The relationship between symbiotic bacteria and helminths

Bacteria have been described from most classes of helminths, although in many cases it is not known whether they are obligatory or facultative. Many of the bacteria described from helminths, particularly those worms residing within the intestinal tract, are ectosymbiotic on the parasites’ tegumental surface. In contrast, endosymbiotic bacteria occurring in helminth organs and tissues are more poorly known but still appear to be widespread. Nevertheless, it remains to be determined if symbionts occur in all stages of the life cycle, and the exact mechanisms (vertical, horizontal or mixed transmission) by which the helminths acquire their bacteria. These holobionts have been most extensively studied in filarial and entomopathogenic nematodes, where a single bacterial species association occurs (Forst & Clarke, Reference Forst, Clarke and Gaugler2002; Taylor et al., Reference Taylor, Bandi and Hoerauf2005). Bacteria from other helminths have been studied less extensively, but are known to harbour a diverse range of species (Cable & Tinsley, Reference Cable and Tinsley1992; Hughes-Stamm et al., Reference Hughes-Stamm, Cribb and Jones1999; Bakke et al., Reference Bakke, Cable and Østbø2006; Ziino et al., Reference Ziino, Giuffrida, Bilei and Panebianco2009). In particular, intestinal helminths are known to host a rich bacterial community (Hsu et al., Reference Hsu, Johansson and Donahue1986; Krecek et al., Reference Krecek, Sayre, Els, Van Niekerk and Malan1987; Poddubnaya & Izvekova, Reference Poddubnaya and Izvekova2005; Korneva, Reference Korneva2008; Hayes et al., Reference Hayes, Bancroft, Goldrick, Portsmouth, Roberts and Grencis2010), and it would appear that the digestive tract is a major site for interactions between bacteria and these parasites.

The nature and formation of helminth bacterial communities has been studied most extensively in fish. The density and biomass of bacteria on the tegument of cestodes of freshwater fish varies according to the vertebrate–helminth combination, where the ecology of the fish influences the bacterial population that the parasite worm acquires from it. In the pike, a predator that feeds all year round, Trianophorous nodulosus from the intestines has a relatively abundant bacterial community. However, on the tegument of another intestinal tapeworm, Eubothrium rugosum, from the burbot, a predator and facultative benthophage with a peak feeding period during the winter, the bacterial population was substantially lower than that found for T. nodulosus. Similarly, Ligula intestinalis found in the body cavity of bream, also had a relatively depauperate bacterial community on its tegumental surface, although the biomass of these microorganisms was still comparable to that found on the surface of E. rugosum (Izvekova & Lapteva, Reference Izvekova and Lapteva2004). The density of symbiotic bacterial populations may also vary seasonally in fish hosts. Fewer bacteria were found on the surface of the intestinal trematode Culaeatrema inconstans in the spring, and the ectoparasitic monogenean Gyrodactylus salaris in the winter, which may be due to the cooler temperatures impeding bacterial colonization and growth (Lasee & Sutherland, Reference Lasee and Sutherland1993; Bakke et al., Reference Bakke, Cable and Østbø2006).

Differences in the diversity of symbiotic bacteria harboured by tapeworms from fish hosts have also been reported, and may be associated with variations in intestinal conditions (for example, whether the host is a benthophage or a detrithophage) or the environmental conditions found in dissimilar aquatic habitats (Poddubnaya & Izvekova, Reference Poddubnaya and Izvekova2005). Different bacterial species, dominated by Gram-negative forms, colonize the superficial or deep areas of cestode teguments, with a preponderance of nanobacteria in the superficial layers, possibly related to their association in the formation of mucosal biofilms and their regulation of functional activity (Plotnikov & Korneva Reference Plotnikov and Korneva2008). The distribution of bacteria along the surface of monozoic tapeworms found in fish intestines changes, with most microorganisms occurring in the middle and posterior parts of the cestode's body, possibly because these sections are most closely associated with nutrient absorption by the parasite (Poddubnaya & Izvekova, Reference Poddubnaya and Izvekova2005). Similarly, bacteria on the surface of the intestinal trematode Gyliauchen nahaensis are only found on the dorsal surface and excretory papillae regions that are exposed to the gut lumen and the nutrients retained within it (Hughes-Stamm et al., 1999), whereas bacterial density varied with both parasite specimen and body region of the ectoparasitic Gyrodactylus avalonia on the skin of fish (Cusack & Cone, Reference Cusack and Cone1985).

