Drug susceptibility of matrix-encapsulated Candida albicans nano-biofilms

A Facile High-Throughput Model of Surface-Independent Staphylococcus aureus Biofilms by Spontaneous Aggregation

The canonical model of biofilm formation begins with the attachment and growth of microbial cells on a surface. While these in vitro models reasonably mimic biofilms formed on foreign bodies such as catheters and implants, this is not the case for biofilms formed in cystic fibrosis and chronic wound infections, which appear to present as aggregates not attached to a surface.

Many microbes in their natural habitats are found in biofilm ecosystems attached to surfaces and not as free-floating (planktonic) organisms. Furthermore, it is estimated that nearly 80% of human infections are associated with biofilms. Biofilms are traditionally defined as three-dimensional, structured microbial communities that are attached to a surface and encased in a matrix of exopolymeric material. While this view of biofilm largely arises from in vitro studies under static or flow conditions, in vivo observations have indicated that this view of biofilms is essentially true only for foreign-body infections on catheters or implants where biofilms are attached to the biomaterial. In mucosal infections such as chronic wounds or cystic fibrosis or joint infections, biofilms can be found unattached to a surface and as three-dimensional aggregates. In this work, we describe a high-throughput model of aggregate biofilms of methicillin-resistant Staphylococcus aureus (MRSA) using 96-well plate hanging-drop technology. We show that MRSA forms surface-independent biofilms, distinct from surface-attached biofilms, that are rich in exopolymeric proteins, polysaccharides, and extracellular DNA (eDNA), express biofilm-related genes, and exhibit heightened antibiotic resistance. We also show that the surface-independent biofilms of clinical isolates of MRSA from cystic fibrosis and central catheter-related infections demonstrate morphological differences. Overall, our results show that biofilms can form by spontaneous aggregation without attachment to a surface, and this new in vitro system can model surface-independent biofilms that may more closely mimic the corresponding physiological niche during infection.

IMPORTANCE The canonical model of biofilm formation begins with the attachment and growth of microbial cells on a surface. While these in vitro models reasonably mimic biofilms formed on foreign bodies such as catheters and implants, this is not the case for biofilms formed in cystic fibrosis and chronic wound infections, which appear to present as aggregates not attached to a surface. The hanging-drop model of biofilms of methicillin-resistant Staphylococcus aureus (MRSA), the major causative organism of skin and soft tissue infections, shows that these biofilms display morphological and antibiotic response patterns that are distinct from those of their surface-attached counterparts, and biofilm growth is consistent with their in vivo location. The simplicity and throughput of this model enable adoption to investigate other single or polymicrobial biofilms in a physiologically relevant setting.

Nano-biofilm Arrays as a Novel Universal Platform for Microscale Microbial Culture and High-Throughput Downstream Applications

Biofilms are the predominant mode of microbial growth and it is now fully accepted that a majority of infections in humans are associated with a biofilm etiology. Biofilms are defined as attached and structured microbial communities surrounded by a protective exopolymeric matrix. Importantly, sessile microorganisms growing within a biofilm are highly resistant to antimicrobial agents. Thus, there is an urgent need to develop new and improved anti-biofilm therapies. Unfortunately, most of the current techniques for in-vitro biofilm formation are not compatible with high throughput screening techniques that can speed up discovery of new drugs with anti-biofilm activity. To try to overcome this major impediment, our group has developed a novel technique consisting of micro-scale culture of microbial biofilms on a microarray platform. Using this technique, hundreds to thousands of microbial biofilms, each with a volume of approximately 30-50 nanolitres, can be simultaneously formed on a standard microscope slide. Despite more than three orders of magnitude of miniaturization over conventional biofilms, these nanobiofilms display similar growth, structural and phenotypic properties, including antibiotic drug resistance. These nanobiofilm chips are amenable to automation, drastically reducing assay volume and costs. This technique platform allows for true high-throughput screening in search for new anti-biofilm drugs.

