In vitro cultivation of cells and a microsporidian parasite of Biomphalaria glabrata (Pulmonata: Basommatophora)

An optimized protocol for routine development of cell culture from adult oyster, Crassostrea madrasensis

Marine molluscan cell lines, required for virus screening and cultivation, form essential tools for developing health management strategies for these animals in the blue economy. Moreover, they are also crucial to develop cultivated seafood. As there is no valid marine molluscan cell line, primary cell cultures are relied upon for all investigations. A sound protocol for generating primary cell cultures from molluscs is entailed, but existing protocols often involve heavy antibiotic usage and depuration that invariably affect gene expression and cell health. This work presents an easy-to-adopt, time-saving protocol using non-depurated mollusc Crassostrea madrasensis, which requires only initial antibiotic treatment and minimal exposure or no use of antibiotics in the cell culture medium. The important experimental considerations for arriving at this protocol have been elucidated. Accordingly, sodium hypochlorite and neomycin sulfate were chosen for disinfecting tissues. The study is the first to use shrimp cell culture medium (SCCM) as a cell culture medium for molluscan cell culture. Despite being osmoconformers, the oysters exhibited stable intracellular osmotic conditions and pH, which, when provided in vitro, promoted effective cardiomyocyte formation. The cell viability could be enhanced using 10% fetal bovine serum (FBS), but healthy cell culture could also be obtained using SCCM without FBS. The optimized culture conditions allowed for regular beating cardiomyocyte clusters that could be retained for a month. Limited cell proliferation, as shown by the BrdU assay, demands further interventions, such as possibly producing induced pluripotent stem cells. The optimized protocol and culture conditions also align with some requirements for producing cultivated meat from marine molluscs.

Status in molluscan cell line development in last one decade (2010–2020): impediments and way forward

Despite the attempts that have started since the 1960s, not even a single cell line of marine molluscs is available. Considering the vast contribution of marine bivalve aquaculture to the world economy, the prevailing viral threats, and the dismaying lack of advancements in molluscan virology, the requirement of a marine molluscan cell line is indispensable. This synthetic review discusses the obstacles in developing a marine molluscan cell line concerning the choice of species, the selection of tissue and decontamination, and cell culture media, with emphasis given on the current decade 2010-2020. Detailed accounts on the experiments on the virus cultivation in vitro and molluscan cell immortalization, with a brief note on the history and applications of the molluscan cell culture, are elucidated to give a holistic picture of the current status and future trends in molluscan cell line development.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00539-x.

Molluscan cells in culture: primary cell cultures and cell lines

In vitro cell culture systems from molluscs have significantly contributed to our basic understanding of complex physiological processes occurring within or between tissue-specific cells, yielding information unattainable using intact animal models. In vitro cultures of neuronal cells from gastropods show how simplified cell models can inform our understanding of complex networks in intact organisms. Primary cell cultures from marine and freshwater bivalve and gastropod species are used as biomonitors for environmental contaminants, as models for gene transfer technologies, and for studies of innate immunity and neoplastic disease. Despite efforts to isolate proliferative cell lines from molluscs, the snail Biomphalaria glabrata Say, 1818 embryonic (Bge) cell line is the only existing cell line originating from any molluscan species. Taking an organ systems approach, this review summarizes efforts to establish molluscan cell cultures and describes the varied applications of primary cell cultures in research. Because of the unique status of the Bge cell line, an account is presented of the establishment of this cell line, and of how these cells have contributed to our understanding of snail host-parasite interactions. Finally, we detail the difficulties commonly encountered in efforts to establish cell lines from molluscs and discuss how these difficulties might be overcome.

Morphological and molecular biological characterization of Pleistophora aegyptiaca sp. nov. infecting the Red Sea fish Saurida tumbil

