MogR regulates flagellar synthesis, cell motility and biofilm formation in the Bacillus cereus group

Abstract

Many bacterial species move by means of rotating flagella. The flagellum is a complex organelle, built up by dozens of different proteins, and its synthesis is usually tightly regulated by the cell. Motility plays a role mostly in the initial phase of an infection. The combination of motility and chemotaxis gives bacteria the opportunity to detect and search for nutrients and also provide the opportunity to colonize, when the environment is favorable. When the bacteria have found their target, adherence occurs and the initial phase of infection has started. In Listeria monocytogenes, a highly flagellated Gram-positive food-borne pathogen which causes life-threatening infections in humans, flagellar gene expression is down-regulated when the bacteria encounter temperatures above 37°C. This has never been observed before in any bacteria, and at 37°C, rendering the bacteria non-motile. A regulator protein, MogR, has been identified that directly binds to the flaA promoter region and thus functions as a repressor of motility gene expression. It is suggested that MogR, located immediately upstream of the motility gene cluster in L. monocytogenes, is essential for temperature-dependent transcription of the motility genes. The Bacillus cereus group is the only other set of organisms known to encode MogR homologs except for L. monocytogenes. Therefore, it has been speculated that organisms in the Bacillus cereus group and Listeria share a novel regulatory mechanism for flagella production, where MogR functions as a master regulator to control expression of flagellar motility genes. Bacillus anthracis, Bacillus cereus and Bacillus thuringiensis are members of the Bacillus cereus (sensu lato) group within the gender Bacillus. The bacteria are Gram-positive, rod shaped and spore-forming. While B. anthracis is well known for causing the lethal disease anthrax and has been used as a biological warfare weapon, B. cereus is an important source of food poisoning and a growing problem for the dairy industry, while B. thuringiensis has been used worldwide as a biopesticide. Results from the current thesis show that a MogR overexpression strain becomes non-motile at 37C after observing motility during growth curves, and a larger number of genes, 109 in total, were down-regulated in the MogR overexpression strain by microarray analysis. The results from the microarray analysis showed that a total of 26 genes, including mogR itself, were upregulated in the MogR overexpression strain as compared with a vector control strain. MogR was shown to positively affect the transcription of a gene encoding for a predicted collagen adhesion protein, an ortholog to B. cereus ATCC 14579 BC1060 which is located upstream of a riboswitch, that gets positively regulated upon binding of c-di-GMP, a well-known second messenger molecule. Also, an ortholog to B. cereus ATCC14579 BC0422, annotated as a methyl-accepting chemotaxis protein (located upstream of a c-di-GMP ”off” riboswitch) was down-regulated in the MogR overexpression strain. Results from biofilm assays indicate that the MogR overexpression strain also produce significantly more biofilm than the empty vector control strain, particularly at 37°C.