Biofilms Impact on Drinking Water Quality

A paradigm shift currently occurs in microbiology, with significant impacts in a variety of environmental, medical and industrial applications. The old misconception of free floating microbes is invalidated by a different knowledge pattern: the great majority of terrestrial microorganisms live in communities associated to surfaces, called biofilms (Costerton et al, 1987; Flemming, 2008; Muntean, 2009). This organisation mode is associated to all surfaces in contact with water in drinking water processing, storage and distribution. Such biofilms are represented by structured consortia of sessile microorganisms characterized by surface attachment, self-produced exopolymeric matrix, structural, functional and metabolic heterogeneity, capable of intercellular communication by quorum-sensing and plurispecific composition.


Introduction
A paradigm shift currently occurs in microbiology, with significant impacts in a variety of environmental, medical and industrial applications.The old misconception of free floating microbes is invalidated by a different knowledge pattern: the great majority of terrestrial microorganisms live in communities associated to surfaces, called biofilms (Costerton et al, 1987;Flemming, 2008;Muntean, 2009).This organisation mode is associated to all surfaces in contact with water in drinking water processing, storage and distribution.Such biofilms are represented by structured consortia of sessile microorganisms characterized by surface attachment, self-produced exopolymeric matrix, structural, functional and metabolic heterogeneity, capable of intercellular communication by quorum-sensing and plurispecific composition.
Biofouling in drinking and industrial water systems has detrimental effects such as microbiological and chemical deterioration in water quality, corrosion inducing, drinking water treatment yield loss, efficiency reducing in cooling and heating exchange and transport, as well as in membrane processes (White et al, 1999;Flemming et al, 2002;LeChevallier and Au, 2004;Coetser and Cloete, 2005).
Biofilms are playing a major role in drinking and waste water treatment processes due to their enhanced properties of mineralization, bioaccumulation and bioadsorbtion.Despite the beneficial effects of the biological filter known as schmutzdecke in slow sand filtration or of the bio-sand filters, biofilms occurrence in other treatment stages, in drinking water networks and reservoirs represents a continuous challenge to water professionals.Drinking water associated biofilms induce residual disinfectants depletion and may cause aesthetic problems consisting in colour, odour and taste degradation due to chemical compounds released and more important, they pose a threat to human and animal health by hosting pathogenic or toxins producing bacteria, viruses, protozoa, algae, fungi and invertebrates.The great majority of water related health problems are the result of microbial contamination (Riley et al, 2011).Considering these aspects, naturally occurring biofilms in contact with drinking water were identified and described as microbial reservoirs for further contamination (Szewzyk et al, 2000;Wingender and Flemming, 2011).
The complex structure of drinking water associated biofilms is influenced by the microbial composition of source water and sediments (LeChevallier et al, 1987;Szewzyk et al, 2000;Emtiazi et al, 2004).They may enter the distribution network, escaping the treatment and disinfection processes (known as breakthrough) and multiply in bulk water or biofilms.The two modes of multiplication are defined as regrowth (recovery of disinfectant injured cells) and aftergrowth (microbial growth in a distribution system) processes (Characklis, 1988;van der Kooij, 2003).Pathogenic microorganisms of concern may also emerge in drinking water systems by intrusion, due to external contamination events in different steps of water treatment, storage and transportation: cross connections, backflow events, pipe breaks, negative pressure and because of improper flushing and disinfection procedures.

