Host-Bacterial Crosstalk Determines Staphylococcus aureus Nasal Colonization

Comparative models for human nasal infections and immunity

The human olfactory system is a mucosal surface and a major portal of entry for respiratory and neurotropic pathogens into the body. Understanding how the human nasopharynx-associated lymphoid tissue (NALT) halts the progression of pathogens into the lower respiratory tract or the central nervous system is key for developing effective cures. Although traditionally mice have been used as the gold-standard model for the study of human nasal diseases, mouse models present important caveats due to major anatomical and functional differences of the human and murine olfactory system and NALT. We summarize the NALT anatomy of different animal groups that have thus far been used to study host-pathogen interactions at the olfactory mucosa and to test nasal vaccines. The goal of this review is to highlight the strengths and limitations of each animal model of nasal immunity and to identify the areas of research that require further investigation to advance human health.

Fighting Staphylococcus aureus Biofilms with Monoclonal Antibodies

Staphylococcus aureus (S. aureus) is a notorious pathogen and one of the most frequent causes of biofilm-related infections. The treatment of S. aureus biofilms is hampered by the ability of the biofilm structure to shield bacteria from antibiotics as well as the host's immune system. Therefore, new preventive and/or therapeutic interventions, including the use of antibody-based approaches, are urgently required. In this review, we describe the mechanisms by which anti-S. aureus antibodies can help in combating biofilms, including an up-to-date overview of monoclonal antibodies currently in clinical trials. Moreover, we highlight ongoing efforts in passive vaccination against S. aureus biofilm infections, with special emphasis on promising targets, and finally indicate the direction into which future research could be heading.

Relationship between nasal Carrier isolates and clinical isolates in children with Staphylococcus aureus infections

AIM

To assess the relationship between Staphylococcus aureus (S. aureus) strains colonizing the anterior nares and clinical isolate colonizing other, non-nasal infectious sites in children with S. aureus infections.

METHODS

Fifty-six hospitalized children with S. aureus infection were screened and 22 pairs of nasal carrier isolates and non-nasal clinical isolates were characterized by polymerase chain reaction (PCR) assay for the detection of methicillin resistance (mecA) gene, Panton-Valentine leukocidin virulence (PVL) gene, and multilocus sequence typing (MLST) for the purpose of identifying sequence types of S. aureus.

RESULTS

In this study, Sequence Type (ST) 59 was found to be the predominant clonal type in the nasal carrier isolates, with statistically significant differences in positive mecA and PVL expression compared with other STs. In general, there was consistence between the STs detected in the nose and other, non-nasal sites for each patient (Kappa = 0.950), where 19 pairs (86.4%) of colonization isolates and their corresponding non-nasal clinical isolates were indistinguishable in mecA, PVL, and ST expression.

CONCLUSION

ST59 is reported here as a dominant and virulent methicillin-resistant S. aureus (MRSA) clone which may has become a leading sequence type among virulent MRSAs in Sichuan area. Overall there is a strong correlation between colonization and infection in pediatric patients that may be genetically indistinguishable and endogenous. Therefore, nasal swabs as a routine test for children, the elimination of nasal carriage may be considered as a prevention strategy for some systemic S. aureus infections.

The effect of bacterial chemotaxis on host infection and pathogenicity

Chemotaxis enables microorganisms to move according to chemical gradients. Although this process requires substantial cellular energy, it also affords key physiological benefits, including enhanced access to growth substrates. Another important implication of chemotaxis is that it also plays an important role in infection and disease, as chemotaxis signalling pathways are broadly distributed across a variety of pathogenic bacteria. Furthermore, current research indicates that chemotaxis is essential for the initial stages of infection in different human, animal and plant pathogens. This review focuses on recent findings that have identified specific bacterial chemoreceptors and corresponding chemoeffectors associated with pathogenicity. Pathogenicity-related chemoeffectors are either host and niche-specific signals or intermediates of the host general metabolism. Plant pathogens were found to contain an elevated number of chemotaxis signalling genes and functional studies demonstrate that these genes are critical for their ability to enter the host. The expanding body of knowledge of the mechanisms underlying chemotaxis in pathogens provides a foundation for the development of new therapeutic strategies capable of blocking infection and preventing disease by interfering with chemotactic signalling pathways.