One hundred three out of 225 (45.8%) of the Red Sea fish Saurida tumbil were infected with microsporidian parasites. The infection was recorded as tumor-like masses (whitish macroscopic cysts) or xenomas often up to 2 cm in diameter and embedded in the peritoneal cavity. Generally, the infection was increased during winter 63.8% (86 out of 135) and fall to 18.9% (17 out of 90) in summer. Light microscopic study revealed that xenomas were encapsulated by a fibrous layer encircling numerous sporophorous vesicles filled with mature spores measuring 1.7 ± 0.6 (1.5-2.7 μm) × 1.5 ± 0.3 μm (1.2-1.8 μm) in size. Ultrastructural microscopic study showed the presence of smooth membranes of the sarcoplasmic reticulum forming a thick, amorphous coat surrounding various developmental stages of the parasite. The various recognizable stages of the parasite were uninuclear, binucleated, and multinucleated meronts followed by detachment of the plasmalemma of the sporont from the sporophorous vesicle producing sporoblasts. Mature spores consist of a spore coat and spore contents. The spore contents consist of the uninucleated sporoplasm and a posterior vacuole located at the posterior end. The polar tube consists of a straight shaft and a coiled region (26-32 coils) arranged in many rows along the inside periphery of the spore. The polaroplast consisted of an anterior region of closely and loosely packed membranes. Molecular analysis based on the small subunit rDNA gene was performed to determine the phylogenetic position of the present species. The percentage identity between this species and a range of other microsporidia predominantly from aquatic hosts demonstrated a high degree of similarity (>92%) with eight Pleistophora species. Comparison of the nucleotide sequences and divergence showed that the sequence of the present microsporidium was most similar to that of Pleistophora anguillarum (99.8% identity) differing in 13 nucleotide positions. So, the present species was recorded and phylogenetically positioned as a new species of Pleistophora.

An evaluation of protozoal characteristics in relation to biological control of pests

Of the unicellular eukaryotes, formerly Protozoa, now considered to belong to five separate phyla, only the neogregarines and microsporidia are serious contenders for a role in biological control of invertebrate pests. Ciliates of the genus Lambornella, which penetrate their hosts via cuticular cysts, have potential in mosquito control but have not been investigated in depth.

‘Protozoa’ generally kill their hosts by overwhelming numbers, destroying the normal function of organs or depleting the host of essential reserves. Because they are slow-acting pathogens they cannot be used on their own when pests have already reached a high level of abundance nor can they be relied upon when the damage threshold of pests is low. Their principal use will be as the slow-acting component of a 2-pathogen or pathogen-plus-chemical formulation, which can be used when a degree of damage is tolerable.

A comparison is made between two microsporidia in lepidopteran hosts, Vairimorpha necatrix and Nosema pyrausta. The former causes high mortality in a wide range of hosts, when bacterial septicaemia ensues after disruption of the gut wall by the microsporidian invasion process. Some larvae may survive this period and live to damage crops, but none survives to adulthood. There is no transovarial transmission and the parasite is rarely found in natural populations. V. necatrix could be used as a microbial pesticide for short-term control. N. pyrausta is restricted to a single host, the European corn borer. It has low pathogenicity, causing some larval mortality especially under conditions of environmental stress. Most hosts survive to adults but show reduced longevity and fecundity. The parasite is transmitted transovarially and is highly prevalent in the field. It is not considered pathogenic enough to be used as a microbial pesticide but is an important factor in regulating natural populations. These examples illustrate the inverse relationship between pathogenicity and prevalence and show how cycles of host population abundance may be driven by pathogens of moderate to low pathogenicity.

Two kinds of transovarial transmission mechanisms are discussed. With the microsporidia of winter moth, vegetative stages and spores, even when abundant in egg yolk, do not gain access to larval tissues but are confined to the meconium in larvae just before eclosion. Larvae are not infected when they hatch but the spores are carried over in the eggs to the next period of larval feeding activity. In contrast, some genera of microsporidia in haematophagous diptera, e.g. Amblysopora in mosquitoes, actually infect the cells of developing larvae, which are already infected when they hatch.

The prospects for biological control with ‘Protozoa’ are reviewed for vectors of medical importance and for pests of pasture, field crops, forests and stored products. Particular attention is given to the use of microsporidia in combination with low concentrations of compatible chemical insecticides and with other pathogens (e.g. viruses).

Spores for field application can be produced in natural or experimental hosts by feeding or intrahaemocoelic inoculation. Yields vary according to the species of parasite and host. Examples are Nosema locustae in Melanoplus bivittatus yielding 3·9 × 109 spores/grasshopper enough to treat more than 1 hectare of rangeland, and Vairimorpha necatrix in Heliothis zea, yielding 1·67 × 109 spores per larva, with 2·5 × 1012 spores/hectare required for field application. In vitro culture is at present a laboratory tool only, with yields too low for economic returns.