Drinking water biofilms, emerging pathogens and opportunistic pathogens
The most alarming consequences as a result of biofouling in drinking water distribution systems consist in the presence, multiplication and dispersion into water of bacterial pathogens, opportunistic pathogens, parasitic protozoa, viruses and toxins releasing fungi and algae.They may appear as primary colonizers promoting the adhesion at the interface and subsequent biofilm formation (Costerton, 1994), but more often as secondary colonizers in ecological microniches offered by the existent attached community.
Emerging pathogens are those that have appeared in a human population for the first time, or have occurred previously but are increasing in incidence or expanding to areas where they have not previously been reported, usually over the last 20 years.They include: bacteria (pathogenic E. coli, Helicobacter pylori, Campylobacter jejuni, Mycobacterium avium complex), parasitic protozoa (Cryptosporidium spp., Cyclospora cayetanensis, Toxoplasma gonidii), viruses (noroviruses, hepatitis E) and toxic cyanobacteria (Hunter et al, 2003).Opportunistic pathogens are commonly members of water microbiota that would be normally harmless to a healthy individual but can infect a compromised host (US EPA, 2002).Such microorganisms were detected worldwide in drinking water and associated biofilms and in raw water and sediments.In this context, establishing health-based targets, drinking water quality assurance implies effective preventive measures, such as sources water protection in order to reduce contamination risk in these strategic environments, as well as corrective actions stipulated in water safety plans prepared by water suppliers.
When cases of illness are registered, epidemiological studies are conducted in order to demonstrate similarities in genetic profiles of strains isolated from clinical and the environmental specimens, to track the source of infection.Drinking water and associated biofilms are often among the prime candidates tested when gastrointestinal diseases and different types of infections are recorded.
Developing countries are facing a major lack of safe drinking water, the transmission via faecal-oral route causing enormous numbers of severe water related illness and cases of deaths, especially in infants (Riley et al, 2011).Microbial source tracking approach is particularly important for drinking water sources, in order to identify the origin of contamination.Increasing population's access to clean drinking water and sanitation facilities is one of the first priorities of local and global authorities.
Even if the access to high quality drinking water is provided, neither humans in rich countries have definitely won the fight against microbes, yet.Tap water quality assurance is facing new challenges, consisting mainly in biofouling issues, emergent waterborne pathogens, toxins releasing and opportunistic pathogens occurrence.Besides households, different branches of industry, especially food and pharmaceutical, health care facilities, schools, nursing homes and other critical areas are carefully included in monitoring assays.Recently is seriously considered the ability of some species, referred as opportunistic pathogens, to induce disease under certain circumstances in immunocompromised individuals: immunosuppressed, malnourished, diabetic, burn, cancer, AIDS, on haemodialysis, respiratory, with organ transplants patients (Rusin et al, 1997;Payment and Robertson, 2004).Sensitive subpopulations such as young children, elderly persons or pregnant women are also vulnerable to infections caused by opportunistic pathogens (Reynolds et al, 2008).Other special categories of exposed subjects consist in patients with indwelling cannulae and catheters, implant devices and contact lenses wearers.Opportunistic pathogens are becoming a major issue, causing from allergy or superficial infections to life-threatening systemic infections, since the ascending trend in congenital and acquired immunodeficiency affecting global population.Such species presence is often investigated, in addition to routine monitoring, within drinking water and associated biofilms, as wide-occurring bacteria of concern in the continuous increasing category of hospitalised and ambulatory immunocompromised persons (Glasmacher et al, 2003).
Even in drinking water carefully treated and distributed at high standards, pathogenic contamination and disease outbreaks may occur (Szewzyk et al, 2000;Wingender & Flemming, 2011) demonstrating the imperative requirement for comprehensive water safety plans implementation.

Drinking water quality assessment -Microbiological aspects and biofilms
Tap water supposes not to be and is not sterile, microbial load in bulk water consisting mainly in inoffensive heterotrophs, presumably coming from associated biofilms by detachment during dispersion.Routine monitoring of raw water sources, finishing water at the exit from treatment plant, drinking water in pipe networks, service reservoirs and finally at the consumers implies periodically investigations of a number of water samples collected with a frequency depending on the population deserved.According to European regulations, microbial indicators assessed by standardised conventional culturing techniques are: colony count at 37⁰C, colony count at 22⁰C, total coliforms, Escherichia coli, intestinal enterococci and Clostridium perfringens.The greatest microbial risk being associated with ingestion of water contaminated with human or animal faeces, thus the potential presence of pathogenic bacteria, viruses and cysts of protozoan parasites; faecal indices (E.coli, intestinal enterococci and C. perfringens) presence is routinely investigated.
The shortcomings of water quality monitoring based on faecal indicators and heterotrophic plate count, resulting in underestimation of drinking water microbial populations in numbers and composition are discussed worldwide considering the following:

•
Only a small volume approximated to represent from 2 x 10 -7 to 5 x 10 -7 % of delivered drinking water is examined in routine monitoring (Allen, 2011); • In drinking water systems, the high majority of bacteria, estimated at 95%, are located attached at the surfaces, while only 5% are found in water phase and detected by sampling as commonly used for quality control (Flemming et al, 2002).Other studies are indicating bacterial numbers characterizing the biomass in pipe biofilms being 25 times more abundant than the suspended cells (Servais et al, 2004).Although, the common notion that biofilms dominate the distribution systems has been proven to be not true under all conditions by Srinivasan (2008), whose findings suggest that bulk bacteria may dominate in network sections containing chlorine residuals lower than 0.1mg/L and having residence time longer than 12 hours.
• A significant percent of water and biofilm bioburden may be in a viable but non cultivable state, unable to grow on artificial growth media but alive and capable of renewed activity and so hygienically relevant (Moritz et al, 2010).A small fraction of waterborne microorganisms (0.01%) are estimated to be culturable heterotrophic bacteria (Watkins and Jian, 1997;Exner et al, 2003); • Limitations of detection methods (Lehtola et al, 2006;2007;September et al, 2007).The investigation of drinking water associated biofilms from four European countries (France, Great Britain, Portugal and Latvia) confirmed E. coli presence by culturing techniques in one out of five pipes whereas all networks except one were positive for E. coli using the PNA FISH methods; their viability was also demonstrated.E.coli contributed with percents from 0.001% to 0.1% in the total bacterial numbers (Juhna et al., 2007); • Faecal indicators are the best predictors of potential risks, but their concentrations rarely correlate perfectly with those of pathogens (Payment and Locas, 2011).Although in freshwater significant correlations have been established between faecal indices and pathogenic species, their presence in drinking water showed limited or no correlation with different species of pathogenic or opportunistic pathogenic bacteria, viruses, protozoa and fungi.Water quality assessment based only on the investigation of faecal indicators' presence proved to be insufficient when many waterborne outbreaks emerged.Still, until more reliable indices and methods of detection will be wider implemented, the well-known standardised procedures are applied in routine monitoring across the globe.
Many studies targeting attached microbial communities have been performed for quantification of the total number of germs, by different methods.They offer an unspecific overview upon microbial load, bringing certain information about drinking water treatment process efficiency and distribution system integrity.Still, the real composition and dynamics of microbial populations within drinking water associated biofilms represents a continuous challenge.Experimental biofilm succession monitored for a long term development indicated a stable population state after 500 days in a model drinking water distribution system.A homogenous composition of the population in the mature biofilm could mask a dynamic situation at a smaller scale (Martiny et al., 2003).Quantitative and prescriptive evaluation is the next target of scientific community.Prediction of microorganisms' behaviour in the distribution system water and biofilms requires greater understanding of the effects in microbial attachment, detachment, survival, multiplication and viability of three groups of abiotic and biotic factors: substratum physicochemical properties (type of materials), biofilm composition (microbial intra-and interspecific interactions) and bulk water characteristics (disinfectants residuals, oxygen and nutrients concentrations, system hydraulics, temperature).