Bringing together what belongs together: Optimizing murine infection models by using mouse-adapted Staphylococcus aureus strains

Staphylococcus (S.) aureus is a leading cause of bacterial infection world-wide, and currently no vaccine is available for humans. Vaccine development relies heavily on clinically relevant infection models. However, the suitability of mice for S. aureus infection models has often been questioned, because experimental infection of mice with human-adapted S. aureus requires very high infection doses. Moreover, mice were not considered to be natural hosts of S. aureus. The latter has been disproven by our recent findings, showing that both laboratory mice, as well as wild small mammals including mice, voles, and shrews, are naturally colonized with S. aureus. Here, we investigated whether mouse-and vole-derived S. aureus strains show an enhanced virulence in mice as compared to the human-adapted strain Newman. Using a step-wise approach based on the bacterial genotype and in vitro assays for host adaptation, we selected the most promising candidates for murine infection models out of a total of 254 S. aureus isolates from laboratory mice as well as wild rodents and shrews. Four strains representing the clonal complexes (CC) 8, 49, and 88 (n = 2) were selected and compared to the human-adapted S. aureus strain Newman (CC8) in murine pneumonia and bacteremia models. Notably, a bank vole-derived CC49 strain, named DIP, was highly virulent in BALB/c mice in pneumonia and bacteremia models, whereas the other murine and vole strains showed virulence similar to or lower than that of Newman. At one tenth of the standard infection dose DIP induced disease severity, bacterial load and host cytokine and chemokine responses in the murine bacteremia model similar to that of Newman. In the pneumonia model, DIP was also more virulent than Newman but the effect was less pronounced. Whole genome sequencing data analysis identified a pore-forming toxin gene, lukF-PV(P83)/lukM, in DIP but not in the other tested S. aureus isolates. To conclude, the mouse-adapted S. aureus strain DIP allows a significant reduction of the inoculation dose in mice and is hence a promising tool to develop clinically more relevant infection models.

Staphylococcus aureus Nasal Colonization: An Update on Mechanisms, Epidemiology, Risk Factors, and Subsequent Infections

Up to 30% of the human population are asymptomatically and permanently colonized with nasal Staphylococcus aureus. To successfully colonize human nares, S. aureus needs to establish solid interactions with human nasal epithelial cells and overcome host defense mechanisms. However, some factors like bacterial interactions in the human nose can influence S. aureus colonization and sometimes prevent colonization. On the other hand, certain host characteristics and environmental factors can predispose to colonization. Nasal colonization can cause opportunistic and sometimes life-threatening infections such as surgical site infections or other infections in non-surgical patients that increase morbidity, mortality as well as healthcare costs.

Risk factors for and molecular characteristics of methicillin-resistant Staphylococcus aureus nasal colonization among healthy children in southern Taiwan, 2005-2010

BACKGROUND/PURPOSE

Nasal colonization of Staphylococcus aureus is a well-defined risk factor for subsequent infection. This study investigated the prevalence of methicillin-resistant S. aureus (MRSA) in southern Taiwan and aimed to identify the host factors for S. aureus colonization and the virulence factor of Panton-Valentine Leukocidin (PVL) genes.

METHODS

In a hospital-based study in Kaohsiung from Oct. 2005 to Dec. 2010, we performed nasal swab in the healthy children aged 2-60 months. We examined the relationship between the demographic characteristics and S. aureus nasal colonization. MRSA isolates were further analyzed for antimicrobial susceptibility and molecular characteristics.

