Staphylococcus aureus AgrA binding to the RNAIII-agr regulatory region

Response of Staphylococcus aureus physiology and Agr quorum sensing to low-shear modeled microgravity

Staphylococcus aureus is commonly isolated from astronauts returning from spaceflight. Previous analysis of omics data from S. aureus low Earth orbit cultures indicated significantly increased expression of the Agr quorum sensing system and its downstream targets in spaceflight samples compared to ground controls. In this current study, the rotary cell culture system (RCCS) was used to investigate the effect of low-shear modeled microgravity (LSMMG) on S. aureus physiology and Agr activity. When cultured in the same growth medium and temperature as the previous spaceflight experiment, S. aureus LSMMG cultures exhibited decreased agr expression and altered growth compared to normal gravity control cultures, which are typically oriented with oxygenation membrane on the bottom of the high aspect rotating vessel (HARV). When S. aureus was grown in an inverted gravity control orientation (oxygenation membrane on top of the HARV), reduced Agr activity was observed relative to both traditional control and LSMMG cultures, signifying that oxygen availability may affect the observed differences in Agr activity. Metabolite assays revealed increased lactate and decreased acetate excretion in both LSMMG and inverted control cultures. Secretomics analysis of LSMMG, control, and inverted control HARV culture supernatants corroborated these results, with inverted and LSMMG cultures exhibiting a decreased abundance of Agr-regulated virulence factors and an increased abundance of proteins expressed in low-oxygen conditions. Collectively, these studies suggest that the orientation of the HARV oxygenation membrane can affect S. aureus physiology and Agr quorum sensing in the RCCS, a variable that should be considered when interpreting data using this ground-based microgravity model.IMPORTANCES. aureus is commonly isolated from astronauts returning from spaceflight and from surfaces within human-inhabited closed environments such as spacecraft. Astronaut health and immune function are significantly altered in spaceflight. Therefore, elucidating the effects of microgravity on S. aureus physiology is critical for assessing its pathogenic potential during long-term human space habitation. These results also highlight the necessity of eliminating potential confounding factors when comparing simulated microgravity model data with actual spaceflight experiments.

Repeated Exposure of Vancomycin to Vancomycin-Susceptible Staphylococcus aureus (VSSA) Parent Emerged VISA and VRSA Strains with Enhanced Virulence Potentials

The emergence of resistance against the last-resort antibiotic vancomycin in staphylococcal infections is a serious concern for human health. Although various drug-resistant pathogens of diverse genetic backgrounds show higher virulence potential, the underlying mechanism behind this is not yet clear due to variability in their genetic dispositions. In this study, we investigated the correlation between resistance and virulence in adaptively evolved isogenic strains. The vancomycin-susceptible Staphylococcus aureus USA300 was exposed to various concentrations of vancomycin repeatedly as a mimic of the clinical regimen to obtain mutation(s)-accrued-clonally-selected (MACS) strains. The phenotypic analyses followed by expression of the representative genes responsible for virulence and resistance of MACS strains were investigated. MACS strains obtained under 2 and 8 µg/ml vancomycin, named Van2 and Van8, respectively; showed enhanced vancomycin minimal inhibitory concentrations (MIC) to 4 and 16 µg/ml, respectively. The cell adhesion and invasion of MACS strains increased in proportion to their MICs. The correlation between resistance and virulence potential was partially explained by the differential expression of genes known to be involved in both virulence and resistance in MACS strains compared to parent S. aureus USA300. Repeated treatment of vancomycin against vancomycin-susceptible S. aureus (VSSA) leads to the emergence of vancomycin-resistant strains with variable levels of enhanced virulence potentials.

RNAIII is linked with the pentose phosphate pathway through the activation of RpiRc in Staphylococcus aureus

Staphylococcus aureus RNAIII is a dual-function regulatory RNA that controls the expression of multiple virulence genes and especially the transition from adhesion to the production of exotoxins. However, its contribution to S. aureus central metabolism remains unclear. Using MS2-affinity purification coupled with RNA sequencing, we uncovered more than 50 novel RNAIII-mRNA interactions. Among them, we demonstrate that RNAIII is a major activator of the rpiRc gene, encoding a regulator of the pentose phosphate pathway (PPP). RNAIII binds the 5' UTR of rpiRc mRNA to favor ribosome loading, leading to an increased expression of RpiRc and, subsequently, of two PPP enzymes. Finally, we show that RNAIII and RpiRc are involved in S. aureus fitness in media supplemented with various carbohydrate sources related to PPP and glycolysis. Collectively, our data depict an unprecedented phenotype associated with the RNAIII regulon, especially the direct implication of RNAIII in central metabolic activity modulation. These findings show that the contribution of RNAIII in Staphylococcus aureus adaptation goes far beyond what was initially reported.

IMPORTANCE

Staphylococcus aureus is a major human pathogen involved in acute and chronic infections. Highly recalcitrant to antibiotic treatment, persistent infections are mostly associated with the loss of RNAIII expression, a master RNA regulator responsible for the switch from colonization to infection. Here, we used the MS2 affinity purification coupled with RNA sequencing approach to identify novel mRNA targets of RNAIII and uncover novel functions. We demonstrate that RNAIII is an activator of the expression of genes involved in the pentose phosphate pathway and is implicated in the adjustment of bacterial fitness as a function of carbohydrate sources. Taken together, our results demonstrate an unprecedented role of RNAIII that goes beyond the knowledge gained so far and contributes to a better understanding of the role of RNAIII in bacterial adaptation expression and the coordination of a complex regulatory network.

Physalin H, physalin B, and isophysalin B suppress the quorum-sensing function of Staphylococcus aureus by binding to AgrA

The virulence of Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA), depends on the expression of toxins and virulence factors controlled by the quorum-sensing (QS) system, encoded on the virulence accessory gene regulator (agr) locus. The aim of this study was to identify a phytochemical that inhibits Agr-QS function and to elucidate its mechanism. We screened 577 compounds and identified physalin H, physalin B, and isophysalin B--phytochemicals belonging to physalins found in plants of the Solanaceae family--as novel Agr-QS modulators. Biological analyses and in vitro protein-DNA binding assays suggested that these physalins suppress gene expression related to the Agr-QS system by inhibiting binding of the key response regulator AgrA to the agr promoters, reducing the function of hemolytic toxins downstream of these genes in MRSA. Furthermore, although physalin F suppressed gene expression in the Agr-QS system, its anti-hemolytic activity was lower than that of physalins H, B, and isophysalin B. Conversely, five physalins isolated from the same plant with the ability to suppress Agr-QS did not reduce bacterial Agr-QS activity but inhibited AgrA binding to DNA in vitro. A docking simulation revealed that physalin interacts with the DNA-binding site of AgrA in three docking states. The carbonyl oxygens at C-1 and C-18 of physalins, which can suppress Agr-QS, were directed to residues N201 and R198 of AgrA, respectively, whereas these carbonyl oxygens of physalins, without Agr-QS suppression activity, were oriented in different directions. Next, 100-ns molecular dynamics simulations revealed that the hydrogen bond formed between the carbonyl oxygen at C-15 of physalins and L186 of AgrA functions as an anchor, sustaining the interaction between the carbonyl oxygen at C-1 of physalins and N201 of AgrA. Thus, these results suggest that physalin H, physalin B, and isophysalin B inhibit the interaction of AgrA with the agr promoters by binding to the DNA-binding site of AgrA, suppressing the Agr-QS function of S. aureus. Physalins that suppress the Agr-QS function are proposed as potential lead compounds in the anti-virulence strategy for MRSA infections.

