Characterization of biofilm formation by clinically relevant serotypes of group A streptococci

Interplay between group A Streptococcus and host innate immune responses

SUMMARYGroup A Streptococcus (GAS), also known as Streptococcus pyogenes, is a clinically well-adapted human pathogen that harbors rich virulence determinants contributing to a broad spectrum of diseases. GAS is capable of invading epithelial, endothelial, and professional phagocytic cells while evading host innate immune responses, including phagocytosis, selective autophagy, light chain 3-associated phagocytosis, and inflammation. However, without a more complete understanding of the different ways invasive GAS infections develop, it is difficult to appreciate how GAS survives and multiplies in host cells that have interactive immune networks. This review article attempts to provide an overview of the behaviors and mechanisms that allow pathogenic GAS to invade cells, along with the strategies that host cells practice to constrain GAS infection. We highlight the counteractions taken by GAS to apply virulence factors such as streptolysin O, nicotinamide-adenine dinucleotidase, and streptococcal pyrogenic exotoxin B as a hindrance to host innate immune responses.

Group A Streptococcus adaptation to diverse niches: lessons from transcriptomic studies

Group A Streptococcus (GAS) is a major human pathogen, causing diseases ranging from mild superficial infections of the skin and pharyngeal epithelium to severe systemic and invasive diseases. Moreover, post infection auto-immune sequelae arise by a yet not fully understood mechanism. The ability of GAS to cause a wide variety of infections is linked to the expression of a large set of virulence factors and their transcriptional regulation in response to various physiological environments. The use of transcriptomics, among others -omics technologies, in addition to traditional molecular methods, has led to a better understanding of GAS pathogenesis and host adaptation mechanisms. This review focusing on bacterial transcriptomic provides new insight into gene-expression patterns in vitro, ex vivo and in vivo with an emphasis on metabolic shifts, virulence genes expression and transcriptional regulators role.

Role of serotype and virulence determinants of Streptococcus pyogenes biofilm bacteria in internalization and persistence in epithelial cells in vitro

Streptococcus pyogenes causes a multitude of local and systemic infections, the most common being pharyngitis in children. Recurrent pharyngeal infections are common and are thought to be due to the re-emergence of intracellular GAS upon completion of antibiotic treatment. The role of colonizing biofilm bacteria in this process is not fully clear. Here, live respiratory epithelial cells were inoculated with broth-grown or biofilm bacteria of different M-types, as well as with isogenic mutants lacking common virulence factors. All M-types tested adhered to and were internalized into epithelial cells. Interestingly, internalization and persistence of planktonic bacteria varied significantly between strains, whereas biofilm bacteria were internalized in similar and higher numbers, and all strains persisted beyond 44 hours, showing a more homogenous phenotype. The M3 protein, but not the M1 or M5 proteins, was required for optimal uptake and persistence of both planktonic and biofilm bacteria inside cells. Moreover, the high expression of capsule and SLO inhibited cellular uptake and capsule expression was required for intracellular survival. Streptolysin S was required for optimal uptake and persistence of M3 planktonic bacteria, whereas SpeB improved intracellular survival of biofilm bacteria. Microscopy of internalized bacteria showed that planktonic bacteria were internalized in lower numbers as individual or small clumps of bacteria in the cytoplasm, whereas GAS biofilm bacteria displayed a pattern of perinuclear localization of bacterial aggregates that affected actin structure. Using inhibitors targeting cellular uptake pathways, we confirmed that planktonic GAS mainly uses a clathrin-mediated uptake pathway that also required actin and dynamin. Clathrin was not involved in biofilm internalization, but internalization required actin rearrangement and PI3 kinase activity, possibly suggesting macropinocytosis. Together these results provide a better understanding of the potential mechanisms of uptake and survival of various phenotypes of GAS bacteria relevant for colonization and recurrent infection.

Organotypic 3D Co-Culture of Human Pleura as a Novel In Vitro Model of Staphylococcus aureus Infection and Biofilm Development

Bacterial pleural infections are associated with high mortality. Treatment is complicated due to biofilm formation. A common causative pathogen is Staphylococcus aureus (S. aureus). Since it is distinctly human-specific, rodent models do not provide adequate conditions for research. The purpose of this study was to examine the effects of S. aureus infection on human pleural mesothelial cells using a recently established 3D organotypic co-culture model of pleura derived from human specimens. After infection of our model with S. aureus, samples were harvested at defined time points. Histological analysis and immunostaining for tight junction proteins (c-Jun, VE-cadherin, and ZO-1) were performed, demonstrating changes comparable to in vivo empyema. The measurement of secreted cytokine levels (TNF-α, MCP-1, and IL-1β) proved host-pathogen interactions in our model. Similarly, mesothelial cells produced VEGF on in vivo levels. These findings were contrasted by vital, unimpaired cells in a sterile control model. We were able to establish a 3D organotypic in vitro co-culture model of human pleura infected with S. aureus resulting in the formation of biofilm, including host-pathogen interactions. This novel model could be a useful microenvironment tool for in vitro studies on biofilm in pleural empyema.

Trans-cinnamaldehyde-Biosurfactant Complex as a Potent Agent against Enterococcus faecalis Biofilms

Enterococcus faecalis is an opportunistic microbial pathogen frequently associated with diverse infections, including those of the skin and teeth, as well as those from surgical wounds. It forms robust biofilms that are highly tolerant to most antimicrobials and first-line antibiotics. Therefore, investigating alternative strategies to eradicate its biofilms is a critical need. We recently demonstrated that trans-cinnamaldehyde (TC) potently kills E. faecalis biofilm cells and prevents biofilm recovery, and yet, the extreme hydrophobicity of TC hampers clinical translation. Here, we report that a complex of TC with an FDA-approved biosurfactant (acidic sophorolipid/ASL) significantly reduces the bacterial viability and biomass of E. faecalis biofilms, compared to TC alone. A confocal laser-scanning microscopic analysis demonstrated that the TC-ASL treatment significantly decreased the biofilm thickness and volume. In conclusion, our study highlights the anti-biofilm potential of the newly developed TC-ASL.

Consistent Biofilm Formation by Streptococcus pyogenes emm 1 Isolated From Patients With Necrotizing Soft Tissue Infections

Objectives

Biofilm formation has been demonstrated in muscle and soft tissue samples from patients with necrotizing soft tissue infection (NSTI) caused by Streptococcus pyogenes, but the clinical importance of this observation is not clear. Although M-protein has been shown to be important for in vitro biofilm formation in S. pyogenes, the evidence for an association between emm type and biofilm forming capacity is conflicting. Here we characterize the biofilm forming capacity in a collection of S. pyogenes isolates causing NSTI, and relate this to emm type of the isolates and clinical characteristics of the patients.

Methods

Bacterial isolates and clinical data were obtained from NSTI patients enrolled in a multicenter prospective observational study. Biofilm forming capacity was determined using a microtiter plate assay.

Results

Among 57 cases, the three most frequently encountered emm types were emm1 (n = 22), emm3 (n = 13), and emm28 (n = 7). The distribution of biofilm forming capacity in emm1 was qualitatively (narrow-ranged normal distribution) and quantitatively (21/22 isolates in the intermediate range) different from other emm types (wide ranged, multimodal distribution with 5/35 isolates in the same range as emm1). There were no significant associations between biofilm forming capacity and clinical characteristics of the patients.