RESULTS

Among 3020 healthy children, 840 (27.8%) children had S. aureus nasal colonization. Of 840 isolates, 246 (29.3%) isolates were MRSA. MRSA colonization was significantly associated with age 2-6 months, day care attendance, and influenza vaccination. Breastfeeding was a protective factor against MRSA colonization. Most MRSA isolates were susceptible to trimethoprim-sulfamethoxazole and doxycycline. Ninety-four percent of MRSA isolates carried either type IV staphylococcal cassette chromosome mec (SCCmec) or SCCmec V_T and 87% belonged to the local community strains, namely clonal complex 59/SCCmec IV or V_T. MRSA isolates with PVL-negative was associated with children with passive smoking.

CONCLUSIONS

Between 2005 and 2010, 27.8% and 8.14% of healthy children in southern Taiwan had nasal carriage of S. aureus and MRSA, respectively. Most MRSA isolates were local community strains. Several demographic factors associated with nasal MRSA colonization were identified.

Lung colonization by Aspergillus fumigatus is controlled by ZNF77

Aspergillus fumigatus is a critical pathogen of humans. Exposure to A. fumigatus conidia occurs frequently but is normally cleared from the respiratory airways. In contrast, individuals with respiratory diseases are often highly colonized by fungi. Here, we use genome-edited epithelial cells to show that the genetic variant rs35699176 in ZNF77 causes loss of integrity of the bronchial epithelium and increases levels of extracellular matrix proteins. These changes promote A. fumigatus conidial adhesion, germination and growth. RNA-seq and LC/MS-MS analysis reveal rs35699176 upregulates vesicle trafficking leading to an increment of adhesion proteins. These changes make cells carrying rs35699176 more receptive to A. fumigatus in the early stages of infection. Moreover, patients with fungal asthma carrying rs35699176^+/- have higher A. fumigatus loads in their respiratory airway. Our results indicate ZNF77 as a key controller of Aspergillus colonization and suggest its utility as a risk-marker for patient stratification.

Evaluation of the Pig-Tailed Macaque (Macaca nemestrina) as a Model of Human Staphylococcus aureus Nasal Carriage

Staphylococcus aureus nasal carriage is a common condition affecting both healthy and immunocompromised populations and provides a reservoir for dissemination of potentially infectious strains by casual contact. The factors regulating the onset and duration of nasal S. aureus colonization are mostly unknown, and a human-relevant animal model is needed. Here, we screened 17 pig-tailed macaques (Macaca nemestrina) for S. aureus carriage, and 14 of 17 animals tested positive in the nose at one or both screening sessions (8 weeks apart), while the other 3 animals were negative in the nose but positive in the pharynx at least once. As in humans, S. aureus colonization was densest in the nose, and treatment of the nostrils with mupirocin ointment effectively cleared the nostrils and 6 extranasal body sites. Experimental nasal S. aureus colonization was established with 104 CFU/nostril, and both autologous and nonautologous strains survived over 40 days without any apparent adverse effects. A human nasal S. aureus isolate (strain D579, sequence type 398) was carried in 4 of 6 animals for over 3 weeks. Nostrils that did eradicate experimentally applied S. aureus exhibited neutrophilic innate immunity marked by elevated nasal interleukin-1β (IL-1β), IL-8, and monocyte chemotactic protein 1 levels and a 10-fold decreased IL-1 receptor antagonist/IL-1β ratio within 7 days postinoculation, analogous to the human condition. Taken together, pig-tailed macaques represent a physiological model of human S. aureus nasal carriage that may be utilized for testing natural colonization and decolonization mechanisms as well as novel classes of anti-S. aureus therapeutics.

