Anti-methicillin-resistant Staphylococcus aureus assay of azalomycin F5a and its derivatives

Drug Combinations to Prevent Antimicrobial Resistance: Various Correlations and Laws, and Their Verifications, Thus Proposing Some Principles and a Preliminary Scheme

Antimicrobial resistance (AMR) has been a serious threat to human health, and combination therapy is proved to be an economic and effective strategy for fighting the resistance. However, the abuse of drug combinations conversely accelerates the spread of AMR. In our previous work, we concluded that the mutant selection indexes (SIs) of one agent against a specific bacterial strain are closely related to the proportions of two agents in a drug combination. To discover probable correlations, predictors and laws for further proposing feasible principles and schemes guiding the AMR-preventing practice, here, three aspects were further explored. First, the power function (y = axb, a > 0) correlation between the SI (y) of one agent and the ratio (x) of two agents in a drug combination was further established based on the mathematical and statistical analyses for those experimental data, and two rules a1 × MIC1 = a2 × MIC2 and b1 + b2 = −1 were discovered from both equations of y = a1xb1 and y = a2xb2 respectively for two agents in drug combinations. Simultaneously, it was found that one agent with larger MPC alone for drug combinations showed greater potency for narrowing itself MSW and preventing the resistance. Second, a new concept, mutation-preventing selection index (MPSI) was proposed and used for evaluating the mutation-preventing potency difference of two agents in drug combination; a positive correlation between the MPSI and the mutant prevention concentration (MPC) or minimal inhibitory concentration (MIC) was subsequently established. Inspired by this, the significantly positive correlation, contrary to previous reports, between the MIC and the corresponding MPC of antimicrobial agents against pathogenic bacteria was established using 181 data pairs reported. These results together for the above three aspects indicate that the MPCs in alone and combination are very important indexes for drug combinations to predict the mutation-preventing effects and the trajectories of collateral sensitivity, and while the MPC of an agent can be roughly calculated from its corresponding MIC. Subsequently, the former conclusion was further verified and improved via antibiotic exposure to 43 groups designed as different drug concentrations and various proportions. The results further proposed that the C/MPC for the agent with larger proportion in drug combinations can be considered as a predictor and is the key to judge whether the resistance and the collateral sensitivity occur to two agents. Based on these above correlations, laws, and their verification experiments, some principles were proposed, and a diagram of the mutation-preventing effects and the resistant trajectories for drug combinations with different concentrations and ratios of two agents was presented. Simultaneously, the reciprocal of MPC alone (1/MPC), proposed as the stress factors of two agents in drug combinations, together with their SI in combination, is the key to predict the mutation-preventing potency and control the trajectories of collateral sensitivity. Finally, a preliminary scheme for antimicrobial combinations preventing AMR was further proposed for subsequent improvement research and clinic popularization, based on the above analyses and discussion. Moreover, some similar conclusions were speculated for triple or multiple drug combinations.

Pharmacokinetics of Azalomycin F, a Natural Macrolide Produced by Streptomycete Strains, in Rats

As antimicrobial resistance has been increasing, new antimicrobial agents are desperately needed. Azalomycin F, a natural polyhydroxy macrolide, presents remarkable antimicrobial activities. To investigate its pharmacokinetic characteristics in rats, the concentrations of azalomycin F contained in biological samples, in vitro, were determined using a validated high-performance liquid chromatography–ultraviolet (HPLC-UV) method, and, in vivo, samples were assayed by an ultra-high performance liquid chromatography–tandem mass spectrometric (UPLC–MS/MS) method. Based on these methods, the pharmacokinetics of azalomycin F were first investigated. Its plasma concentration-time courses and pharmacokinetic parameters in rats were obtained by a non-compartment model for oral (26.4 mg/kg) and intravenous (2.2 mg/kg) administrations. The results indicate that the oral absolute bioavailability of azalomycin F is very low (2.39 ± 1.28%). From combinational analyses of these pharmacokinetic parameters, and of the results of the in-vitro absorption and metabolism experiments, we conclude that azalomycin F is absorbed relatively slowly and with difficulty by the intestinal tract, and subsequently can be rapidly distributed into the tissues and/or intracellular f of rats. Azalomycin F is stable in plasma, whole blood, and the liver, and presents plasma protein binding ratios of more than 90%. Moreover, one of the major elimination routes of azalomycin F is its excretion through bile and feces. Together, the above indicate that azalomycin F is suitable for administration by intravenous injection when used for systemic diseases, while, by oral administration, it can be used in the treatment of diseases of the gastrointestinal tract.

