Surface water is an indispensable resource for human life. However, mankind has dramatically experienced the role of water in the transmission of infectious diseases because it may act as a reservoir of different types of pathogens ranging from viruses to bacteria, fungi, and parasites. This happens not only in developing countries because of the scarcity of resources and methods for its qualitative evaluation and purification but also in rich countries where microepidemics are reported almost daily worldwide. Nowadays the world has become integrated and global, and the hope that it may be possible to successfully eradicate an infectious disease is nothing more than an illusory oversimplification. This situation is further complicated by the knowledge that anthropogenic activities and current and predicted climate changes will greatly affect any disease that has an environmentally sensitive stage or vector, as in the case of waterborne infections.
Several methodologies, resembling those employed in clinical microbiology, have long been adopted in the study of the biology and in the monitoring of surface water microorganisms, including pathogens. A vast amount of information has been collected till date; in particular, the data obtained starting from the second half of the 20th century have clearly demonstrated that a substantial discrepancy exists between culture and nonculture methods, the latter proving much more sensitive than the former.
One of the most interesting consequences of the molecular approach to the study of surface water microorganisms is that the concept of bacterial cell death has been revisited by several authors also in the light of the discovery of previously unrecognized survival strategies adopted by bacteria to persist in natural environments under nonoptimal conditions. It is now evident from such studies that viability does not necessarily mean culturability, and that nonculturable bacteria cannot always be considered dead (i.e. incapable of sustaining a new infectious process in humans). This means, therefore, that the methodologies currently adopted based on microbial cultivation may be unable to detect the real load of pathogens in waters with severe repercussions for human health.
The goal of this issue of Current Opinion in Biotechnology is to discuss relevant aspects of the biology of waterborne pathogens as well as to describe methodologies for their accurate detection currently in use or under development.
In this issue, molecular mechanisms that allow human pathogens to persist in surface waters are reviewed by various scientists. Huq and colleagues address the subject of biofilm formation by human pathogens in natural environments as a survival strategy adopted in response to environmental stress. The role of cell-to-cell communication (i.e. quorum sensing) is discussed as a key factor for the initiation of biofilm formation. Occurrence of the viable but nonculturable (VBNC) state in Vibrio cholerae, when organized in a biofilm structure, is considered the main reason the real load of these human pathogens was underestimated or undetectable in the past, when inappropriate detection methods were employed.
The VBNC state issue is also discussed by Signoretto and Canepari who describe this phenomenon in Gram-positive bacteria such as enterococci, Listeria, and Micrococcus. Data are reported indicating that, for enterococci at least, the VBNC state is the main mode of persistence in waters. The knowledge of the basic molecular aspects of the VBNC state is considered crucial for the identification of specific bacterial targets to be used for accurate, realistic detection of these microorganisms in natural environments.
The survival of pathogenic bacteria in waters as well as the role of the aquatic environment in the evolution of virulence traits and pathogen transmission to humans is discussed by Vezzulli and colleagues, using V. cholerae as an example. Starting from recent evidence showing the existence of V. cholerae ligands involved in colonization of both human intestine and chitin surfaces, these authors suggest that bacterial traits involved in both pathogenicity and out-of-body life may be promising targets for the development of new strategies for combating infectious diseases.
Waters are not only a reservoir of human pathogens but also a reservoir of antibiotic-resistant bacteria either shed into the water from animals and humans or originate from the autochthonous flora. Industrially produced antibiotic molecules derived from therapeutic/prophylactic intervention in both humans and farm animals reared on an industrial scale are present in waters with the resulting effects of altering the indigenous microflora and/or selecting antibiotic-resistant bacteria. This interesting and highly topical issue is exhaustively reviewed and discussed by Baquero and colleagues together with suggestions for preventive interventions allowing a reduction of the resistant bacterial load as well as of the active antimicrobial agents in wastewater.
Series of articles are devoted to the analysis of innovative approaches for a more realistic evaluation of the presence of pathogens and their density in waters. Among the various possible approaches, nucleic-acid-based techniques are of greatest interest. Although major advances in molecular identification of pathogens have been achieved as a result of the advent of new biotechnology methods, significant difficulties that emerge in designing, upgrading, and checking the efficiency of such methods using in silico analyses. This fundamental issue is magnificently treated by Richard Christen who, instead of reviewing the published literature, utilizes the innovative approach of presenting the available sequences and oligomers used to identify pathogenic bacteria. Using a few case studies to review the present difficulties, he gives some advice on how to retrieve and properly analyze public sequences.