Conclusions

The biofilm forming capacity of emm1 isolates was uniform and differed significantly from other emm types. The impact of biofilm formation in NSTI caused by S. pyogenes on clinical outcomes remains uncertain.

Clinically relevant in vitro biofilm models: A need to mimic and recapitulate the host environment

Biofilm-associated infections are difficult to treat and eradicate because of their increased antimicrobial tolerance. In vitro biofilm models have enabled the high throughput testing of an array of differing novel antimicrobials and treatment strategies. However, biofilms formed in these oftentimes basic in vitro systems do not resemble biofilms seen in vivo. As a result, translatability from the lab to the clinic is poor or limited. To improve translatability, in vitro models must better recapitulate the host environment. This review describes and critically evaluates new and innovative in vitro models that better mimic the environments of a variety of clinically important, biofilm-associated infections of the skin, oropharynx, lungs, and infections related to indwelling implants and medical devices. This review highlights that many of these models represent considerable advances in the field of biofilm research and help to translate laboratory findings into the clinical practice.

Streptococcus pyogenes TrxSR Two-Component System Regulates Biofilm Production in Acidic Environments

Bacteria form biofilms for their protection against environmental stress and produce virulence factors within the biofilm. Biofilm formation in acidified environments is regulated by a two-component system, as shown by studies on isogenic mutants of the sensor protein of the two-component regulatory system in Streptococcus pyogenes. In this study, we found that the LiaS histidine kinase sensor mediates biofilm production and pilus expression in an acidified environment through glucose fermentation. The liaS isogenic mutant produced biofilms in a culture acidified by hydrochloric acid but not glucose, suggesting that the acidified environment is sensed by another protein. In addition, the trxS isogenic mutant could not produce biofilms or activate the mga promoter in an acidified environment. Mass spectrometry analysis showed that TrxS regulates M protein, consistent with the transcriptional regulation of emm, which encodes M protein. Our results demonstrate that biofilm production during environmental acidification is directly under the control of TrxS.

Development and assessment of a high-throughput biofilm and biomass testing platform

OBJECTIVE

To develop and evaluate a simple platform technology for developing static biofilms in a 96-well microtitre plate for various downstream applications. The technology allows monitoring of growth rate, biofilm formation and quantifying biofilm biomass by using crystal violet (CV) and safranin O (SO) staining over seven-day time periods for pathogens including clinical isolates most commonly associated with hard-to-treat wound infections.

METHOD

A total of 157 bacteria including Acinetobacter, Enterobacter, Klebsiella, Pseudomonas and Staphylococcus spp. were used in the study. Bacterial growth was measured at 600nm optical density (OD). Biofilm formation was monitored and assessed quantitatively with CV at 570nm and SO staining at 492nm for one-, two-, three- and seven-day incubation periods.

RESULTS

Bacterial growth rate and static biofilm biomass in the 96-well plates varied for various strains tested. Both CV and SO staining showed similar results in the biomass, with SO assay displaying more reproducible data throughout the study. Most of the strains were metabolically active even at the seven-day incubation period. Microbial adherences of all bacterial strains on the plastic surface was assessed with CV staining: 28 Acinetobacter, 17 Staphylococcus, 12 Pseudomonas and four Enterobacter strains were strong biofilm producers. Moderate biofilm-producing strains included 27 Staphylococcus, 14 Acinetobacter, eight Pseudomonas and three Enterobacter. Weak biofilm-producing strains included: 33 Staphylococcus, six Enterobacter, two Pseudomonas and one Acinetobacter. Only one Pseudomonas aeruginosa strain did not develop biofilm.

CONCLUSION

Our results demonstrate the feasibility of using 96-well microtitre plates as a high-throughput platform for quantitative measurement and assessment of biofilm development over time. Studying microbial adherence or biofilm biomass generated on various surfaces using a high-throughput system could provide valuable information for in vitro testing and developing therapeutics for biofilm infections. Employing the biofilm testing platform described in this study makes it possible to simultaneously develop different biofilms formed by specific pathogens, and study potential association between the quantity of bacterial biomass and strength of a biofilm formed by specific wound pathogens. In addition, the described testing approach could provide an optimal model for standardised and high-throughput screening of candidate antibiofilm therapeutics.

An optimised GAS-pharyngeal cell biofilm model

Group A Streptococcus (GAS) causes 700 million infections and accounts for half a million deaths per year. Biofilm formation has been implicated in both pharyngeal and dermal GAS infections. In vitro, plate-based assays have shown that several GAS M-types form biofilms, and multiple GAS virulence factors have been linked to biofilm formation. Although the contributions of these plate-based studies have been valuable, most have failed to mimic the host environment, with many studies utilising abiotic surfaces. GAS is a human specific pathogen, and colonisation and subsequent biofilm formation is likely facilitated by distinct interactions with host tissue surfaces. As such, a host cell-GAS model has been optimised to support and grow GAS biofilms of a variety of GAS M-types. Improvements and adjustments to the crystal violet biofilm biomass assay have also been tailored to reproducibly detect delicate GAS biofilms. We propose 72 h as an optimal growth period for yielding detectable biofilm biomass. GAS biofilms formed are robust and durable, and can be reproducibly assessed via staining/washing intensive assays such as crystal violet with the aid of methanol fixation prior to staining. Lastly, SEM imaging of GAS biofilms formed by this model revealed GAS cocci chains arranged into three-dimensional aggregated structures with EPS matrix material. Taken together, we outline an efficacious GAS biofilm pharyngeal cell model that can support long-term GAS biofilm formation, with biofilms formed closely resembling those seen in vivo.

Genetics, Structure, and Function of Group A Streptococcal Pili

Streptococcus pyogenes (Group A Streptococcus; GAS) is an exclusively human pathogen. This bacterial species is responsible for a large variety of infections, ranging from purulent but mostly self-limiting oropharynx/skin diseases to streptococcal sequelae, including glomerulonephritis and rheumatic fever, as well as life-threatening streptococcal toxic-shock syndrome. GAS displays a wide array of surface proteins, with antigenicity of the M protein and pili utilized for M- and T-serotyping, respectively. Since the discovery of GAS pili in 2005, their genetic features, including regulation of expression, and structural features, including assembly mechanisms and protein conformation, as well as their functional role in GAS pathogenesis have been intensively examined. Moreover, their potential as vaccine antigens has been studied in detail. Pilus biogenesis-related genes are located in a discrete section of the GAS genome encoding fibronectin and collagen binding proteins and trypsin-resistant antigens (FCT region). Based on the heterogeneity of genetic composition and DNA sequences, this region is currently classified into nine distinguishable forms. Pili and fibronectin-binding proteins encoded in the FCT region are known to be correlated with infection sites, such as the skin and throat, possibly contributing to tissue tropism. As also found for pili of other Gram-positive bacterial pathogens, GAS pilin proteins polymerize via isopeptide bonds, while intramolecular isopeptide bonds present in the pilin provide increased resistance to degradation by proteases. As supported by findings showing that the main subunit is primarily responsible for T-serotyping antigenicity, pilus functions and gene expression modes are divergent. GAS pili serve as adhesins for tonsillar tissues and keratinocyte cell lines. Of note, a minor subunit is considered to have a harpoon function by which covalent thioester bonds with host ligands are formed. Additionally, GAS pili participate in biofilm formation and evasion of the immune system in a serotype/strain-specific manner. These multiple functions highlight crucial roles of pili during the onset of GAS infection. This review summarizes the current state of the art regarding GAS pili, including a new mode of host-GAS interaction mediated by pili, along with insights into pilus expression in terms of tissue tropism.