Lichen-like association of Chlamydomonas reinhardtii and Aspergillus nidulans protects algal cells from bacteria

Organismal interactions within microbial consortia and their responses to harmful intruders remain largely understudied. An important step toward the goal of understanding functional ecological interactions and their evolutionary selection is the study of increasingly complex microbial interaction systems. Here, we discovered a tripartite biosystem consisting of the fungus Aspergillus nidulans, the unicellular green alga Chlamydomonas reinhardtii, and the algicidal bacterium Streptomyces iranensis. Genetic analyses and MALDI-IMS demonstrate that the bacterium secretes the algicidal compound azalomycin F upon contact with C. reinhardtii. In co-culture, A. nidulans attracts the motile alga C. reinhardtii, which becomes embedded and surrounded by fungal mycelium and is shielded from the algicide. The filamentous fungus Sordaria macrospora was susceptible to azalomycin F and failed to protect C. reinhardtii despite chemotactically attracting the alga. Because S. macrospora was susceptible to azalomycin F, this data imply that for protection the fungus needs to be resistant. Formation of the lichen-like association between C. reinhardtii and A. nidulans increased algal growth. The protection depends on the increased amounts of membrane lipids provided by resistant fungi, thereby generating a protective shelter against the bacterial toxin. Our findings reveal a strategy whereby algae survive lethal environmental algicides through cooperation with fungi.

Azalomycin F5a Eradicates Staphylococcus aureus Biofilm by Rapidly Penetrating and Subsequently Inducing Cell Lysis

Antimicrobial resistance has emerged as a serious threat to public health. Bacterial biofilm, as a natural lifestyle, is a major contributor to resistance to antimicrobials. Azalomycin F_5a, a natural guanidine-containing polyhydroxy macrolide, has remarkable activities against Gram-positive bacteria, including Staphylococcus aureus, a major causative agent of hospital-acquired infections. To further evaluate its potential to be developed as a new antimicrobial agent, its influence on S. aureus biofilm formation was evaluated using the crystal violet method, and then its eradication effect against mature biofilms was determined by confocal laser scanning microscopy, the drop plate method, and regrowth experiments. The results showed that azalomycin F_5a could significantly inhibit S. aureus biofilm formation, and such effects were concentration dependent. In addition, it can also eradicate S. aureus mature biofilms with the minimum biofilm eradication concentration of 32.0 μg/mL. As extracellular deoxyribonucleic acid (eDNA) plays important roles in the structural integrity of bacterial biofilm, its influence on the eDNA release in S. aureus biofilm was further analyzed using gel electrophoresis. Combined with our previous works, these results indicate that azalomycin F_5a could rapidly penetrate biofilm and causes damages to the cell membrane, leading to an increase in DNase release and eventually eradicating S. aureus biofilm.

Guanidine-Containing Polyhydroxyl Macrolides: Chemistry, Biology, and Structure-Activity Relationship

Antimicrobial resistance has been seriously threatening human health, and discovering new antimicrobial agents from the natural resource is still an important pathway among various strategies to prevent resistance. Guanidine-containing polyhydroxyl macrolides, containing a polyhydroxyl lactone ring and a guanidyl side chain, can be produced by many actinomycetes and have been proved to possess many bioactivities, especially broad-spectrum antibacterial and antifungal activities. To explore the potential of these compounds to be developed into new antimicrobial agents, a review on their structural diversities, spectroscopic characterizations, bioactivities, acute toxicities, antimicrobial mechanisms, and the structure-activity relationship was first performed based on the summaries and analyses of related publications from 1959 to 2019. A total of 63 guanidine-containing polyhydroxyl macrolides were reported, including 46 prototype compounds isolated from 33 marine and terrestrial actinomycetes and 17 structural derivatives. Combining with their antimicrobial mechanisms, structure-activity relationship analyses indicated that the terminal guanidine group and lactone ring of these compounds are vital for their antibacterial and antifungal activities. Further, based on their bioactivities and toxicity analyses, the discovery of guanidyl side-chain targeting to lipoteichoic acid of Staphylococcus aureus indicated that these compounds have a great potency to be developed into antimicrobial and anti-inflammatory drugs.

