Microgravity Alters the Physiological Characteristics of Escherichia coli O157:H7 ATCC 35150, ATCC 43889, and ATCC 43895 under Different Nutrient Conditions

The aim of this study is to provide understanding of microgravity effects on important food-borne bacteria, Escherichia coli O157:H7 ATCC 35150, ATCC 43889, and ATCC 43895, cultured in nutrient-rich or minimal medium. Physiological characteristics, such as growth (measured by optical density and plating), cell morphology, and pH, were monitored under low-shear modeled microgravity (LSMMG; space conditions) and normal gravity (NG; Earth conditions). In nutrient-rich medium, all strains except ATCC 35150 showed significantly higher optical density after 6 h of culture under LSMMG conditions than under NG conditions (P < 0.05). LSMMG-cultured cells were approximately 1.8 times larger than NG-cultured cells at 24 h; therefore, it was assumed that the increase in optical density was due to the size of individual cells rather than an increase in the cell population. The higher pH of the NG cultures relative to that of the LSMMG cultures suggests that nitrogen metabolism was slower in the latter. After 24 h of culturing in minimal media, LSMMG-cultured cells had an optical density 1.3 times higher than that of NG-cultured cells; thus, the higher optical density in the LSMMG cultures may be due to an increase in both cell size and number. Since bacteria actively grew under LSMMG conditions in minimal medium despite the lower pH, it is of some concern that LSMMG-cultured E. coli O157:H7 may be able to adapt well to acidic environments. These changes may be caused by changes in nutrient metabolism under LSMMG conditions, although this needs to be demonstrated in future studies.