Within the endotherm intestine, the vertebrate host's bacterial community is one of the main ecological factors influencing the establishment of helminths (Stefanski, Reference Stefanski1965). For example, certain strains of Trichuris muris eggs are dependent on the presence of particular gut-associated bacteria to induce hatching. Two pathways are involved in the bacterial induction of such hatching, either requiring direct contact between egg and bacteria or independent of contact, initiated only by bacterial excretory/secretory products (Hayes et al., Reference Hayes, Bancroft, Goldrick, Portsmouth, Roberts and Grencis2010; Koyama, Reference Koyama2013, Reference Koyama2016). Nevertheless, Trichuris suis eggs will not hatch in response to the same bacterial species that trigger T. muris, suggesting that individual nematode species have adapted to different host-specific gut bacteria (Vejzagić et al., Reference Vejzagić, Adelfio, Keiser, Kringel, Thamsborg and Kapal2015).

Once established within the endotherm intestines, helminths do not appear to have carried any bacteria from the source vertebrate host with them during the process of infection, certainly eggs of the nematode Ascaridia galli do not retain the bacterial community of the adult worm after passage from the vertebrate host (Baron et al., Reference Baron, Hansen and Lord1960). Instead, helminths appear to acquire a few specific symbiotic bacterial species from the wide range present in the digestive tract, although this is likely to be a greater number of symbionts than those associated with filarial or entomopathogenic nematodes. For example, Heligmosomoides polygyrus L3 larvae, shortly after infecting the mouse intestines, have been found to have a uniquely complex but depauperate bacterial community, not derived from the gut microbiota. In contrast, once fully developed into adult worms they hosted a bacterial population that was closely related to that found in the mouse intestinal tract (Walk et al., Reference Walk, Blum, Ewing, Weinstock and Young2010). Nevertheless, it would appear that the location of the helminth in the digestive tract of the animal host may also influence the bacteria it acquires. Adult trematodes of Clinostomum marginatum from the mouth of the heron host possessed a diverse bacterial population on their tegumental surface, but specimens recovered from the oesophagus retained no bacteria (Aho et al., Reference Aho, Uglem, Moore and Larson1991).

Intestinal bacterial communities fluctuate seasonally and in different regions of the mammalian gut, which may influence the distribution of helminths once established (Stefanski, Reference Stefanski1965). Nevertheless, it has been found that the bacterial population declines, with proportionally fewer anaerobic bacteria and a shift in abundance from the class Bacilli to the class Clostridia, in mammalian intestines after infections with Hymenolepis spp. tapeworms (Mettrick & Podesta, Reference Mettrick and Podesta1974; McKenney et al., Reference McKenney, Williamson, Yoder, Rawls, Bilbo and Parher2015). These changes have been related to the lowering of the luminal pH due to the worms’ hydrogen ion secretory mechanism (Podesta & Mettrick, Reference Podesta and Mettrick1974). Interestingly, when completely germ-free mice are infected experimentally with Hymenolepis nana, significantly fewer parasites establish an infection, the body length and rate of maturity of adult cestodes is lower, and they are expelled at a faster rate from the mouse host (Przyjalkowski, Reference Przyjalkowski1977). A similar trend was also found for Hymenolepis diminuta infections in germ-free rats, although these results were less clear-cut in this host–parasite combination (Houser & Burns, Reference Houser and Burns1968). These studies suggest that the intestinal bacterial population plays a key role in the parasite's development and that the tapeworm may be deliberately changing conditions in the intestines in order to ensure that the density and composition of the bacterial community favours those particular species that provide the greatest benefit. Similar findings have been reported for the nematodes H. polygyrus and T. muris, which raise the abundance of Lactobacillus spp. in mammalian intestines, probably in order to promote their own survival due to the bacteria's effect on the gut immune response (Walk et al., Reference Walk, Blum, Ewing, Weinstock and Young2010; Reynolds et al., Reference Reynolds, Smith, Filbey, Harcus, Hewitson, Redpath, Valdez, Yebra, Finlay and Maizels2014; Holm et al., Reference Holm, Sorobetea, Kiilerich, Ramayo-Caldas, Estelle, Ma, Madsen, Kristiansen and Svensson-frej2015). Such changes appear to be replicated in the wild, where different helminth species are associated with specific shifts in bacterial species composition or abundance in the intestines of wood mice (Kreisinger et al., Reference Kreisinger, Bastien, Hauffe, Marchesi and Perkins2015).

