The influence of spaceflight and simulated microgravity on bacterial motility and chemotaxis

Abstract

As interest in space exploration rises, there is a growing need to quantify the impact of microgravity on the growth, survival, and adaptation of microorganisms, including those responsible for astronaut illness. Motility is a key microbial behavior that plays important roles in nutrient assimilation, tissue localization and invasion, pathogenicity, biofilm formation, and ultimately survival. Very few studies have specifically looked at the effects of microgravity on the phenotypes of microbial motility. However, genomic and transcriptomic studies give a broad general picture of overall gene expression that can be used to predict motility phenotypes based upon selected genes, such as those responsible for flagellar synthesis and function and/or taxis. In this review, we focus on specific strains of Gram-negative bacteria that have been the most studied in this context. We begin with a discussion of Earth-based microgravity simulation systems and how they may affect the genes and phenotypes of interest. We then summarize results from both Earth- and space-based systems showing effects of microgravity on motility-related genes and phenotypes.

Fig. 1. Ground-based systems for simulating microgravity.

HARV, STLV, and RWPV images from Synthecon, Houston, TX with permission from Bill Anderson. RPM image reprinted from Wuest et al. with no modification under the CC BY 3.0 License (https://creativecommons.org/licenses/by/3.0/).

Fig. 2. 3D orientation of a rotating wall vessel (RWV) with the direction of the gravity vector included.

Left: Simulated microgravity showing a typical microbe path with gravity vectors canceling in the completely vertical orientation. Right: Normal gravity control with a horizontal orientation.

Fig. 3. Simulated shear stress values along the walls in an RPM.

Shear stress values vary throughout with no clear steady state. Reprinted from Wuest et al. with no alterations under the CC BY 4.0 License (https://creativecommons.org/licenses/by/4.0/).

Fig. 4. HARV bioreactors injected with crystal violet.

(Left) Under simulated microgravity, the dye stays along the outer wall before gradually migrating inward. (Right) The dye spread under normal gravity. Reprinted from Crabbe et al. with permission from John Wiley and Sons with no alterations.

Fig. 5. Motility and chemotaxis gene expression categorized by study and organism.

Color indicators: Green: upregulation, Yellow: no change reported, Red: downregulation. Dashes indicate no data available. Normal Gravity Rotation control is represented by NG.

Fig. 6. Bacterial flagellar motor (E. coli) with associated protein components.

Eco02040 reprinted with modification under the CC BY 4.0 License (https://creativecommons.org/licenses/by/4.0/) based on KEGG, Kanehisa et al..

Fig. 7. Thin-section transmission electron microscopy images of E. coli.

Left: Sample cultures on Earth. Right: Sample cultured in space exhibiting an irregular cell shape. Red arrows indicate extracellular vesicles. Images taken with a Phillips CM 100 TEM at an accelerating voltage of 80 kV. Figure and modified caption reprinted from Zea et al. under the CC BY 4.0 License (https://creativecommons.org/licenses/by/4.0/).

Fig. 8. Heat map depicting the clustering patterns of the eight treatments by KEGG pathways associated with the proposed function of V. fischeri genes at 12 and 24 h.

Gene changes governing flagellar assembly and bacterial chemotaxis labeled at the top. Colors represent the differential abundance of individual genes listed by V. fischeri identification number (VF-ID) for both wild type (WT) and Δhfq mutants under simulated microgravity (M) and normal gravity (G) conditions. Figure and modified caption reprinted from Duscher et al. under the conditions of the CC BY 4.0 License (https://creativecommons.org/licenses/by/4.0/).