Quantitative Hemolysis Assays

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.

Novel Anti-virulence Compounds Disrupt Exotoxin Expression in MRSA

Hemolysins are lytic exotoxins expressed in most strains of S. aureus , but hemolytic activity varies between strains. We have previously reported several novel anti-virulence compounds that disrupt the S. aureus transcriptome, including hemolysin gene expression. This report delves further into our two lead compounds, loratadine and a structurally related brominated carbazole, and their effects on hemolysin production in MRSA. To gain understanding into how these compounds affect hemolysis, we analyzed these exotoxins at the DNA, RNA, and protein level after in vitro treatment. While lysis of red blood cells varied between strains, DNA sequence variation did not account for it. We hypothesized that our compounds would modulate gene expression of multiple hemolysins in a laboratory strain and a clinically relevant hospital-acquired strain of MRSA, both with SCC mec type II. RNA-seq analysis of differential gene expression in untreated and compound-treated cultures revealed hundreds of differentially expressed genes, with a significant enrichment in genes involved in hemolysis. The brominated carbazole and loratadine both displayed the ability to reduce hemolysis in the laboratory strain, but displayed differential activity in a hospital-acquired strain. These results corroborate gene expression studies as well as western blots of alpha hemolysin. Together, this work suggests that small molecules may alter exotoxin production in MRSA, but that the directionality and/or magnitude of the difference is likely strain-dependent.

Piperine, a phytochemical prevents the biofilm city of methicillin-resistant Staphylococcus aureus: A biochemical approach to understand the underlying mechanism

Methicillin-resistant Staphylococcus aureus (MRSA), a drug-resistant human pathogen causes several nosocomial as well as community-acquired infections involving biofilm machinery. Hence, it has gained a wide interest within the scientific community to impede biofilm-induced MRSA-associated health complications. The current study focuses on the utilization of a natural bioactive compound called piperine to control the biofilm development of MRSA. Quantitative assessments like crystal violet, total protein recovery, and fluorescein-di-acetate (FDA) hydrolysis assays, demonstrated that piperine (8 and 16 μg/mL) could effectively compromise the biofilm formation of MRSA. Light and scanning electron microscopic image analysis confirmed the same. Further investigation revealed that piperine could reduce extracellular polysaccharide production by down-regulating the expression of icaA gene. Besides, piperine could reduce the cell-surface hydrophobicity of MRSA, a crucial factor of biofilm formation. Moreover, the introduction of piperine could interfere with microbial motility indicating the interaction of piperine with the quorum-sensing components. A molecular dynamics study showed a stable binding between piperine and AgrA protein (regulator of quorum sensing) suggesting the possible meddling of piperine in quorum-sensing of MRSA. Additionally, the exposure to piperine led to the accumulation of intracellular reactive oxygen species (ROS) and potentially heightened cell membrane permeability in inhibiting microbial biofilm formation. Besides, piperine could reduce the secretion of diverse virulence factors from MRSA. Further exploration revealed that piperine interacted with extracellular DNA (e-DNA), causing disintegration by weakening the biofilm architecture. Conclusively, this study suggests that piperine could be a potential antibiofilm molecule against MRSA-associated biofilm infections.

Cuminaldehyde and Tobramycin Forestall the Biofilm Threats of Staphylococcus aureus: A Combinatorial Strategy to Evade the Biofilm Challenges

