Perkinsus marinus in bioreactor: growth and a cost-reduced growth medium

Perkinsus marinus (Perkinsea) is an osmotrophic facultative intracellular marine protozoan responsible for "Dermo" disease in the eastern oyster, Crassostrea virginica. In 1993 in vitro culture of P. marinus was developed in the absence of host cells. Compared to most intracellular protozoan parasites, the availability of P. marinus to grow in the absence of host cells has provided the basis to explore its use as a heterologous expression system. As the genetic toolbox is becoming available, there is also the need for larger-scale cultivation and lower-cost media formulations. Here, we took an industrial approach to scaled-up growth from a small culture flask to bioreactors, which required developing new cultivation parameters, including aeration, mixing, pH, temperature control, and media formulation. Our approach also enabled more real-time data collection on growth. The bioreactor cultivation method showed similar or accelerated growth rates of P. marinus compared to culture in T-flasks. Redox measurements indicated sufficient oxygen availability throughout the cultivation. Replacing fetal bovine serum with chicken serum showed no differences in the growth rate and a 60% reduction in the medium cost. This study opens the door to furthering P. marinus as a valid heterologous expression system by showing the ability to grow in bioreactors.

ONE-SENTENCE SUMMARY

Perkinsus marinus, a microbial parasite of oysters that could be useful for developing vaccines for humans, has been shown to grow well in laboratory equipment that can be expanded to commercial scale using a less expensive growth formula than usual laboratory practice.

The combined influence of sub-optimal temperature and salinity on the in vitro viability of Perkinsus marinus, a protistan parasite of the eastern oyster Crassostrea virginica

Perkinsus marinus is a major cause of mortality in eastern oysters along the Gulf of Mexico and Atlantic coasts. It is also well documented that temperature and salinity are the primary environmental factors affecting P. marinus viability and proliferation. However, little is known about the effects of combined sub-optimal temperatures and salinities on P. marinus viability. This in vitro study examined those effects by acclimating P. marinus at three salinities (7, 15, 25 ppt) to 10 degrees C to represent the lowest temperatures generally reached in the Gulf of Mexico, and to 2 degrees C to represent the lowest temperatures reached along the mid-Atlantic coasts and by measuring changes in cell viability and density on days 1, 30, 60 and 90 following acclimation. Cell viability and density were also measured in 7 ppt cultures acclimated to each temperature and then transferred to 3.5 ppt. The largest decreases in cell viability occurred only with combined low temperature and salinity, indicating that there is clearly a synergistic effect. The largest decreases in cell viability occurred only with both low temperature and salinity after 30 days (3.5 ppt, 2 degrees C: 0% viability), 60 days (3.5 ppt, 10 degrees C: 0% viability) and 90 days (7 ppt, 2 degrees C: 0.6+/-0.7%; 7 ppt, 10 degrees C: 0.2+/-0.2%).

Determination of the effects of temperature on viability, metabolic activity and proliferation of two Perkinsus species, and its significance to understanding seasonal cycles of perkinsosis

The range of water temperatures in which Perkinsus species can survive and proliferate remains ill-defined, particularly at lower temperatures. The in vitro viability, metabolic activity, and proliferation of 3 isolates each of P. marinus and P. olseni trophozoites at 28°C, and at 15 and 4°C, after transfer from 28°C, were compared. Both species showed declines in metabolic activity and proliferation from 28°C to 15°C. At 4°C, both species had viability after 30 days incubation time (P. marinus 49%, P. olseni 58%), but limited metabolic activity and no proliferation. Perkinsus marinus viability was further compared when transferred directly from 28°C, 18°C and progressively from 18°C (0·5°C/day) to 2, 4 and 6°C and maintained for up to 4 months. Viability was highest under progressive transfer (77% and 54% after 30 and 60 days exposure to test temperatures). The decrease in P. marinus viability at the lower temperatures in vitro only partially explains decreasing parasite infection intensities in eastern oysters in the colder months of the year. Moreover, the significant decrease in parasite infection intensities in late winter and early spring, as temperatures increase, is likely due to an active process of elimination by oyster host defences.

