Contaminated water as a source of Helicobacter pylori infection: A review.

Over the preceding years and to date, the definitive mode of human infection by Helicobacter pylori has remained largely unknown and has thus gained the interest of researchers around the world. Numerous studies investigated possible sources of transmission of this emerging carcinogenic pathogen that colonizes >50% of humans, in many of which contaminated water is mentioned as a major cause. The infection rate is especially higher in developing countries, where contaminated water, combined with social hardships and poor sanitary conditions, plays a key role. Judging from the growing global population and the changing climate, the rate is expected to rise. Here, we sum up the current views of the water transmission hypothesis, and we discuss its implications.

Introduction

Water crisis and risk of infectious diseases in the developing world

On July 28, 2010, the General Assembly of the United Nations voted to recognize access to clean water and sanitation as a human right (URL: http://www.un.org/News/Press/docs/2010/ga10967.doc.htm), a long-awaited decision that had been advocated and endorsed by the scientific community [1]. This recent UN resolution came at a time in which water is increasingly becoming at the heart of geopolitical and socioeconomic conflicts, notably in the developing world and in particular as a consequence of climate change [2,3].

In developing countries, many communities lack access to a reliable source of clean water (Fig. 1A) or sanitation services (Fig. 1B) [4]. Instead, those communities find themselves having no other choice but to depend on the surrounding sources of continuously flowing water, such as nearby rivers and streams as their sole everyday water source (Fig. 2A). On the other hand, isolated communities living in low-populated deserted geographical areas, located hundreds of miles away from a nearby river branch or stream, are obliged to rely on municipal water wells as their main supply for drinking and irrigation (Fig. 2B). An alarmingly rising number of those individuals suffer from numerous gastrointestinal tract-related problems [5-8], some of which can be directly linked to Helicobacter pylori infection, which can result into chronic infection and even cancer [9,10].

Fig. 1

Global patterns of (A) percent population without sustainable access to an improved water source (B) percent population with access to sanitation. Cartograms or map projections were downloaded from http://www.worldmapper.org (© Copyright SASI Group, University of Sheffield; and Mark Newman, University of Michigan).

Fig. 2

Example of suboptimal water sources in developing countries. (A) A running water source in Giza, Egypt (Photo credit: Radwa Raed Sharaf); (B) An exposed water well in an Al-Bahariya Oasis, Egypt (Photo credit: Mohamed Mahdy Khalifa).

When waterborne diseases are discussed, acute infections related to diarrhea and malnutrition (e.g., infections by Vibrio cholerae, Escherichia coli, and Salmonella enterica) often come to the front scene [3,11], but it is less common to consider chronic diseases, such as those resulting from H. pylori infection, as water-related public health threats. Still, the increase in H. pylori-associated gastrointestinal conditions could only raise an obvious question of whether contaminated water is a route of transmission of this pathogen, being a common factor among the infected patients [12]. This question gains particular importance given the continuously changing pattern of human demography expected to redraw the global map of H. pylori epidemiology [13].

In this article, we briefly introduce H. pylori and its epidemiology, we review evidence suggesting contaminated water as a source of infection with emphasis on recent evidence confirming viability of the bacteria isolated from water sources, and we discuss the potential implications of this route of transmission on global health and health policies.

Helicobacter pylori and its transmission

H. pylori, a bacterium initially observed in 1893 ([14] cited in [15]), has not been recognized as an infectious agent until 1982—in the seminal work of Nobel Laureates, Warren and Marshall [16-18]. H. pylori colonizes various regions of the upper digestive system, mainly the stomach and duodenum, causing stomach and duodenal ulcers and certain stomach cancers [9,19,20]. The infection is surprisingly common, and the bacteria are believed to colonize more than half of the world’s population [21].

H. pylori bacteria grow only under microaerophilic conditions on rich media [22]. An interesting feature of these bacteria is their ability to adapt to harsh conditions. They are capable of becoming virtually metabolically inactive, with minimal synthesis of DNA and RNA through a conversion from spiral into coccoid forms, offering a survival advantage in cases when chances of survival are slim [23] to none [24,25]. The coccoid form has been further classified into three categories, a dying form, a viable culturable form, and a viable-but-non-culturable state (VBNC), found to be metabolically active but not actively growing [26,27].

The nature of H. pylori and its infection niche, the human stomach, suggest ingestion as the most likely means of acquisition of this pathogen [28]. Nevertheless, its specific route of transmission has been widely debated among researchers to be oral–oral, gastro–oral, or fecal–oral (recently reviewed in [13] and [29]).