Staphylococcus aureus, an opportunistic Gram-positive pathogen, is known for causing various infections in humans, primarily by forming biofilms. The biofilm-induced antibiotic resistance has been considered a significant medical threat. Combinatorial therapy has been considered a reliable approach to combat antibiotic resistance by using multiple antimicrobial agents simultaneously, targeting bacteria through different mechanisms of action. To this end, we examined the effects of two molecules, cuminaldehyde (a natural compound) and tobramycin (an antibiotic), individually and in combination, against staphylococcal biofilm. Our experimental observations demonstrated that cuminaldehyde (20 μg/mL) in combination with tobramycin (0.05 μg/mL) exhibited efficient reduction in biofilm formation compared to their individual treatments (p < 0.01). Additionally, the combination showed an additive interaction (fractional inhibitory concentration value 0.66) against S. aureus. Further analysis revealed that the effective combination accelerated the buildup of reactive oxygen species (ROS) and increased the membrane permeability of the bacteria. Our findings also specified that the cuminaldehyde in combination with tobramycin efficiently reduced biofilm-associated pathogenicity factors of S. aureus, including fibrinogen clumping ability, hemolysis property, and staphyloxanthin production. The selected concentrations of tobramycin and cuminaldehyde demonstrated promising activity against the biofilm development of S. aureus on catheter models without exerting antimicrobial effects. In conclusion, the combination of tobramycin and cuminaldehyde presented a successful strategy for combating staphylococcal biofilm-related healthcare threats. This combinatorial approach holds the potential for controlling biofilm-associated infections caused by S. aureus.

Unveiling Synergistic Potency: Exploring Butyrolactone I to Enhance Gentamicin Efficacy against Methicillin-Resistant Staphylococcus aureus (MRSA) Strain USA300

Staphylococcus aureus, including MRSA strains, poses significant health risks, imposing a significant disease burden and mortality. We investigate butyrolactone I (BL-1), a marine-derived metabolite from Aspergillus terreus, enhancing aminoglycoside efficacy against MRSA. A promising synergy is observed with BL-1 and various aminoglycosides, marked by low fractional inhibitory concentration indexes (FICIs < 0.5). Comprehensive studies utilizing USA300 MRSA and gentamicin reveal a remarkable one-fourth reduction in minimum inhibitory concentration (MIC) with 20 μg/mL BL-1. A relative abundance assay indicates that BL-1 enhances gentamicin uptake while restraining extracellular presence, involving intricate transmembrane signaling and molecular interactions. RNA-Seq analysis yielded an unexpected revelation, unveiling a distinctive gene expression profile and distinguishing it from other treatment approaches. Furthermore, meticulous analyses validated the extensive perturbations induced by BL-1 exposure, affecting diverse biological functions, encompassing glycolysis, amino acid metabolisms, substance transmembrane transport, and virulence generation. These valuable insights inspired further confirmation of bacterial virulence and the modulation of membrane permeability resulting from BL-1 treatment. Phenotypic validations corroborated our observations, revealing reduced membrane permeability and hemolytic toxicity, albeit demanding a deeper comprehension of the intricate interplay underlying these actions. Our study contributes crucial mechanistic insights to the development of therapeutic strategies against this notorious pathogen and the judicious employment of aminoglycosides. Additionally, it elucidates marine-derived metabolites' ecological and functional roles, exemplified by fungal quorum sensing signals. These compounds could give producers a competitive edge, inhibiting microorganism proliferation and suggesting novel approaches for combating resistant pathogens.

A novel indolylbenzoquinone compound HL-J6 suppresses biofilm formation and α-toxin secretion in methicillin-resistant Staphylococcus aureus

Eradication of methicillin-resistant Staphylococcus aureus (MRSA) is challenging due to multi-drug resistance of strains and biofilm formation, the latter of which is an important barrier to the penetration of antibiotics and host defences. As such, there is an urgent need to discover and develop novel agents to fight MRSA-associated infection. In this study, HL-J6, a novel indolylbenzoquinone compound, was shown to inhibit S. aureus strains, with a minimum inhibitory concentration against MRSA252 of 2 µg/mL. Moreover, HL-J6 exhibited potent antibiofilm activity in vitro and was able to kill bacteria in biofilm. In the mouse models of wound infection, HL-J6 treatment reduced the MRSA load significantly and inhibited biofilm formation on the wounds. The potent targets of its antibiofilm activity were explored by real-time reverse transcriptase polymerase chain rection, which indicated that HL-J6 downregulated the transcription levels of sarA, atlAE and icaADBC. Moreover, Western blot results showed that HL-J6 reduced the secretion level of α-toxin, a major virulence factor. These findings indicate that HL-J6 is a promising lead compound for the development of novel drugs against MRSA biofilm infections.

