recovery was significantly slower than all planktonic populations. Results from this work have implications for our understanding of biofilm-related infections and their resilience to antimicrobial treatment. SIGNIFICANCE AND IMPACT OF THE STUDY: Removal of biofilms from surfaces and infection sites via disaggregation and induction of dispersion may reverse their antibiotic tolerant state. However, little is known of the recovery of the cells upon disaggregation from biofilms. Driven by this gap in knowledge we quantified the effect of ciprofloxacin on disaggregated biofilms of Pseudomonas aeruginosa, including those previously exposed to ciprofloxacin. Our results provide further insight into bacterial resilience, regrowth, and antimicrobial efficacy, as reduction in cell viability does not directly correlate with the metabolic activity of bacteria at the time of the exposure to antimicrobials. Thus, despite a perceived reduction in viability, the potential for cell persistence and regrowth remains and recovery is quicker upon subsequent exposure to antimicrobial, supporting the increase in resilience and recurrence of infections.

Biofilm Formation is not Necessary for Development of Quinolone-Resistant “Persister” cells in an Attached Staphylococcus Epidermidis Population

Staphylococcus epidermidis is a common pathogen in device-associated infections which is able to attach onto polymeric surfaces and develop multilayered biofilms. Attached S. epidermidis displays reduced susceptibility to antimicrobial agents. In this study we investigated the influence of ciprofloxacin and the group IV quinolones gatifloxacin, gemifloxacin, and moxifloxacin with the minimal attachment killing (MAK) assay. MAK concentrations were determined for three biofilm-positive wild-type strains and their isogenic biofilm-negative mutants Depending on strain and investigated quinolone, it was possible to distinguish between a heterogeneous MAK (MAKhetero), and a homogeneous resistance (MAKhomo) which corresponds to the model of a few persisting cells under antibiotic treatment. A lower MAKhomo was detected for the biofilm-negative mutants as well as for the corresponding wild-types for some of the tested quinolones, which seems to be a result of higher bacterial inocula, whereas the MAKhetero concentrations were comparable for mutants and wild-types for nearly all of the tested antibiotics and strains. These data indicate that biofilm formation is not necessary for persistence of attached S. epidermidis cells under treatment with quinolones and could explain therapeutic failure in foreign body-associated infections due to biofilm-negative S. epidermidis isolates. The individual resistance phenotypes of investigated strains indicate that the determination of MAK concentrations might help to predict the therapy outcome of foreign body-associated infections with both biofilm-positive and biofilm-negative S. epidermidis. Thus, the relatively high activity displayed by group IV quinolones against individual attached staphylococcal isolates indicates a possible treatment option with the respective quinolones for foreign body-associated infections due to these isolates. (Int J Artif Organs 2008; 31: 752–60)

Control of Biofilms with the Fatty Acid Signaling Molecule cis-2-Decenoic Acid

Biofilms are complex communities of microorganisms in organized structures attached to surfaces. Importantly, biofilms are a major cause of bacterial infections in humans, and remain one of the most significant challenges to modern medical practice. Unfortunately, conventional therapies have shown to be inadequate in the treatment of most chronic biofilm infections based on the extraordinary innate tolerance of biofilms to antibiotics. Antagonists of quorum sensing signaling molecules have been used as means to control biofilms. QS and other cell-cell communication molecules are able to revert biofilm tolerance, prevent biofilm formation and disrupt fully developed biofilms, albeit with restricted effectiveness. Recently however, it has been demonstrated that Pseudomonas aeruginosa produces a small messenger molecule cis-2-decenoic acid (cis-DA) that shows significant promise as an effective adjunctive to antimicrobial treatment of biofilms. This molecule is responsible for induction of the native biofilm dispersion response in a range of Gram-negative and Gram-positive bacteria and in yeast, and has been shown to reverse persistence, increase microbial metabolic activity and significantly enhance the cidal effects of conventional antimicrobial agents. In this manuscript, the use of cis-2-decenoic acid as a novel agent for biofilm control is discussed. Stimulating the biofilm dispersion response as a novel antimicrobial strategy holds significant promise for enhanced treatment of infections and in the prevention of biofilm formation.