Deciphering the dynamics of methicillin-resistant Staphylococcus aureus biofilm formation: from molecular signaling to nanotherapeutic advances

Methicillin-resistant Staphylococcus aureus (MRSA) represents a global threat, necessitating the development of effective solutions to combat this emerging superbug. In response to selective pressures within healthcare, community, and livestock settings, MRSA has evolved increased biofilm formation as a multifaceted virulence and defensive mechanism, enabling the bacterium to thrive in harsh conditions. This review discusses the molecular mechanisms contributing to biofilm formation across its developmental stages, hence representing a step forward in developing promising strategies for impeding or eradicating biofilms. During staphylococcal biofilm development, cell wall-anchored proteins attach bacterial cells to biotic or abiotic surfaces; extracellular polymeric substances build scaffolds for biofilm formation; the cidABC operon controls cell lysis within the biofilm, and proteases facilitate dispersal. Beside the three main sequential stages of biofilm formation (attachment, maturation, and dispersal), this review unveils two unique developmental stages in the biofilm formation process for MRSA; multiplication and exodus. We also highlighted the quorum sensing as a cell-to-cell communication process, allowing distant bacterial cells to adapt to the conditions surrounding the bacterial biofilm. In S. aureus, the quorum sensing process is mediated by autoinducing peptides (AIPs) as signaling molecules, with the accessory gene regulator system playing a pivotal role in orchestrating the production of AIPs and various virulence factors. Several quorum inhibitors showed promising anti-virulence and antibiofilm effects that vary in type and function according to the targeted molecule. Disrupting the biofilm architecture and eradicating sessile bacterial cells are crucial steps to prevent colonization on other surfaces or organs. In this context, nanoparticles emerge as efficient carriers for delivering antimicrobial and antibiofilm agents throughout the biofilm architecture. Although metal-based nanoparticles have been previously used in combatting biofilms, its non-degradability and toxicity within the human body presents a real challenge. Therefore, organic nanoparticles in conjunction with quorum inhibitors have been proposed as a promising strategy against biofilms. As nanotherapeutics continue to gain recognition as an antibiofilm strategy, the development of more antibiofilm nanotherapeutics could offer a promising solution to combat biofilm-mediated resistance.

AgrA directly binds to the promoter of vraSR and downregulates its expression in Staphylococcus aureus

Staphylococcus aureus is an important human pathogen and vancomycin is widely used for the treatment of S. aureus infections. The global regulator agr is known as a well-described virulence regulator. Previous studies have found that agr-dysfunction strains are more likely to develop into vancomycin-resistant strains, but the mechanism for this phenomenon remains unknown. VraSR is a two-component regulatory system related to vancomycin resistance. In this study, we found that the expression levels of vraR were higher in agr-dysfunction clinical strains than in the agr-functional strains. We knocked out agr in a clinical strain, and quantitative reverse transcription PCR and β-galactosidase activity assays revealed that agr repressed transcription of vraR. After vancomycin exposures, population analysis revealed larger subpopulations displaying reduced susceptibility in agr knockout strain compared with wild-type strain, and this pattern was also observed in agr-dysfunction clinical strains compared with the agr-functional strains. Electrophoretic mobility experiment demonstrated binding of purified AgrA to the promoter region of vraR. In conclusion, our results indicated that the loss of agr function in S. aureus may contribute to the evolution of reduced vancomycin susceptibility through the downregulation of vraSR.

Deciphering agr quorum sensing in Staphylococcus aureus: insights and therapeutic prospects

The emergence of superbugs like methicillin-resistant Staphylococcus aureus exposed the limitations of treating microbial infections using antibiotics. At present, the discovery of novel and convincing therapeutic methods are being executed increasingly as possible substitutes to conventional antibiotic therapies. The quorum sensing helps Staphylococcus aureus become more viable through their signaling mechanisms. In recent years, targeting the prominent factors of quorum sensing has obtained remarkable attention as a futuristic approach to dealing with bacterial pathogenicity. The standard antibiotic therapy intends to inhibit the organism by targeting specific molecules and afford a chance for the evolution of antibiotic resistance. This prompts the development of novel therapeutic strategies like inhibiting quorum sensing that can limit bacterial virulence by decreasing the selective pressure, thereby restricting antibiotic resistance evolution. This review furnishes new insights into the accessory gene regulator quorum sensing in Staphylococcus aureus and its inhibition by targeting the genes that regulate the operon. Further, this review comprehensively explores the inhibitors reported up to date and their specific targets and discusses their potentially ineffective alternative therapy against methicillin-resistant Staphylococcus aureus.

A novel small-molecule compound S-342-3 effectively inhibits the biofilm formation of Staphylococcus aureus

IMPORTANCE

Biofilms are an important virulence factor in Staphylococcus aureus and are characterized by a structured microbial community consisting of bacterial cells and a secreted extracellular polymeric matrix. Inhibition of biofilm formation is an effective measure to control S. aureus infection. Here, we have synthesized a small molecule compound S-342-3, which exhibits potent inhibition of biofilm formation in both MRSA and MSSA. Further investigations revealed that S-342-3 exerts inhibitory effects on biofilm formation by reducing the production of polysaccharide intercellular adhesin and preventing bacterial adhesion. Our study has confirmed that the inhibitory effect of S-342-3 on biofilm is achieved by downregulating the expression of genes responsible for biofilm formation. In addition, S-342-3 is non-toxic to Galleria mellonella larvae and A549 cells. Consequently, this study demonstrates the efficacy of a biologically safe compound S-342-3 in inhibiting biofilm formation in S. aureus, thereby providing a promising antibiofilm agent for further research.

Engineered probiotic Lactobacillus plantarum WCSF I for monitoring and treatment of Staphylococcus aureus infection

IMPORTANCE

Bacterial infection and the emergence of drug-resistant strains are major problems in clinical treatment. Staphylococcus aureus, which typically infects the skin and blood of animals, is also a potential intestinal pathogen that needs to be addressed by the emergence of a new treatment approach. Probiotic therapy is the most likely alternative to antibiotic therapy to solve the problem of bacterial drug resistance in clinical practice. In this study, the engineered Lactobacillus plantarum can not only sense the signal AIP to detect S. aureus but also kill S. aureus by secreting the lysostaphin enzyme. Our strategy employed an Agr quorum-sensing genetic circuit to simultaneously detect and treat pathogenic bacteria, which provided a theoretical possibility for solving practical clinical bacterial infection cases in the future.

Anti-Biofilm Activity of Carnosic Acid from Salvia rosmarinus against Methicillin-Resistant Staphylococcus aureus

The Salvia rosmarinus "Eretto Liguria" ecotype was studied as a source of valuable bioactive compounds. LC-MS analysis of the methanolic extract underlined the presence of diterpenoids, triterpenoids, polyphenolic acids, and flavonoids. The anti-virulence activity of carnosic acid along with the other most abundant compounds against methicillin-resistant Staphylococcus aureus (MRSA) was evaluated. Only carnosic acid induced a significant reduction in the expression of agrA and rnaIII genes, which encode the key components of quorum sensing (QS), an intracellular signaling mechanism controlling the virulence of MRSA. At a concentration of 0.05 mg/mL, carnosic acid inhibited biofilm formation by MRSA and the expression of genes involved in toxin production and made MRSA more susceptible to intracellular killing, with no toxic effects on eukaryotic cells. Carnosic acid did not affect biofilm formation by Pseudomonas aeruginosa, a human pathogen that often coexists with MRSA in complex infections. The selected ecotype showed a carnosic acid content of 94.3 ± 4.3 mg/g. In silico analysis highlighted that carnosic acid potentially interacts with the S. aureus AgrA response regulator. Our findings suggest that carnosic acid could be an anti-virulence agent against MRSA infections endowed with a species-specific activity useful in multi-microbial infections.

The RgaS-RgaR two-component system promotes Clostridioides difficile sporulation through a small RNA and the Agr1 system

The ability to form a dormant spore is essential for the survival of the anaerobic pathogen, Clostridioides difficile, outside of the mammalian gastrointestinal tract. The initiation of sporulation is governed by the master regulator of sporulation, Spo0A, which is activated by phosphorylation. Multiple sporulation factors control Spo0A phosphorylation; however, this regulatory pathway is not well defined in C. difficile. We discovered that RgaS and RgaR, a conserved orphan histidine kinase and orphan response regulator, function together as a cognate two-component regulatory system to directly activate transcription of several genes. One of these targets, agrB1D1, encodes gene products that synthesize and export a small quorum-sensing peptide, AgrD1, which positively influences expression of early sporulation genes. Another target, a small regulatory RNA now known as SpoZ, impacts later stages of sporulation through a small hypothetical protein and an additional, unknown regulatory mechanism(s). Unlike Agr systems in many organisms, AgrD1 does not activate the RgaS-RgaR two-component system, and thus, is not responsible for autoregulating its own production. Altogether, we demonstrate that C. difficile utilizes a conserved two-component system that is uncoupled from quorum-sensing to promote sporulation through two distinct regulatory pathways.