Perkinsus marinus, a protozoan parasite of the eastern oyster, has a requirement for dietary sterols

Perkinsus marinus, a protozoan parasite of the eastern oyster, Crassostrea virginica, causes high mortality in its host along the Atlantic and Gulf coasts of North America. P. marinus meronts cultured in vitro in medium containing complete lipid supplement (cod liver oil, cholesterol and alpha tocopherol acetate in detergent) are able to synthesize a wide variety of lipids, yet cultures cannot be maintained in lipid-free medium. To determine P. marinus lipid requirements meronts were inoculated into media containing different combinations of lipid components in detergent. Treatments included complete lipid supplement (positive control), detergent only (negative control), cholesterol in detergent, alpha tocopherol acetate in detergent and cholesterol+alpha tocopherol acetate in detergent. Meronts proliferated in the positive control medium and media containing cholesterol or cholesterol+alpha tocopherol acetate, but failed to proliferate in the negative control medium and the medium containing just alpha tocopherol acetate. Gas chromatography analysis of P. marinus meronts grown in medium with added (13)C sodium acetate (0.5 mg mL(-1)) revealed the presence of fatty acids containing (13)C, but the only sterol present was cholesterol containing no (13)C. These results suggest that P. marinus cannot synthesize sterols and must sequester them from its host.

Assessment of the Cell Viability of CulturedPerkinsus marinus(Perkinsea), a Parasitic Protozoan of the Eastern Oyster,Crassostrea virginica, Using SYBRgreen-Propidium Iodide Double Staining and Flow Cytometry

A flow cytometry (FCM) assay using SYBRgreen and propidium iodide double staining was tested to assess viability and morphological parameters of Perkinsus marinus under different cold- and heat-shock treatments and at different growth phases. P. marinus meront cells, cultivated at 28 degrees C, were incubated in triplicate for 30 min at -80 degrees C, -20 degrees C, 5 degrees C, and 20 degrees C for cold-shock treatments and at 32 degrees C, 36 degrees C, 40 degrees C, 44 degrees C, 48 degrees C, 52 degrees C, and 60 degrees C for heat-shock treatments. A slight and significant decrease in percentage of viable cells (PVC), from 93.6% to 92.7%, was observed at -20 degrees C and the lowest PVC was obtained at -80 degrees C (54.0%). After 30 min of heat shocks at 40 degrees C and 44 degrees C, PVC decreased slightly but significantly compared to cells maintained at 28 degrees C. When cells were heat shocked at 48 degrees C, 52 degrees C, and 60 degrees C heavy mortality occurred and PVC decreased to 33.8%, 8.0%, and 3.4%, respectively. No change in cell complexity and size was noted until cells were heat shocked at >or=44 degrees C. High cell mortality was detected at stationary phase of P. marinus cell culture. Cell viability dropped below 40% in 28-day-old cultures and ranged 11-25% in 38 to 47-day-old cultures. Results suggest that FCM could be a useful tool for determining viability of cultured P. marinus cells.

Analysis of extracellular proteins of two Perkinsus spp. isolated from the softshell clam Mya arenaria in vitro

Biochemical characterization of the extracellular proteins (ECP) of two softshell clam Perkinsus spp. cloned isolates, Perkinsus chesapeaki isolate G-117 and Perkinsus marinus H-49, was performed and compared to that of the oyster-derived P. marinus isolate P-1. G-117 and H-49 demonstrated distinct differences in enzyme activities; however, all three isolates shared common bands. Substrate-impregnated gels showed H-49 to possess proteolytic activities while G-117 did not. Inhibition studies revealed that H-49 ECP contain serine proteases similar to those described for P-1. The G-117 ECP lacked proteolytic activity but showed a higher production of lipolytic enzymes than H-49 or P-1. Optimal in vitro growth temperatures for the two clam isolates were generally lower than those for P-1. G-117 showed faster growth at lower salinities than either H-49 or P-1. Clam Perkinsus spp. isolates appear to be better adapted to lower salinities and temperatures than the P. murinus isolate of the eastern oyster.