Streptomyces as a Prominent Resource of Future Anti-MRSA Drugs

Methicillin-resistant Staphylococcus aureus (MRSA) pose a significant health threat as they tend to cause severe infections in vulnerable populations and are difficult to treat due to a limited range of effective antibiotics and also their ability to form biofilm. These organisms were once limited to hospital acquired infections but are now widely present in the community and even in animals. Furthermore, these organisms are constantly evolving to develop resistance to more antibiotics. This results in a need for new clinically useful antibiotics and one potential source are the Streptomyces which have already been the source of several anti-MRSA drugs including vancomycin. There remain large numbers of Streptomyces potentially undiscovered in underexplored regions such as mangrove, deserts, marine, and freshwater environments as well as endophytes. Organisms from these regions also face significant challenges to survival which often result in the production of novel bioactive compounds, several of which have already shown promise in drug development. We review the various mechanisms of antibiotic resistance in MRSA and all the known compounds isolated from Streptomyces with anti-MRSA activity with a focus on those from underexplored regions. The isolation of the full array of compounds Streptomyces are potentially capable of producing in the laboratory has proven a challenge, we also review techniques that have been used to overcome this obstacle including genetic cluster analysis. Additionally, we review the in vivo work done thus far with promising compounds of Streptomyces origin as well as the animal models that could be used for this work.

Synergistic combination of two antimicrobial agents closing each other's mutant selection windows to prevent antimicrobial resistance

Antimicrobial resistance seriously threatened human health. Combination therapy is generally an effective strategy to fight resistance, while some data on its effects are conflicting. To explore the reasons, the fractional inhibitory concentration indexes (FICIs) of three designed combinations against methicillin-resistant Staphylococcus aureus (MRSA) were determined using checkerboard method, and their minimal concentrations inhibiting colony formation by 99% (MIC_99%s) and mutant prevention concentrations (MPCs) alone or in combinations including different proportions were first determined using agar plates. The results indicated that different proportions of a combination had presented different MPCs and mutant selection window (MSWs), and also showed that the smaller the FICIs of two agents in combinations were, the more probable their MSWs were to close each other. As two agents of a combination had different pharmacokinetic characters, the ratios of two agents in blood and infectious sites were likely different even though a specific proportion was administrated, which would lead to different effects preventing resistance. Thereby, these experimental results theoretically indicated that synergistic combination closing each other’s MSWs had a great potency to prevent resistance according to the hypotheses of MSW and MPC, and deduced that in vivo synergistic validity of a combination was likely a key to prevent resistance. Moreover, a synergistic combination of roxithromycin/doxycycline with the FICIs of 0.26–0.50 and 0.28–0.38 respectively against MRSA 01 and 02 was obtained, and the MSWs of these two agents could be simultaneously closed each other in a certain range of proportions, but for others. Meanwhile, its effect preventing resistance needs to be further verified.

Mechanism of Azalomycin F5a against Methicillin-Resistant Staphylococcus aureus

To investigate the mechanism of azalomycin F_5a against methicillin-resistant Staphylococcus aureus (MRSA), the conductivity of MRSA suspension and the adenylate kinase activity of MRSA culture were determined with the intervention of azalomycin F_5a, which were significantly increased compared to those of blank controls. This inferred that azalomycin F_5a could lead to the leakage of cellular substances possibly by increasing permeability to kill MRSA. As phospholipid bilayer was mainly responsible for cell-membrane permeability, the interaction between azalomycin F_5a and cell-membrane lipids was further researched by determining the anti-MRSA activities of azalomycin F_5a combined with cell-membrane lipids extracted from test MRSA or with 1,2-dipalmitoyl-sn-glycero-3-phospho-glycerol (DPPG) for possible molecular targets lying in MRSA cell-membrane. The results indicated that the anti-MRSA activity of azalomycin F_5a remarkably decreased when it combined with membrane lipids or DPPG. This indicated that cell-membrane lipids especially DPPG might be important targets of azalomycin F_5a against MRSA.

An Iterative Module in the Azalomycin F Polyketide Synthase Contains a Switchable Enoylreductase Domain

Detailed analysis of the modular Type I polyketide synthase (PKS) involved in the biosynthesis of the marginolactone azalomycin F in mangrove Streptomyces sp. 211726 has shown that only nineteen extension modules are required to accomplish twenty cycles of polyketide chain elongation. Analysis of the products of a PKS mutant specifically inactivated in the dehydratase domain of extension-module 1 showed that this module catalyzes two successive elongations with different outcomes. Strikingly, the enoylreductase domain of this module can apparently be "toggled" off and on : it functions in only the second of these two cycles. This novel mechanism expands our understanding of PKS assembly-line catalysis and may explain examples of apparent non-colinearity in other modular PKS systems.