Occurrence of bacteria in drinking water and associated biofilms
Among the nuisance bacteria regularly found in drinking water and biofilms, species that are not characteristic to the water environment may appear due to contamination events, with major impacts upon human health.When compared with planktonic counterparts, biofilm bacteria and other inhabitants display superior characteristics due to specialization within this emergent structure and to complex relationships established (Costerton, 1994).Community belonging, from a microbial perspective represents a benefit materialized in increasing the chances of survival in this oligotrophic environment by offering ecological microniches, establishing intra-and interspecific cooperation relationships by communication via quorum sensing and perpetuating individuals' resistance to disinfection agents.Even species not able to survive and most of them incapable of growth and multiplication in water were identified in associated biofilms; recent studies have demonstrated their ability to grow in those microniches.For example, Legionella pneumophila survives but does not multiply in sterile drinking water, its proliferation being dependent on parasitic relationship with other microorganisms: 14 species of amoebae, two species of ciliated protozoa, and one slime mould -L.pneumophila being described as protozoonotic bacteria (Murga et al, 2001;Fields et al, 2002;Declerck, 2010).In many outbreaks, the presence of pathogenic bacteria was not detected by routine monitoring, the correlation with faecal indicators found in tap water samples being defective.E. coli bacillus, the most popular faecal indicator, was chosen inter alia based on its incapacity of growth in water.Recent studies have shown its ability to multiplicate in drinking water associated biofilms under strictly anaerobic conditions (Latimer et al., 2010), so the indicative value of the faecal index of choice becomes questionable.
One of the advantages offered by drinking water biofilm organization to its members is represented by the enhanced resistance to disinfection residuals.The four hypothetical mechanisms of biofilm resistance involve slow antimicrobial penetration, deployment of adaptative stress responses, physiological heterogeneity in biofilm population and the presence of phenotypic variants or persister cells (Chambless et al, 2005).Another benefit from the microbial perspective consists in the emergence of genetically encoded resistance to biocides and antibiotics, and the spread of antimicrobial resistance genes in bacterial populations via mobile genetic elements, by lateral gene transfer.Integrons are genetic elements possessing a site-specific recombination system for assembling of resistance genes in gene cassettes.They play a major role in the rapid spread of antibiotic resistance in clinical environments.Gene cassettes encoding resistance to quaternary ammonium compounds (qac) and integron-integrase (intlI) genes characteristics for class 1 integron were recently recovered from environmental samples, including biofilms from a groundwater treatment plant (Gillings et al, 2009).The proximity of individuals in biofilm consortia and the extremely short generation times in bacteria multiplication are prerequisites for intensive rates of lateral gene transfer and thus resistance spreading and perpetuation in diverse natural or artificial ecosystems.Experimental studies emphasized on bacteria ability of colonization, survival and multiplication in water associated biofilms, followed by dispersion in water phase in a planktonic state.The findings of Banning et al. (2003) suggested that the ability of P. aeruginosa to survive longer than E. coli in water associated biofilm could not be attributed to the association with the biofilm, rather than to the ability to utilize a wider range of organic molecules as carbon and energy sources compared to other Enterobacteriaceae.An increment in available nutrients may reduce E. coli survival in enhanced competition for nutrients and increased antagonism by the indigenous microbial population.Lehtola et al. (2007) investigated the survival of faecal indices versus pathogenic bacteria and viruses, in drinking water biofilms experimentally infested with E. coli, L. pneumophila, Mycobacterium avium and canine calcivirus (as a surrogate for human norovirus).The results proved that pathogenic bacteria and virus particles entering water distribution systems can survive in biofilms for weeks, even in conditions of high-shear turbulent flow and may pose a risk to the consumers.Meanwhile, E. coli registered a limited survival to a few days in water and in biofilms, being a poor indicator of certain pathogens in biofilms.The study also showed that standard culture methods may seriously underestimate the real numbers of bacteria in water and biofilms.
Comparative evaluation of classical techniques involving bacterial growth on specific selective media and molecular methods based on 16s rDNA sequence identity reveals a high discrepancy between what was expected to grow and the species isolated from specific selective growth media.Bacterial analyses of water based on selective isolation and culturing approach is recommended to be interpreted with caution (September et al, 2007).
Experimental studies revealed also low detectable numbers by culture-based technique in case of potable water biofilms infected with Campylobacter jejuni (Lehtola et al, 2006).C. jejuni and C. coli waterborne epidemics registered in Finland (Hänninen et al, 2000) and France (Gallay et al, 2000) were associated to consumption of contaminated tap water with origin in polluted sources.
Severe outbreaks such as cholera caused by the ingestion of water contaminated with Vibrio cholerae, typhoid and paratyphoid enteric fevers caused by Salmonella enterica subsp.enterica serovar Typhi, respective serovar Paratyphi, shigellosis due to infections with Shigella species still occur in countries with insufficient access to safe water.But even in developed countries, outbreak events involving emerging pathogenic bacteria like Legionella pneumophila, waterborne E.coli O157:H7 and foodborne E.coli O104:H4 demonstrate the microbes' versatility and the fragility of humanity's victory over the nature.

Occurrence of protozoa in drinking water and associated biofilms
The food web in drinking water microbial consortia is based on heterotrophic bacteria, the next trophic level being represented by protozoa.Species of parasitic protozoa, including free living amoebae associated with infections in humans have been isolated from source waters and drinking water (table 2).Their presence represents a double threat to human health, being also related to amoeba-resisting bacteria, such as Legionella spp.and Mycobacterium spp., which proliferate in protozoa thus increasing the probability of causing diseases in humans (Marciano-Cabral et al, 2010).Some protozoa, for example Giardia spp.and Cryptosporidium spp.may persist to hostile environment in drinking water, resist to different disinfection procedures and accumulate in biofilms under the form of cysts, respective oocysts.Experimental introducing Cryptosporidium oocysts for the prediction of behaviour in drinking water distribution system showed surface attachment and subsequent intermittent detachment, with exposure to high doses of chlorine (20mg/L) needed for the removal of substantial numbers of oocysts attached to pipe walls (Warneke et al, 2006).The study of Helmi et al. (2008) investigating the interaction of Giardia lamblia and Cryptosporidium parvum (oo)cysts in drinking water biofilms, revealed that protozoa are able to attach in biofilm matrix from the first day and survive extended periods of time, longer for Cryptosporidium.Viable (oo)cysts were recovered from biofilm and water phase for the whole period of investigation, of 34 days, turbulent shear stress influencing the detachment.Table 2. Protozoa detected in raw water sources, water treatment plants (DWTP) and drinking water networks (WDS) and associated biofilms.

