Arsenic contamination and microbial drug-resistance: lessons from Leishmania antimony resistance
Naouel Eddaikra1, Youssef El-Fakhry2, Khatima Aït-Oudhia3, Bruno Oury4, Denis Sereno4,5*
1 Laboratoire d’Eco-épidemiologie Parasitaire et Génétique des Populations, Institute Pasteur of Algeria, Route du Petit Staoueli Dely Brahim, Alger, Algeria.
2 Life and Earth Sciences Dept Faculty of Sciences II and Public Health Faculty Fanar, Lebanon.
3 Ecole Nationale Supérieure Vétérinaire, BP 161, Hassan Badi El-Harrach, Alger, Algeria.
4 Unité mixte de recherche IRD 224 MiVegec (Maladies infectieuses et Vecteurs : écologie, génétique, évolution et contrôle), Institut de Recherche pour le Développement (IRD), BP 64501, 34394 Montpellier cedex 5, France.
5 Unité mixte de recherche IRD 177 InterTryp ("Interactions Hôtes-Vecteurs-Parasites-Environnement dans les maladies tropicales négligées dues aux trypanosomatides"), Institut de Recherche pour le Développement (IRD), BP 64501, 34394 Montpellier cedex 5, France.
* Corresponding author:
Bacterial antibiotic resistance is an increasing public health concern. Because antibiotics in clinical use to treat human bacterial infection have been and are still structurally unrelated to heavy metals, evidences that heavy metals may select for antibiotic-resistance have started to be gathered only in the early 80’s, with the discovery that mechanisms operating to confer tetracyclin resistance in Echerichia coli are of same nature than those conferring heavy-metal resistance [1]. These are efflux mechanisms that are generally not specific for a given antibiotic and can confer cross-resistance between heavy metals and antibiotic [2]. On the other hand, many genes coding for metal resistance have been found to be present on bacterial plasmids together with antibiotic resistance ones [3]. The selection for sub-lethal concentration of heavy metals leads to the selection of multiresistant antibiotic plasmids [4]. Pentavalent antimony, a heavy metal used to treat all forms of leishmaniosis was developed in the 1950s. In 1997, the first indications of therapeutic failure linked to Leishmania drug resistance originated in northern Bihar province of India [5]. Antimony (Sb) shares many common chemical properties with arsenic (As) and in vitro studies have demonstrated that common mechanisms are involved in the occurrence of As and Sb cross-resistance [6]. Therefore, the hypothesis of cross selection between environmental sources of both metal was raised [7] and data supporting the presence of As in drinking water in area with high rate of clinical antimony failure were gathered in India [8].
Arsenic is naturally present in the environment, due to geologic sources. In addition, anthropogenic activities significantly contribute to its release and accumulation in the environment, particularly in certain agricultural locations. Besides India, As contamination is a worldwide problem whose cartography of risks begins to be published [9-10]. Obviously, the origin of Arsenic contamination involved in the selection for Leishmania antimony resistance is not yet fully known. Leishmaniosis is a vector born disease that means that the parasite alternates between a vertebrate (mammalian) host and an insect vector Phlebotomine sandfly. Perry and coll in 2011 proposed drinking water as the source of As acting in the selection for antimony resistance in India. They have experimentally ascertained the cross-resistance fate of leishmania parasites in mice [11]. Nevertheless, in heavily polluted areas such as mining sites, swamps and urban environments, sandflies that feed on plants could be contaminated by local atmospheric dust, and wild or feral animal, reservoirs of leishmania, can be exposed to As in contaminated water.
Perry and collaborators in 2015 [12] reported no statistical correlation between As content in drinking water and clinical outcome. This observation is finally not surprising, because treatment failure is a multifactorial event whose parasite resistance represent only one determinant. Secondly, the clinical efficiency of SSG treatment requires an efficient T-cell response and the treatment efficiency is improved by the adjunction of IFN-g. Arsenic is a well-known cause of immunosuppression that can impact the outcome of the antimony treatment. Thirdly, because during the period of the treatment (i.e 2006-2010), the As concentration in drinking water was not measured and can be currently different. So water drinking might not be the sole source of As. Therefore, the effect of environmental As contamination on the selection of Leishmania for antimony resistance still remains an open question that will require further investigation to probe it. Nevertheless, this study highlights the fact that drinking water contaminated by As significantly elevates the risk of mortality by all causes including visceral leishmaniosis. This observation might have some importance for the delineation of risk areas for visceral leishmaniasis. In particular if As in drinking water is a confirmed negative predictive factor for survival visceral leishmaniasis and supporting the emergence of leishmania antimony resistance. Finally, this observation, if further confirmed might have to be evaluated in endemic foci of cutaneous and muco-cutaneous leishmaniosis but also over other trypanosomatid born diseases, like the Chagas diseases in South America and sleeping sickness in Africa. In particular, the role of As contamination for the selection of parasite-resistance await further investigations in sleeping sickness that is caused by T. brucei gambiense and T. brucei rhodesiense. Indeed, Melarsoprol an arsenical derivative is in clinical use for patients with at both late and chronic stage of the disease that involve central nervous system. In this case the link should be more obvious, but the source of As contamination will await further investigation. At last, if the presence of As in drinking water is now identified as potential intercurrent selecting agent favoring antimony cross resistance, the presence of substantial amount of As in herbal of the traditional pharmacopea, in use to treat infectious diseases, should also represent a potential source of As playing a role in the selection of drug resistant parasites [13]. Altogether the report of Perry and collaborator that can be considered as the first steps paving the way of studies on the influence of heavy-metal pollution on parasite drug resistance will open new perspectives to probe drug resistance issues of other widespread diseases, like malaria or Tuberculosis for instance.