Local gentamicin delivery from resorbable viscous hydrogels is therapeutically effective

BACKGROUND

Local delivery can achieve the high antimicrobial concentrations necessary to kill biofilm-related microbes. Degradation times for resorbable carriers are too long. Hydrogels (gels of hydrophilic polymer in water) can degrade faster but release antimicrobials too quickly. We previously developed hydrogels based on the copolymer poly(N-isopropylacrylamide-co-dimethyl-γ-butyrolactone acrylate-co-Jeffamine® M-1000 acrylamide) (PNDJ) with delivery times of several days with complete degradation in less than 6 weeks.

QUESTIONS/PURPOSES

We asked: (1) What is the elution profile of gentamicin from PNDJ hydrogels? (2) Is gentamicin released from gentamicin-loaded PNDJ (G-PNDJ) hydrogel effective for treatment of orthopaedic infection? (3) Does local gentamicin delivery from G-PNDJ hydrogel cause renal dysfunction?

METHODS

(1) Two formulations of G-PNDJ, lower dose (1.61 wt%) and higher dose (3.14 wt%), five samples each, were eluted in buffered saline under infinite sink conditions. (2) Infections were induced in 16 New Zealand White rabbits by inserting a Kirschner wire in a devascularized radius segment and inoculating with 7.5×10(6) colony-forming units Staphylococcus aureus. At 3 weeks, débridement was performed and a new Kirschner wire was placed in the dead space. Treatment was randomized to higher-dose G-PNDJ or no hydrogel. No systemic antimicrobials were used. Positive culture and acute inflammation on histology were used to determine the presence of infection 4 weeks postdébridement. (3) 3.14 wt% G-PNDJ, 0.75, 1.5, or 3.0 mL, was injected subcutaneously in nine Sprague-Dawley rats, three of each dose. Serum gentamicin, blood urea nitrogen, and creatinine were measured on Days 1, 3, 7, 14, and 28.

RESULTS

(1) Gentamicin release was sustained over 7 days with the higher-dose formulation release profile similar to release from high-dose antimicrobial-loaded bone cement. (2) Four weeks postdébridement, infection was present in eight of eight no-hydrogel rabbits but zero of eight rabbits treated with G-PNDJ hydrogel (p<0.001). (3) Blood urea nitrogen and creatinine were transiently elevated (p<0.05) only for the two of three rats receiving the 3.0-mL dose on Days 3 and 7.

CONCLUSIONS

Gentamicin is delivered from PNDJ hydrogel with low systemic exposure and decreased treatment failure for orthopaedic infection. Transient renal dysfunction occurs at high doses. Biodistribution and toxicity testing are needed for G-PNDJ to be clinically usable.

CLINICAL RELEVANCE

Resorbable viscous hydrogels for local antimicrobial delivery may improve outcomes for one-stage management of implant infections when uncemented reconstructions are performed.

Ureteral Stents and Foley Catheters-Associated Urinary Tract Infections: The Role of Coatings and Materials in Infection Prevention

Urinary tract infections affect many patients, especially those who are admitted to hospital and receive a bladder catheter for drainage. Catheter associated urinary tract infections are some of the most common hospital infections and cost the health care system billions of dollars. Early removal is one of the mainstays of prevention as 100% of catheters become colonized. Patients with ureteral stents are also affected by infection and antibiotic therapy alone may not be the answer. We will review the current evidence on how to prevent infections of urinary biomaterials by using different coatings, new materials, and drug eluting technologies to decrease infection rates of ureteral stents and catheters.