It is apparent that studies undertaken in germ-free hosts may provide an important source of information on the role of symbiotic bacteria for helminths. Using gnotobiotic techniques, parasitism in mammalian and avian animals devoid of bacterial populations can be studied, although results from these experimental conditions are likely only relevant for those helminths that acquire their symbiotic bacteria from the host. Nevertheless, certain helminths will not establish, are unable to grow, grow less successfully or are less pathogenic in the absence of bacteria in these hosts (Doll & Franker, Reference Doll and Franker1963; Wescott, Reference Wescott1970; Przjalkowski, Reference Przyjalkowski1977), indicating the key role such microorganisms play in the biology of these particular species. However, in contrast, others can develop normally (Reid & Bottero, Reference Reid and Botero1967; Weinstein et al., Reference Weinstein, Newton, Sawyer and Sommerville1969), or may develop in a germ-free atypical host in which, under normal circumstances, they would be unable to establish (Newton et al., Reference Newton, Weinstein and Jones1959). Furthermore, helminths that grow normally under germ-free conditions, such as Schistosoma mansoni (Bezerra et al., Reference Bezerra, Vieira, Pleasants, Nicoli, Coelho and Bambirra1985), are known to harbour a bacterial population (Ottens & Dickerson, Reference Ottens and Dickerson1972), suggesting that these symbionts may not have a nutritional or developmental role but rather one associated with defence, other non-critical function or are using the worm as a ‘reservoir’ (see below). Alternatively symbiotic bacteria may be facultative rather than obligatory under these conditions and the helminth's functional biology, although not performing at an optimal level, may be adequate enough to allow normal development. There are few applicable generalizations of the manner in which helminth species may respond to these conditions; certainly groups such as cestodes or nematodes demonstrate a wide range of species-specific and, probably, strain-specific responses to these germ-free hosts, suggesting that the presence of bacteria has a complex influence on helminths in endothermic vertebrates.

The relationship between bacteria and helminths may not always be to the advantage of the worm. Species of schistosomes, resident in the blood vessels of mammalian hosts, can harbour a number of bacterial species, particular Salmonella, both within their guts and on their teguments, with bacteria associating more with female than with male worms (Melhem & LoVerde, Reference Melhem and LoVerde1984; Tuazon et al., Reference Tuazon, Nash, Cheever and Neva1985). No known role in the functional biology of the parasite is considered to occur with these bacteria. However, Salmonella within the parasite's gut were protected from the immune response of the mammalian host, and periodic regurgitation of the bacteria by the schistosome contributed to prolonged septicaemia. Animals will harbour an infection of this bacterium for a more prolonged period when the helminth was also present and the schistosomes were thus providing a long-term focus for bacterial multiplication in the portal–mesenteric blood vessels. The use of a helminth as a ‘reservoir’ by bacteria is not unusual for symbiotic interactions. Within marine environments, sponges can similarly be found harbouring pathogenic bacteria that cause diseases in neighbouring corals and sea fans, although they themselves remain unaffected (Webster & Taylor, Reference Webster and Taylor2012).

In the same manner, helminths, particularly trematodes, may act as vectors for endosymbiotic bacteria that can cause serious diseases in vertebrates after horizontal transmission from the adult trematode once it has established within the definitive host (Vaughan et al., Reference Vaughan, Tkach and Greiman2012). Vertical transmission of bacteria through each stage of the trematodes’ life cycle occurs, although the prevalence can be variable and at times may be low. For example, Neorickettsia risticii prevalence within individual Plagiorchis elegans cercariae ranged from 11 to 90%, which were acquired while developing within intramolluscan stages that themselves maintained a 100% bacterial prevalence (Greiman et al., Reference Greiman, Tkach and Vaughan2013). No positive or negative effects of these bacteria on the biology of the hosting trematode have been determined so far.

