Environmental bacteria as triggers to brain disease: Possible mechanisms of toxicity and associated human risk.

AIMS: Brain disease, in its many forms, has recently demonstrated a great socio-economic impact and represents one of the hardest challenges of present research. Although each pathology of this highly heterogenous group is characterized by individual features, there is an increasing number of common toxicological mechanisms that have been evidenced. This review aims to summarize the state-of-art knowledge concerning the role of environmental bacteria in brain diseases focusing on different mechanisms of action that could be interacting in an additive or synergistic way. MATERIALS AND METHODS: For this wide-range subject, we focused on two emerging types of bacterial-derived brain exposure and damage and specifically treated representative examples: i) environmental bacterial-derived compounds in the form of the cyanobacterial product BMAA (ß-N-methylamino-L-alanine) toxin and its isomers DAB (2,4-diaminobutyric acid) and AEG (N-(2-aminoethyl)glycine) and ii) toxicity related to bacterial infections in the form of the emerging Lyme neuroborreliosis (LNB), determined by Borrelia burgdorferi (Bb). KEY FINDINGS: Defined as pleiotropic contaminants, BMAA and Bb act through multiple toxicological pathways including inflammation, oxidative stress and excitotoxicity. Multiple investigations in in vitro and in vivo models have underlined the involved mechanisms of action but further investigations are needed to clarify the role of possible cocktail effects and underline possible new targets of intervention. SIGNIFICANCE: Environmental bacteria represent emerging risk factors because of environmental changes, anthropogenic activities and human lifestyle evolutions. Future directions and research ambitions are here discussed in order to evaluate human risk and possible ways of intervention and prevention.

Environmental Free-Living Amoebae Can Predate on Diverse Antibiotic-Resistant Human Pathogens.

Here, we sought to test the resistance of human pathogens to unaltered environmental free-living amoebae. Amoebae are ubiquitous eukaryotic microorganisms and important predators of bacteria. Environmental amoebae have also been proposed to serve as both potential reservoirs and training grounds for human pathogens. However, studies addressing their relationships with human pathogens often rely on a few domesticated amoebae that have been selected to feed on rich medium, thereby possibly overestimating the resistance of pathogens to these predatory phagocytes. From an open-air composting site, we recovered over 100 diverse amoebae that were able to feed on Acinetobacter baumannii and Klebsiella pneumoniae. In a standardized and quantitative assay for predation, the isolated amoebae showed a broad predation spectrum, killing clinical isolates of A. baumannii, K. pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus. Interestingly, A. baumannii, which was previously reported to resist predation by laboratory strains of Acanthamoeba, was efficiently consumed by closely related environmental amoebae. The isolated amoebae were capable of feeding on highly virulent carbapenem-resistant or methicillin-resistant clinical isolates. In conclusion, the natural environment is a rich source of amoebae with broad-spectrum bactericidal activities, including against antibiotic-resistant isolates. IMPORTANCE Free-living amoebae have been proposed to play an important role in hosting and disseminating various human pathogens. The resistance of human pathogens to predation by amoebae is often derived from in vitro experiments using model amoebae. Here, we sought to isolate environmental amoebae and to test their predation on diverse human pathogens, with results that challenge conclusions based on model amoebae. We found that the natural environment is a rich source of diverse amoebae with broad-spectrum predatory activities against human pathogens, including highly virulent and antibiotic-resistant clinical isolates.