In vitro efficacy of cold atmospheric pressure plasma on S. sanguinis biofilms in comparison of two test models

Dental plaque critically affects the etiology of caries, periodontitis and periimplantitis. The mechanical removal of plaque can only be performed partially due to limited accessibility. Therefore, plaque still represents one of the major therapeutic challenges. Even though antiseptic mouth rinses reduce the extent of biofilm temporarily, plaque removal remains incomplete and continuous usage can even result in side effects. Here we tested argon plasma produced by kinpen09 as one option to inactivate microorganisms and to eliminate plaque. S. sanguinis biofilms cultivated in either the European Biofilm Reactor (EUREBI) or in 24 well plates were treated with argon plasma. In both test systems a homogeneous, good analyzable and stable biofilm was produced on the surface of titan plates within 72 h (>6,9 log10 CFU/ml). Despite the significantly more powerful biofilm production in EUREBI, the difference of 0.4 log10 CFU/ml between EUREBI and the 24 well plates was practically not relevant. For that reason both test models were equally qualified for the analysis of efficacy of cold atmospheric pressure plasma. We demonstrate a significant reduction of the biofilm compared to the control in both test models. After plasma application of 180 s the biofilm produced in EUREBI or in 24 well plates was decreased by 0.6 log10 CFU/ml or 0.5 log10 CFU/ml, respectively. In comparison to recently published studies analyzing the efficacy of kinpen09, S. sanguinis produces a hardly removable biofilm. Future investigations using reduced distances between plasma source and biofilm, various compositions of plasma and alternative plasma sources will contribute to further optimization of the efficacy against S. sanguinis biofilms.

The Composition and Metabolic Phenotype of Neisseria gonorrhoeae Biofilms

Neisseria gonorrhoeae has been shown to form biofilms during cervical infection. Thus, biofilm formation may play an important role in the infection of women. The ability of N. gonorrhoeae to form membrane blebs is crucial to biofilm formation. Blebs contain DNA and outer membrane structures, which have been shown to be major constituents of the biofilm matrix. The organism expresses a DNA thermonuclease that is involved in remodeling of the biofilm matrix. Comparison of the transcriptional profiles of gonococcal biofilms and planktonic runoff indicate that genes involved in anaerobic metabolism and oxidative stress tolerance are more highly expressed in biofilm. The expression of aniA, ccp, and norB, which encode nitrite reductase, cytochrome c peroxidase, and nitric oxide reductase respectively, is required for mature biofilm formation over glass and human cervical cells. In addition, anaerobic respiration occurs in the substratum of gonococcal biofilms and disruption of the norB gene required for anaerobic respiration, results in a severe biofilm attenuation phenotype. It has been demonstrated that accumulation of nitric oxide (NO) contributes to the phenotype of a norB mutant and can retard biofilm formation. However, NO can also enhance biofilm formation, and this is largely dependent on the concentration and donation rate or steady-state kinetics of NO. The majority of the genes involved in gonococcal oxidative stress tolerance are also required for normal biofilm formation, as mutations in the following genes result in attenuated biofilm formation over cervical cells and/or glass: oxyR, gor, prx, mntABC, trxB, and estD. Overall, biofilm formation appears to be an adaptation for coping with the environmental stresses present in the female genitourinary tract. Therefore, this review will discuss the studies, which describe the composition and metabolic phenotype of gonococcal biofilms.

Transcriptional profiling identifies the metabolic phenotype of gonococcal biofilms

Neisseria gonorrhoeae, the etiologic agent of gonorrhea, is frequently asymptomatic in women, often leading to chronic infections. One factor contributing to this may be biofilm formation. N. gonorrhoeae can form biofilms on glass and plastic surfaces. There is also evidence that biofilm formation may occur during natural cervical infection. To further study the mechanism of gonococcal biofilm formation, we compared transcriptional profiles of N. gonorrhoeae biofilms to planktonic profiles. Biofilm RNA was extracted from N. gonorrhoeae 1291 grown for 48 h in continuous-flow chambers over glass. Planktonic RNA was extracted from the biofilm runoff. In comparing biofilm with planktonic growth, 3.8% of the genome was differentially regulated. Genes that were highly upregulated in biofilms included aniA, norB, and ccp. These genes encode enzymes that are central to anaerobic respiratory metabolism and stress tolerance. Downregulated genes included members of the nuo gene cluster, which encodes the proton-translocating NADH dehydrogenase. Furthermore, it was observed that aniA, ccp, and norB insertional mutants were attenuated for biofilm formation on glass and transformed human cervical epithelial cells. These data suggest that biofilm formation by the gonococcus may represent a response that is linked to the control of nitric oxide steady-state levels during infection of cervical epithelial cells.

