Defining Established and Emerging Microbial Risks in the Aquatic Environment: Current Knowledge, Implications, and Outlooks

Hindawi Publishing Corporation International Journal of Microbiology Volume 2011, Article ID 462832, 15 pages doi:10.1155/2011/462832 Review Article Defining Established and Emerging Microbial Risks in the Aquatic Environment: Current Knowledge, Implications, and Outlooks Neil J. Rowan Department of Nursing and Health Science, School of Science, Athlone Institute of Technology, Dublin Road, Athlone, Co. Westmeath, Ireland Correspondence should be addressed to Neil J. Rowan, Received 22 March 2010; Accepted 27 July 2010 Academic Editor: Max Teplitski Copyright © 2011 Neil J. Rowan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This timely review primarily addresses important but presently undefined microbial risks to public health and to the natural environment. It specifically focuses on current knowledge, future outlooks and offers some potential alleviation strategies that may reduce or eliminate the risk of problematic microbes in their viable but nonculturable (VBNC) state and Cryptosporidium oocysts in the aquatic environment. As emphasis is placed on water quality, particularly surrounding efficacy of decontamination at the wastewater treatment plant level, this review also touches upon other related emerging issues, namely, the fate and potential ecotoxicological impact of untreated antibiotics and other pharmaceutically active compounds in water. Deciphering best published data has elucidated gaps between science and policy that will help stakeholders work towards the European Union’s Water Framework Directive (2000/60/EC), which provides an ambitious legislative framework for water quality improvements within its region and seeks to restore all water bodies to “good ecological status” by 2015. Future effective risk-based assessment and management, post definition of the plethora of dynamic inter-related factors governing the occurrence, persistence and/or control of these presently undefined hazards in water will also demand exploiting and harnessing tangential advances in allied disciplines such as mathematical and computer modeling that will permit efficient data generation and transparent reporting to be undertaken by well-balanced consortia of stakeholders. 1. Viable But Nonculturable Forms of Waterborne Bacteria 1.1. Background. Since the introduction of the concept or sublethally injured or viable but nonculturable (VBNC) cells by Byrd and Colwell in the 1980’s [1], there is increasing evidence for the existence of such a state in microbes, particularly in the aquatic environment that elicits a myriad of interrelated sub-lethal microbial stresses such as nutrient starvation and osmotic stress [2, 3] (Table 1). This is a cause for concern because of evidence that microbial pathogens in such a state may still retain their capacity to cause infections after ingestion by fish, animals, or by humans, despite their inability to grow under conditions employed in laboratorybased procedures for determining their presence in water [4]. Albeit currently unknown in terms of its severity or scope, it is now generally appreciated that heavily stressed pathogenic microbial species existing in a VBNC (or not immediately culturable state) may potentially pose as yet an undefined risk to public health, which is attested by the fact that there is increasing evidence to support the viewpoint that stressed cells in this quiescent state may actually be more virulent than well-fed laboratory-tamed microorganisms due to augmented virulence factor expression. Xu et al. [5] were the first to bring experimental evidence of the existence of VBNC state in pathogenic bacteria, where they showed that E. coli and V. cholera cells that were suspended in artificial seawater quickly lost their ability to grow on the culture media normally used for their detection. 1.2. Definition. According to Oliver [16], a bacterium in the VBNC state is defined as “a cell which is metabolically active, which being incapable of undergoing the cellular division required for growth in or on a medium normally supporting grown of that cell.” Besnard et al. [17] suggest that the transition to the VBNC state in L. monocytogenes 2 International Journal of Microbiology Table 1: Methods used to detect VBNC state in waterborne microorganisms. Method(s) Employed Failure of microbial growth in culture media Use of redox probes to detect microbial respiratory chain activity Incorporation of radio-labelled substrates in culture media Resuscitation in embryo of egg yolk Detection in immunodeficient mice Addition of antioxidants to culture media RNA-based genotypic approaches (16S/23S rRNA, mRNA) cDNA microarrays In situ hybridisation (FISH), microradiography, epi-fluorescence microscopy, flow cytometry Rapid enzyme assays Oligonucleotide probes and tagged green fluorescent protein Microbial quorum sensing Reporting author(s)∗ [5] [6, 7] [8] [4] [9] [10] [11] [2] [12] [13] [14] [15] ∗ This is a representative list of authors citing use of named methods for detection of VBNC state in waterborne organisms and therefore does not convey all published work in this area. represents a survival strategy that bacteria can adopt under adverse conditions (starvation, salt stress, etc.). VBNC microorganisms are considered to represent a subpopulation of cells that are unable to grow in the usual culture media and cannot resuscitate by traditional resuscitation techniques, but yet remain physically active for several functions such as cellular elongation [18], respiratory chain activity [6, 7, 17], or incorporation of radio-labelled substrates [8]. For example, Cappelier and coworkers [4] recently reported that avirulent VBNC cells of L. monocytogenes incubated in filtered sterilized distilled water need the presence of an embryo to be recovered in egg yolk and regain virulence after recovery. The VBNC state was observed after a 25 to 47 days incubation period (concentration of culturable cells less than 1 colony forming unit per mL). L. monocytogenes isolated from salmon, patients and the environment. L. monocytogenes were tested for virulence in a cell plaque assay and by intraperitoneally inoculation in immunodeficient RAG1 mice. Moreover, Moreno et al. [22] described successions in cellular alterations in Helicobacter pylori NCTC 11637 after inoculation into chlorinated drinking water. They concluded that H. pylori could survive disinfection practices normally used in drinking water treatment in the VBNC form, which would allow them to reach final consumption points and, at the same time, enable them to be undetectable by culture methods. Whereas Kastberg et al. [23] recently reported that L. monocytogenes cells, whether planktonic or attached, were homogenous with respect to sensitivity to acidic disinfectants at the single-cell level. 1.3. VBNC State a (...truncated)