Effect of in-water administration of quorum system inhibitors in broilers' productive performance and intestinal microbiome in a mild necrotic enteritis challenge

The poultry industry has been facing the impact of necrotic enteritis (NE), a disease caused by the bacterium Clostridium perfringens producing the haemolytic toxin NetB. NE severity may vary from mild clinical to prominent enteric signs causing reduced growth rates and affecting feed conversion ratio. NetB production is controlled by the Agr-like quorum-sensing (QS) system, which coordinates virulence gene expression in response to bacterial cell density. In this study, the peptide-containing cell-free spent media (CFSM) from Enterococcus faecium was tested in NE challenged broilers in two battery cage and one floor pen studies. Results showed a significant reduction of NE mortality. Metagenomic sequencing of the jejunum microbiome revealed no impact of the CFSM on the microbial community, and growth of C. perfringens was unaffected by CFSM in vitro. The expression of QS-controlled virulence genes netB, plc and pfoA was found to be significantly repressed by CFSM during the mid-logarithmic stage of C. perfringens growth and this corresponded with a significant decrease in haemolytic activity. Purified fractions of CFSM containing bioactive peptides were found to cause reduced haemolysis. These results showed that bioactive peptides reduce NE mortality in broilers by interfering with the QS system of C. perfringens and reducing bacterial virulence. Furthermore, the microbiome of C. perfringens-challenged broilers is not affected by quorum sensing inhibitor containing CFSM.

PtrA regulates prodigiosin synthesis and biological functions in Serratia marcescens FZSF02

Serratia marcescens is a gram-negative bacterium that is able to produce many secondary metabolites, such as the prominent red pigment prodigiosin (PG). In this work, a ptrA-disrupted mutant strain with reduced PG production was selected from Tn5 transposon mutants. RT-qPCR results indicated that ptrA promoted elevated transcription of the pig gene cluster in S. marcescens FZSF02. Furthermore, we found that ptrA also controls several other important biological functions of S. marcescens, including swimming and swarming motilities, biofilm formation, hemolytic activity, and stress tolerance. In conclusion, this study demonstrates that ptrA is a PG synthesis-promoting factor in S. marcescens and provides a brief understanding of the regulatory mechanism of ptrA in S. marcescens cell motility and hemolytic activity.

New insight into the virulence and inflammatory response of Staphylococcus aureus strains isolated from diabetic foot ulcers

Staphylococcus aureus strains isolated from diabetic foot ulcers (DFUs) have less virulence, but still cause severe infections. Furthermore, hypovirulent S. aureus strains appear to be localized in the deep tissues of diabetic foot osteomyelitis, indicating that the unique environment within DFUs affects the pathogenicity of S. aureus. In this study, the cell-free culture medium (CFCM) of S. aureus strains isolated from DFUs exhibited higher cytotoxicity to human erythrocytes than those isolated from non-diabetic patients with sepsis or wounds. Among these S. aureus strains isolated from DFUs, β-toxin negative strains have less virulence than β-toxin positive strains, but induced a higher expression of inflammatory cytokines. Our study and previous studies have shown that the synergistic effect of phenol-soluble modulin α and β-toxin contributes to the higher hemolytic activity of β-toxin positive strains. However, lysis of human erythrocytes by the CFCM of β-toxin negative strains was greatly inhibited by an autolysin inhibitor, sodium polyanethole sulfonate (SPS). A high level of glucose greatly reduced the hemolytic activity of S. aureus, but promoted the expression of interleukin-6 (IL-6) in human neutrophils. However, 5 mM glucose or glucose-6-phosphate (G6P) increased the hemolytic activity of SA118 (a β-toxin negative strain) isolated from DFUs. Additionally, patients with DFUs with growth of S. aureus had lower level of serum IL-6 than those with other bacteria, and the CFCM of S. aureus strains significantly reduced lipopolysaccharide-induced IL-6 expression in human neutrophils. Therefore, the virulence and inflammatory response of S. aureus strains isolated from DFUs are determined by the levels of glucose and its metabolites, which may explain why it is the predominant bacteria isolated from DFUs.