Physiology of the helminth–bacterial association

Detailed studies on the nature of the ecophysiological relationship between helminths and bacteria have been examined in only a few holobionts. The most extensive investigations have been undertaken on the single bacterial associations of filarial and entomopathogenic nematodes. As these particular interactions have been well documented and reviewed, only a brief overview of relevant ecological effects will be presented; more extensive information can be found within the cited papers. In filarial nematodes, Wolbachia bacteria are endosymbiotic partners of only certain species of vector-transmitted Onchocercinae and Dirofilariinae. They are vertically transmitted from adult female worms and appear to be obligatory symbionts. For these helminths, treatment with antibacterial agents adversely affects the nematode, resulting in delayed moulting, reduced growth rates, embryonic failure and death. Such effects do not occur in similarly treated nematodes that do not harbour symbiotic bacteria, indicative of the dependent nature of the relationship. In particular, this bacterium has been found to biosynthesize nutrients or metabolites during nematode embryogenesis and development, to prevent apoptosis of host cells, and to have a pro-inflammatory effect that facilitates nematode entry via mast cell activation and increased vascular permeability (Taylor et al., Reference Taylor, Bandi and Hoerauf2005, Reference Taylor, Mediannikov, Raoult and Greub2012).

In contrast, entomopathogenic nematodes of the family Steinernematidae and Heterorhabditidae form an endosymbiotic relationship with Xenorhabdus and Photorhabdus Gram-negative bacteria that are carried in the gut of juvenile larvae. When the juvenile nematode penetrates into a suitable invertebrate host, typically an insect, the bacteria are released to proliferate and produce a wide range of toxins and hydrolytic exoenzymes, which cause the death of the host and its bioconversion into a ‘nutrient soup’ for the nematode's growth and reproduction (Forst & Clarke, Reference Forst, Clarke and Gaugler2002). Metabolites from the bacteria are known to have antibiotic and nematicidial properties to inhibit the growth of competing microorganisms and to repel nematode species that vie for food and space (Webster et al., Reference Webster, Chen, Hu, Li and Gaugler2002). Similarly, in order to deter invertebrate saprophages, such as ants, utilizing the cadaver before the nematode has completed development, some bacteria produce a substance known as ‘ant-deterrent factor’ effective against a wide range of species (Zhou et al., Reference Zhou, Kaya, Heungens and Goodrich-Blair2002). Once the nematodes have completed reproduction in the cadaver their infective juveniles are recolonized by the bacteria before they disseminate into the soil (Forst & Clarke, Reference Forst, Clarke and Gaugler2002).

Within other kinds of symbiotic relationships, extensive evidence for a nutritional role has been found for those bacteria associated with the tegument of helminths found in vertebrates. Adults of the trematode C. marginatum from the mouth of herons have a rich bacterial community on their teguments and absorb glucose by an active transport system. After treatment with antibiotics the trematodes lost both their bacteria and the glucose transport system associated with them. Nevertheless, adult worms of this species, recovered from the throat, do not possess symbiotic bacteria and only take up glucose by facilitated diffusion, suggesting that environmental cues, such as the presence of symbiotic bacteria, during the trematode's development influence the formation of any particular cutaneous sugar transport functions (Uglem et al., Reference Uglem, Larson, Aho and Lee1991).

Pseudophyllidean and Caryophyllidean cestodes from fish intestines have extensive bacterial communities on their teguments, which are capable of carbohydrate hydrolysis. These bacteria may decrease the energy expenditure of tapeworm digestion by supplying, over a wide range of pH conditions, the hydrolytic enzymes, particularly α-amylase, used in this process, with the greatest contribution being made by those bacteria capable of amylolytic and proteolytic activity, which are more firmly bound to the tegument surface (Izvekova, Reference Izvekova2005a, Reference Izvekovab). In addition, the nutritional composition of the fish diet was considered likely to influence the level of contribution that these bacterial enzymes make to cestode digestion, with higher α-amylase activity being detected in lower-carbohydrate gut conditions (Izvekova & Komova, Reference Izvekova and Komova2005).