Antivirulence activities of retinoic acids against Staphylococcus aureus

Multidrug-resistant bacteria such as Staphylococcus aureus constitute a global health problem. Gram-positive S. aureus secretes various toxins associated with its pathogenesis, and its biofilm formation plays an important role in antibiotic tolerance and virulence. Hence, we investigated if the metabolites of vitamin A1 might diminish S. aureus biofilm formation and toxin production. Of the three retinoic acids examined, 13-cis-retinoic acid at 10 μg/mL significantly decreased S. aureus biofilm formation without affecting its planktonic cell growth (MIC >400 μg/mL) and also inhibited biofilm formation by Staphylococcus epidermidis (MIC >400 μg/mL), but less affected biofilm formation by a uropathogenic Escherichia coli strain, a Vibrio strain, or a fungal Candida strain. Notably, 13-cis-retinoic acid and all-trans-retinoic acid significantly inhibited the hemolytic activity and staphyloxanthin production by S. aureus. Furthermore, transcriptional analysis disclosed that 13-cis-retinoic acid repressed the expressions of virulence- and biofilm-related genes, such as the two-component arlRS system, α-hemolysin hla, nuclease (nuc1 and nuc2), and psmα (phenol soluble modulins α) in S. aureus. In addition, plant and nematode toxicity assays showed that 13-cis-retinoic acid was only mildly toxic at concentrations many folds higher than its effective antibiofilm concentrations. These findings suggest that metabolites of vitamin A1, particularly 13-cis-retinoic acid, might be useful for suppressing biofilm formation and the virulence characteristics of S. aureus.

The RgaS-RgaR two-component system promotes Clostridioides difficile sporulation through a small RNA and the Agr1 system

The ability to form a dormant spore is essential for the survival of the anaerobic, gastrointestinal pathogen Clostridioides difficile outside of the mammalian gastrointestinal tract. The initiation of sporulation is governed by the master regulator of sporulation, Spo0A, which is activated by phosphorylation. Multiple sporulation factors control Spo0A phosphorylation; however, this regulatory pathway is not well defined in C. difficile. We discovered that RgaS and RgaR, a conserved orphan histidine kinase and orphan response regulator, function together as a cognate two-component regulatory system to directly activate transcription of several genes. One of these targets, agrB1D1, encodes gene products that synthesize and export a small quorum-sensing peptide, AgrD1, which positively influences expression of early sporulation genes. Another target, a small regulatory RNA now known as SrsR, impacts later stages of sporulation through an unknown regulatory mechanism(s). Unlike Agr systems in many organisms, AgrD1 does not activate the RgaS-RgaR two-component system, and thus, is not responsible for autoregulating its own production. Altogether, we demonstrate that C. difficile utilizes a conserved two-component system that is uncoupled from quorum-sensing to promote sporulation through two distinct regulatory pathways.

Cinnamaldehyde Targets the LytTR DNA-Binding Domain of the Response Regulator AgrA to Attenuate Biofilm Formation of Listeria monocytogenes

The Agr quorum sensing (QS) system is known to contribute to biofilm formation in Listeria monocytogenes. Cinnamaldehyde, a natural food preservative, is considered an inhibitor of Agr-mediated QS in L. monocytogenes. However, the exact mechanism by which cinnamaldehyde acts on Agr remains unclear. In this study, we assessed the effects of cinnamaldehyde on the histidine kinase AgrC and the response regulator AgrA in the Agr system. AgrC kinase activity was not influenced by cinnamaldehyde, and binding between AgrC and cinnamaldehyde was not observed when microscale thermophoresis (MST) was performed, indicating that AgrC was not the target of cinnamaldehyde. AgrA is specifically bound to the agr promoter (P2) to activate the transcription of the Agr system. However, AgrA-P2 binding was prevented by cinnamaldehyde. The interaction between cinnamaldehyde and AgrA was further confirmed with MST. Two conserved amino acids, Asn-178 and Arg-179, located in the LytTR DNA-binding domain of AgrA, were identified as the key sites for cinnamaldehyde-AgrA binding by alanine mutagenesis and MST. Coincidentally, Asn-178 was also involved in the AgrA-P2 interaction. Taken together, these results suggest that cinnamaldehyde acts as a competitive inhibitor of AgrA in AgrA-P2 binding, which leads to suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. IMPORTANCE Listeria monocytogenes can form biofilms on various food contact surfaces, posing a serious threat to food safety. Biofilm formation of L. monocytogenes is positively regulated by the Agr quorum sensing system. Thus, an alternative strategy for controlling L. monocytogenes biofilms is interfering with the Agr system. Cinnamaldehyde is considered an inhibitor of the L. monocytogenes Agr system; however, its exact mechanism of action is still unclear. Here, we found that AgrA (response regulator), rather than AgrC (histidine kinase), was the target of cinnamaldehyde. The conserved Asn-178 in the LytTR DNA-binding domain of AgrA was involved in cinnamaldehyde-AgrA and AgrA-P2 binding. Therefore, the occupation of Asn-178 by cinnamaldehyde suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. Our findings could provide a better understanding of the mechanism by which cinnamaldehyde inhibits L. monocytogenes biofilm formation.

Chemical and biomolecular insights into the Staphylococcus aureus agr quorum sensing system: Current progress and ongoing challenges

Staphylococcus aureus is a ubiquitous bacterium that has become a major threat to human health due to its extensive toxin production and tremendous capacity for antibiotic resistance (e.g., MRSA "superbug" infections). Amid a worsening antibiotic resistance crisis, new strategies to combat this deadly microbe that remove the selective pressure of traditional approaches are in high demand. S. aureus utilizes an accessory gene regulator (agr) quorum sensing network to monitor its local cellular population and trigger a devastating communal attack, like an invading horde, once a threshold cell density has been reached. The role of the agr system in a range of disease types is still being unraveled. Herein, we discuss the present-day biochemical understanding of agr along with unresolved details, describe its connection to the progression of infection, and review how chemical strategies have been implemented to study and intercept this signaling pathway. This research is illuminating the potential of agr as an anti-virulence target in S. aureus and should inform the study of similar, yet less studied, agr systems in related bacterial pathogens.

A clash of quorum sensing vs quorum sensing inhibitors: an overview and risk of resistance

Indiscriminate use of antibiotics to treat microbial pathogens has caused emergence of multiple drug resistant strains. Most infectious diseases are caused by microbes that are capable of intercommunication using signaling molecules, which is known as quorum sensing (QS). Such pathogens express their pathogenicity through various QS-regulated virulence factors. Interference of QS could lead to decisive results in controlling such pathogenicity. Hence, QS inhibition has become an attractive new approach for the development of novel drugs. Many quorum sensing inhibitors (QSIs) of diverse origins have been reported. It is imperative that more such anti-QS compounds be found and studied, as they have significant effect on microbial pathogenicity. This review attempts to give a brief account of QS mechanism, its inhibition and describes some compounds with anti-QS potential. Also discussed is the possibility of emergence of quorum sensing resistance.

Antimicrobial Treatment of Staphylococcus aureus Biofilms

Staphylococcus aureus is a microorganism frequently associated with implant-related infections, owing to its ability to produce biofilms. These infections are difficult to treat because antimicrobials must cross the biofilm to effectively inhibit bacterial growth. Although some antibiotics can penetrate the biofilm and reduce the bacterial load, it is important to understand that the results of routine sensitivity tests are not always valid for interpreting the activity of different drugs. In this review, a broad discussion on the genes involved in biofilm formation, quorum sensing, and antimicrobial activity in monotherapy and combination therapy is presented that should benefit researchers engaged in optimizing the treatment of infections associated with S. aureus biofilms.