Pathogenic Mechanisms of Streptococcal Necrotizing Soft Tissue Infections

Necrotizing skin and soft tissue infections (NSTIs) are severe life-threatening and rapidly progressing infections. Beta-hemolytic streptococci, particularly S. pyogenes (group A streptococci (GAS)) but also S. dysgalactiae subsp. equisimilis (SDSE, most group G and C streptococcus), are the main causative agents of monomicrobial NSTIs and certain types, such as emm1 and emm3, are over-represented in NSTI cases. An arsenal of bacterial virulence factors contribute to disease pathogenesis, which is a complex and multifactorial process. In this chapter, we summarize data that have provided mechanistic and immuno-pathologic insight into host-pathogens interactions that contribute to tissue pathology in streptococcal NSTIs. The role of streptococcal surface associated and secreted factors contributing to the hyper-inflammatory state and immune evasion, bacterial load in the tissue and persistence strategies, including intracellular survival and biofilm formation, as well as strategies to mimic NSTIs in vitro are discussed.

Singularities of Pyogenic Streptococcal Biofilms - From Formation to Health Implication

Biofilms are generally defined as communities of cells involved in a self-produced extracellular matrix adhered to a surface. In biofilms, the bacteria are less sensitive to host defense mechanisms and antimicrobial agents, due to multiple strategies, that involve modulation of gene expression, controlled metabolic rate, intercellular communication, composition, and 3D architecture of the extracellular matrix. These factors play a key role in streptococci pathogenesis, contributing to therapy failure and promoting persistent infections. The species of the pyogenic group together with Streptococcus pneumoniae are the major pathogens belonging the genus Streptococcus, and its biofilm growth has been investigated, but insights in the genetic origin of biofilm formation are limited. This review summarizes pyogenic streptococci biofilms with details on constitution, formation, and virulence factors associated with formation.

MFAP9: Characterization of an extracellular thermostable antibacterial peptide from marine fungus with biofilm eradication potential

An extracellular thermostable antibacterial peptide designated as MFAP9 was purified from marine Aspergillus fumigatus BTMF9 by ammonium sulfate precipitation followed by ion exchange chromatography on a DEAE-sepharose column. The molecular weight of MFAP9 was found to be∼3 kDa in SDS-PAGE gel corresponding a single intensity peak in MALDI-TOF. The distinct peak with a retention time of 32.5 min appeared in high performance liquid chromatography (HPLC), further confirming the purity. Isoelectric focusing, two-dimensional gel electrophoresis and peptide mass fingerprinting were performed for the characterization of MFAP9. Functional analysis of purified MFAP9 exhibited strong antibacterial activity against Bacillus circulans (NCIM 2107) with MIC and MBC values of 0.525 μg/mL and 4.2 μg/mL, respectively. The in vitro antibiofilm effect of MFAP9 was analyzed against bacteria which have strong biofilm forming potential. The antibiofilm effect of MFAP9 treatment on Bacillus pumilus was examined using scanning electron microscopy. MFAP9 was found to be active at high temperatures and a wide range of pH (28). In addition, it showed varied sensitivity towards proteolytic enzymes. The peptide was nontoxic to human RBCs at higher concentrations. These results indicate that MFAP9 is an antibacterial peptide, suitable for the development of novel anti-infective agent with strong antibiofilm potential.

Effect of Coptis chinensis on Biofilm Formation and Antibiotic Susceptibility in Mycobacterium abscessus

Mycobacterium abscessus infections are notoriously difficult to be treated and newer treatment options are required. Coptis chinensis (C. chinensis) and its main compound berberine are frequently used to treat bacterial and viral infections. In this study, the susceptibility of M. abscessus to C. chinensis extract and berberine was assessed by minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) evaluation. The effects of C. chinensis and berberine on biofilm formation and antibiotic susceptibility in M. abscessus were observed. C. chinensis at concentrations of MIC (1.5 mg/mL) and 2 × MIC (3.0 mg/mL) and berberine at ½ × MIC (0.125 mg/mL) demonstrated a strong inhibition of biofilm formation. Concentration of C. chinensis at ½ × MIC resulted in a significant reduction in MICs of trimethoprim/sulfamethoxazole (TMP/SXT), clarithromycin (CLA), and linezolid (LZD). Similarly, ½ × MIC berberine had a significant effect on the MIC reductions of nine antibiotics including TMP/SXT, CLA, and LZD. Notably, the resistance level MIC of LZD against M. abscessus was reversed to a susceptible level by treatment with either C. chinensis or berberine. Therefore, C. chinensis and berberine have the potential to produce a synergistic antimycobacterial effect, reduce biofilm formation, and decrease antibacterial resistance to LZD in M. abscessus.

Deciphering Streptococcal Biofilms

Streptococci are a diverse group of bacteria, which are mostly commensals but also cause a considerable proportion of life-threatening infections. They colonize many different host niches such as the oral cavity, the respiratory, gastrointestinal, and urogenital tract. While these host compartments impose different environmental conditions, many streptococci form biofilms on mucosal membranes facilitating their prolonged survival. In response to environmental conditions or stimuli, bacteria experience profound physiologic and metabolic changes during biofilm formation. While investigating bacterial cells under planktonic and biofilm conditions, various genes have been identified that are important for the initial step of biofilm formation. Expression patterns of these genes during the transition from planktonic to biofilm growth suggest a highly regulated and complex process. Biofilms as a bacterial survival strategy allow evasion of host immunity and protection against antibiotic therapy. However, the exact mechanisms by which biofilm-associated bacteria cause disease are poorly understood. Therefore, advanced molecular techniques are employed to identify gene(s) or protein(s) as targets for the development of antibiofilm therapeutic approaches. We review our current understanding of biofilm formation in different streptococci and how biofilm production may alter virulence-associated characteristics of these species. In addition, we have summarized the role of surface proteins especially pili proteins in biofilm formation. This review will provide an overview of strategies which may be exploited for developing novel approaches against biofilm-related streptococcal infections.

