Effect of simulated microgravity on E. coli K12 MG1655 growth and gene expression

Abstract

This study demonstrates the effects of simulated microgravity on E. coli K 12 MG1655 grown on LB medium supplemented with glycerol. Global gene expression analysis indicated that the expressions of hundred genes were significantly altered in simulated microgravity conditions compared to that of normal gravity conditions. Under these conditions genes coding for adaptation to stress are up regulated (sufE and ssrA) and simultaneously genes coding for membrane transporters (ompC, exbB, actP, mgtA, cysW and nikB) and carbohydrate catabolic processes (ldcC, ptsA, rhaD and rhaS) are down regulated. The enhanced growth in simulated gravity conditions may be because of the adequate supply of energy/reducing equivalents and up regulation of genes involved in DNA replication (srmB) and repression of the genes encoding for nucleoside metabolism (dfp, pyrD and spoT). In addition, E. coli cultured in LB medium supplemented with glycerol (so as to protect the cells from freezing temperatures) do not exhibit multiple stress responses that are normally observed when cells are exposed to microgravity in LB medium without glycerol.

Figure 1. Growth of E. coli at 30°C under microgravity conditions in a clinostat (□) and under normal gravity (▪) conditions.

Figure 2. DNA microarray analysis of clinostat-induced gene expression in E. coli.

The Volcano plot depicts gene expression in E. coli culture at 0.8 OD (OD_{600 nm}) cultured in the presence of 10% glycerol under microgravity conditions compared to the control. Genes that are represented on the right side of the volcano-axis are up regulated and those that are on left side of the axis are down regulated. Out of the 4377 genes (O) analysed, 53genes were upregulated (•) and 47were down regulated (•).Only those genes that showed more than 1.5 fold change in expression and a P value <0.05 were identified as either up- or down-regulated. The x-axis represents the fold change and the dark vertical lines represent cut-offs at 1.5 fold decrease and increase. The y-axis represents the p-values and the dark horizontal line indicates a p value cut-off of 0.05.

Figure 3. Effect of microgravity on the expression of genes hyaE, mdtD, srmB, pyrD, rhaD, yicL and ldcC using RNA from E. coli grown in a clinostat (▪)and compared with E. coli grown in normal gravity conditions (□).

The OD of the the cultures was 0.8 (OD_{600 nm}) The P values were pyrD, rhaD (P<0.001), yicL (p<0.05), srmB, hyaE, mdtD and ldcC (P<0.1) respectively.

Figure 4. Distribution of down regulated genes (%) based on biological process classification reported by Gene ontology term functional categories using DAVID version 2.0 software.