Occurrence of viruses in drinking water and associated biofilms
Sources of drinking water were investigated for the presence of enteric viruses, especially when gastrointestinal outbreaks occurred, and the results revealed episodes of faecal contamination in raw water.Epidemiological studies conducted supported the association between drinking water consumption and illness (table 3).The presence of enteric viruses associated with inadequate water supplies, poor sanitation and hygiene is mostly affecting developing countries (Ashbolt et al, 2004).Episodes of gastroenteritis caused especially by noroviruses attributed to contaminated drinking water have been reported also in developed countries.Inefficient raw water treatment and secondary contamination of distribution systems with sewage are of high concern, enteric viruses being generally more resistant than enteric bacteria to widely used free chlorine, chlorine dioxide and monochloramine disinfectants (LeChevallier & Au, 2004).Although no complete investigations regarding faecal indicators presence were performed in the considered studies (especially for intestinal enterococci as index of viruses), coliforms and E. coli have been detected in water samples in many cases of gastroenteritis outbreaks investigated (Parshionikar et al, 2003;Hewitt et al, 2007;Koh et al, 2011).
There is no evidence of viruses ability of multiplication in environmental biofilms, but they may survive for extended periods of time trapped in the matrix, similarly to protozoan (oo)cysts, and be detached in water column, where remain inactive until they find a host.Experimental studies using pilot scale systems demonstrated the ability of viruses to attach and accumulate into drinking water biofilms within one hour after inoculation, while their detachment in water phase is influenced by flow velocity (Lehtola et al, 2007;Helmi et al, 2008).The viral genomes were detected in biofilms over the whole period of both experiments (for 21, respectively 34 days).Helmi and co-workers, investigating the poliovirus infectivity, recovered the infectious viruses only for 6 days, when flow velocity increment from laminar to turbulent regimen was applied, concluding that detection of viral genome in biofilms is not sufficient to assess a risk associated with the presence of infectious particles.

Occurrence of fungi in drinking water and associated biofilms
Initially considered to be airborne, fungal infections in immunocompromised patients hospitalized in controlled atmospheric conditions raised the hypothesis of waterborne origin of aspergillosis (Anaissie & Costa, 2001).Opportunistic pathogens, potentially causing superficial or systemic infections, allergenic or toxigenic species of fungi (yeasts and moulds) have been isolated from drinking water worldwide, their presence being primary attributed to the ability of surfaces colonization as biofilms (table 4).In some studies, the correlation with standard hygiene indicators was not found (Göttlich, 2002), other authors described negative correlations between bacteria and filamentous fungi, which may be explained either by competition for nutrients either by inhibiting toxins produced (Gonçalves et al, 2006) while in other investigations positive significant correlations were found between the presence of filamentous fungi, yeasts and bacteria in drinking water (Sammon et al, 2010).Regarding filamentous fungi behaviour in water distribution systems, deposition is attributed to highly resistant spores, while mycotoxins, taste and odour changing compounds producing implies germination and hyphal growth in biofilms.The occurrence of fungi in drinking water systems may have significant impact due to health effects of mycotoxins (such as aflatoxins): mutagenic, teratogenic, oestrogenic, carcinogenic and allergenic, although no reports of disease attributed to mycotoxins produced in the water distribution systems have been reported (Sonigo et al, 2011).