Hydroquinones Inhibit Biofilm Formation and Virulence Factor Production in Staphylococcus aureus

Staphylococcus aureus is one of the major pathogens responsible for antimicrobial resistance-associated death. S. aureus can secrete various exotoxins, and staphylococcal biofilms play critical roles in antibiotic tolerance and the persistence of chronic infections. Here, we investigated the inhibitory effects of 18 hydroquinones on biofilm formation and virulence factor production by S. aureus. It was found that 2,5-bis(1,1,3,3-tetramethylbutyl) hydroquinone (TBHQ) at 1 µg/mL efficiently inhibits biofilm formation by two methicillin-sensitive and two methicillin-resistant S. aureus strains with MICs of 5 µg/mL, whereas the backbone compound hydroquinone did not (MIC > 400 µg/mL). In addition, 2,3-dimethylhydroquinone and tert-butylhydroquinone at 50 µg/mL also exhibited antibiofilm activity. TBHQ at 1 µg/mL significantly decreased the hemolytic effect and lipase production by S. aureus, and at 5−50 µg/mL was non-toxic to the nematode Caenorhabditis elegans and did not adversely affect Brassica rapa seed germination or growth. Transcriptional analyses showed that TBHQ suppressed the expression of RNAIII (effector of quorum sensing). These results suggest that hydroquinones, particularly TBHQ, are potentially useful for inhibiting S. aureus biofilm formation and virulence.

Inhibition of Staphylococcus aureus Biofilm Formation and Virulence Factor Production by Petroselinic Acid and Other Unsaturated C18 Fatty Acids

Staphylococcus aureus is a major human pathogen that secretes several toxins associated with the pathogenesis of sepsis and pneumonia. Its antibiotic resistance is notorious, and its biofilms play a critical role in antibiotic tolerance. We hypothesized fatty acids might inhibit S. aureus biofilm formation and the expressions of its virulence factors. Initially, the antibiofilm activities of 27 fatty acids against a methicillin-sensitive S. aureus strain were investigated. Of the fatty acids tested, three C18 unsaturated fatty acids, that is, petroselinic, vaccenic, and oleic acids at 100 μg/mL, inhibited S. aureus biofilm formation by more than 65% without affecting its planktonic cell growth (MICs were all > 400 μg/mL). Notably, petroselinic acid significantly inhibited biofilm formation of two methicillin-resistant S. aureus strains and two methicillin-sensitive S. aureus strains. In addition, petroselinic acid significantly suppressed the production of three virulence factors, namely, staphyloxanthin, lipase, and α-hemolysin. Transcriptional analysis showed that petroselinic acid repressed the gene expressions of quorum sensing regulator agrA, effector of quorum sensing RNAIII, α-hemolysin hla, nucleases nuc1 and nuc2, and the virulence regulator saeR. Furthermore, petroselinic acid dose-dependently inhibited S. aureus biofilm formation on abiotic surfaces and porcine skin. These findings suggest that fatty acids, particularly petroselinic acid, are potentially useful for controlling biofilm formation by S. aureus.

IMPORTANCE Fatty acids with a long carbon chain have recently attracted attention because of their antibiofilm activities against microbes. Here, we report the antibiofilm activities of 27 fatty acids against S. aureus. Of the fatty acids tested, three C18 unsaturated fatty acids (petroselinic, vaccenic, and oleic acids) significantly inhibited biofilm formation by S. aureus. Furthermore, petroselinic acid inhibited the production of several virulence factors in S. aureus. The study also reveals that the action mechanism of petroselinic acid involves repression of quorum-sensing-related and virulence regulator genes. These findings show that natural and nontoxic petroselinic acid has potential use as a treatment for S. aureus infections, including infections by methicillin-resistant S. aureus strains, and in food processing facilities.

A Journey into Animal Models of Human Osteomyelitis: A Review

Osteomyelitis is an infection of the bone characterized by progressive inflammatory destruction and apposition of new bone that can spread via the hematogenous route (hematogenous osteomyelitis (HO)), contiguous spread (contiguous osteomyelitis (CO)), and direct inoculation (osteomyelitis associated with peripheral vascular insufficiency (PVI)). Given the significant financial burden posed by osteomyelitis patient management, the development of new preventive and treatment methods is warranted. To achieve this objective, implementing animal models (AMs) of infection such as rats, mice, rabbits, avians, dogs, sheep, goats, and pigs might be of the essence. This review provides a literature analysis of the AMs developed and used to study osteomyelitis. Historical relevance and clinical applicability were taken into account to choose the best AMs, and some study methods are briefly described. Furthermore, the most significant strengths and limitations of each species as AM are discussed, as no single model incorporates all features of osteomyelitis. HO’s clinical manifestation results in extreme variability between patients due to multiple variables (e.g., age, sex, route of infection, anatomical location, and concomitant diseases) that could alter clinical studies. However, these variables can be controlled and tested through different animal models.

Evaluation of methicillin-resistant Staphylococcus virulence genes and antibiotics susceptibility in Iranian population

Background

Methicillin resistance Staphylococcus aureus (MRSA) is one most important pathogens for human health. The ability of this organism for producing different kinds of disease is related to its virulence gene. The frequency of hemolysin alpha (hla), hemolysin beta (hlb), and exfoliative toxin A (eta) virulence genes of MRSA was evaluated, and the association of these genes with antibiotics susceptibility was investigated.

Materials and Methods

In a cross-sectional study, a total of 695 Staphylococcus clinical samples from seven different provinces of Iran were evaluated. MRSA was detected by cefoxitin disk. Virulence genes were detected by polymerase chain reaction. Susceptibility to clindamycin and ciprofloxacin was evaluated according to the Clinical and Laboratory Standards Institute guideline.

Results

From a total of 695 samples, 170 (24.46%) were found to be MRSA. 142, 82, and 132 samples of MRSA were hla, hlb, and eta positive, respectively. hla gene was significantly found more frequently in patients at least 18 years (P = 0.02). 105 (68.6%) and 93 (59.6%) of MRSA samples were resistance to ciprofloxacin and clindamycin, respectively. hlb gene was significantly more resistant to clindamycin (P = 0.04) and ciprofloxacin (P = 0.01). Logistic regression analysis displayed hlb-positive MRSA strains were significantly associated with ciprofloxacin (odds ratio [OR]: 3.6, 95% confidence interval [CI] = 1.637-8.00) and clindamycin (OR: 1.93, 95% CI 1.00-3.68).

Conclusion

MRSA strains from Staphylococcus aureus which isolated from hospitalized Iranian patients are significantly resistant to clindamycin and ciprofloxacin and it is may be because of hlb virulence gene. These samples consist of both community-acquired MRS) and health-care associated MRSA, so we could not use this finding as a guide for local antibiotics usage.

Peptidomimetics as Potential Anti-Virulence Drugs Against Resistant Bacterial Pathogens

The uncontrollable spread of superbugs calls for new approaches in dealing with microbial-antibiotic resistance. Accordingly, the anti-virulence approach has arisen as an attractive unconventional strategy to face multidrug-resistant pathogens. As an emergent strategy, there is an imperative demand for discovery, design, and development of anti-virulence drugs. In this regard, peptidomimetic compounds could be a valuable source of anti-virulence drugs, since these molecules circumvent several shortcomings of natural peptide-based drugs like proteolytic instability, immunogenicity, toxicity, and low bioavailability. Some emerging evidence points to the feasibility of peptidomimetics to impair pathogen virulence. Consequently, in this review, we shed some light on the potential of peptidomimetics as anti-virulence drugs to overcome antibiotic resistance. Specifically, we address the anti-virulence activity of peptidomimetics against pathogens' secretion systems, biofilms, and quorum-sensing systems.