Assessing the Role of Pharyngeal Cell Surface Glycans in Group A Streptococcus Biofilm Formation

Group A Streptococcus (GAS) causes 700 million infections and accounts for half a million deaths per year. Antibiotic treatment failure rates of 20–40% have been observed. The role host cell glycans play in GAS biofilm formation in the context of GAS pharyngitis and subsequent antibiotic treatment failure has not been previously investigated. GAS serotype M12 GAS biofilms were assessed for biofilm formation on Detroit 562 pharyngeal cell monolayers following enzymatic removal of all N-linked glycans from pharyngeal cells with PNGase F. Removal of N-linked glycans resulted in an increase in biofilm biomass compared to untreated controls. Further investigation into the removal of terminal mannose and sialic acid residues with α1-6 mannosidase and the broad specificity sialidase (Sialidase A) also found that biofilm biomass increased significantly when compared to untreated controls. Increases in biofilm biomass were associated with increased production of extracellular polymeric substances (EPS). Furthermore, it was found that M12 GAS biofilms grown on untreated pharyngeal monolayers exhibited a 2500-fold increase in penicillin tolerance compared to planktonic GAS. Pre-treatment of monolayers with exoglycosidases resulted in a further doubling of penicillin tolerance in resultant biofilms. Lastly, an additional eight GAS emm-types were assessed for biofilm formation in response to terminal mannose and sialic acid residue removal. As seen for M12, biofilm biomass on monolayers increased following removal of terminal mannose and sialic acid residues. Collectively, these data demonstrate that pharyngeal cell surface glycan structures directly impact GAS biofilm formation in a strain and glycan specific fashion.

Streptococcus agalactiae clinical isolates in Northwest Iran: antibiotic susceptibility, molecular typing, and biofilm formation

Background: Group B Streptococcus (S. agalactiae) is one of the colonizing bacteria in pregnant women which can be a causative agent of meningitis and neonatal sepsis. This organism has also been increasingly related to invasive infections in non-pregnant adults. Objective: In present study, we aimed to characterize the clonality of biofilm-producing S. agalactiae isolates from various sources from two different clinical laboratories in Tehran, Iran. Materials and Methods: S. agalactiae isolates were collected from community-acquired (CA) and hospital-acquired (HA) infections in pregnant and non-pregnant adults. The antimicrobial susceptibility patterns and biofilm formation ability were determined. In addition, pulse field gel electrophoresis (PFGE) was used to verify the clonal diversity of isolates. Results: Out of the 87 isolates, 15 (16.6%) formed biofilm. The antibiotic resistance rate was 98.85% for clindamycin, 98.85% for tetracycline, followed by 29.88% for erythromycin, 9.19% for moxifloxacin and 6.89% for levofloxacin. The PFGE patterns revealed a total of 16 different clusters consisting of 6 single types (STs). Conclusion: This study evaluated the biofilm formation of clinical S. agalactiae, which may be a step towards understanding its role in pathological processes. Biofilm formation was significant only in the hypervirulent ST-17 clone. Intraclonal spread of isolates indicates that a local lineage of isolates is responsible for infection by these bacteria.

A Role of Epithelial Cells and Virulence Factors in Biofilm Formation by Streptococcus pyogenes In Vitro

Biofilm formation by Streptococcus pyogenes (group A streptococcus [GAS]) in model systems mimicking the respiratory tract is poorly documented. Most studies have been conducted on abiotic surfaces, which poorly represent human tissues. We have previously shown that GAS forms mature and antibiotic-resistant biofilms on physiologically relevant epithelial cells. However, the roles of the substratum, extracellular matrix (ECM) components, and GAS virulence factors in biofilm formation and structure are unclear. In this study, biofilm formation was measured on respiratory epithelial cells and keratinocytes by determining biomass and antibiotic resistance, and biofilm morphology was visualized using scanning electron microscopy. All GAS isolates tested formed biofilms that had similar, albeit not identical, biomass and antibiotic resistance for both cell types. Interestingly, functionally mature biofilms formed more rapidly on keratinocytes but were structurally denser and coated with more ECM on respiratory epithelial cells. The ECM was crucial for biofilm integrity, as protein- and DNA-degrading enzymes induced bacterial release from biofilms. Abiotic surfaces supported biofilm formation, but these biofilms were structurally less dense and organized. No major role for M protein, capsule, or streptolysin O was observed in biofilm formation on epithelial cells, although some morphological differences were detected. NAD-glycohydrolase was required for optimal biofilm formation, whereas streptolysin S and cysteine protease SpeB impaired this process. Finally, no correlation was found between cell adherence or autoaggregation and GAS biofilm formation. Combined, these results provide a better understanding of the role of biofilm formation in GAS pathogenesis and can potentially provide novel targets for future treatments against GAS infections.

Current Understanding of Group A Streptococcal Biofilms

Background:

It has been proposed that GAS may form biofilms. Biofilms are microbial communities that aggregate on a surface, and exist within a self-produced matrix of extracellular polymeric substances. Biofilms offer bacteria an increased survival advantage, in which bacteria persist, and resist host immunity and antimicrobial treatment. The biofilm phenotype has long been recognized as a virulence mechanism for many Gram-positive and Gram-negative bacteria, however very little is known about the role of biofilms in GAS pathogenesis.

Objective:

This review provides an overview of the current knowledge of biofilms in GAS pathogenesis. This review assesses the evidence of GAS biofilm formation, the role of GAS virulence factors in GAS biofilm formation, modelling GAS biofilms, and discusses the polymicrobial nature of biofilms in the oropharynx in relation to GAS.

Conclusion:

Further study is needed to improve the current understanding of GAS as both a mono-species biofilm, and as a member of a polymicrobial biofilm. Improved modelling of GAS biofilm formation in settings closely mimicking in vivo conditions will ensure that biofilms generated in the lab closely reflect those occurring during clinical infection.

Morphology and Ultrastructure of Group A Streptococcus Biofilms

Light and electron microscopy enables researchers to study the ultrastructure of GAS biofilms formed on abiotic surfaces. Chains of streptococci surrounded by a bluish film are seen under a light microscope after alcian blue staining of preparations grown on coverslips. The extracellular matrix (indicator of biofilm maturity) becomes visible on ultrathin sections in transmission electron microscopy after additional staining with alcian blue; filamentous structures, characteristic of biofilm, are observed in intercellular spaces. The data obtained by scanning electron microscopy also demonstrate the presence of biofilm.

Pathogenicity Characterization of Prevalent-Type Streptococcus dysgalactiae subsp. equisimilis Strains

Streptococcus dysgalactiae subsp. equisimilis (SDSE) is an emerging human pathogen that causes severe invasive streptococcal diseases. Recent reports have shown that SDSE exhibits high pathogenicity with different mechanisms from that of Streptococcus pyogenes, although the two streptococci possess some common virulence factors such as streptolysin, streptokinase, and cell-binding proteins. To date, only a few studies have examined the variety of mechanisms expressing the pathogenicity of SDSE. Among nine SDSE clinical isolates sequenced in this study, we present in vitro and in vivo analyses of KNZ01 and KNZ03, whose emm and multilocus species types (MLSTs) are prevalent in Japan and other countries. For the comparison of pathogenicity, we also utilized the ATCC 12394 strain. The whole-genome analysis showed that KNZ03 and ATCC 12394 are categorized into an identical clonal complex by MLST and are phylogenetically close. However, the three strains exhibited different characteristics for pathogenicity in vitro; ATCC 12394 showed significant cytotoxicity to human keratinocytes and release of streptolysin O (SLO) compared to KNZ01 and KNZ03; KNZ03 exhibited significantly high hemolytic activity, but did not secrete SLO. KNZ01 and KNZ03 adhered to human keratinocytes at a higher rate than ATCC 12394; KNZ03 showed a higher rate of survival after a brief (30 min) incubation with human neutrophils compared to the other two strains; also, KNZ01 grew more rapidly in the presence of human serum. In vivo subcutaneous infection commonly resulted in ulcer formation in the three strains 7 days after infection. KNZ01-infected mice showed significant body weight loss 2 days after infection. Besides, on post-infection day 2, only KNZ01 remained in the cutaneous tissues of mice. Scanning electron microscopy analysis revealed that KNZ01 formed an extracellular structure (biofilm), which was probably composed of cell wall-anchoring proteins, in the presence of glucose and human serum. The extracellular structure of ATCC 12394 was also changed dramatically in response to culture conditions, whereas that of KNZ03 did not. Our study proposed that each SDSE strain possesses different virulence factors characteristics for mediating pathogenicity in humans.