[Treatment of infected total knee arthroplasty. When does implant salvage make sense?]

Infection of a total knee arthroplasty can be classified as acute, chronic and haematogenic with and without implant loosening. A differentiated treatment concept for all types of infection is necessary. Furthermore, specific treatment has to be initiated early, as any delay is associated with a worsening of the prognosis. Treatment of infection with implant salvage may be one therapeutic option if the implant is not loose. According to the current literature, therapy with retention of the prosthesis may be promising: (1) in the case of early infection (<3 weeks of ongoing symptoms), (2) with unconstrained implants, (3) in the case of infection with a single organism that is susceptible to antibiotic therapy, (4) if soft tissue coverage is not affected, and (5) if the immune system is not compromised. Chronic infections, (semi-)constrained implants and soft tissue defects have to be considered as contraindications and implants should be removed. Early and consequent therapy with operative débridement and specific long-term antibiotic therapy are necessary to achieve implant salvage. The additional application of antibiotics addressing bacterial biofilms have helped to improve the prognosis. Due to the fact that revision arthroplasty is often associated with limited function after infection of the total knee joint, retention of the implant has to be considered a therapeutic alternative in early infection.

Alginate production affects Pseudomonas aeruginosa biofilm development and architecture, but is not essential for biofilm formation

Extracellular polymers can facilitate the non-specific attachment of bacteria to surfaces and hold together developing biofilms. This study was undertaken to qualitatively and quantitatively compare the architecture of biofilms produced byPseudomonas aeruginosastrain PAO1 and its alginate-overproducing (mucA22) and alginate-defective (algD) variants in order to discern the role of alginate in biofilm formation. These strains, PAO1, Alg+PAOmucA22and Alg−PAOalgD, tagged with green fluorescent protein, were grown in a continuous flow cell system to characterize the developmental cycles of their biofilm formation using confocal laser scanning microscopy. Biofilm Image Processing (bip) and Community Statistics (comstat) software programs were used to provide quantitative measurements of the two-dimensional biofilm images. All three strains formed distinguishable biofilm architectures, indicating that the production of alginate is not critical for biofilm formation. Observation over a period of 5 days indicated a three-stage development pattern consisting of initiation, establishment and maturation. Furthermore, this study showed that phenotypically distinguishable biofilms can be quantitatively differentiated.

Bacterial adhesion: seen any good biofilms lately?

The process of surface adhesion and biofilm development is a survival strategy employed by virtually all bacteria and refined over millions of years. This process is designed to anchor microorganisms in a nutritionally advantageous environment and to permit their escape to greener pastures when essential growth factors have been exhausted. Bacterial attachment to a surface can be divided into several distinct phases, including primary and reversible adhesion, secondary and irreversible adhesion, and biofilm formation. Each of these phases is ultimately controlled by the expression of one or more gene products. Ultrastructurally, the mature bacterial biofilm resembles an underwater coral reef containing pyramidal or mushroom-shaped microcolonies of organisms embedded within an extracellular glycocalyx, with channels and cavities to allow the exchange of nutrients and waste. The biofilm protects its inhabitants from predators, dehydration, biocides, and other environmental extremes while regulating population growth and diversity through primitive cell signals. From a physiological standpoint, surface-bound bacteria behave quite differently from their planktonic counterparts. Recognizing that bacteria naturally occur as surface-bound and often polymicrobic communities, the practice of performing antimicrobial susceptibility tests using pure cultures and in a planktonic growth mode should be questioned. That this model does not reflect conditions found in nature might help explain the difficulties encountered in the management and treatment of biomedical implant infections.

