Summary: Chitosan, an aminopolysaccharide biopolymer, has a unique chemical structure as a linear polycation with a high charge density, reactive hydroxyl and amino groups as well as extensive hydrogen bonding. It displays excellent biocompatibility, physical stability and processability. The term ‘chitosan’ describes a heterogenous group of polymers combining a group of physicochemical and biological characteristics, which allow for a wide scope of applications that are both fascinating and as yet uncharted. The increased awareness of the potentials and industrial value of this biopolymer lead to its utilization in many applications of technical interest, and increasingly in the biomedical arena. Although not primarily used as an antimicrobial agent, its utility as an ingredient in both food and pharmaceutical formulations lately gained more interest, when a scientific understanding of at least some of the pharmacological activities of this versatile carbohydrate began to evolve. However, understanding the various factors that affect its antimicrobial activity has become a key issue for a better usage and a more efficient optimization of chitosan formulations. Moreover, the use of chitosan in antimicrobial systems should be based on sufficient knowledge of the complex mechanisms of its antimicrobial mode of action, which in turn would help to arrive at an appreciation of its entire antimicrobial potential.

Chitosan is a polysaccharide biopolymer that combines a unique set of versatile physicochemical and biological characteristics which allow for a wide range of applications. Although its antimicrobial activity is well documented, its mode of action has hitherto remained only vaguely defined. In this work we investigated the antimicrobial mode of action of chitosan using a combination of approaches, including in vitro assays, killing kinetics, cellular leakage measurements, membrane potential estimations, and electron microscopy, in addition to transcriptional response analysis. Chitosan, whose antimicrobial activity was influenced by several factors, exhibited a dose-dependent growth-inhibitory effect. A simultaneous permeabilization of the cell membrane to small cellular components, coupled to a significant membrane depolarization, was detected. A concomitant interference with cell wall biosynthesis was not observed. Chitosan treatment ofStaphylococcus simulans22 cells did not give rise to cell wall lysis; the cell membrane also remained intact. Analysis of transcriptional response data revealed that chitosan treatment leads to multiple changes in the expression profiles ofStaphylococcus aureusSG511 genes involved in the regulation of stress and autolysis, as well as genes associated with energy metabolism. Finally, a possible mechanism for chitosan's activity is postulated. Although we contend that there might not be a single classical target that would explain chitosan's antimicrobial action, we speculate that binding of chitosan to teichoic acids, coupled with a potential extraction of membrane lipids (predominantly lipoteichoic acid) results in a sequence of events, ultimately leading to bacterial death.

Staphylococcus aureus (S. aureus) is a pathobiont of humans as well as a multitude of animal species. The high prevalence of multi-resistant and more virulent strains of S. aureus necessitates the development of new prevention and treatment strategies for S. aureus infection. Major advances towards understanding the pathogenesis of S. aureus diseases have been made using conventional mouse models, i.e., by infecting naïve laboratory mice with human-adapted S.aureus strains. However, the failure to transfer certain results obtained in these murine systems to humans highlights the limitations of such models. Indeed, numerous S. aureus vaccine candidates showed promising results in conventional mouse models but failed to offer protection in human clinical trials. These limitations arise not only from the widely discussed physiological differences between mice and humans, but also from the lack of attention that is paid to the specific interactions of S. aureus with its respective host. For instance, animal-derived S. aureus lineages show a high degree of host tropism and carry a repertoire of host-specific virulence and immune evasion factors. Mouse-adapted S.aureus strains, humanized mice, and microbiome-optimized mice are promising approaches to overcome these limitations and could improve transferability of animal experiments to human trials in the future.

Background: The growing threat of microbial resistance against traditional antibiotics has prompted the development of several antimicrobial nanoparticles (NPs), including silver NPs (AgNPs). In this article, a simple and eco-friendly method for the synthesis of AgNPs using the cranberry powder aqueous extract is reported. Materials and methods: Cranberry powder aqueous extracts (0.2%, 0.5%, and 0.8% w/v) were allowed to interact for 24 hours with a silver nitrate solution (10 mM) at 30°C at a ratio of 1:10. The formation of AgNPs was confirmed by ultraviolet-visible spectroscopy and their concentrations were determined using atomic absorption spectroscopy. The prepared NPs were evaluated by transmission electron microscopy, measurement of ζ-potential, and Fourier-transform infrared spectroscopy. The in vitro antimicrobial properties of AgNPs were then investigated against several microbial strains. Finally, in vivo appraisal of both wound-healing and antimicrobial properties of either plain AgNPs (prepared using 0.2% extract) or AgNP-Pluronic F-127 gel was conducted in a rat model after induction of a Staphylococcus aureus ATCC 6538P wound infection. Results: The formation of AgNPs was confirmed by ultraviolet-visible spectroscopy, where a surface-plasmon resonance absorption peak was observed between 432 and 438 nm. Both size and concentration of the formed AgNPs increased with increasing concentration of the extracts. The developed NPs were stable, almost spherical, and polydisperse, with a size range of 1.4–8.6 nm. The negative ζ-potential values, as well as Fourier-transform infrared spectroscopy analysis, indicated the presence of a capping agent adsorbed onto the surface of the particles. In vitro antimicrobial evaluation revealed a size-dependent activity of the AgNPs against the tested organisms. Finally, AgNPs prepared using 0.2% extract exhibited a substantial in vivo healing potential for full-thickness excision wounds in rats. Conclusion: AgNPs were successfully synthesized from a silver nitrate solution through a simple green route, using cranberry powder aqueous extract as a reducing as well as capping agent.

Rats are a reservoir of human- and livestock-associated methicillin-resistant Staphylococcus aureus (MRSA). However, the composition of the natural S. aureus population in wild and laboratory rats is largely unknown. Here, 144 nasal S. aureus isolates from free-living wild rats, captive wild rats and laboratory rats were genotyped and profiled for antibiotic resistances and human-specific virulence genes. The nasal S. aureus carriage rate was higher among wild rats (23.4%) than laboratory rats (12.3%). Free-living wild rats were primarily colonized with isolates of clonal complex (CC) 49 and CC130 and maintained these strains even in husbandry. Moreover, upon livestock contact, CC398 isolates were acquired. In contrast, laboratory rats were colonized with many different S. aureus lineages—many of which are commonly found in humans. Five captive wild rats were colonized with CC398-MRSA. Moreover, a single CC30-MRSA and two CC130-MRSA were detected in free-living or captive wild rats. Rat-derived S. aureus isolates rarely harbored the phage-carried immune evasion gene cluster or superantigen genes, suggesting long-term adaptation to their host. Taken together, our study revealed a natural S. aureus population in wild rats, as well as a colonization pressure on wild and laboratory rats by exposure to livestock- and human-associated S. aureus, respectively.