These three routes of transmission, reviewed elsewhere [13,28], are not mutually exclusive and may all be simultaneously involved in the infection process [30,31]. In this article, we focus on the oral ingestion of contaminated water or water-related items. This route of transmission can be fairly argued [12] since water biofilms have been suggested [27] to provide the bacteria with a protective habitat necessary to endure the water handling process. In addition, groundwater supply, being the sole source of water in many geographic areas, ideally fits into the oral–fecal, and perhaps the gastro–oral, models of infection. By time and throughout their life, inhabitants of those geographic areas consume large volumes, which statistically cause their chances of becoming infected to skyrocket.

Water as a source of infection

The hypothesis of water being a route of transmission of H. pylori [7,12,32] is supported by epidemiologic studies that have observed a higher prevalence of H. pylori infection [33-35] and a more rapid acquisition rate [36,37] in developing countries, which, in most instances, suffer from problems related to the sanitary distribution of water among the population (Fig. 1).

Evidence supporting the water transmission hypothesis comes largely from two groups of studies: (i) epidemiologic studies showing association between prevalence of H. pylori and water-related sources (See Table 1 for landmark studies representing this group) and (ii) studies that detected or isolated H. pylori from water sources (Table 2).

Table 1

Example of landmark epidemiologic studies suggesting possible water transmission.

Year published	Location	# Cases	Design/Methods	Main finding(s) and significance	Refs.
1991	Peru	407 children (<12 years)	Epidemiologic study using 13C Urea breath test	First report suggesting water as a risk factor for H. pylori	[38]
2002	Kazakhstan	288 Unrelated healthy individuals	Cross-sectional seroepidemiologic study between May–August 1999	Statistical and epidemiologic evidence that water and poor sanitation, rather than ethnicity or crowding, are risk factors for H. pylori infection: drinking river water is the highest risk	[41]
2008	Japan	224 Children (<6 years)	Three-year follow-up study	In one district using deep groundwater, the prevalence rate among children was 0%, and these children maintained their uninfected status throughout. Other districts with normal prevalence rate used river water	[46]
2012	Malaysia	161 Subjects (including 82 controls)	Case-control study using gastric histology to detect H. pylori	Increased risk of H. pylori is associated with unsanitary practices. Also the use of well water and overall poor hygiene were associated with a higher risk of infection (OR = 3.38, 95% CI: 1.76–6.46)	[69]
2013	Six Latin American countries	1859 adults	Urea breath test	The odds of H. pylori infection correlated with the lack of indoor plumbing (OR 1.3: 1.0–1.8)	[70]

# Cases: Number of human subjects.

OR: Odds ratio.

CI: Confidence interval.

Refs.: References.

Table 2

Key studies detecting H. pylori in water samples and confirming the water transmission hypothesis.

Year published	Location	Water source	Detection method	Main finding(s) and significance	Refs.
1993	Maryland, USA	Laboratory microcosms	Autoradiography (to assess viability of VBNC forms)	This study provides evidence for the metabolic activity of VBNC H. pylori in water, which supports a possible waterborne route of infection for H. pylori.	[42]
2001	Japan	Tap, well, river, and seawater	Membrane filtration followed by polymerase chain reaction	Detection of H. pylori. DNA in well water	[45]
May 2003	Wisconsin, USA	Any	Culture-based method: development of selective medium for H. pylori	A selective HP-agar medium was developed for the isolation of H. pylori from mixed microbial population in water that provides faster growth and superior selectivity	[71]
2003	North Carolina, USA	Fresh water	Membrane diffusion chambers followed by plate counts and Live/Dead Baclight assay	H. pylori can persist in the VBNC state, which represents a public health hazard.	[59]
January 2004	Portugal and United Kingdom	Various	Different culture media and growth conditions	This work demonstrates the possibility of optimizing culture-based techniques for recovery of H. pylori from water	[72]
April 2006	Portugal and United Kingdom	Well	N/A	This study suggests the detection of the pathogen in well water described by other authors can be related to the increased ability of H. pylori to integrate into biofilms under conditions of low shear stress. It will also allow a more rational selection of locations to perform molecular or plate culture analysis for the detection of H. pylori in drinking water-associated biofilms.	[47]
2011	Basra, Iraq	Treated municipal drinking water	Modified Columbia Urea Agar	Successful cultivation and identification of 14 H. pylori samples	[56]
2012	Missouri, USA	N/A	A lanthanum-based concentration method coupled with quantitative real-time PCR	The authors succeeded in developing a detection method for water samples with low concentrations of H. pylori and E. coli.	[73]
2012	Spain	Wastewater	A combination of culture methods following filtration of the samples and molecular techniques, mostly PCR and fluorescent immunohistochemistry	The authors successfully identified the presence of H. pylori in 6 out of 45 wastewater samples.	[74]
2012	Karachi, Pakistan	Drinking tap water samples	Concentration of samples via membrane filtration and PCR on DNA isolated from residue on membranes	The authors obtained a positive result in 4% of samples (2 out of 50 total samples).	[54]
2013	Isfahan, Iran	Various water sources including tap water, bottled mineral water from different brands and samples from publicly available water coolers	Culture on supplemented Brucella agar followed by Gram staining and biochemical tests. Positive results confirmed by PCR amplification of ureC gene	Culture methods successfully detected H. pylori in five out of 200 samples while PCR amplification of ureC gene was successful in 14 samples. The authors suggest that PCR-positive, culture-negative samples may have coccoid forms of H. pylori; in our opinion, this could be also due to the presence of other ureC-carrying bacteria, or other Helicobacter species.	[57]

Refs.: References.