Anti-Biofilm Effects of Torilis japonica Ethanol Extracts against Staphylococcus aureus

The spread of antibiotic-resistant strains of Staphylococcus aureus, a gram-positive opportunistic pathogen, has increased due to the frequent use of antibiotics. Inhibition of the quorum-sensing systems of biofilm-producing strains using plant extracts represents an efficient approach for controlling infections. Torilis japonica is a medicinal herb showing various bioactivities; however, no studies have reported the anti-biofilm effects of T. japonica extracts against drug-resistant S. aureus. In this study, we evaluated the inhibitory effects of T. japonica ethanol extract (TJE) on biofilm production in methicillin-sensitive S. aureus (MSSA) KCTC 1927, methicillin-resistant S. aureus (MRSA) KCCM 40510, and MRSA KCCM 40511. Biofilm assays showed that TJE could inhibit biofilm formation in all strains. Furthermore, the hemolysis of sheep blood was found to be reduced when the strains were treated with TJE. The mRNA expression of agrA, sarA, icaA, hla, and RNAIII was evaluated using reverse transcription-polymerase chain reaction to determine the effect of TJE on the regulation of genes encoding quorum sensing-related virulence factors in MSSA and MRSA. The expression of hla reduced in a concentration-dependent manner upon treatment with TJE. Moreover, the expression levels of other genes were significantly reduced compared to those in the control group. In conclusion, TJE can suppress biofilm formation and virulence factor-related gene expression in MSSA and MRSA strains. The extract may therefore be used to develop treatments for infections caused by antibiotic-resistant S. aureus.

Significant variability exists in the cytotoxicity of global methicillin-resistant Staphylococcus aureus lineages

Staphylococcus aureus is a major human pathogen where the emergence of antibiotic resistant lineages, such as methicillin-resistant S. aureus (MRSA), is a major health concern. While some MRSA lineages are restricted to the healthcare setting, the epidemiology of MRSA is changing globally, with the rise of specific lineages causing disease in healthy people in the community. In the past two decades, community-associated MRSA (CA-MRSA) has emerged as a clinically important and virulent pathogen associated with serious skin and soft-tissue infections (SSTI). These infections are primarily cytotoxin driven, leading to the suggestion that hypervirulent lineages/multi-locus sequence types (STs) exist. To examine this, we compared the cytotoxicity of 475 MRSA isolates representing five major MRSA STs (ST22, ST93, ST8, ST239 and ST36) by employing a monocyte-macrophage THP-1 cell line as a surrogate for measuring gross cytotoxicity. We demonstrate that while certain MRSA STs contain highly toxic isolates, there is such variability within lineages to suggest that this aspect of virulence should not be inferred from the genotype of any given isolate. Furthermore, by interrogating the accessory gene regulator (Agr) sequences in this collection we identified several Agr mutations that were associated with reduced cytotoxicity. Interestingly, the majority of isolates that were attenuated in cytotoxin production contained no mutations in the agr locus, indicating a role of other undefined genes in S. aureus toxin regulation.

Potential Antimicrobial Activity of Galloyl-Flavonoid Glycosides From Woodfordia uniflora Against Methicillin-Resistant Staphylococcus aureus

Antibiotic-resistant infections are a growing problem; to combat multi-drug resistant bacterial infections, antibiotics with novel mechanisms of action are needed. Identification of potent bioactive natural products is an attractive avenue for developing novel therapeutic strategies against bacterial infections. As part of our ongoing research to explore bioactive natural products from diverse resources, we investigated the antimicrobial compounds from Woodfordia uniflora, a flowering shrub unique to the Dhofar region of Oman. The plant has been used as a remedy for skin infections in Oman. However, to date, no study has examined the antimicrobial compounds in W. uniflora. Phytochemical analysis of the methanolic extract of W. uniflora leaves in combination with LC/MS-based analysis allowed us to isolate and identify four flavonoid-type analogs (1–4), procyanidin B3-3-O-gallate (1), rhamnetin 3-O-(6″-galloyl)-β-D-glucopyranoside (2), rhamnetin 3-O-α-L-rhamnopyranoside (3), and quercetin 3-O-(6″-galloyl)-β-D-glucopyranoside (4). The isolates have a novel mechanism of action; the compounds inhibit biofilm formation in methicillin-resistant Staphylococcus aureus (MRSA) and synergize with methicillin. Our metabolite analysis revealed that this synergizing activity by compounds was achieved by remodeling metabolism including central carbon metabolism and glutamine biosynthesis that resulted in abnormal cell formation and reduction in biofilm formation of MRSA. Taken together, these findings provide experimental evidence that rhamnetin 3-O-(6″-galloyl)-β-D-glucopyranoside (2) and quercetin 3-O-(6″-galloyl)-β-D-glucopyranoside (4) can be considered as potential therapeutic agents for the treatment of methicillin-resistant S. aureus-associated diseases.

Peptidome: Chaos or Inevitability

Thousands of naturally occurring peptides differing in their origin, abundance and possible functions have been identified in the tissue and biological fluids of vertebrates, insects, fungi, plants and bacteria. These peptide pools are referred to as intracellular or extracellular peptidomes, and besides a small proportion of well-characterized peptide hormones and defense peptides, are poorly characterized. However, a growing body of evidence suggests that unknown bioactive peptides are hidden in the peptidomes of different organisms. In this review, we present a comprehensive overview of the mechanisms of generation and properties of peptidomes across different organisms. Based on their origin, we propose three large peptide groups-functional protein "degradome", small open reading frame (smORF)-encoded peptides (smORFome) and specific precursor-derived peptides. The composition of peptide pools identified by mass-spectrometry analysis in human cells, plants, yeast and bacteria is compared and discussed. The functions of different peptide groups, for example the role of the "degradome" in promoting defense signaling, are also considered.

Effect of biofilm on the survival of Staphylococcus aureus isolated from raw milk in high temperature and drying environment

Microbial contamination in dairy products is a momentous factor affecting food safety. Studies have shown that Staphylococcus aureus, which is an important causative agent of a range of infectious and foodborne diseases, may remain in raw milk after a series of complex processing processes. Although most S. aureus possess biofilm formation capacity, there are few studies concerning the role of biofilm formation of this bacterium in stress tolerance and longtime survival in the dairy products. In this study, we selected 5 S. aureus (RMSA1, RMSA2, RMSA3, RMSA4 and RMSA5) isolates from raw milk and investigated their virulence and biofilm characteristics. Results from biofilm assays showed that all 6 S. aureus strains (5 dairy isolates and 1 human-derived model strain NCTC8325) could form complete biofilms in vitro. The reverse transcription-PCR experiments confirmed that multiple genes related to virulence factors and biofilm formation were expressed in the 6 strains. Furthermore, we simulated the high temperature (at 60 °C for 30 min) and drying pressure (at 37 °C for 24 h) during dairy processing to detect the survival rate of strains culturedunderbiofilm or planktonic condition. The data showed that under high temperature and dry conditions, the survival rates of strains cultured under biofilm conditions were much higher than that of strains cultured under planktonic conditions. In addition, the adversity resistance associated with biofilm formation was more obvious in the milk-isolated strains compared with strain NCTC8325. This study provides evidence regarding the mechanisms of stress resistance of S. aureus strains isolated from raw milk and contribute to prevention of dairy product contamination caused by this bacterium.

Quorum-sensing regulation of virulence factors in bacterial biofilm

Chronic polymicrobial wound infections are often characterized by the presence of bacterial biofilms. They show considerable structural and functional heterogeneity, which influences the choice of antimicrobial therapy and wound healing dynamics. The hallmarks of biofilm-associated bacterial infections include elevated antibiotic resistance and extreme pathogenicity. Biofilm helps bacteria to evade the host defense mechanisms and persist longer in the host. Quorum-sensing (QS)-mediated cell signaling primarily regulates biofilm formation in chronic infections and plays a major role in eliciting virulence. This review focuses on the QS mechanisms of two major bacterial pathogens, Staphylococcus aureus and Pseudomonas aeruginosa and explains how they interact in the wound microenvironment to regulate biofilm development and virulence. The review also provides an insight into the treatment modalities aimed at eradicating polymicrobial biofilms. This information will help us develop better diagnostic modalities and devise effective treatment regimens to successfully manage and overcome severe life-threatening bacterial infections.