Cellular chaining influences biofilm formation and structure in group A Streptococcus

Group A Streptococcal (GAS) biofilm formation is an important pathological feature contributing to the antibiotic tolerance and progression of various GAS infections. Although a number of bacterial factors have been described to promote in vitro GAS biofilm formation, the relevance of in vitro biofilms to host-associated biofilms requires further understanding. In this study, we demonstrate how constituents of the host environment, such as lysozyme and NaCl, can modulate GAS bacterial chain length and, in turn, shape GAS biofilm morphology and structure. Disruption of GAS chains with lysozyme results in biofilms that are more stable. Based on confocal microscopy, we attribute the increase in biofilm stability to a dense and compact three-dimensional structure produced by de-chained cells. To show that changes in biofilm stability and structure are due to the shortening of bacterial chains and not specific to the activity of lysozyme, we demonstrate that augmented chaining induced by NaCl or deletion of the autolysin gene mur1.2 produced defects in biofilm formation characterized by a loose biofilm architecture. We conclude that GAS biofilm formation can be directly influenced by host and environmental factors through the modulation of bacterial chain length, potentially contributing to persistence and colonization within the host. Further studies of in vitro biofilm models incorporating physiological constituents such as lysozyme may uncover new insights into the physiology of in vivo GAS biofilms.

Probiotic potency of Lactobacillus plantarum KX519413 and KX519414 isolated from honey bee gut

The Indian honey bee Apis cerana indica, which harbors an abundant and diverse range of lactic acid bacteria (LAB) in their gut with beneficial effects, was used as the source for the isolation of LAB. In the present study, two LAB isolates from honey bee gut were selected primarily based on their phenotypic and selective biochemical characterization, followed by PCR and identified using 16S rRNA sequencing as Lactobacillus plantarum and were registered in National Centre for Biotechnology Information under accession number KX519413 and KX519414. The probiotic potency of test strains indicated their survivability at acidic pH, bile salts and viability in simulated gastric juice enabling them to withstand gastrointestinal tract conditions. Evaluation of cell surface properties suggested that they possess an important defense mechanism against the pathogen since they are hydrophobic, auto-aggregative and have co-aggregative ability. Further, efficient exopolysaccharide production by them indicates not only their ability to enrich biofilm formation and auto-aggregation, but also enhances bacterial adhesion and colonization on the host intestinal tract. The present study concluded that L. plantarum from the gut of Apis cerana indica possesses probiotic potency, and potential candidates for use as food besides application in nutraceutical and pharmaceutical industries.

Streptococcus pyogenes Capsule Promotes Microcolony-Independent Biofilm Formation

Biofilms play an important role in the pathogenesis of group A streptococcus (GAS), a Gram-positive pathogen responsible for a wide range of infections and with a significant public health impact. Although most GAS serotypes are able to form biofilms, there is a large amount of heterogeneity between individual strains in biofilm formation, as measured by standard crystal violet assays. It is generally accepted that biofilm formation includes the initial adhesion of bacterial cells to a surface followed by microcolony formation, biofilm maturation, and extensive production of extracellular matrix that links together proliferating cells and provides a scaffold for the three-dimensional (3D) biofilm structure. However, our studies show that for GAS strain JS95, microcolony formation is not an essential step in static biofilm formation, and instead, biofilm can be effectively formed from slow-growing or nonreplicating late-exponential- or early-stationary-phase planktonic cells via sedimentation and fixation of GAS chains. In addition, we show that the GAS capsule specifically contributes to the alternative sedimentation-initiated biofilms. Microcolony-independent sedimentation biofilms are similar in morphology and 3D structure to biofilms initiated by actively dividing planktonic bacteria. We conclude that GAS can form biofilms by an alternate noncanonical mechanism that does not require transition from microcolony formation to biofilm maturation and which may be obscured by biofilm phenotypes that arise via the classical biofilm maturation processes.IMPORTANCE The static biofilm assay is a common tool for easy biomass quantification of biofilm-forming bacteria. However, Streptococcus pyogenes biofilm formation as measured by the static assay is strain dependent and yields heterogeneous results for different strains of the same serotype. In this study, we show that two independent mechanisms, for which the protective capsule contributes opposing functions, may contribute to static biofilm formation. We propose that separation of these mechanisms for biofilm formation might uncover previously unappreciated biofilm phenotypes that may otherwise be masked in the classic static assay.

The 5'-nucleotidase S5nA is dispensable for evasion of phagocytosis and biofilm formation in Streptococcus pyogenes

5’-nucleotidases are widespread among all domains of life. The enzymes hydrolyze phosphate residues from nucleotides and nucleotide derivatives. In some pathobiontic bacteria, 5’-nucleotidases contribute to immune evasion by dephosphorylating adenosine mono-, di-, or tri-phosphates, thereby either decreasing the concentration of pro-inflammatory ATP or increasing the concentration of anti-inflammatory adenosine, both acting on purinergic receptors of phagocytic cells. The strict human pathogen Streptococcus pyogenes expresses a surface-associated 5’-nucleotidase (S5nA) under infection conditions that has previously been discussed as a potential virulence factor. Here we show that deletion of the S5nA gene does not significantly affect growth in human blood, evasion of phagocytosis by neutrophils, formation of biofilms and virulence in an infection model with larvae of the greater wax moth Galleria mellonella in S. pyogenes serotypes M6, M18 and M49. Hence, the surface-associated 5’-nucleotidase S5nA seems dispensable for evasion of phagocytosis and biofilm formation in S. pyogenes.