Nevertheless, under certain high-carbohydrate conditions ectosymbiotic bacteria may act as competitors for nutritional substrates, only producing hydrolytic enzymes again when carbohydrate levels drop (Reference IzvekovaIzvekova, 2005a). Enzyme activity on the cestode tegumental surface may be higher than that found on the intestinal mucosa, possibly due to the parasite surface being capable of maintaining elevated numbers of bacteria (Reference IzvekovaIzvekova, 2006b).This enzyme activity is considered to be associated with fish feeding. During active feeding the contribution of symbiotic bacteria to hydrolysis increases, while at low feeding rates the bacteria compete with fish and cestode for available nutrients (Reference IzvekovaIzvekova, 2006a). Exogenous α-amylase has also been demonstrated on the surface of mammalian tapeworms (Pappas & Read, Reference Pappas and Read1975) and it is possible that this enzyme activity may also potentially derive from a bacterial source.

Studies on the temperature adaptation of ectosymbiotic bacteria, as determined from the thermal characteristics of these proteinase enzymes, was found to vary between different fish–helminth associations. The bacterial enzyme activity derived from tapeworms demonstrated different thermal optima, depending on whether the parasite was from a fish host that undertook a high degree of winter feeding, and often differed from the activity levels found on the surface of the host intestines. These results suggest that the composition of bacterial species on the tegument of cestodes differs according to environmental conditions (Kuz'mina & Pervushina, Reference Kuz'mina and Pervushina2003).

A nutritional role for endosymbiotic bacteria may also be occurring in Ascaris suum. This intestinal nematode of endotherms has a diverse bacterial community within its own guts and many of these bacteria secrete the hormone serotonin, which is considered to stimulate glycogen metabolism (Hsu et al., Reference Hsu, Johansson and Donahue1986). When A. suum is incubated with antibiotics, both bacterial and serotonin levels become significantly reduced in the worm, suggesting that these microorganisms are providing an important supplementary source of this hormone and thereby boosting the parasite's own relatively low levels of endogenous production (Shahkolahi & Donahue, Reference Shahkolahi and Donahue1993). It is possible these bacteria may also be providing substantial amounts of supplementary histamine for the nematode. This monoamine, like serotonin, is associated with stimulating glycogenolysis (Brownlee et al., Reference Brownlee, Fairweather, Johnston and Shaw1994) and in bacterial-free conditions significantly lower concentrations are found in parasite tissues (Philips et al., Reference Philips, Sturman and West1975; Hsu et al., Reference Hsu, Johansson and Donahue1986).

Bacterial symbionts in the vertebrate intestines can also have a protective role. Ascaris spp. nematodes from pigs and horses have an ectosymbiotic relationship with a number of Gram-negative bacteria that include Pseudomonas pyocyanea. This bacterium, in particular, demonstrates an antibiotic action on Gram-positive bacteria, notably Bacillus mesentericus. In the absence of P. pyocyanea, B. mesentericus will cause lysis of the nematode tegument and ultimately death of the parasite (Emanuiloff, Reference Emanuiloff1958). Interestingly, in mammalian hosts when either Pseudomonas aeruginosa or B. mesentericus populations in the digestive tract were artificially elevated, the establishment of the intestinal phase of Trichinella spiralis was affected, being lower with B. mesentericus or higher with P. aeruginosa exposure (Stefanski & Przjalkowski, Reference Stefanski and Przyjalkowski1965, Reference Stefanski and Przyjalkowski1966). Similarly, increased parasite establishment was obtained with other Gram-negative bacteria but not the Gram-positive Lactobacillus sp. (Stefanski & Przjalkowski, Reference Stefanski and Przyjalkowski1966).

Symbiotic bacteria of invertebrates under anthropogenic stress

Microbial symbionts are clearly important to helminth health. It seems likely that, under conditions of environmental stress imposed by anthropogenic factors, the interactions between all members in the symbiotic partnership could be at risk. Insights into the potential changes that may take place under climate change and pollution exposure are of vital importance. Fortunately, because animal evolution has been continuously influenced by symbiosis with microorganisms, fundamental features of the animal–bacteria interface are conserved among all organisms, allowing knowledge from well-studied invertebrate symbiosis models to be applied broadly (Chaston & Goodrich-Blair, Reference Chaston and Goodrich-Blair2010). Thus, important pointers to the way helminth symbiotic relationships are affected by anthropogenic stress may be derived from the more extensively studied responses of marine invertebrate holobiont systems to degraded environmental conditions.

Environmental stressors may influence these symbiont communities by an indirect effect via the host, a direct effect on the bacterial population or a mixture of both (Newton & McKenzie, 1995). Studies on marine invertebrates indicate that pollution and temperature stress may have a profound effect on the symbiont–host relationship, with significant consequences for the host organism.