N/A.: Not applicable.

Water was first suggested as a source of H. pylori infection in 1991 by Klein and coworkers, who observed that Peruvian children with an external source of drinking water were more likely to be infected with H. pylori than children with an internal source [38]. Subsequently, H. pylori cells were detected in the water provided to cities nearby Lima, Peru in 1996 [39] and in municipal water, treated wastewater, and well water in Sweden in 1998 [40]. A few years later, Nurgalieva and coworkers noted that drinking river water was a high risk factor for H. pylori infection in Kazakhstan [41]. Accordingly, they stated that transmission of H. pylori could be waterborne [41].

Shahamat and colleagues hypothesized that the VBNC form of H. pylori persists in water [42], and in a number of studies [36,38,43,44], untreated municipal water was considered as a main cause of the increased H. pylori prevalence in the areas subjected to research. Effectively, in 2001, H. pylori’s DNA was detected in a Japanese well, whose consumers were infected [45], while a more recent study from Japan suggested river water-associated incidence [46].

The water transmission possibility was studied in depth in a thesis published in 2005, in which Azevedo strongly argues that drinking water can pose a substantial threat of H. pylori infection based on the fulfillment of several essential criteria [32]. These criteria include the ability of H. pylori to adhere to different materials and to co-aggregate with other bacteria and form complex structures on pipes or other surfaces in contact with water [32]. The notion about the inability of the bacterium to survive alone in running water, but to develop a symbiotic relationship and form complex structures on contact surfaces [47], makes it rational to assume that groundwater is a reservoir for H. pylori due to its stagnant nature.

Surprisingly, it is not uncommon to detect H. pylori’s DNA in water [48,49]. In fact, Lu and coworkers went as far as culturing the bacteria from the untreated municipal water using immunomagnetic separation (IMS), which was further confirmed by polymerase chain reaction (PCR) and a set of microbiological tests [44]. However, as Azevedo pointed out [32], the improved handling of water in more developed countries, coupled with sanitary conditions, which mandate proper disinfection, has effectively impeded the transmittance of H. pylori over the course of the last 20 years [32]. Nevertheless, H. pylori was shown to retain its viability in chlorinated water [50,51].

Furthermore, older findings by West and coworkers show that H. pylori is capable of survival in different types of aquatic environments under an array of physical variables [52]. West et al. conclude that the bacterium, unlike other pathogens, is unusually tolerant to pH fluctuations [52]. In support of this finding, a study regarding the occupational health hazards, conducted years later (2008) in India, indicated that the sewage and sanitary workers experience a high risk of H. pylori infection [53]. This could only be linked to the constant exposure of these workers to contaminated water in their line of work, in the absence of strict regulations and protocols to ensure their safety. In the same study, the author reported a rising blood level of IgG antibodies, targeted against the bacterium, with increased age [53].

In light of accruing evidence from studies published before 2005, Bellack and colleagues suggested a conceptual model for water’s role in H. pylori transmission. Their model is based on the assumption that humans and animals can be long-term carriers of the bacteria and that they can transfer it to water, which is a short-term reservoir, via the fecal route [12]. Accordingly, their model suggests the requirement for continuous water contamination by human or animal feces with the high likelihood of fecal–oral transmission to humans consuming contaminated water, in which bacteria survive for limited time. However, Bellack’s model stopped short at direct evidence of viable bacteria isolated from water sources. Such evidence has lately been available from different sources, where direct isolation of viable H. pylori from water has been reported in developing countries, with less optimal water hygiene, suggesting that bacterial isolation is more likely to be successful when the microbial burden is relatively high. Examples include studies in Pakistan [54,55], Iraq [56], and Iran [57] (see Table 2).

Of note, not all investigators support the water hypothesis, and some have actually designed experiments to debunk it. Janzon and coworkers, for example, reported their failure to detect H. pylori DNA in water in spite of using a highly sensitive real-time PCR assay and in spite of adopting a series of controls in their study [58]. Although this conflict has not been resolved, it is possible that these contradictions are related to the variability in bacterial load in water samples. After all, “absence of evidence is not evidence of absence” (quote attributed to US astronomer Carl Sagan).