On-chip phenotypic investigation of combinatory antibiotic effects by generating orthogonal concentration gradients

Combinatory therapy using two or more kinds of antibiotics is attracting considerable attention for inhibiting multi-drug resistant pathogenic bacteria. Although the therapy mostly leads to more powerful antimicrobial effects than using a single antibiotic (synergy), interference may arise from certain antibiotic combinations, resulting in the antimicrobial effect being suppressed (antagonism). Here, we present a microfluidic-based phenotypic screening chip to investigate combinatory antibiotic effects by automatically generating two orthogonal concentration gradients on a bacteria-trapping agarose gel. Computational simulations and fluorescence experiments together verify the simultaneous establishment of 121 concentration combinations, facilitating on-chip drug testing with stability and efficiency. Against Gram-negative bacteria, Pseudomonas aeruginosa, our chip allows the measurement of phenotypic growth levels, and enables various types of analyses for all antibiotic pairs to be conducted in 7 h. Furthermore, by providing a specific amount of susceptibility data, our chip enables the two reference models, Loewe additivity and Bliss independence, to be implemented, which classify the antibiotic interaction types into synergy or antagonism. These results suggest the efficacy of our chip as a cell-based drug screening platform for exploring the underlying pharmacological patterns of antibiotic interactions, with potential applications in guidance in clinical therapies and in screening other cell-type agents.

nBioChip, a Lab-on-a-Chip Platform of Mono- and Polymicrobial Biofilms for High-Throughput Downstream Applications

With an estimated 80% of infections being associated with a biofilm mode of growth and the ensuing recalcitrance of these biofilms with respect to conventional antibiotic treatment leading to high mortality rates, there is a dire and unmet need for the development of novel approaches to prevent, treat, and control these infections. Both bacteria and fungi are capable of forming biofilms that are inherently fragile and often polymicrobial in nature, which further complicates treatment. In this work, we showcase a nanobiofilm chip as a convenient platform for culturing several hundreds of mono- or polymicrobial biofilms and for susceptibility testing. This platform enables true ultra-high-throughput screening for antimicrobial drug discovery or diagnostics or for addressing fundamental issues in microbiology.

Current in vitro techniques for the culture of microorganisms, and particularly of delicate microbial biofilms, are still mostly limited to low-density plates and manual labor and are not amenable to automation and true high-throughput (HT) applications. We have developed a novel fully automated platform for the formation of mono- and polymicrobial biofilms of Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans at the nanoscale level. The nBioChip is robotically printed, robustly handled, and scanned using a standard microarray reader. Using this technique, hundreds to thousands of identical nanobiofilms encapsulated in hydrogel spots were cultured on microscope slides. The spots can withstand the washing steps involved in screening assays. The miniaturized biofilms demonstrated characteristics similar to those displayed by conventionally formed macroscopic biofilms, including (i) three-dimensional architectural features, (ii) synthesis of exopolymeric matrix material, and (iii) elevated resistance to antibiotic treatment. On the basis of our results, the nBioChip can generate reliable high-throughput antimicrobial susceptibility testing (HT-AST) in 12 to 18 h. The chip serves as a proof-of-concept universal platform for high-throughput drug screening and other downstream applications and furthers understanding of microbial interactions in mixed-species communities at the nanoscale level.

IMPORTANCE With an estimated 80% of infections being associated with a biofilm mode of growth and the ensuing recalcitrance of these biofilms with respect to conventional antibiotic treatment leading to high mortality rates, there is a dire and unmet need for the development of novel approaches to prevent, treat, and control these infections. Both bacteria and fungi are capable of forming biofilms that are inherently fragile and often polymicrobial in nature, which further complicates treatment. In this work, we showcase a nanobiofilm chip as a convenient platform for culturing several hundreds of mono- or polymicrobial biofilms and for susceptibility testing. This platform enables true ultra-high-throughput screening for antimicrobial drug discovery or diagnostics or for addressing fundamental issues in microbiology.

Enhanced effect in combination of curcumin- and ketoconazole-loaded methoxy poly (ethylene glycol)-poly (ε-caprolactone) micelles

In order to enhance water-solubility and realize controlled release while keeping synergistic effects of ketoconazole and curcumin, drug-loaded methoxy poly (ethylene glycol)-b-poly (ε-caprolactone) micelles were prepared through thin membrane hydration method. Transmission electric microscopy and dynamitic light scattering characterization revealed the formation of ketoconazole- and curcumin-loaded micelles with an average size of 44.70nm and 39.56nm, respectively. The drug-loaded micelles endowed the two drugs' slow controlled release with water-solubility enhanced to 85 and 82000 folds higher than the corresponding raw drugs, respectively. In vitro antifungal activity test, chequerboard test and inhibition zone test indicated that efficacy of ketoconazole-loaded micelles was improved by introduction of curcumin-loaded micelles with a low fractional inhibitory concentration index (0.073). Biofilm formation inhibition assay also demonstrated that participation of curcumin-loaded micelles obviously strengthened the inhibition of fungal biofilms formation induced by ketoconazole-loaded micelles. The high synergistic activity of combinations is encouraging and the MPEG-PCL micelle is a potential drug delivery system for the combination of ketoconazole and curcumin.