Phosphorylated OmpR Is Required for Type 3 Fimbriae Expression in Klebsiella pneumoniae Under Hypertonic Conditions

OmpR/EnvZ is a two-component system that senses osmotic signals and controls downstream gene expression in many species of Enterobacteriaceae. However, the role of OmpR/EnvZ in Klebsiella pneumoniae remains unknown. In this study, we found that production of MrkA, the major subunit of type 3 fimbriae, was decreased under hypertonic conditions. A deletion mutant of ompR and a site-directed mutant with a single amino acid substitution of aspartate 55 to alanine (D55A), which mimics the unphosphorylated form of OmpR, markedly reduced MrkA production under hypertonic conditions. These results indicate that K. pneumoniae type 3 fimbriae expression is activated by the phosphorylated form of OmpR (OmpR∼P). Although no typical OmpR∼P binding site was found in the P_mrkA sequence, mrkA mRNA levels and P_mrkA activity were decreased in the ΔompR and ompR_D55A strains compared with the wild type (WT) strain, indicating that OmpR∼P mediates type 3 fimbriae expression at the transcriptional level. Previous reports have demonstrated that a cyclic-di-GMP (c-di-GMP) related gene cluster, mrkHIJ, regulates the expression of type 3 fimbriae. We found that both the ompR and ompR_D55A mutants exhibited decreased mrkHIJ mRNA levels, intracellular c-di-GMP concentration, and bacterial biofilm amount, but increased total intracellular phosphodiesterase activity in response to hypertonic conditions. These results indicate that OmpR∼P regulates type 3 fimbriae expression to influence K. pneumoniae biofilm formation via MrkHIJ and modulation of intracellular c-di-GMP levels. Taken together, we herein provide evidence that OmpR∼P acts as a critical factor in the regulation of the c-di-GMP signaling pathway, type 3 fimbriae expression, and biofilm amount in K. pneumoniae in response to osmotic stresses.

Alkaline Phosphatase Activity of Staphylococcus aureus Grown in Biofilm and Suspension Cultures

Staphylococcus aureus is known for its resistance to antibiotic treatment as well as the ability to form biofilms. Biofilm formation has been seen in S. aureus infections, yet, the mechanism of biofilm formation is not completely understood. Many molecules, such as DNA and polysaccharides, have been identified in the biofilm microenvironment, but little is known about the enzymes involved in the process. In this paper, alkaline phosphatase (ALP) activity was investigated in S. aureus grown either in biofilm or suspension cultures, achieved using DNase I. A significant increase of ALP activity was observed in S. aureus biofilm culture compared to its suspension counterpart. Treatment of sodium orthovanadate, an ALP inhibitor, significantly decreased biofilm formation. Its inhibition was on par with DNase I treatment at specific doses. Thus, ALP may play an important role in the biofilm formation. Likewise, ALP inhibition may be a novel target for anti-biofilm therapeutics.

Optical and Electron Microscopic Study of the Morphology and Ultrastructure of Biofilms Formed by Streptococcus pyogenes

Our study confirmed the capacity of S. pyogenes strains to form biofilms on abiotic surfaces. Chains of streptococci surrounded by bluish film were seen under a microscope after alcian blue staining of the preparations grown on slides. On ultrathin sections in transmission electron microscope, the extracellular matrix (indicator of biofilm maturity) became visible after staining with alcian blue. Microscopy of the sections shows structures characteristic of a biofilm in spaces between the cells. Scanning electron microscopy also demonstrates the presence of a biomembrane. Importantly that type 1M strain forming in fact no membranes when cultured on plastic plates (Costar) formed biofilms on the glass. It seems that the conditions for the biofilm formation on the plastic and on the glass differ, due to which the exopolymeric matrices formed on different surfaces vary by biochemical composition.

Antagonism Against Fish Pathogens by Cellular Components/Preparations of Bacillus coagulans (MTCC-9872) and It's In Vitro Probiotic Characterisation

Bacterial fish pathogens are pervasive in aquaculture. Control of bacterial fish pathogen is a difficult task among aquaculture practitioners. A large number of antibiotics are used for the control of prevalent bacterial pathogens in aquaculture. This may lead to drug resistance among pathogens and further treatment will be ineffective. Here, we can use probiotic bacteria as a biocontrol agent in fish disease and it is a novel field. In this study, antimicrobial potential of the bacterium Bacillus coagulans (MTCC-9872) has been evaluated through in vitro antagonistic activity of cellular preparations/components against potent pathogens. The cellular preparations/components such as Ethyl acetate extract, whole-cell product, heat-killed whole-cell product, and filtered broth were exhibited bactericidal activity against the tested pathogens. Bactericidal activity varied among different cellular preparation/components. The tested bacterium effectively produced biofilm as significant as tested positive control in a microtitre plate and effectively adhered on to the glass slide. In addition, the bacterium was capable of producing extracellular enzymes necessary for the digestion of food materials and was capable to grow in fish mucus from Oreochromis niloticus. The bacterium tolerated bile juice secreted by the host. Moreover, intraperitoneal injection of the bacterium did not induce any pathological signs, symptoms or mortalities in Oreochromis niloticus and revealed the safety of this bacterium in the fish.

A Method for Quantification of Epithelium Colonization Capacity by Pathogenic Bacteria

Most bacterial infections initiate at the mucosal epithelium lining the gastrointestinal, respiratory, and urogenital tracts. At these sites, bacterial pathogens must adhere and increase in numbers to effectively breach the outer barrier and invade the host. If the bacterium succeeds in reaching the bloodstream, effective dissemination again requires that bacteria in the blood, reestablish contact to distant endothelium sites and form secondary site foci. The infectious potential of bacteria is therefore closely linked to their ability to adhere to, colonize, and invade epithelial and endothelial surfaces. Measurement of bacterial adhesion to epithelial cells is therefore standard procedure in studies of bacterial virulence. Traditionally, such measurements have been conducted with microtiter plate cell cultures to which bacteria are added, followed by washing procedures and final quantification of retained bacteria by agar plating. This approach is fast and straightforward, but yields only a rough estimate of the adhesive properties of the bacteria upon contact, and little information on the ability of the bacterium to colonize these surfaces under relevant physiological conditions. Here, we present a method in which epithelia/endothelia are simulated by flow chamber-grown human cell layers, and infection is induced by seeding of pathogenic bacteria on these surfaces under conditions that simulate the physiological microenvironment. Quantification of bacterial adhesion and colonization of the cell layers is then performed by in situ time-lapse fluorescence microscopy and automatic detection of bacterial surface coverage. The method is demonstrated in three different infection models, simulating Staphylococcus aureus endothelial infection and Escherichia coli intestinal- and uroepithelial infection. The approach yields valuable information on the fitness of the bacterium to successfully adhere to and colonize epithelial surfaces and can be used to evaluate the influence of specific virulence genes, growth conditions, and antimicrobial treatment on this process.

Iron enhances the peptidyl deformylase activity and biofilm formation in Staphylococcus aureus

Staphylococcus aureus plays a major role in persistent infections and many of these species form structured biofilms on different surfaces which is accompanied by changes in gene expression profiles. Further, iron supplementation plays a critical role in the regulation of several protein(s)/enzyme function, which all aid in the development of active bacterial biofilms. It is well known that for each protein, deformylation is the most crucial step in biosynthesis and is catalyzed by peptidyl deformylase (PDF). Thus, the aim of the current study is to understand the role of iron in biofilm formation and deformylase activity of PDF. Hence, the PDF gene of S. aureus ATCC12600 was PCR amplified using specific primers and sequenced, followed by cloning and expression in Escherichia coli DH5α. The deformylase activity of the purified recombinant PDF was measured in culture supplemented with/without iron where the purified rPDF showed K_m of 1.3 mM and V_max of 0.035 mM/mg/min, which was close to the native PDF (K_m = 1.4 mM, V_max = 0.030 mM/mg/min). Interestingly, the K_m decreased and PDF activity increased when the culture was supplemented with iron, corroborating with qPCR results showing 100- to 150-fold more expression compared to control in S. aureus and its drug-resistant strains. Further biofilm-forming units (BU) showed an incredible increase (0.42 ± 0.005 to 6.3 ± 0.05 BU), i.e., almost 15-fold elevation in anaerobic conditions, indicating the significance of iron in S. aureus biofilms.