Tryptophan interferes with the quorum sensing and cell surface hydrophobicity of Staphylococcus aureus: a promising approach to inhibit the biofilm development

Staphylococcus aureus, a Gram-positive bacterium has been implicated in a plethora of human infections by virtue of its biofilm-forming ability. Inhibition in microbial biofilm formation has been found to be a promising approach towards compromising microbial pathogenesis. In this regard, various natural and synthetic molecules have been explored to attenuate microbial biofilm. In this study, the role of an amino acid, L-tryptophan was examined against the biofilm-forming ability of S. aureus. The compound did not execute any antimicrobial characteristics, instead, showed strong antibiofilm activity with the highest biofilm inhibition at a concentration of 50 µg/mL. Towards understanding the underlying mechanism of the same, efforts were given to examine whether tryptophan could inhibit biofilm formation by interfering with the quorum-sensing property of S. aureus. A molecular docking analysis revealed an efficient binding between the quorum-sensing protein, AgrA, and tryptophan. Moreover, the expression of the quorum-sensing gene (agrA) got significantly reduced under the influence of the test compound. These results indicated that tryptophan could interfere with the quorum-sensing property of the organism thereby inhibiting its biofilm formation. Further study revealed that tryptophan could also reduce the cell surface hydrophobicity of S. aureus by downregulating the expression of dltA. Moreover, the tested concentrations of tryptophan did not show any significant cytotoxicity. Hence, tryptophan could be recommended as a potential antibiofilm agent to manage the biofilm-associated infections caused by S. aureus.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02924-3.

Secondary Metabolites Governing Microbiome Interaction of Staphylococcal Pathogens and Commensals

Various Staphylococcus species colonize skin and upper airways of warm-blooded animals. They compete successfully with many other microorganisms under the hostile and nutrient-poor conditions of these habitats using mechanisms that we are only beginning to appreciate. Small-molecule mediators, whose biosynthesis requires complex enzymatic cascades, so-called secondary metabolites, have emerged as crucial components of staphylococcal microbiome interactions. Such mediators belong to a large variety of compound classes and several of them have attractive properties for future drug development. They include, for instance, bacteriocins such as lanthipeptides, thiopeptides, and fibupeptides that inhibit bacterial competitor species; signaling molecules such as thiolactone peptides that induce or inhibit sensory cascades in other bacteria; or metallophores such as staphyloferrins and staphylopine that scavenge scant transition metal ions. For some secondary metabolites such as the aureusimines, the exact function remains to be elucidated. How secondary metabolites shape the fitness of Staphylococcus species in the complex context of other microbial and host defense factors remains a challenging field of future research. A detailed understanding will help to harness staphylococcal secondary metabolites for excluding the pathogenic species Staphylococcus aureus from the nasal microbiomes of at-risk patients, and it will be instrumental for the development of advanced anti-infective interventions.

Staquorsin: A Novel Staphylococcus aureus Agr-Mediated Quorum Sensing Inhibitor Impairing Virulence in vivo Without Notable Resistance Development

The emergence of microbial resistance to the available antibiotics is a major public health concern, especially with the limited rate of developing new antibiotics. The utilization of anti-virulence agents is a non-conventional approach that can be used to combat microbial infection. In Staphylococcus aureus, many virulence factors are regulated by the Agr-mediated quorum sensing (QS). We developed a chemical compound that acts a potential Agr-inhibitor without reducing bacterial viability. The compound was designated staquorsin for Staphylococcus aureus QS inhibitor. In silico analyses confirmed the binding of staquorsin to the AgrA active site with an absolute binding score comparable to savirin, a previously described AgrA inhibitor. However, staquorsin turned out to be superior over savarin in not affecting the S. aureus viability in concentrations up to 600 μM. On the other hand, savirin inhibited S. aureus growth in concentrations as low as 25 μM. Moreover, staquorsin proved to be a potent inhibitor of the Agr system by inhibiting hemolysins, lipase production, and affecting biofilms formation and detachment. On the molecular level it significantly inhibited the effector transcript RNA III. In vivo testing, using the murine skin abscess model, confirmed the ability of staquorsin to modulate S. aureus virulence by effectively controlling the infection. Twenty passages of S. aureus in the presence of 40 μM staquorsin have not resulted in loss of activity as evidenced by maintaining its ability to reduce hemolysin production and RNA III transcript levels. In conclusion, we hereby describe a novel anti-virulence compound inhibiting the S. aureus Agr-system and its associated virulence factors. It is active both in vitro and in vivo, and its frequent use does not lead to the development of resistance. These findings model staquorsin as a promising drug candidate to join the fierce battle against the formidable pathogen S. aureus.

Non-Native Peptides Capable of Pan-Activating the agr Quorum Sensing System across Multiple Specificity Groups of Staphylococcus epidermidis

Staphylococcus epidermidis is a leading cause of hospital-acquired infections. Traditional antibiotics have significantly reduced efficacy against this pathogen due to its ability to form biofilms on abiotic surfaces and drug resistance. The accessory gene regulator (agr) quorum sensing system is directly involved in S. epidermidis pathogenesis. Activation of agr is achieved via binding of the autoinducing peptide (AIP) signal to the extracellular sensor domain of its cognate receptor, AgrC. Divergent evolution has given rise to four agr specificity groups in S. epidermidis defined by the unique AIP sequence used by each group (AIPs-I-IV) with observed cross-group activities. As agr agonism has been shown to reduce biofilm growth in S. epidermidis, the development of pan-group activators of the agr system is of interest as a potential antivirulence strategy. To date, no synthetic compounds have been identified that are capable of appreciably activating the agr system of more than one specificity group of S. epidermidis or, to our knowledge, of any of the other Staphylococci. Here, we report the characterization of the structure-activity relationships for agr agonism by S. epidermidis AIP-II and AIP-III and the application of these new SAR data and those previously reported for AIP-I for the design and synthesis of the first multigroup agr agonists. These non-native peptides were capable of inducing the expression of critical biofilm dispersal agents (i.e., phenol-soluble modulins) in cell culture and represent new tools to study the role of quorum sensing in S. epidermidis infections.

Potential role of probiotics in reducing Clostridioides difficile virulence: Interference with quorum sensing systems

Opportunistic pathogenic bacteria may cause disease after the normally protective microbiome is disrupted (typically by antibiotic exposure). Clostridioides difficile is one such pathogen having a severe impact on healthcare facilities and increasing costs of medical care. The search for new therapeutic strategies that are not reliant on additional antibiotic exposures are currently being explored. One such strategy is to disrupt the production of C. difficile virulence factors by interfering with quorum sensing (QS) systems. QS has been well studied in other bacteria, but our understanding in C. difficile is not so well understood. Some probiotic strains or combinations of strains have been shown to be effective in the treatment or primary prevention of C. difficile infections and may possess multiple mechanisms of action. One mechanism of probiotics might be the inhibition of QS, but their role has not been clearly defined yet. A literature search was conducted using standard databases (PubMed, Google Scholar) from database inception to August 2020. The objective of this paper is to update our understanding of how QS leads to toxin production by C. difficile, which is important in pathogenesis, and how QS inhibitors or probiotics may disrupt this pathway. We found two main QS systems for C. difficile (Agr and Lux systems) that are involved in C. difficile pathogenesis by regulating toxin production, motility and adherence. Probiotics and other QS inhibitors targeting QS systems may represent important new directions of therapy and prevention of CDI.