For sponge–bacterial associations both elevated seawater temperatures and chemical pollution cause changes in the normally stable microbial communities, which result in a corresponding decline in sponge health. This includes gross tissue necrosis of the host, due to both the breakdown of symbiotic functions caused by the disappearance of many symbionts and the establishment of alien microbial populations that may include potential pathogens (Webster et al., Reference Webster, Webb, Ridd, Hill and Negri2001, Reference Webster, Cobb and Negri2008; Lemoine et al., Reference Lemoine, Buell, Hill, Hill, Custodio, Lobo-Hajdu, Hajdu and Muricy2007; Selvin et al., Reference Selvin, Priya, Kiran, Thangavelu and Bai2009; Lopez-Legentil et al., Reference Lopez-Legentil, Erwin, Pawlik and Song2010; Tian et al., Reference Tian, Wang, Bougouffa, Gao, Cai, Zhang, Bajic and Qian2014a, Reference Tian, Lee, Wang, Cai, Bougouffa, Chiu, Wu and Qianb). Nevertheless, the larval sponge holobiont demonstrates a high tolerance to thermal stress, exceeding the threshold of the adult, capable of maintaining a highly stable symbiosis at seawater temperatures that are predicted to occur under climate change (Webster et al., Reference Webster, Botte, Soo and Whelan2011).

Coral holobionts are also susceptible to environmental change. Temperature stress results in changes to the composition and metabolism of coral-associated bacteria. There is an elevated level of bacterial pathogens and a decline in antibiotic-producing bacteria, causing adverse health effects on the host and ultimately leading to bleaching of the coral (Ritchie, Reference Ritchie2006: Lesser et al., Reference Lesser, Falcon, Rodriguez-Roman, Enriquez, Hoegh-Guldberg and Iglesias-Prieto2007). Similarly ocean acidification also causes changes in the bacterial community of corals, although the host shows no visible signs of stress in the short term (Webster et al., Reference Webster, Negri, Flores, Humphrey, Soo, Botte, Vogel and Uthicke2013). In contrast, exposure to chemical pollution (tributyltin) caused a reduction in newly settled coral branchlets, although symbionts appeared unaffected, suggesting that this chemical's toxicity was more effective against the host than its microbial community (Smith et al., Reference Smith, Negri, Philip, Webster and Heyward2003).

Other marine invertebrates have been less extensively studied but indicate that similar complex effects are apparent. For example, amongst numerous bivalve molluscs recovered from metal-polluted sites, only a species that contained symbiotic chemosynthetic bacteria had the highest accumulated concentrations of copper and zinc, possibly because their symbiotic bacteria may be a source of trace metal accumulation (Silva et al., Reference Silva, Smith and Rainbow2006). These bacteria could be partially responsible for observed differences in metal accumulation of macro-organisms because of their ability to bind metals, either at the cell surface or transported within for various intracellular functions (Ehrlich, Reference Ehrlich1997), although the long-term effects of such toxicant bioaccumulation for the host invertebrate remain unknown. Similarly, the tolerance of hydrothermal vent mussels to cadmium exposure may be associated with thio- and methanotropic symbiotic bacteria in the gills, which possess significant superoxide dismutase activity, elevating the level of antioxidant enzymatic protection (Company et al., Reference Company, Serafim, Cosson, Camus, Shillito, Fiala-Medioni and Bebianno2006).

In contrast, exposure of echinoderms and their symbiotic bacterial community to oil pollution resulted in a transient decrease in subcuticular bacterial numbers after a single exposure, and a gradual continual decline with repeated exposures, although no obvious effect on the host starfish were apparent (Newton & McKenzie, 1995). However, studies on the symbiotic bacterial populations of a polychaete at a copper and zinc polluted harbour found a change in the composition of the bacterial community, with an increase in the abundance of Alteromonadales, species known to reduce and metabolize heavy metals, which may be a factor in the high degree of tolerance polychaetes demonstrate to metal pollution (Neave et al., Reference Neave, Streton-Joyce, Glasby, McGuiness, Parry and Gibb2012).

These studies suggest that invertebrate holobionts under anthropogenic stress can result in substantial changes in the composition and density of the symbiotic bacterial community, with often a corresponding increase in the number of pathogenic bacterial species. Typically, these changes result in adverse effects on the invertebrate host, but occasional positive effects may occur.