Antibacterial activity of autochthonous bacteria isolated from Anabas testudineus (Bloch, 1792) and it's in vitro probiotic characterization

Antimicrobial potentials of bacteria isolated from Anabas testudineus have been evaluated through in vitro antagonistic activity against potent fish pathogens. The cellular components and filtered culture medium were effective against six fish pathogens. Altogether 110 strains were isolated from the fish gut, out of which 10 strains were selected through well diffusion method. From them, a strain HGA8B having cumulative maximum score was selected as candidate probiotic. The whole-cell product, heat-killed whole-cell product, Ethyl acetate extract, and the filtered broth were exhibited bactericidal activity against the tested pathogens. In addition, the isolated bacterium was capable of producing extracellular enzymes important for the digestion of food materials and was capable of growth in fish mucus from Oreochromis niloticus. The strain tolerated bile juice secreted by the host and effectively produced biofilm. Analysis of 16S rDNA sequence revealed that isolated strain HGA8B was Bacillus sp. (MF351637). Furthermore, intraperitoneal injection of the bacterium did not induce any pathological signs, symptoms or mortalities in Oreochromis niloticus and revealed the safety of this bacterium as a candidate probiotic in aquaculture.

IscR Regulation of Type 3 Fimbriae Expression in Klebsiella pneumoniae CG43

In Klebsiella pneumoniae, we have previously shown that IscR, an Fe–S cluster-containing transcriptional factor, plays a dual role in controlling capsular polysaccharide biosynthesis and iron-acquisition systems by switching between its holo and apo forms. In this study, the effect of IscR on type 3 fimbriae expression and biofilm formation was investigated. We found that production of the major subunit of type 3 fimbriae, MrkA, was increased in the ΔiscR and iscR_3CA strains, a strain expressing a mutant IscR that mimics apo-IscR, at both the translational and transcriptional levels. Based on the fact that type 3 fimbriae expression is the major factor affecting biofilm formation, increased biofilm formation was also found in ΔiscR or iscR_3CA, suggesting that holo-IscR represses biofilm formation. However, the repression of type 3 fimbriae expression by IscR is indirect. To further understand the regulatory mechanism of IscR, the effect of IscR on the expression of mrkHIJ, which encodes cyclic di-GMP (c-di-GMP)-related regulatory proteins that control type 3 fimbriae expression, was studied. We found that holo-IscR could directly repress mrkHI transcription, indicating that MrkHI is required for IscR regulation of type 3 fimbriae expression. Finally, deletion of iscR attenuated K. pneumoniae virulence in a peritonitis model of mouse infection, while the absence of the [2Fe–2S] cluster of IscR had no effect on K. pneumoniae virulence during infection. Taken together, our results demonstrate the underlying mechanism of the [2Fe–2S] cluster of IscR in controlling type 3 fimbriae expression and its effect on K. pneumoniae pathogenesis.

Investigation of the Fim1 putative pilus locus of Streptococcus equi subspecies equi

The Gram-positive bacterium Streptococcus equi subspecies equi (S. equi) is the causative agent of strangles, among the most frequently diagnosed infectious diseases of horses worldwide. Genome analysis of S. equi strain 4047 (Se4047) identified a putative operon, Fim1, with similarity to the pilus loci of other Gram-positive bacteria. The Fim1 locus was present in all strains of S. equi and its close relative S. equi subspecies zooepidemicus (S. zooepidemicus) that have been studied to date. In this study we provide evidence that the putative structural pilus proteins, SEQ_0936 and CNE, are produced on the cell surface during in vitro growth and in vivo infection. Although the proteins encoded within the Fim1 locus are not essential for attachment or biofilm formation, over-transcription of SEQ_0936 and CNE enhanced attachment to equine tissue in vitro. Our data suggest that whilst the Fim1 locus does not produce a polymerized pilus structure, the products of the Fim1 locus may fulfil an adhesive function. The putative pilus-associated regulator, tetR, which contains a nonsense mutation in S. equi, was able to regulate transcription of the Fim1 locus following repair and over-transcription, confirming its predicted role in the operon.

Collagen-like proteins of pathogenic streptococci

The collagen domain, which is defined by the presence of the Gly-X-Y triplet repeats, is amongst the most versatile and widespread known structures found in proteins from organisms representing all three domains of life. The streptococcal collagen-like (Scl) proteins are widely present in pathogenic streptococci, including Streptococcus pyogenes, S. agalactiae, S. pneumoniae, and S. equi. Experiments and bioinformatic analyses support the hypothesis that all Scl proteins are homotrimeric and cell wall-anchored. These proteins contain the rod-shaped collagenous domain proximal to cell surface, as well as a variety of outermost non-collagenous domains that generally lack predicted functions but can be grouped into one of six clusters based on sequence similarity. The well-characterized Scl1 proteins of S. pyogenes show a dichotomous switch in ligand binding between human tissue and blood environments. In tissue, Scl1 adhesin specifically recognizes the wound microenvironment, promotes adhesion and biofilm formation, decreases bacterial killing by neutrophil extracellular traps, and modulates S. pyogenes virulence. In blood, ligands include components of complement and coagulation-fibrinolytic systems, as well as plasma lipoproteins. In all, the Scl proteins signify a large family of structurally related surface proteins, which contribute to the ability of streptococci to colonize and cause diseases in humans and animals.

Global Analysis and Comparison of the Transcriptomes and Proteomes of Group A Streptococcus Biofilms

Prokaryotes are thought to regulate their proteomes largely at the level of transcription. However, the results from this first set of global transcriptomic and proteomic analyses of paired microbial samples presented here show that this assumption is false for the majority of genes and their products in S. pyogenes. In addition, the tenuousness of the link between transcription and translation becomes even more pronounced when microbes exist in a biofilm or a stationary planktonic state. Since the transcriptome level does not usually equal the proteome level, the validity attributed to gene expression studies as well as proteomic studies in microbial analyses must be brought into question. Therefore, the results attained by either approach, whether RNA-seq or shotgun proteomics, must be taken in context and evaluated with particular care since they are by no means interchangeable.

To gain a better understanding of the genes and proteins involved in group A Streptococcus (GAS; Streptococcus pyogenes) biofilm growth, we analyzed the transcriptome, cellular proteome, and cell wall proteome from biofilms at different stages and compared them to those of plankton-stage GAS. Using high-throughput RNA sequencing (RNA-seq) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) shotgun proteomics, we found distinct expression profiles in the transcriptome and proteome. A total of 46 genes and 41 proteins showed expression across the majority of biofilm time points that was consistently higher or consistently lower than that seen across the majority of planktonic time points. However, there was little overlap between the genes and proteins on these two lists. In line with other studies comparing transcriptomic and proteomic data, the overall correlation between the two data sets was modest. Furthermore, correlation was poorest for biofilm samples. This suggests a high degree of regulation of protein expression by nontranscriptional mechanisms. This report illustrates the benefits and weaknesses of two different approaches to global expression profiling, and it also demonstrates the advantage of using proteomics in conjunction with transcriptomics to gain a more complete picture of global expression within biofilms. In addition, this report provides the fullest characterization of expression patterns in GAS biofilms currently available.