A Novel Aza-Derivative Inhibits agr Quorum Sensing Signaling and Synergizes Methicillin-Resistant Staphylococcus aureus to Clindamycin

Increasing antibiotic resistance and diminishing pharmaceutical industry investments have increased the need for molecules that can treat infections caused by dangerous pathogens such as methicillin-resistant Staphylococcus aureus (MRSA). Quorum Sensing (QS) is a signaling mechanism that regulates bacterial virulence in pathogens. A report demonstrating that the anti-inflammatory drug Diflunisal reduces MRSA virulence factors' expression prompted us to design, synthesize and test 16 aza-analogs as inhibitors of S. aureus virulence factors controlled by the accessory gene regulator (agr) QS system. At first, we evaluated by qRT-PCR the activity of compounds on rnaIII expression, a QS related gene. Azan-7 was the most active molecule tested and it did not show cytotoxic activity in human cell lines. Moreover, we demonstrated that it did not affect bacterial proliferation. Regulation of MRSA virulence genes by Azan-7 was investigated using qRT-PCR and RNAseq. Azan-7 significantly reduced hla, psmα, hysA, agrA, cap1A, and cap1C gene expression. In silico docking demonstrated that Azan-7 binds the response regulator AgrA. This data was confirmed by electrophoretic mobility shift assay (EMSA) reporting that Azan-7 binding to AgrA protein strongly reduced the AgrA-DNA complex formation at the P3 promoter region involved in the regulation of rnaIII transcription. Azan-7 inhibited MRSA-mediated haemolysis, reduced survival of the pathogen at low pH levels, and increased macrophage killing. In addition, Azan-7 enhanced MRSA susceptibility to clindamycin both in planktonic growth and biofilm. Azan-7 did not induce resistance over 10 days in culture. It was equally active against all the AgrA MRSA subtypes encountered among clinical isolates, but it was not active against Staphylococcus epidermidis, although the AgrA proteins show an approximate 80% homology. These results demonstrate that Azan-7 inhibits the expression of MRSA virulence factors by interfering in the QS and synergizes MRSA biofilm with clindamycin, indicating the compound as a promising candidate for the treatment of MRSA infections.

Salicylic acid stabilizes Staphylococcus aureus biofilm by impairing the agr quorum-sensing system

Salicylic acid (SAL) has recently been shown to induce biofilm formation in Staphylococcus aureus and to affect the expression of virulence factors. This study was aimed to investigate the effect of SAL on the regulatory agr system and its impact on S. aureus biofilm formation. The agr quorum-sensing system, which is a central regulator in S. aureus pathogenicity, plays a pivotal role in the dispersal of S. aureus mature biofilms and contributes to the creation of new colonization sites. Here, we demonstrate that SAL impairs biofilm dispersal by interfering with agr expression. As revealed by our work, protease and surfactant molecule production is diminished, and bacterial cell autolysis is also negatively affected by SAL. Furthermore, as a consequence of SAL treatment, the S. aureus biofilm matrix revealed the lack of extracellular DNA. In silico docking and simulation of molecular dynamics provided evidence for a potential interaction of AgrA and SAL, resulting in reduced activity of the agr system. In conclusion, SAL stabilized the mature S. aureus biofilms, which may prevent bacterial cell dissemination. However, it may foster the establishment of infections locally and consequently increase bacterial persistence leading to therapeutic failure.

Biofilms by bacterial human pathogens: Clinical relevance - development, composition and regulation - therapeutical strategies

Notably, bacterial biofilm formation is increasingly recognized as a passive virulence factor facilitating many infectious disease processes. In this review we will focus on bacterial biofilms formed by human pathogens and highlight their relevance for diverse diseases. Along biofilm composition and regulation emphasis is laid on the intensively studied biofilms of Vibrio cholerae, Pseudomonas aeruginosa and Staphylococcus spp., which are commonly used as biofilm model organisms and therefore contribute to our general understanding of bacterial biofilm (patho-)physiology. Finally, therapeutical intervention strategies targeting biofilms will be discussed.

Staphylococcal Biofilms: Challenges and Novel Therapeutic Perspectives

Staphylococci, like Staphylococcus aureus and S. epidermidis, are common colonizers of the human microbiota. While being harmless in many cases, many virulence factors result in them being opportunistic pathogens and one of the major causes of hospital-acquired infections worldwide. One of these virulence factors is the ability to form biofilms-three-dimensional communities of microorganisms embedded in an extracellular polymeric matrix (EPS). The EPS is composed of polysaccharides, proteins and extracellular DNA, and is finely regulated in response to environmental conditions. This structured environment protects the embedded bacteria from the human immune system and decreases their susceptibility to antimicrobials, making infections caused by staphylococci particularly difficult to treat. With the rise of antibiotic-resistant staphylococci, together with difficulty in removing biofilms, there is a great need for new treatment strategies. The purpose of this review is to provide an overview of our current knowledge of the stages of biofilm development and what difficulties may arise when trying to eradicate staphylococcal biofilms. Furthermore, we look into promising targets and therapeutic methods, including bacteriocins and phage-derived antibiofilm approaches.

The cytoplasmic loops of AgrC contribute to the quorum-sensing activity of Staphylococcus aureus

In Staphylococcus aureus, the accessory gene regulator (agr) quorum-sensing system is thought to play an important role in biofilm formation. The histidine kinase AgrC is one of the agr system components and activated by the self-generated auto-inducing peptide (AIP), which is released continuously into the extracellular environment during bacterial growth. The extracellular loops (Extra-loops) of AgrC are crucial for AIP binding. Here, we reported that the cytoplasmic loops (Cyto-loops) of AgrC are also involved in Agr activity. We identified S. aureus ST398 clinical isolates containing a naturally occurring single amino acid substitution (lysine to isoleucine) at position 73 of an AgrC Cyto-loop that exhibited significantly stronger biofilm formation and decreased Agr activity compared to the wild-type strain. A constructed strain containing the K73I point mutation in AgrC Cyto-loop continued to show a growth dependent induction of the agr system, although the growth dependent induction was delayed by about 6 h compared to the wild-type. In addition, a series of strains containing deletion mutants of the AgrC Cyto- and Extra-loops were constructed and revealed that the removal of the two Cyto-loops and Extra-loops 2 and 3 totally abolished the Agr activity and the growth-dependence on the agr system induction. Remarkably, the Extra-loop 1 deletion did not affect the Agr activity. In conclusion, the AgrC Cyto-loops play a crucial role in the S. aureus quorum-sensing activity.

Bacterial chatter in chronic wound infections

One of the hallmark characteristics of chronic diabetic wounds is the presence of biofilm-forming bacteria. Bacteria encapsulated in a biofilm may coexist as a polymicrobial community and communicate with each other through a phenomenon termed quorum sensing (QS). Here, we describe the QS circuits of bacterial species commonly found in chronic diabetic wounds. QS relies on diffusion of signaling molecules and the local concentration changes of these molecules that bacteria experience in wounds. These biochemical signaling pathways play a role not only in biofilm formation and virulence but also in wound healing. They are, therefore, key to understanding the distinctive nature of these infections. While several in vivo and in vitro models exist to study QS in wounds, there has been limited progress in understanding the interplay between QS molecules and host factors that contribute to wound healing. Lastly, we examine the potential of targeting QS for both diagnosis and therapeutic intervention purposes.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Novel targets of pentacyclic triterpenoids in Staphylococcus aureus: A systematic review

BACKGROUND

Staphylococcus aureus is an important pathogen both in community-acquired and healthcare-associated infections, and has successfully evolved numerous strategies for resisting the action to practically all antibiotics. Resistance to methicillin is now widely described in the community setting (CMRSA), thus the development of new drugs or alternative therapies is urgently necessary. Plants and their secondary metabolites have been a major alternative source in providing structurally diverse bioactive compounds as potential therapeutic agents for the treatment of bacterial infections. One of the classes of natural secondary metabolites from plants with the most bioactive compounds are the triterpenoids, which comprises structurally diverse organic compounds. In nature, triterpenoids are often found as tetra- or penta-cyclic structures.

AIM

This review highlights the anti-staphylococcal activities of pentacyclic triterpenoids, particularly α-amyrin (AM), betulinic acid (BA) and betulinaldehyde (BE). These compounds are based on a 30-carbon skeleton comprising five six-membered rings (ursanes and lanostanes) or four six-membered rings and one five-membered ring (lupanes and hopanes).

METHODS

Electronic databases such as ScienceDirect, PubMed and Scopus were used to search scientific contributions until March 2018, using relevant keywords. Literature focusing on the antimicrobial and antibiofilms of effects of pentacyclic triterpenoids on S. aureus were identified and summarized.