Potential impacts of anthropogenic stress on symbiotic bacteria of helminths

The effects of anthropogenic changes to ecosystems and their influence on parasites are well documented (Marcogleise, Reference Marcogliese2001; Vidal-Martinez et al., Reference Vidal-Martinez, Pech, Sures, Purucker and Poulin2010). Various forms of pollution, including chemicals, metals and pharmaceuticals, such as antibiotics, may impair helminth functional biology (Riggs et al., Reference Riggs, Lemly and Esch1987; Sebelova et al., Reference Sebelova, Kuperman and Gelnar2002; Morley, Reference Morley2009, Reference Morley2012). Similarly, environmental stress caused by extreme changes in temperature, pH and salinity may also affect helminths (Möller, Reference Möller1978; Mills, Reference Mills1980; MacLeod & Poulin, Reference MacLeod and Poulin2012). Such effects are usually attributed to the direct action of the stressor on the helminth's biology. Yet, there is rarely supporting evidence for such assumptions and it is not unreasonable, given the key role symbiotic bacteria play in the biology of certain helminths, that in some cases a decline in helminth health may be indirect, associated with a change in the composition or function of their bacterial community.

For example, the selective occurrence of symbiotic bacteria in certain parasite species under polluted conditions may affect xenobiotic accumulation, especially metals. Increased metal accumulation in helminths compared to their fish hosts has been a characteristic result of numerous studies of host–parasite associations in polluted environments (Sures, Reference Sures2003). Although the exact mechanisms causing this have yet to be fully determined for most species, the ability of bacteria to accumulate metals may play a role in helminth bioaccumulation as it does for free-living invertebrates. Complex exposure to multiple stressors, including inorganic and organic pollutants, temperature or pH, may alter helminth bacterial communities, compromising their abilities to accumulate metals. Similarly, pollution exposure or environmental stress may change the bacterial community of vertebrate intestines, either in density or species composition (Syvokienë & Mickënienë, Reference Syvokienë and Mickënienë1999; Izvekova et al., Reference Izvekova, Izvekov and Plotnikov2007). Such changes may impact a helminth's ability to establish and develop in a host that no longer contains the necessary symbiotic bacteria for optimal functioning, or contains an elevated abundance of pathogenic bacteria, thereby altering the prevalence and intensity of parasites within the vertebrate populations. Certainly, for entomopathogenic nematodes, an elevated abundance of pathogenic bacteria within the insect host results in competition with the helminth's symbiotic bacteria for nutrients, thereby affecting the worm's development (Poinar et al., Reference Poinar, Thomas and Lighthart1990).

Nevertheless, the most obvious influence that changes in ecosystem health may bring is between those helminths that possess symbiotic bacteria and those that apparently do not. The presumed competitive advantages of symbionts may be diminished or completely eliminated if the bacteria are more vulnerable to degraded conditions than the host helminth. With the functional biology of these helminths compromised, changes in the composition of parasite communities may occur, as non-symbiont-using helminths are able to take advantage and increase in occurrence. Such a scenario may be more likely under conditions of chemical toxicity, such as antibiotic pollution, where bacteria are known to be significantly affected (Morley, Reference Morley2009). This may be particularly relevant to vertebrate intestinal microbial communities, which appear to be the source of many symbiotic bacteria for helminths that reside in this organ.

However, this situation can only apply if the symbiosis is inflexible or highly specialized towards a single microbe and therefore unable to adapt to environmental change. Nevertheless, studies on entomopathogenic nematodes and their single-species bacterial symbionts suggests that parasites may coevolve in response to thermal environments, as gradual improvements in nematode virulence and establishment at high temperatures may be associated with a greater growth rate of their symbiotic bacteria, due to selection for thermophilic mutants (Grewal et al., Reference Grewal, Gaugler and Shupe1996).