Occurrence of algae in drinking water and associated biofilms
Algae are assumed not to be characteristic to water distribution system biofilms due to the absence of light (Wingender and Flemming, 2011), but algal biomass is a major component of biofilms in surface source waters, water treatment and storage, in areas exposed to air and light.Experimental research designed by Chrisostomou et al (2009) emphasized on airdispersed phytoplankton diversity and colonization potential of algal taxa in drinking water reservoir systems.Algal communities are associated to biofilms and may support bacterial growth, for example Legionella species (Declerck, 2010).Few recent studies investigating the presence of algae in drinking water are available (table 5).Table 5. Algae identified in drinking water distribution systems (WDS).
Algal toxins, of which the most dangerous for humans is cyanobacterial microcystin, are considered chemical hazards in drinking water, especially when open-air reservoirs are used in water storage (Lymperopoulou et al, 2011).Algal growth and eutrophication in surface waters are widely investigated, with respect to ecological effects.In drinking water sources and throughout the water treatment process, distribution and storage, algal blooms raise issues about toxins releasing and aesthetic problems inducing, such as colour and smell.Algal removal in drinking water treatment is recommended to be carefully performed, in order not to disrupt the cells and release toxins in drinking water (LeChevallier and Au, 2004).Epidemiological studies are conducted worldwide in order to demonstrate the evidence of algal toxins in the environment and to evaluate their relatedness to illness in humans.Possible linkages between algae toxins in drinking water and health effects, including liver problems and diarrhoea in children were indicated by a survey in Namibia, although microcystin never exceeded the tolerable daily intake (Gunnarsson and Sanseovic, 2001).

Drinking water and associated biofilms -Chemical aspects
Detrimental effects of biofouling in drinking water distribution systems include chemical aspects, involving organic and inorganic compounds produced by the microorganisms inhabiting water phase, biofilms and sediments.Different volatile compounds, organic and inorganic acids, metal oxides and enzymes resulted in microbial metabolism or decay may cause aesthetic problems in water: colour, taste and odours and may also have an impact on the substratum, leading to microbially influenced corrosion.

Drinking water aesthetic problems
Aesthetic and organoleptic characteristics of water may be affected by a series of chemical substances, resulting in colour, odour and taste degradation.Such substances originate in microbial activity and decomposition in source waters and in distribution systems, disinfectants used in water treatment, materials used in pipes and joints in water networks.
A list of these substances, related to microbial activity and decay that may be produced in the journey of drinking water from drinking water sources to the tap, that may influence consumers perception, is presented in table 6 (after the UK Environment Agency, 2004).These chemical compounds are usually attributed to microbial biofilms associated to drinking water processing and distribution.
Investigating the sources of taste and odour in drinking water in order to find their sources and mitigation strategies, Peter (2008) concluded that low concentrations in chlorine residuals, stagnant water, plastic pipes and particles accumulation in distribution systems may increase the generation of taste and odour compounds by favouring biofilm formation and microbial activity.Other sources of aesthetic problems in water may reside in the activity of bacteria involved in sulphur cycle, producing sulphur odours and yellow discoloration (US EPA, 2002).Oxidation and reduction of soluble metals may produce metal oxides, leading to consumer complaints about the metallic taste and yellow, black or brown staining water (Cerrato et al, 2006).

Microorganisms -Surface interactions and microbially influenced corrosion
Biofouling proved to be interdependent on surface characteristics.Investigations of microbial reversible and irreversible attachment in primary or secondary colonization and in drinking water biofilms composition concluded as following:

•
The hydrophobic/hydrophilic properties of the substrate are influencing biofilm formation.Exopolysacharides produced by some bacteria facilitate cell adhesion to hydrophilic surfaces, while exopolymers of other bacteria may show a preference for hydrophobic substrata (Beech et al, 2005).
Regarding the influence of the substratum on biofilm composition, copper materials appear to be colonized just by L. pneumophila in low numbers, inhibiting P. aeruginosa integration, while drinking water biofilms on elastomeric and polyethylene materials proved to be a better support for pseudomonads (Moritz et al, 2010).
• Pipe materials may be corroded, influencing disinfection effectiveness: corrosion products i n i r o n p i p e s r e a c t w i t h f r e e c h l o r i n e a n d lead to residual disinfectants depletion.LeChevallier et al. (1987) detected high concentrations of coliforms only in tubercles formed on iron pipes and suggested few possible explanations: coliform growth stimulated by iron oxides; nutrient syntropy; sourface roughness; protection from disinfection.Iron pipes may be a better support for fungi also, when compared to PVC pipes (Doggett, 2000).
Drinking water flowing through PVC pipes contains three times the aqueous concentration of soluble manganese and 35 times the concentration of total manganese than present in the drinking water transported by iron pipes (Cerrato et al, 2006).Kiss, 1986;Drăgan-Bularda & Samuel, 2006;Muntean, 2009).Their representative species may belong to microbial communities of source waters and sediments, enter drinking water treatment plants and distribution systems in a planktonic state and adhere to surfaces or become members of established biofilms (Costerton, 1994).Their metabolites have significant impacts on drinking water quality, either being released in bulk water where may react with other compounds, for example with disinfectants, leading to toxic disinfection-by products (as trihalomethanes), or by remaining in biofilm matrix where acting upon pipes surfaces and inducing corrosion.the binding capacity of biofilm matrix which may lead to deposits accumulation with clogging effects (Beech et al, 2005;Coetser and Cloete, 2005).