IMPORTANCE Prokaryotes are thought to regulate their proteomes largely at the level of transcription. However, the results from this first set of global transcriptomic and proteomic analyses of paired microbial samples presented here show that this assumption is false for the majority of genes and their products in S. pyogenes. In addition, the tenuousness of the link between transcription and translation becomes even more pronounced when microbes exist in a biofilm or a stationary planktonic state. Since the transcriptome level does not usually equal the proteome level, the validity attributed to gene expression studies as well as proteomic studies in microbial analyses must be brought into question. Therefore, the results attained by either approach, whether RNA-seq or shotgun proteomics, must be taken in context and evaluated with particular care since they are by no means interchangeable.

Microbiome, biofilms, and pneumonia in the ICU

PURPOSE OF REVIEW

Lower respiratory tract infections remain one of the leading causes of death in the world. Recently, the introduction of molecular methods based on DNA sequencing and microarrays for the identification of nonculturable microorganisms and subspecies variations has challenged the previous 'one bug - one disease' paradigm, providing us with a broader view on human microbial communities and their role in the development of infectious diseases. The purpose of this review is to describe recent understanding of the role of microbiome and bacterial biofilm in the development of lung infections, and, at the same time, to present new areas of research opportunities.

RECENT FINDINGS

The review describes recent literature in cystic fibrosis patients, chronic obstructive pulmonary disease patients, and literature in mechanically ventilated patients that helped to elucidate the role of microbiome and biofilm formation in the development of pneumonia.

SUMMARY

The characterization of the human microbiome and biofilms has changed our understanding of lower respiratory tract infections. More comprehensive, sensitive, and fast methods for bacterial, fungal, and viral detection are warranted to establish the colonization of the lower respiratory tract in healthy individuals and sick patients. Future research might explore the global bacterial, fungal, and viral pulmonary ecosystems and their interdependence to target novel preventive approaches and therapeutic strategies in chronic and acute lung infections.

Unique Footprint in the scl1.3 Locus Affects Adhesion and Biofilm Formation of the Invasive M3-Type Group A Streptococcus

The streptococcal collagen-like proteins 1 and 2 (Scl1 and Scl2) are major surface adhesins that are ubiquitous among group A Streptococcus (GAS). Invasive M3-type strains, however, have evolved two unique conserved features in the scl1 locus: (i) an IS1548 element insertion in the scl1 promoter region and (ii) a nonsense mutation within the scl1 coding sequence. The scl1 transcript is drastically reduced in M3-type GAS, contrasting with a high transcription level of scl1 allele in invasive M1-type GAS. This leads to a lack of Scl1 expression in M3 strains. In contrast, while scl2 transcription and Scl2 production are elevated in M3 strains, M1 GAS lack Scl2 surface expression. M3-type strains were shown to have reduced biofilm formation on inanimate surfaces coated with cellular fibronectin and laminin, and in human skin equivalents. Repair of the nonsense mutation and restoration of Scl1 expression on M3-GAS cells, restores biofilm formation on cellular fibronectin and laminin coatings. Inactivation of scl1 in biofilm-capable M28 and M41 strains results in larger skin lesions in a mouse model, indicating that lack of Scl1 adhesin promotes bacterial spread over localized infection. These studies suggest the uniquely evolved scl1 locus in the M3-type strains, which prevents surface expression of the major Scl1 adhesin, contributed to the emergence of the invasive M3-type strains. Furthermore these studies provide insight into the molecular mechanisms mediating colonization, biofilm formation, and pathogenesis of group A streptococci.

Composition and Activity of the Non-canonical Gram-positive SecY2 Complex

The accessory Sec system in Streptococcus gordonii DL1 is a specialized export system that transports a large serine-rich repeat protein, Hsa, to the bacterial surface. The system is composed of core proteins SecA2 and SecY2 and accessory Sec proteins Asp1–Asp5. Similar to canonical SecYEG, SecY2 forms a channel for translocation of the Hsa adhesin across the cytoplasmic membrane. Accessory Sec proteins Asp4 and Asp5 have been suggested to work alongside SecY2 to form the translocon, similar to the associated SecY, SecE, and SecG of the canonical system (SecYEG). To test this theory, S. gordonii secY2, asp4, and asp5 were co-expressed in Escherichia coli. The resultant complex was subsequently purified, and its composition was confirmed by mass spectrometry to be SecY2-Asp4-Asp5. Like SecYEG, the non-canonical complex activates the ATPase activity of the SecA motor (SecA2). This study also shows that Asp4 and Asp5 are necessary for optimal adhesion of S. gordonii to glycoproteins gp340 and fibronectin, known Hsa binding partners, as well as for early stage biofilm formation. This work opens new avenues for understanding the structure and function of the accessory Sec system.

Biofilm in group A streptococcal necrotizing soft tissue infections

Necrotizing fasciitis caused by group A streptococcus (GAS) is a life-threatening, rapidly progressing infection. At present, biofilm is not recognized as a potential problem in GAS necrotizing soft tissue infections (NSTI), as it is typically linked to chronic infections or associated with foreign devices. Here, we present a case of a previously healthy male presenting with NSTI caused by GAS. The infection persisted over 24 days, and the surgeon documented the presence of a "thick layer biofilm" in the fascia. Subsequent analysis of NSTI patient tissue biopsies prospectively included in a multicenter study revealed multiple areas of biofilm in 32% of the patients studied. Biopsies associated with biofilm formation were characterized by massive bacterial load, a pronounced inflammatory response, and clinical signs of more severe tissue involvement. In vitro infections of a human skin tissue model with GAS NSTI isolates also revealed multilayered fibrous biofilm structures, which were found to be under the control of the global Nra gene regulator. The finding of GAS biofilm formation in NSTIs emphasizes the urgent need for biofilm to be considered as a potential complicating microbiological feature of GAS NSTI and, consequently, emphasizes reconsideration of antibiotic treatment protocols.

Streptococcus pyogenes adhesion and colonization

Streptococcus pyogenes (group A Streptococcus, GAS) is a human-adapted pathogen responsible for a wide spectrum of disease. GAS can cause relatively mild illnesses, such as strep throat or impetigo, and less frequent but severe life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. GAS is an important public health problem causing significant morbidity and mortality worldwide. The main route of GAS transmission between humans is through close or direct physical contact, and particularly via respiratory droplets. The upper respiratory tract and skin are major reservoirs for GAS infections. The ability of GAS to establish an infection in the new host at these anatomical sites primarily results from two distinct physiological processes, namely bacterial adhesion and colonization. These fundamental aspects of pathogenesis rely upon a variety of GAS virulence factors, which are usually under strict transcriptional regulation. Considerable progress has been made in better understanding these initial infection steps. This review summarizes our current knowledge of the molecular mechanisms of GAS adhesion and colonization.