Development of a New Application for Comprehensive Viability Analysis Based on Microbiome Analysis by Next-Generation Sequencing: Insights into Staphylococcal Carriage in Human Nasal Cavities

The nasal carriage rate of Staphylococcus aureus in human is 25 to 30%, and S. aureus sporadically causes severe infections. However, the mechanisms underlying staphylococcal carriage remain largely unknown. In the present study, we constructed an rpoB-based microbiome method for staphylococcal species discrimination. Based on a microbiome scheme targeting viable cell DNA using propidium monoazide (PMA) dye (PMA microbiome method), we also developed a new method to allow the comprehensive viability analysis of any bacterial taxon. To clarify the ecological distribution of staphylococci in the nasal microbiota, we applied these methods in 46 nasal specimens from healthy adults. PMA microbiome results showed that Staphylococcaceae and Corynebacteriaceae were the most predominant viable taxa (average relative abundance: 0.435262 and 0.375195, respectively), and Staphylococcus epidermidis exhibited the highest viability in the nasal microbiota. Staphylococcus aureus detection rates from nasal specimens by rpoB-based conventional and PMA microbiome methods were 84.8% (39 of 46) and 69.5% (32 of 46), respectively, which substantially exceeded the values obtained by a culture method using identical specimens (36.9%). Our results suggest that Staphylococcaceae species, especially S. epidermidis, adapted most successfully to human nasal cavity. High detection of S. aureus DNA by microbiome methods suggests that almost all healthy adults are consistently exposed to S. aureus in everyday life. Furthermore, the large difference in S. aureus detection rates between culture and microbiome methods suggests that S. aureus cells frequently exist in a viable but nonculturable state in nasal cavities. Our method and findings will contribute to a better understanding of the mechanisms underlying carriage of indigenous bacteria.IMPORTANCE Metagenomic analyses, such as 16S rRNA microbiome methods, have provided new insights in various research fields. However, conventional 16S rRNA microbiome methods do not permit taxonomic analysis of only the viable bacteria in a sample and have poor resolving power below the genus level. Our new schemes allowed for viable cell-specific analysis and species discrimination, and nasal microbiome data using these methods provided some interesting findings regarding staphylococcal nasal carriage. According to our comprehensive viability analysis, the high viability of Staphylococcus species, especially Staphylococcus epidermidis, in human nasal carriage suggests that this taxon has adapted most successfully to human nasal tissue. Also, a higher detection rate of S. aureus DNA by microbiome methods (84.8%) than by a culture method (36.9%) suggests that almost all healthy adults are consistently exposed to Staphylococcus aureus in the medium and long term. Our findings will contribute to a better understanding of the mechanisms underlying the carriage of indigenous bacteria.

Cessation from Smoking Improves Innate Host Defense and Clearance of Experimentally Inoculated Nasal Staphylococcus aureus

Staphylococcus aureus nasal carriage is transient in most humans and usually benign, but dissemination of S. aureus to extranasal sites causes the majority of clinical infections, and S. aureus is a major cause of serious infections in the United States. A better understanding of innate nasal decolonization mechanisms is urgently needed, as are relevant models for studying S. aureus clearance. Here, we screened a population of healthy smokers for nasal S. aureus carriage and compared the participants' abilities to clear experimentally applied nasal S. aureus before and after completion of a smoking cessation program. We determined that cigarette smoking increases the mean nasal S. aureus load (2.6 × 10⁴ CFU/swab) compared to the load observed in healthy nonsmokers (1.7 × 10³ CFU/swab) and might increase the rate of S. aureus nasal carriage in otherwise-healthy adults: 22 of 99 smokers carried S. aureus at the screening visit, while only 4 of 30 nonsmokers screened positive during the same time period. Only 6 of 19 experimental inoculation studies in active smokers resulted in S. aureus clearance within the month of follow-up, while in the cessation group, 6 of 9 subjects cleared nasal S. aureus and carriage duration averaged 21 ± 4 days. Smoking cessation associated with enhanced expression of S. aureus-associated interleukin-1β (IL-1β) and granulocyte colony-stimulating factor (G-CSF) in nasal fluids. Participants who failed to clear S. aureus exhibited a higher nasal S. aureus load and elevated nasal interleukin-1 receptor antagonist (IL-1RA) expression at the preexperiment study visits. We conclude that smokers exhibit higher S. aureus loads than nonsmokers and that innate immune pathways, including G-CSF expression and signaling through the IL-1 axis, are important mediators of nasal S. aureus clearance.

