Outer-membrane damage in sublethally heated Escherichia coli K-12.

Exponentially grown cells of Escherichia coli K-12 heated at 48 degrees C in potassium phosphate buffer at pH 7.0 were structrually injured before death. During heating for 60 min about 20% of the cellular lipopolysaccharide (LPS) was released from the outer membrane into the heating medium. Removal of 30% of the cellular LPS, by washing the cells in buffer containing ethylenediaminetetraacetic acid (EDTA), caused no significant increase in the rate of death and structural injury produced by heating. The addition of EDTA to the heating medium produced only a slight increase in the rate of thermal death but a large increase in the rate of structural injury. By a combination of heating at 48 degrees C and washing with EDTA, a maximum of 50% of the LPS was released from cells. These results taken together suggest that structural injury and loss of LPS are not the direct causes of death. The addition of 5 m M Mg2+ to the heating medium protected the cells from death and structural injury caused by heating at 48 degrees C.