Many naturally occurring symbioses are, however, typically complex, with multiple symbionts exhibiting coordinating, competing or independent influences on host physiology, or in which individual symbionts affect multiple interacting hosts. Such diversity has an advantage in it may allow an organism to associate preferentially with different symbionts that demonstrate optimal performance under diverse environmental conditions, such as temperature, as illustrated by sea anemones and their symbionts (Hussa & Goodrich-Blair, Reference Hussa and Goodrich-Blair2013). Helminths, particularly those residing in the microbe-rich intestines of vertebrates or as ectoparasites, may also demonstrate symbiont-switching as environmental conditions change. Alternatively, multiple symbiont carriage may be maintained due to differences in temperature tolerance or other abiotic factors between the symbionts. Thus symbionts may exhibit redundancy in their contributions to the host under certain conditions, only becoming necessary when environmental circumstances change. Such a mechanism may explain seasonal changes in symbiont predominance and the avoidance of direct competition between bacteria (Hussa & Goodrich-Blair, Reference Hussa and Goodrich-Blair2013).

Changes in symbiotic bacterial community composition due to environmental stressors are likely to lead to significant changes in microbial function, with potentially significant implications for helminth functional biology. Any shift away from a symbiont community towards one composed of bacterial pathogens (e.g. Webster et al., Reference Webster, Cobb and Negri2008) may ultimately facilitate a decline in helminth health, due to the breakdown of symbiotic functions, increased levels of stress and associated compromised defence mechanisms, and thus a subsequent collapse in their populations on or within free-living hosts.

Concluding remarks

The increasingly recognized importance of helminth parasites to the biology and ecology of their free-living hosts under anthropogenic pressure presents an opportunity to finally address the nature of the bacterial–helminth holobiont across a range of parasite species, and to determine how important the bacteria may be for the health of the parasite. Webster & Blackall (Reference Webster and Blackall2009), in discussing the knowledge gaps in sponge–microbial symbioses, highlighted a number of research areas that required further elucidation. Many of these are relevant to helminth–bacterial interactions. As such, these focal points should also be key future research areas for ecological helminthology and include:

1. 1 The significance of microbial associations to helminth health, particularly by enhancing our understanding of the specific functions of helminth symbionts and their relationship to other ‘symbiotic’ processes, such as disease. In particular, the critical functional aspects and cellular and molecular interactions between helminth and symbiont to determine the role bacteria play in the parasite's biology and ecology. By understanding the nature of the communication between the two organisms, this should help determine how the symbiosis occurs and what compels the association to persist, particularly the helminth's ability to make critical distinctions between symbionts, food and pathogens. Such studies would also help to enlighten the existing paradox of those helminths found to associate with bacteria and those that have not; to determine whether associations are mainly facultative or obligatory, restricted to particular helminth class or life stage; if the kinds of free-living host or the location where the helminth resides are influential; or finally the influence of abiotic factors associated with environmental conditions that the helminth life cycle passes through.

2. 2 The consistency and stability of the symbiosis across wide environmental gradients. At this time it is difficult to determine if the uniform microbial community found on multiple sponge species from different oceans (Webster & Blackall, Reference Webster and Blackall2009) is a replicable feature for bacterial–helminth associations. The available data for a number of these associations would suggest not, and that different helminth species and classes interact with differing bacteria. However, as the number of studies undertaken so far have been limited, and did not always attempt to identify specific bacterial symbionts, studies of stability or instability over wider environmental scales are needed, particularly extreme environments such as high-latitude habitats. Correlations of stability with environmental gradients such as temperature, light, salinity or eutrophication would greatly enhance our understanding of the influence of specific symbiotic associations on helminth fitness.

3. 3 The sensitivity of symbiosis to particular kinds of stress. As documented in the present article, this has been actively studied for sponges and corals, and the potential role of symbiotic bacteria in helminth responses to stress has been evaluated. However, studies are now needed to examine any mechanisms linking symbiotic community structure to helminth fitness under specific degraded environment scenarios. In addition, there is no knowledge as to the adaptive capacity of bacteria–helminth symbioses, and how the resistance of the relationships will be affected by genetic turnover, functional redundancy, expression of stress-regulated genes and lateral gene transfer.

Our understanding of helminth–bacteria interactions remains in its infancy. It continues to be unknown how extensive and influential symbiotic associations in helminths are. Nevertheless, at least some of the physiological changes in helminth–host relationships under pollution, climate change or other anthropogenic factors may be due to an imbalance or inhibition in the symbiotic relationships certain helminths have with bacteria. In conditions of rapidly accelerating environmental change, a mechanistic understanding of the sensitivity of helminth holobionts to environmental stress is therefore urgently required.

Conflict of interest

None.