Microorganisms
Some of the recommended strategies in drinking water associated biofilm control are: source waters protection, appropriate treatment, infrastructure contamination prevention, pipes and reservoirs maintenance, corrosion control, appropriate disinfection practices, nutrient levels reducing, water quality monitoring, personnel training, water safety plans implementation.
We are still living in an age of surfaces, even the remark was first said by Oscar Wild's character in 1895.Having in mind the virtual idea of self-cleaning surfaces, researchers in nanotechnology field are targeting innovative repellent materials with a wide range of applications, for the biofouling control.The superhydrophobicity models such as "the lotus effect" characterizing the lotus (Nelumbo nucifera) leaf, offered by natural patterns are investigated at nanoscale.The interdependence between surface roughness, reduced particle adhesion and water repellence proved to be the keystone in the self-cleaning mechanism of many biological surfaces (Barthlott & Neinhuis, 1997).

Conclusions
The present review emphasize on the following recent and relevant findings: • Biofilms associated with drinking water are ubiquitous, harbouring bacterial pathogens, opportunistic pathogens, parasitic protozoa, viruses, toxins releasing fungi and algae; • Microbial consortia in contact with drinking water have significant impacts upon water quality and may threat human health when contamination events occur; • Access to safe water continues to be a target for developing countries, unfulfilled at the moment; • Even in developed countries, where substantial efforts are submitted in order to ensure population's access to a high quality drinking water, microbial versatility represents an endless source of problems, with respect to opportunistic pathogens emergence; • Microbial communities in water networks and biofilms represent complex ecosystems; their ecology is influenced by a series of abiotic and biotic factors: raw water sources quality, temperature, flow rate and system hydraulics, nutrient concentration, pipe material, particles accumulation, ingress and intrusion, water treatment, water disinfection and microbial interactions; Further research is needed in order to understand attached microbial consortia for biofouling prevention and control in drinking water industry, as a matter of public security. www.intechopen.com Enteric bacteria such as Escherichia coli, Klebsiella pneumoniae, K. oxytoca, Enterobacter cloacae, E. aglomerans, Helicobacter pylori, Campylobacter spp., Shigella spp., Salmonella spp., Clostridium perfringens, Enterococcus faecalis, E. faecium, as well as environmental bacteria becoming opportunistic pathogens Legionella pneumophila, Pseudomonas aeruginosa, P. fluorescens, Aeromonas hydrophila, A. caviae, Mycobacterium avium, M. xenopi, together with other waterborne agents have been indicated to live in ecological microniches offered by drinking water associated biofilms (table 1).

Table 1 .
Pathogenic and opportunistic pathogenic bacteria detected in association with drinking water; WDS -water distribution systems, DWTP -drinking water treatment plant.

Table 3 .
Viruses identified in raw water sources and water distribution networks (WDS).

Table 4 .
Fungi identified in drinking water distribution systems (WDS) and associated biofilms.

Table 6 .
Chemical compounds produced by microbial decomposition and metabolism, affecting taste and odour of drinking water.

Table 7 .
Physiological groups of bacteria, their representatives and metabolites.Ecological Water Quality -Water Treatment and Reuse 154 Chemical and enzymatic microbial products resulted in biofilms activity may induce corrosion and related effects by different mechanisms: • oxygen concentration cells and anaerobic sites generation (promoting growth of anaerobic bacteria); • formation of iron concentration cells by the activity of iron and manganese oxidizing bacteria; • metabolites such as acids produced by bacteria have corrosive action upon the surface; • production of depolarizing enzymes within the biofilm matrix, which may persist longer than viable cells; • exopolymers produced by slime forming bacteria stimulate biofilm formation and biomass accumulation; • www.intechopen.com