Future therapies targeted towards eliminating Candida biofilms and associated infections

INTRODUCTION

Candida species are common human commensals and cause either superficial or invasive opportunistic infections. The biofilm form of candida as opposed to its suspended, planktonic form, is predominantly associated with these infections. Alternative or adjunctive therapies are urgently needed to manage Candida infections as the currently available short arsenal of antifungal drugs has been compromised due to their systemic toxicity, cross-reactivity with other drugs, and above all, by the emergence of drug-resistant Candida species due to irrational drug use. Areas covered: Combination anti-Candida therapies, antifungal lock therapy, denture cleansers, and mouth rinses have all been proposed as alternatives for disrupting candidal biofilms on different substrates. Other suggested approaches for the management of candidiasis include the use of natural compounds, such as probiotics, plants extracts and oils, antifungal quorum sensing molecules, anti-Candida antibodies and vaccines, cytokine therapy, transfer of primed immune cells, photodynamic therapy, and nanoparticles. Expert commentary: The sparsity of currently available antifungals and the plethora of proposed anti-candidal therapies is a distinct indication of the urgent necessity to develop efficacious therapies for candidal infections. Alternative drug delivery approaches, such as probiotics, reviewed here is likely to be a reality in clinical settings in the not too distant future.

Plasticity of Candida albicans Biofilms

Candida albicans, the most pervasive fungal pathogen that colonizes humans, forms biofilms that are architecturally complex. They consist of a basal yeast cell polylayer and an upper region of hyphae encapsulated in extracellular matrix. However, biofilms formed in vitro vary as a result of the different conditions employed in models, the methods used to assess biofilm formation, strain differences, and, in a most dramatic fashion, the configuration of the mating type locus (MTL). Therefore, integrating data from different studies can lead to problems of interpretation if such variability is not taken into account. Here we review the conditions and factors that cause biofilm variation, with the goal of engendering awareness that more attention must be paid to the strains employed, the methods used to assess biofilm development, every aspect of the model employed, and the configuration of the MTL locus. We end by posing a set of questions that may be asked in comparing the results of different studies and developing protocols for new ones. This review should engender the notion that not all biofilms are created equal.

Candida albicans Biofilms and Human Disease

In humans, microbial cells (including bacteria, archaea, and fungi) greatly outnumber host cells. Candida albicans is the most prevalent fungal species of the human microbiota; this species asymptomatically colonizes many areas of the body, particularly the gastrointestinal and genitourinary tracts of healthy individuals. Alterations in host immunity, stress, resident microbiota, and other factors can lead to C. albicans overgrowth, causing a wide range of infections, from superficial mucosal to hematogenously disseminated candidiasis. To date, most studies of C. albicans have been carried out in suspension cultures; however, the medical impact of C. albicans (like that of many other microorganisms) depends on its ability to thrive as a biofilm, a closely packed community of cells. Biofilms are notorious for forming on implanted medical devices, including catheters, pacemakers, dentures, and prosthetic joints, which provide a surface and sanctuary for biofilm growth. C. albicans biofilms are intrinsically resistant to conventional antifungal therapeutics, the host immune system, and other environmental perturbations, making biofilm-based infections a significant clinical challenge. Here, we review our current knowledge of biofilms formed by C. albicans and closely related fungal species.

Updates on Therapeutic Strategies Against Candida (and Aspergillus) Biofilm Related Infections

Fungal biofilm related infections are commonly associated with medical devices with biofilms contributing to the virulence of the involved fungal species. If infection does occur, removal of medical device is often warranted. However, this is not always possible. Moreover, biofilm associated infections are often resistant to antifungals and host immunity. Therefore, a need for new agents and strategies to combat these devastating infections is needed. Although no randomized clinical trials have been conducted or are likely to be conducted in the future, the Infectious Disease Society of America (IDSA) and the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) utilized available published data and clinical experience of the infectious disease community to propose strategies to treat biofilm associated devise infections. In this chapter we describe the emerging therapies for biofilm related infections.