Spores can be stored, according to species, dry or in distilled water with antibiotics at 4 °C. This gives good survival for months or years. In field applications feeding-bait formulations are more efficacious than sprays because they concentrate the spores for uptake by the target species and give the spores some protection from harmful ultraviolet radiation.

Pheromone lures have been used for the introduction of spores by males into pest infestations in stored grain. Males are lured to sites dusted with spores and return to the grain after removal of the lure, to contaminate females and larvae. The use of these lures, first as traps to monitor pest population increases, then to effect a controlled pest growth curve by introduction of pathogens, is an attractive innovation. Protozoa are considered safe for field application on the limited evidence available.

Propagation of Nosema eurytremae (Microsporida: Nosematidae) from trematode larvae, in abnormal hosts and in tissue culture

Propagation of Nosema eurytremae, a microsporidian pathogen of trematode larvae, was investigated by inoculation of spores into the haemocoele of insects and by growth in tissue cultures. Locusts and the larvae of three lepidopteran species were good hosts but cockroaches were not. Low replication was obtained in one lepidopteran species after per os infection. Antibiotics controlled bacterial growth in suspensions of microsporidian spores but fungi were unaffected by all antibiotics tested, except at concentrations detrimental to the microsporidia. All stages of the microsporidium developed in cell lines of Xenopus laevis and Aedes pseudoscutellaris when spores were induced to hatch in contact with cell monolayers: the Aedes culture was contaminated by yeasts. Repeated washing of the Xenopus cells with fresh medium, after the sporoplasms of N. eurytremae had penetrated the cells, removed yeast contaminants and sterile cultures were obtained. Replication during 4 passages over 53 days was only 100 to 200-fold compared with the original inoculum but spores harvested from the cultures were infective to a fresh culture and to Pieris brassicae by inoculation into the haemocoele.

Molecular data suggest that microsporidian parasites in freshwater snails are diverse

Microsporidian parasites infect almost all invertebrate and vertebrate hosts and have significant effects on individual and population fitness. Phylogenetic analysis demonstrates that the phylum is highly divergent and that some lineages show strong associations with host taxa. We here examine the diversity and distribution of parasites in gastropod molluscs to test for host-parasite co-association. 16 populations representing 10 species of freshwater snails were screened using microsporidian specific small subunit rDNA primers. Four novel microsporidian parasite sequences were detected within populations of three host species from the genera Bulinus, Biomphalaria and Planorbis. Prevalence ranged from 5 to 84%. Phylogenetic analysis of these novel sequences reveals that they group together as a paraphyletic assemblage in the microsporidian tree basal to the two lineages containing the genera Encephalitozoon and Nosema. Preliminary observation of one microsporidian infection, show parasites distributed in all tissue systems of Bulinus globosus. However, infection is most prevalent in the digestive gland while also in the egg sacs, suggesting that the microsporidium is using a mixed strategy of horizontal and vertical transmission in this population.

Molecular identification of symbionts from the pulmonate snail Biomphalaria glabrata in Brazil

The icthyosporean, Capsaspora owczarzaki, a known predator of Schistosoma mansoni sporocysts in vitro, is more prevalent in laboratory-reared strains of the intermediate snail host, Biomphalaria glabrata resistant to S. mansoni, than from the susceptible M line strain. We examined whether B. glabrata resistant to the NIH-PR-1 strain of S. mansoni from 2 regions in Brazil were also host to C. owczarzaki. Symbiont presence was examined using hemolymph culturing and nested polymerase chain reaction of snail genomic DNA with primers designed to specifically amplify sequences from relatives of the Icthyosporea. All B. glabrata of the resistant Salvador strain from the laboratory of Dr. Lobato Paraense in Rio de Janeiro, Brazil (n = 46) tested negative for symbionts. Three of 18 semiresistant 10-R2 B. glabrata from the laboratory of Dr. Barbosa in Recife, Brazil tested positive for C. owczarzaki. Another icthyosporean, Anurofeca sp., was identified from 1, 10-R2 snail and from 2 of 12 field-collected B. glabrata from Praia do Forte Orange, Ilha de Itamaracá. Snails from 2 other sites, Hotel Colibri, Pontezinha and Praia do Sossego, Ilha de Itamaracá, were negative for Anurofeca. Two genera of ciliates were also identified. Paruroleptus sp. was found in 4, 10-R2 snails and Trichodina sp. was identified in 2 field-collected snails from Praia do Forte Orange and Praia do Sossego.