RESULTS

Pentacyclic triterpenoids can be divided into three representative classes, namely ursane, lupane and oleananes. This class of compounds have been shown to exhibit analgesic, immunomodulatory, anti-inflammatory, anticancer, antioxidant, antifungal and antibacterial activities. In studies of the antimicrobial activities and targets of AM, BA and BE in sensitive and multidrug-resistant S. aureus, these compounds acted synergistically and have different targets from the conventional antibiotics.

CONCLUSION

The inhibitory mechanisms of S. aureus in novel targets and pathways should stimulate further researches to develop AM, BA and BE as therapeutic agents for infections caused by S. aureus. Continued efforts to identify and exploit synergistic combinations by the three compounds and peptidoglycan inhibitors, are also necessary as alternative treatment options for S. aureus infections.

Alternative approaches to treat bacterial infections: targeting quorum-sensing

Introduction: The emergence of multi- and pan-drug-resistant bacteria represents a global crisis that calls for the development of alternative anti-infective strategies. These comprise anti-virulence approaches, which target pathogenicity without exerting a bacteriostatic or bactericidal effect and are claimed to reduce the development of resistance. Because in many pathogens, quorum-sensing (QS) systems control the expression of virulence factors, interference with QS, or quorum-quenching, is often proposed as a strategy with a broad anti-virulence effect.Areas covered: We discuss the role and regulatory targets of QS control in selected Gram-positive and Gram-negative bacteria, focusing on those with clinical importance and QS control of virulence. We present the components of QS systems that form possible targets for the development of anti-virulence drugs and discuss recent research on quorum-quenching approaches to control bacterial infection.Expert opinion: While there has been extensive research on QS systems and quorum-quenching approaches, there is a paucity of in-vivo research using adequate animal models to substantiate applicability. In-vivo research on QS blockers needs to be intensified and optimized to use clinically relevant setups, in order to underscore that such drugs can be used effectively to overcome problems associated with the treatment of severe infections by antibiotic-resistant pathogens.

Controlling biofilms using synthetic biology approaches

Bacterial biofilms are formed by the complex but ordered regulation of intra- or inter-cellular communication, environmentally responsive gene expression, and secretion of extracellular polymeric substances. Given the robust nature of biofilms due to the non-growing nature of biofilm bacteria and the physical barrier provided by the extracellular matrix, eradicating biofilms is a very difficult task to accomplish with conventional antibiotic or disinfectant treatments. Synthetic biology holds substantial promise for controlling biofilms by improving and expanding existing biological tools, introducing novel functions to the system, and re-conceptualizing gene regulation. This review summarizes synthetic biology approaches used to eradicate biofilms via protein engineering of biofilm-related enzymes, utilization of synthetic genetic circuits, and the development of functional living agents. Synthetic biology also enables beneficial applications of biofilms through the production of biomaterials and patterning biofilms with specific temporal and spatial structures. Advances in synthetic biology will add novel biofilm functionalities for future therapeutic, biomanufacturing, and environmental applications.

Iron Metabolism at the Interface between Host and Pathogen: From Nutritional Immunity to Antibacterial Development

Nutritional immunity is a form of innate immunity widespread in both vertebrates and invertebrates. The term refers to a rich repertoire of mechanisms set up by the host to inhibit bacterial proliferation by sequestering trace minerals (mainly iron, but also zinc and manganese). This strategy, selected by evolution, represents an effective front-line defense against pathogens and has thus inspired the exploitation of iron restriction in the development of innovative antimicrobials or enhancers of antimicrobial therapy. This review focuses on the mechanisms of nutritional immunity, the strategies adopted by opportunistic human pathogen Staphylococcus aureus to circumvent it, and the impact of deletion mutants on the fitness, infectivity, and persistence inside the host. This information finally converges in an overview of the current development of inhibitors targeting the different stages of iron uptake, an as-yet unexploited target in the field of antistaphylococcal drug discovery.

The 5' NAD Cap of RNAIII Modulates Toxin Production in Staphylococcus aureus Isolates

Nicotinamide adenosine dinucleotide (NAD) has been found to be covalently attached to the 5' ends of specific RNAs in many different organisms, but the physiological consequences of this modification are largely unknown. Here, we report the occurrence of several NAD-RNAs in the opportunistic pathogen Staphylococcus aureus Most prominently, RNAIII, a central quorum-sensing regulator of this bacterium's physiology, was found to be 5' NAD capped in a range from 10 to 35%. NAD incorporation efficiency into RNAIII was found to depend in vivo on the -1 position of the P3 promoter. An increase in RNAIII's NAD content led to a decreased expression of alpha- and delta-toxins, resulting in reduced cytotoxicity of the modified strains. These effects seem to be caused neither by changes in RNAIII's secondary structure nor by a different translatability upon NAD attachment, as indicated by unaltered patterns in in vitro chemical probing and toeprinting experiments. Even though we did not observe any effect of this modification on RNAIII's secondary structure or translatability in vitro, additional unidentified factors might account for the modulation of exotoxins in vivo Ultimately, the study constitutes a step forward in the discovery of new roles of the NAD molecule in bacteria.IMPORTANCE Numerous organisms, including bacteria, are endowed with a 5' NAD cap in specific RNAs. While the presence of the 5' NAD cap modulates the stability of the modified RNA species, a significant biological function and phenotype have not been assigned so far. Here, we show the presence of a 5' NAD cap in RNAIII from S. aureus, a dual-function regulatory RNA involved in quorum-sensing processes and regulation of virulence factor expression. We also demonstrate that altering the natural NAD modification ratio of RNAIII leads to a decrease in exotoxin production, thereby modulating the bacterium's virulence. Our work unveils a new layer of regulation of RNAIII and the agr system that might be linked to the redox state of the NAD molecule in the cell.

Matrine and baicalin inhibit apoptosis induced by Panton-Valentine leukocidin of Staphylococcus aureus in bovine mammary epithelial cells

It was previously thought that the Panton-Valentine leukocidin (PVL) toxin of Staphylococcus aureus (S. aureus) was not the main cause of cow mastitis. However, in recent years, detection of the gene encoding PVL has been increasing in dairy cow mastitis, which implies that PVL may be related to bovine mastitis. Therefore, we wanted to search for drugs inhibiting PVL or PVL-induced apoptosis. In this report, we investigated the apoptosis mechanism of PVL in bovine mammary epithelial cells (BMEC) and the inhibition mechanism of matrine and baicalin on PVL-induced apoptosis of BMEC. The results demonstrated that BMEC were damaged and underwent apoptosis by a standard PVL-producing strain of S. aureus (ATCC 49775), a PVL knockout mutant Δpvl 49775, complemented mutant C-Δpvl 49775, or recombinant (r)PVL in vitro. The rates of apoptosis and necrosis induced by S. aureus ATCC 49775 and C-Δpvl 49775 were significantly higher than those induced by Δpvl 49775, demonstrating that BMEC apoptosis and necrosis were associated with PVL. In addition, this research found matrine and baicalin could inhibit the apoptosis of BMEC induced by PVL-producing S. aureus and by rPVL. Matrine downregulated protein expression levels of endogenous and exogenous cleaved caspase-3, cleaved caspase-8, and cleaved caspase-9, and the effect was pronounced at a concentration of 50 μg/mL. Baicalin downregulated the expression of cleaved caspase-9. These results suggested that matrine and baicalin may have potential value against cow mastitis caused by the toxin PVL.

Resilient living materials built by printing bacterial spores

Materials can be made multifunctional by embedding them with living cells that perform sensing, synthesis, energy production, and physical movement. A challenge is that the conditions needed for living cells are not conducive to materials processing and require continuous water and nutrients. Here, we present a three dimensional (3D) printer that can mix material and cell streams to build 3D objects. Bacillus subtilis spores were printed within the material and germinated on its exterior surface, including spontaneously in new cracks. The material was resilient to extreme stresses, including desiccation, solvents, osmolarity, pH, ultraviolet light, and γ-radiation. Genetic engineering enabled the bacteria to respond to stimuli or produce chemicals on demand. As a demonstration, we printed custom-shaped hydrogels containing bacteria that can sense or kill Staphylococcus aureus, a causative agent of infections. This work demonstrates materials endued with living functions that can be used in applications that require storage or exposure to environmental stresses.