Staphylococcus aureus infections, some second thoughts

PURPOSE OF REVIEW

Staphylococcus aureus (S. aureus) is well known for its ability to cause life-threatening infections. On the other hand, this bacterium can thrive as a commensal on and in human tissues without causing much problems. How big a threat is S. aureus actually? Furthermore, commensalism is associated with biofilms, where can we find them, and which natural and artificial components activate biofilm formation?

RECENT FINDINGS

Recent findings on S. aureus carriage on skin, mucosa, and in wounds indicate the presence of large numbers of S. aureus, yet its abundance can be without major implications for the host. S. aureus is often present in biofilms, together with other microorganisms, which can stimulate biofilm formation of S. aureus, in addition medicine including antibiotics can do the same.

SUMMARY

S. aureus can cause devastating infections, but when we take into consideration the ubiquitous presence of S. aureus, the risk seems to be relatively low. S. aureus forms biofilms in response to the 'hazards' on the human body, and signal to do so can come from various sources. All this has to be taken into consideration when we treat a patient as this might have enormous impact on the outcome.

Staphylococcus aureus and Methicillin-Resistant Staphylococcus aureus in Workers in the Food Industry

Staphylococcus aureus is part of the common flora on the skin and mucous membranes of mammals and approximately 20–30% of humans are persistently colonized, mainly by mostly susceptible human-adapted isolates. In contrast, colonization with methicillin-resistant S. aureus is rare (approximately 1%), predominantly transient and associated with prior contact to the health care system. Additionally, in recent years livestock-associated S. aureus clones contributed to colonization in humans, especially in those working in close contact to farm animals. A considerable percentage of colonizing S. aureus isolates is equipped with enterotoxin genes. Humans carrying enterotoxigenic isolates represent a contamination source when handling food, thus generating a continuous risk of S. aureus food intoxication. Molecular characterization of isolates colonizing humans and obtained from food, respectively, enables the tracing of food-related outbreaks back to the source of food intoxication. We will summarize current knowledge about the S. aureus population colonizing humans, including those in close contact to animals and food, respectively. Additionally, we will review data on the molecular characterization of S. aureus isolates related to staphylococcal foodborne disease and the elucidation of staphylococcal foodborne outbreaks. Staphylococcal food poisoning is a common foodborne disease, mediated by the ingestion of enterotoxins produced by enterotoxigenic strains of S. aureus. For several outbreaks of foodborne S. aureus disease, colonized personnel could be identified as the source of food contamination. However, because of the widespread occurrence of enterotoxigenic strains as human colonizers and the often transient nature of colonization, the source of contamination cannot always be identified unambiguously. Therefore, compliance with hygiene measures is the most important requirement to prevent food contamination by both human colonization and environmental S. aureus reservoirs.

The commensal lifestyle of Staphylococcus aureus and its interactions with the nasal microbiota

Although human colonization by facultative bacterial pathogens, such as Staphylococcus aureus, represents a major risk factor for invasive infections, the commensal lifestyle of such pathogens has remained a neglected area of research. S. aureus colonizes the nares of approximately 30% of the human population and recent studies suggest that the composition of highly variable nasal microbiota has a major role in promoting or inhibiting S. aureus colonization. Competition for epithelial attachment sites or limited nutrients, different susceptibilities to host defence molecules and the production of antimicrobial molecules may determine whether nasal bacteria outcompete each other. In this Review, we discuss recent insights into mechanisms that are used by S. aureus to prevail in the human nose and the counter-strategies that are used by other nasal bacteria to interfere with its colonization. Understanding such mechanisms will be crucial for the development of new strategies for the eradication of endogenous facultative pathogens.