Role of sodium pyruvate in maintaining the survival and cytotoxicity of Staphylococcus aureus under high glucose conditions

Glucose is a crucial carbon source for the growth of Staphylococcus aureus, but an excess of glucose is detrimental and even leads to cell death. Pyruvate, the central metabolite of glycolysis, has been shown to have anti-inflammatory and antioxidant properties. This study aimed to investigate the protective effect of pyruvate on S. aureus under high glucose conditions. Sodium pyruvate greatly increased the cytotoxicity of S. aureus strain BAA-1717 to human erythrocytes and neutrophils in vitro. However, the cytotoxicity and survival of S. aureus were significantly reduced by high glucose, which was restored to normal levels by the addition of sodium pyruvate. The expression of hlg and lukS in S. aureus was higher in the LB-GP cultures than that in LB-G cultures, but there was no significant difference in cytotoxicity between LB-GP and LB-G cultures. Furthermore, the hemolytic activity of S. aureus supernatants could be inhibited by the cell-free culture medium (CFCM) of LB-G cultures, suggesting that high levels of extracellular proteases were presence in the CFCM of LB-G cultures, resulting in degradation of the hemolytic factors. The expression of sarA, which negatively regulates extracellular protease secretion, was higher in LB-GP cultures than that in LB-G cultures. Additionally, sodium pyruvate increased acetate production in S. aureus, which helps maintain cell viability under acidic environment. In conclusion, pyruvate plays an important role in the survival and cytotoxicity of S. aureus under high glucose conditions. This finding may aid in the development of effective treatments for diabetic foot infections.

Staphylococcus aureus ST1 promotes persistent urinary tract infection by highly expressing the urease

Staphylococcus aureus (SA) is a relatively uncommon cause of urinary tract infections (UTIs) in the general population. Although rare, S. aureus-induced UTIs are prone to potentially life-threatening invasive infections such as bacteremia. To investigate the molecular epidemiology, phenotypic characteristics, and pathophysiology of S. aureus-induced UTIs, we analyzed non-repetitive 4,405 S. aureus isolates collected from various clinical sources from 2008 to 2020 from a general hospital in Shanghai, China. Among these, 193 isolates (4.38%) were cultivated from the midstream urine specimens. Epidemiological analysis showed UTI-derived ST1 (UTI-ST1) and UTI-ST5 are the primary sequence types of UTI-SA. Furthermore, we randomly selected 10 isolates from each of the UTI-ST1, non-UTI-ST1 (nUTI-ST1), and UTI-ST5 groups to characterize their in vitro and in vivo phenotypes. The in vitro phenotypic assays revealed that UTI-ST1 exhibits an obvious decline in hemolysis of human red blood cells and increased biofilm and adhesion in the urea-supplemented medium, compared to the medium without urea, while UTI-ST5 and nUTI-ST1 did not show significant differences between the biofilm-forming and adhesion abilities. In addition, the UTI-ST1 displayed intense urease activities by highly expressing urease genes, indicating the potential role of urease in UTI-ST1 survival and persistence. Furthermore, in vitro virulence assays using the UTI-ST1 ureC mutant showed no significant difference in the hemolytic and biofilm-forming phenotypes in the presence or absence of urea in the tryptic soy broth (TSB) medium. The in vivo UTI model also showed that the CFU of the UTI-ST1 ureC mutant rapidly reduced during UTI pathogenesis 72 h post-infection, while UTI-ST1 and UTI-ST5 persisted in the urine of the infected mice. Furthermore, the phenotypes and the urease expression of UTI-ST1 were found to be potentially regulated by the Agr system with the change in environmental pH. In summary, our results provide important insights into the role of urease in S. aureus-induced UTI pathogenesis in promoting bacterial persistence in the nutrient-limiting urinary microenvironment.