Theses

Date of Award

2020

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biological Sciences

First Advisor

Haiping Cheng

Second Advisor

Joseph Rachlin

Third Advisor

Julio Gallego-Delgado

Abstract

Sinorhizobium meliloti is a nitrogen-fixing flagellated soil bacterium that engages in a mutual symbiotic relationship with the legume Medicago sativa. Using its flagella, S. meliloti is chemotactic in response to varying environmental cues thereby directed to a target, which is typically the root hairs of M. sativa. Lu et al, 2012 suggested that a signal from the plant can switch S. meliloti from a free-living flagellated microbe to a host-invading non-flagellated microbe, although this signal has not been identified. This switch is thought to occur inside curled root hairs of the plant, and after becoming non-flagellated, S. meliloti starts colonizing. This switch is attributed to the ExoS/ChvI two-component system (TCS), which is composed of a membrane sensor ExoS and a response regulator ChvI. Bacteria belonging to the alphaproteobacteria family have proteins pertaining to the TCS that are homologous, and the findings done in this research can have synonymous implications with other bacteria within the same family. In this research, S. meliloti has been shown to make the switch from swimming to non-swimming in media containing serum, where the swimming could not be recovered with increased calcium. Media made with fractionated serum showed swimming colonies in plates containing small molecules, and a complete suppression of swimming in plates containing large molecules. The swimming seen in whole serum could be suppressed with an ion chelator, although chelation could not completely suppress swimming in plates made with the flow-through of fractionated serum. Increasing glucose and albumin concentration had no effect on S. meliloti motility. With this, the H1 hypothesis states S. meliloti will provide an observable change in phenotype from swimming to non-swimming in the presence of serum, and the H0 hypothesis states that there will not be an observable change in phenotype from swimming to non-swimming in the presence of serum.

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