Development of persistent gastrointestinal S. aureus carriage in mice

One fifth to one quarter of the human population is asymptomatically, naturally and persistently colonised by Staphylococcus aureus. Observational human studies indicate that although the whole population is intermittently exposed, some individuals lose S. aureus rapidly. Others become persistent carriers, as assessed by nasal cultures, with many individuals colonised for decades. Current animal models of S. aureus colonisation are expensive and normally require antibiotics. Importantly, these animal models have not yet contributed to our poor understanding of the dichotomy in human colonisation status. Here, we identify a single strain of S. aureus found to be persistently colonising the gastrointestinal tract of BALB/c mice. Phylogenetic analyses suggest it diverged from a human ST15 lineage in the recent past. We show that murine carriage of this organism occurs in the bowel and nares, is acquired early in life, and can persist for months. Importantly, we observe the development of persistent and non-persistent gastrointestinal carriage states in genetically identical mice. We developed a needle- and antibiotic-free model in which we readily induced S. aureus colonisation of the gastrointestinal tract experimentally by environmental exposure. Using our experimental model, impact of adaptive immunity on S. aureus colonisation could be assessed. Vaccine efficacy to eliminate colonisation could also be investigated using this model.

Emergence and genomic diversification of a virulent serogroup W:ST-2881(CC175) Neisseria meningitidis clone in the African meningitis belt

Countries of the African 'meningitis belt' are susceptible to meningococcal meningitis outbreaks. While in the past major epidemics have been primarily caused by serogroup A meningococci, W strains are currently responsible for most of the cases. After an epidemic in Mecca in 2000, W:ST-11 strains have caused many outbreaks worldwide. An unrelated W:ST-2881 clone was described for the first time in 2002, with the first meningitis cases caused by these bacteria reported in 2003. Here we describe results of a comparative whole-genome analysis of 74 W:ST-2881 strains isolated within the framework of two longitudinal colonization and disease studies conducted in Ghana and Burkina Faso. Genomic data indicate that the W:ST-2881 clone has emerged from Y:ST-175(CC175) bacteria by capsule switching. The circulating W:ST-2881 populations were composed of a variety of closely related but distinct genomic variants with no systematic differences between colonization and disease isolates. Two distinct and geographically clustered phylogenetic clonal variants were identified in Burkina Faso and a third in Ghana. On the basis of the presence or absence of 17 recombination fragments, the Ghanaian variant could be differentiated into five clusters. All 25 Ghanaian disease isolates clustered together with 23 out of 40 Ghanaian isolates associated with carriage within one cluster, indicating that W:ST-2881 clusters differ in virulence. More than half of the genes affected by horizontal gene transfer encoded proteins of the 'cell envelope' and the 'transport/binding protein' categories, which indicates that exchange of non-capsular antigens plays an important role in immune evasion.

Human Adaptive Immunity Rescues an Inborn Error of Innate Immunity

The molecular basis of the incomplete penetrance of monogenic disorders is unclear. We describe here eight related individuals with autosomal recessive TIRAP deficiency. Life-threatening staphylococcal disease occurred during childhood in the proband, but not in the other seven homozygotes. Responses to all Toll-like receptor 1/2 (TLR1/2), TLR2/6, and TLR4 agonists were impaired in the fibroblasts and leukocytes of all TIRAP-deficient individuals. However, the whole-blood response to the TLR2/6 agonist staphylococcal lipoteichoic acid (LTA) was abolished only in the index case individual, the only family member lacking LTA-specific antibodies (Abs). This defective response was reversed in the patient, but not in interleukin-1 receptor-associated kinase 4 (IRAK-4)-deficient individuals, by anti-LTA monoclonal antibody (mAb). Anti-LTA mAb also rescued the macrophage response in mice lacking TIRAP, but not TLR2 or MyD88. Thus, acquired anti-LTA Abs rescue TLR2-dependent immunity to staphylococcal LTA in individuals with inherited TIRAP deficiency, accounting for incomplete penetrance. Combined TIRAP and anti-LTA Ab deficiencies underlie staphylococcal disease in this patient.