Entrance of H. pylori into the VBNC state allows H. pylori to persist in water, but the bacteria remain nonetheless difficult to culture [42]. Other investigators attempted to force the bacteria into entering this state within a laboratory setting [59], and despite the great number of viable cells, the culturability declined sharply to less than 10 colony-forming units per milliliter. This could definitely be a strong indication as to what happens under normal circumstances in a real-life setting [59].

What next? From association, detection, and isolation to causation

As noted above, less than a decade ago, the model suggested by Bellack and colleagues for water’s role in H. pylori transmission [12] seemed quite plausible; yet, there was not enough evidence supporting direct microbial viability. The work of Azevedo [32,47,60] and subsequent published studies on direct microbial isolation (e.g., [56,57]) provided such needed evidence. What remains now is to establish direct causation via well-designed experiments that use water, spiked with H. pylori, to cause colonization and/or disease in animal models, fulfilling Koch’s postulates for disease etiology [61-63]. One challenge is the choice of appropriate animal model; another is confirming that the initiation of disease is caused by ingested rather than resident Helicobacter cells. The latter can be made possible by various methods, ranging from direct labeling to inserting traceable genetic markers in exogenous bacteria by genetic manipulation.

Water-contaminated infection sources

As a corollary to the water transmission hypothesis, if water is a reservoir of H. pylori, then any surface exposed to the contaminated water could potentially act as another source of infection. One clear example is harvested raw fruits and vegetables in rural communities. Those crops pose a threat of being a vehicle for the transmission of H. pylori, being contaminated by irrigation water and in some cases municipal water, sought by some as a substitute for organic manure.

Goodman and coworkers noted this possibility and included the unsanitary habits of the Columbian Andes population as another contribution to the infection pool [43]. These habits range from the use of the open fields when lacking a toilet facility to the late afternoon swimming—as an escape from the surrounding hot climate—in the flowing streams and rivers, considered to be dumping sites for the excess irrigation water. The authors’ results are clear-cut: depending on the source of drinking water, whether from a privately owned well, water pumps, or even tap water—as opposed to a nearby stream or river, the risk of infection fits perfectly into place, which was immensely higher in the latter case.

Possible methods of prevention

Knowing the source of infection is a necessary step toward prevention. Salih reports that in recent years, infection with H. pylori in the developing countries has declined owing to the increased awareness of the possible root of the problem and recommends boiling water to prevent infection [64]. Nowadays, it is highly advisable to boil water used for drinking, or even for washing hands and dishes. This simple measure is especially recommended for those who lack a trustworthy water purification system within the community, although compliance is not guaranteed. One can only agree that the process of boiling is an effective combating regimen, since a temperature of merely 30 °C was capable of arresting the growth of various strains of the bacterium as reported by Xia and coworkers [65]. In most cases, such practice was initially promoted by the respective health authorities to fight off more serious forms of infections caused by water-borne microorganisms.

Despite this seemingly obvious assumption, earlier findings of Mitchell and coworkers [36] appear to somewhat contradict the effectiveness of boiling water. Mitchell’s study included a section of Southern China’s population, who were asked to complete a questionnaire. Results indicated a higher prevalence of infection among rural inhabitants, who drank river water as opposed to well water. Surprisingly, most stated that boiling water is included in their everyday routine [36].

Conclusions

In this Review Article, we focused on water as a possible source of transmission of H. pylori and discussed some experimental findings indicating the possibility of detecting viable H. pylori in water. We recognize that this hypothesis has been challenged [58] and that even if confirmed as a reservoir for H. pylori, water may very well be a secondary route of transmission [18,66]. However, given the accruing evidence, it is still important to seriously consider contaminated water as one of the likely candidate sources and deal with it effectively. Ongoing research aims at providing unequivocal evidence of the suggested route of transmission. As soon as this is achieved, efforts can be directed to prevent further infections and properly treat possible transmission vehicles to cut down the number of new cases.

Outlook

The possibility of H. pylori transmission through water has its promises and perils. On the one hand, water transmission is preventable by the implementation of necessary measures of hygiene and water sanitation. On the other hand, availability of drinking water is likely to be a crisis in the following decades, and the burden of this crisis falls unequally on developing countries [4,5,67]. The problem becomes even more serious when considering how the climate change is affecting our planet’s demography [1,2,67]. Eventual migrations may worsen the situation of the developing countries not only by increasing their populations, but also by rendering the availability of treated potable water even dearer [4,66,68].

On dealing with waterborne infections, one might give priority to infectious diseases with high mortality such as cholera and other diarrheal diseases [3]. However, H. pylori causes cancer especially in elder patients and given that life expectancy has increased, and so has poverty, preventing infection-associated cancers (e.g., H. pylori and hepatitis C) should be a priority of health organizations in the decades to come.

Conflict of interest

The authors have declared no conflict of interest.

Compliance with Ethics Requirements

This article does not contain any studies with human or animal subjects.