A practical application of the issue raised by Christen is provided by Brettar and Höfle, who have reviewed recent advances in molecular detection technologies for bacterial pathogens with a view to ensure the safety of drinking water supplies. The urgent need for molecular detection methods for the benefit of human health is shown in this contribution; also taking into account of the problems related to bacterial physiological state (e.g. the VBNC state), the content of organic matter that can sustain bacterial re-growth, and the bacterial organization in the biofilm structure.
Toxic cyanobacteria (blue-green algae) pose a significant hazard to human health, being producers of harmful cyanotoxins; thus, very recently, there has been an increasing interest on these microorganisms. Pearson and Neilan present a detailed analysis and characterization of cyanotoxin gene clusters in order to develop new methodologies, based primarily on nucleic-acid-based detection, for the quantitative monitoring of the time course of blooming cyanobacterial populations.
An alternative method to the nucleic-acid-based ones for bacterial detection in waters is presented by Fiksdal and Tryland, who analyze enzyme assay techniques for the rapid estimation of the levels of coliforms and Escherichia coli in waters, and compare the results obtained with those of culture methods. The authors emphasize that the evaluation of enzyme activity can be more advantageous than the evaluation of culturable cells because enzymatic activity persists longer in stressing environments than culturability, and, thus, may be expressed by both culturable and VBNC cells.
The presence of human viruses in waters is an additional threat to human health. Several virus families, including enterovirus, norovirus, rotavirus, astrovirus, coronavirus, and hepatitis A virus, can survive and persist for a long time in water, maintaining infectivity in many instances. This suggests the need to include virus detection in the evaluation of the microbiological quality of waters. The article by Bosch and co-workers presents an updated review of the main virus families detected in waters, the most important procedures for water-sample processing, and molecular approaches for efficient virus detection.
Bacteria and viruses are not the only pathogens in surface waters but can also host protozoan parasites. It is well known that Cryptosporidium, Giardia, and Amoeba can be responsible for worldwide waterborne diseases. The article by Bouzid and colleagues reviews the fundamental aspects of these infectious agents and their survival in waters, and considers the improvement in specific detection methods in order to evaluate their presence and viability in samples taken from aquatic environments.
In recent years an innovative approach to the monitoring of the microbiological quality of waters on broad surfaces has been introduced and consists in the remote (via satellite) sensing of ocean parameters than can affect the presence of pathogens in waters and the risk of human transmission. This topic is reviewed by Lleo and colleagues who present the state-of-the-art technology and its advantageous application in ecology, environmental sciences, and medicine. Although at present several difficulties restrict the application of satellite sensing, improvements in satellite capabilities and the introduction of new and more sensitive sensors, in the near future may be expected to enhance our ability to explore environmental risk factors and their possible relationships to disease outbreaks.
Our thanks go to all the authors for their invaluable contributions to this issue in terms of data, expertise, and comment. We trust that our readers will appreciate the relevance and the importance of the various topics addressed.
Biographies
Pietro Canepari (MD) is a Professor of Medical Microbiology in the Faculty of Medicine of the University of Verona. His research has ranged from the role of bacterial cell wall components (mainly peptidoglycan) during cell growth and division to the analysis of the mode of action of, and the resistance to antibiotics acting on bacterial cell wall synthesis and assembly. Since the late 1990s his main fields of interest have been first, the analysis of molecular mechanisms allowing the survival of human pathogens in natural environments with a special emphasis on the viable but nonculturable state and second, the optimization of nonculture detection methods.
Carla Pruzzo is a Professor of Microbiology in the Genoa University Faculty of Science. Her main research field has been bacterial adherence in both humans and surface waters. In recent years, her research has focused on the elucidation, at molecular level, of the strategies adopted by pathogenic and potentially pathogenic bacteria in order to survive in coastal water, to spread in this environment and to infect humans. She has carried out studies on the interaction occurring between pathogenic vibrios and their environmental reservoirs (e.g. chitin-containing zooplankton organisms) and hosts (e.g. bivalves) with a special emphasis on the bacterial surface structures involved.
Contributor Information
Pietro Canepari, Email: pietro.canepari@univr.it.
Carla Pruzzo, Email: Carla.Pruzzo@unige.it.