The oral cavity microbiota: between health, oral disease, and cancers of the aerodigestive tract

Many studies show that the human microbiome plays a critical role in the chronic pathologies of obesity, inflammatory bowel diseases, and diabetes. More recently, the interaction between cancer and the microbiome has been highlighted. Most studies have focused on the gut microbiota because it represents the most extensive bacterial community, and the body of evidence correlating it with gut syndromes is increasing. However, in the strict sense, the gastrointestinal (GI) tract begins in the oral cavity, and special attention should be paid to the specific flora of this cavity. This study reviewed the current knowledge about the various microbial ecosystems of the upper part of the GI tract and discussed their potential link to carcinogenesis. The overall composition of the microbial communities, as well as the presence or absence of "key species", in relation to carcinogenesis is addressed. Alterations in the oral microbiota can potentially be used to predict the risk of cancer. Molecular advances and the further monitoring of the microbiota will increase our understanding of the role of the microbiota in carcinogenesis and open new perspectives for future therapeutic and prophylactic modalities.

Apoptosis, Toll-like, RIG-I-like and NOD-like Receptors Are Pathways Jointly Induced by Diverse Respiratory Bacterial and Viral Pathogens

Lower respiratory tract infections are among the top five leading causes of human death. Fighting these infections is therefore a world health priority. Searching for induced alterations in host gene expression shared by several relevant respiratory pathogens represents an alternative to identify new targets for wide-range host-oriented therapeutics. With this aim, alveolar macrophages were independently infected with three unrelated bacterial (Streptococcus pneumoniae, Klebsiella pneumoniae, and Staphylococcus aureus) and two dissimilar viral (respiratory syncytial virus and influenza A virus) respiratory pathogens, all of them highly relevant for human health. Cells were also activated with bacterial lipopolysaccharide (LPS) as a prototypical pathogen-associated molecular pattern. Patterns of differentially expressed cellular genes shared by the indicated pathogens were searched by microarray analysis. Most of the commonly up-regulated host genes were related to the innate immune response and/or apoptosis, with Toll-like, RIG-I-like and NOD-like receptors among the top 10 signaling pathways with over-expressed genes. These results identify new potential broad-spectrum targets to fight the important human infections caused by the bacteria and viruses studied here.

The Active Component of Aspirin, Salicylic Acid, Promotes Staphylococcus aureus Biofilm Formation in a PIA-dependent Manner

Aspirin has provided clear benefits to human health. But salicylic acid (SAL) -the main aspirin biometabolite- exerts several effects on eukaryote and prokaryote cells. SAL can affect, for instance, the expression of Staphylococcus aureus virulence factors. SAL can also form complexes with iron cations and it has been shown that different iron chelating molecules diminished the formation of S. aureus biofilm. The aim of this study was to elucidate whether the iron content limitation caused by SAL can modify the S. aureus metabolism and/or metabolic regulators thus changing the expression of the main polysaccharides involved in biofilm formation. The exposure of biofilm to 2 mM SAL induced a 27% reduction in the intracellular free Fe²⁺ concentration compared with the controls. In addition, SAL depleted 23% of the available free Fe²⁺ cation in culture media. These moderate iron-limited conditions promoted an intensification of biofilms formed by strain Newman and by S. aureus clinical isolates related to the USA300 and USA100 clones. The slight decrease in iron bioavailability generated by SAL was enough to induce the increase of PIA expression in biofilms formed by methicillin-resistant as well as methicillin-sensitive S. aureus strains. S. aureus did not produce capsular polysaccharide (CP) when it was forming biofilms under any of the experimental conditions tested. Furthermore, SAL diminished aconitase activity and stimulated the lactic fermentation pathway in bacteria forming biofilms. The polysaccharide composition of S. aureus biofilms was examined and FTIR spectroscopic analysis revealed a clear impact of SAL in a codY-dependent manner. Moreover, SAL negatively affected codY transcription in mature biofilms thus relieving the CodY repression of the ica operon. Treatment of mice with SAL induced a significant increase of S aureus colonization. It is suggested that the elevated PIA expression induced by SAL might be responsible for the high nasal colonization observed in mice. SAL-induced biofilms may contribute to S. aureus infection persistence in vegetarian individuals as well as in patients that frequently consume aspirin.