Malaria or tuberculosis have a contrasting epidemiological facies, implying that the contact between the pathogen and As present in their environment should substantially differ in quantity and also origin (insect vectors, mammalian reservoir, individual). In addition, unlike trypanosomatid born diseases, formulation of heavy metal have never been in clinical use even if some studies have been performed on animal models for tuberculosis [14]. Therefore, not so much information is available on the susceptibiltiy status of M. tuberculosis or Plasmodium species towards As, Sb or other heavy metals. For Plasmodium, the in vitro Sb and As susceptibility has been investigated with in vitro tests at least once. Plasmodium falciparum was found to be susceptible in the mM range of Sb or As concentration [15]. Surprisingly these concentrations are in the range of those found effective in vitro against Leishmania or Trypanosoma brucei. This observation highlights, that as hypothetised for Leishmania, a contact between Plasmodium and As during parasite developmental life cycle, in its insect vectors and/or in patients, can exert a selective pressure on the parasite for As resistance. Publications on in vitro susceptibility of Mycobacterium towards As are even more scarce than those on Plasmodium. It has been shown that an efflux pump can mediate resistance towards metal (copper) and isoniazid, a drug in clinical used [16]. In addition, mycobacteria rely for the homeostasis of it’s redox capacity on the use of thiol system [17], that is the primary target for As in trypanosomatids [18]. Therefore, As by targeting this system might exert an antimycobacterial activity also and that needs to be evaluated at least in vitro. Interestingly, a report relates spatial dependency of Buruli ulcer prevalence on As-enriched areas [19]. Buruli ulcer is caused by a peculiar species of Mycobacterium, M. ulcerans, whose transmission cycle is largely unknown but involves arthropods as vectors and/or vehicle to infect human. According to the WHO leishmaniosis, the Chagas disease, the African sleeping sickness and buruli ulcer like 13 other diseases affecting more than 1.4 billon people living in developing countries belongs to the group of the Neglected Tropical disease (http://www.who.int/neglected_diseases/diseases/en/). Currently a need for new medication to fight these diseases is required and the development of drug resistance against molecules currently in use to treat these diseases will have a dramatic impact on public health.
Evidences that As contamination has a deleterious impact on the human health by favoring the selection of drug resistant pathogens, have been raised for visceral leishmaniosis but its involvement on other infectious diseases is currently unknown. To further address this question it is important to investigate more in depth the link between the level of As contamination, and the susceptibility of pathogens (Parasites, bacteria…) towards them. This approach might be achieved by comparing susceptibility towards As of microorganisms isolated from their different hosts, in areas free of As contamination with those of pathogens coming from areas polluted at low and high levels. Such surveys will allow to evidence potentially a correlation between the As susceptibility/resistance status and the antibiotic susceptibility/resistance status. These studies are of importance especially for zoonotic vector born diseases for which the contact between pathogen and As can occur for a high diversity of organisms living in large variety of habitats. In addition, urbanization and the adaptation of insect vectors to polluted environment increase the contact risk between pathogen and As in vector born diseases. The determination of the level of As (heavy metal) accumulation in all the organism involve in the pathogen life cycle will help to pinpoint the origin of the contact between the pathogens and As. Finally, because As acts on fundamental mechanism of thiol metabolism, involved in the defense of cell towards reactive oxygen species, it is essential to determine to what extends antibiotic in current clinical use, target these mechanisms of defense that can lead to cross resistance.
Therefore, we feel important to evaluate to which extent As contamination of drinking water have more deleterious effects on human health than previously supposed. This problem have to be pinpointed in leishmaniasis because heavy metal are still in clinical use, but need now to be evaluated in the light of antimicrobial resistance with relevance not only in bacterial born diseases but also on parasitic disease.
Acknowledgement
This study was supported in part by the BEST Grant program of IRD doctoral Fellows (821849H). We are grateful to the Department for Sustain and Training (DSF) from IRD for providing doctoral Fellowship to N. Eddaikra during the period of this study..
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