N-acylhomoserine-lactone-mediated communication between Pseudomonas aeruginosa and Burkholderia cepacia in mixed biofilms

Pseudomonas aeruginosa and Burkholderia cepacia are capable of forming mixed biofilms in the lungs of cystic fibrosis patients. Both bacteria employ quorum-sensing systems, which rely on N-acylhomoserine lactone (AHL) signal molecules, to co-ordinate expression of virulence factors with the formation of biofilms. As both bacteria utilize the same class of signal molecules the authors investigated whether communication between the species occurs. To address this issue, novel Gfp-based biosensors for non-destructive, in situ detection of AHLs were constructed and characterized. These sensors were used to visualize AHL-mediated communication in mixed biofilms, which were cultivated either in artificial flow chambers or in alginate beads in mouse lung tissue. In both model systems B. cepacia was capable of perceiving the AHL signals produced by P. aeruginosa, while the latter strain did not respond to the molecules produced by B. cepacia. Measurements of extracellular proteolytic activities of defined quorum-sensing mutants grown in media complemented with AHL extracts prepared from culture supernatants of various wild-type and mutant strains supported the view of unidirectional signalling between the two strains.

The cep quorum-sensing system of Burkholderia cepacia H111 controls biofilm formation and swarming motility

Burkholderia cepacia and Pseudomonas aeruginosa often co-exist as mixed biofilms in the lungs of patients suffering from cystic fibrosis (CF). Here, the isolation of random mini-Tn5 insertion mutants of B. cepacia H111 defective in biofilm formation on an abiotic surface is reported. It is demonstrated that one of these mutants no longer produces N-acylhomoserine lactones (AHLs) due to an inactivation of the cepR gene. cepR and the cepI AHL synthase gene together constitute the cep quorum-sensing system of B. cepacia. By using a gene replacement method, two defined mutants, H111-I and H111-R, were constructed in which cepI and cepR, respectively, had been inactivated. These mutants were used to demonstrate that biofilm formation by B. cepacia H111 requires a functional cep quorum-sensing system. A detailed quantitative analysis of the biofilm structures formed by wild-type and mutant strains suggested that the quorum-sensing system is not involved in the regulation of initial cell attachment, but rather controls the maturation of the biofilm. Furthermore, it is shown that B. cepacia is capable of swarming motility, a form of surface translocation utilized by various bacteria to rapidly colonize appropriate substrata. Evidence is provided that swarming motility of B. cepacia is quorum-sensing-regulated, possibly through the control of biosurfactant production. Complementation of the cepR mutant H111-R with different biosurfactants restored swarming motility while biofilm formation was not significantly increased. This result suggests that swarming motility per se is not essential for biofilm formation on abiotic surfaces.

Standardized method for in vitro antifungal susceptibility testing of Candida albicans biofilms

Candida albicans is implicated in many biomaterial-related infections. Typically, these infections are associated with biofilm formation. Cells in biofilms display phenotypic traits that are dramatically different from those of their free-floating planktonic counterparts and are notoriously resistant to antimicrobial agents. Consequently, biofilm-related infections are inherently difficult to treat and to fully eradicate with normal treatment regimens. Here, we report a rapid and highly reproducible microtiter-based colorimetric assay for the susceptibility testing of fungal biofilms, based on the measurement of metabolic activities of the sessile cells by using a formazan salt reduction assay. The assay was used for in vitro antifungal susceptibility testing of several C. albicans strains grown as biofilms against amphotericin B and fluconazole and the increased resistance of C. albicans biofilms against these antifungal agents was demonstrated. Because of its simplicity, compatibility with a widely available 96-well microplate platform, high throughput, and automation potential, we believe this assay represents a promising tool for the standardization of in vitro antifungal susceptibility testing of fungal biofilms.

Bacterial biofilms and human disease

The term biofilm is used to denote a polymer-encased community of microbes which accumulates at a surface. Biofilms are responsible for a number of diseases of man and, because of the intrinsic resistance of these structures to antibiotics and host defence systems, such diseases are very difficult to treat effectively. The application of new microscopic and molecular techniques to biofilms has revolutionised our understanding of their structure, composition, organisation and activities. This review will describe the role that biofilms play in human disease and will outline our new millennial view of these complex and fascinating bacterial communities.