R. Tixador

Growth and division of Escherichia coli under microgravity conditions.

The growth rate in glucose minimal medium and time of entry into the stationary phase in pepton cultures were determined during the STS 42 mission of the space shuttle Discovery. Cells were cultured in plastic bags and growth was stopped at six different time points by lowering the temperature to 5 degrees C, and at a single time point, by formaldehyde fixation. Based on cell number determination, the doubling time calculated for the flight samples of glucose cells was shorter (46 min) than for the ground samples (59 min). However, a larger cell size expected for more rapidly growing cells was not observed by volume measurements with the electronic particle counter, nor by electron microscopic measurement of cell dimensions. Only for cells fixed in flight was a larger cell length and percentage of constricted cells found. An optical density increase in the peptone cultures showed an earlier entry into the stationary phase in flight samples, but this could not be confirmed by viability counts. The single sample with cells fixed in flight showed properties indicative of growth stimulation. However, taking all observations together, we conclude that microgravity has no effect on the growth rate of exponentially growing Escherichia coli cells.

Preliminary results of Cytos 2 experiment.

Cytos 2 experiment, carried out during the French-Soviet manned flight (July 1982), has studied the antibiotics sensitivity of bacteria cultivated in vitro during the orbital flight. The results show an increase of the antibiotics resistance and a larger thickness of the cellular envelope for the inflight cells. The increase of antibiotics resistance can be related to a stimulating effect of space on the cell growth rate or to changes of the cellular envelope structure.

Effects of microgravity and hypergravity on the cell: investigations on Paramecium tetraurelia.

Previous space CYTOS experiments have shown that space flights resulted in an increase in growth of Paramecia cultures. Microgravity is the major factor responsible of this response: indeed the stimulatory effect disappeared in inflight cultures placed on a 1 g centrifuge aboard the Spacelab. On the other hand, exposure to different levels of hypergravity on Earth resulted in an opposite response, i.e. to a reduced cell growth rate. A possible mechanism of microgravity on paramecia is discussed.

Effects of Very Low Doses of Ionizing Radiation on the Clonal Life-span in Paramecium Tetraurelia

Effects of very low doses of ionizing radiation were investigated on life-span in Paramecium tetraurelia. After autogamy single cell cultures were place (1) in a control chamber with radiopermeable walls; (2) in a shielded device with 10 cm thick lead walls; (3) in a similar shielding device including a 60Co source giving a dose of 760 mrad per year at the culture level. Life-spans, expressed in days and in mean total number of fissions measured from autogamy to the death of all sublines, were about 200 fissions and 55 days in controls. Life-spans of shielded sublines were increased and lower life-spans were observed in both shielded and irradiated sublines. These findings provide new evidence for a biological effect of very low doses of ionizing radiations and show that fission potential of Paramecium tetraurelia can be affected by variations in the background radiation level.

Respective role of microgravity and cosmic rays on Paramecium tetraurelia cultured aboard Salyut 6

Paramecium tetraurelia cultured aboard Salyut 6 have shown in increase in cell growth rate, cell volume, water content and changes in electrolyte content. Additional experiments, carried out in balloon flight and on earth, showed that the stimulating effect observed on cell proliferation is related to exposure to cosmic rays. Other changes seem to be due to a direct effect of microgravity on cell. Mechanism of gravity action on cell is discussed.