Thursday, 10 May 2018 15:10

Losing a Function Helps Enterococcus faecalis Increase its Fitness

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Published in mBiosphere

Normally, you might assume the ability to consume more varieties of food would be a fitness benefit for a microbe, especially when one considers our commensal microbiota, which are exposed to a wide variety of our omnivorous diet. A new mBio report suggests the opposite: that the commensal bacteria Enterococcus faecalis increases its fitness by ditching one of its metabolic pathways.


mBio: Loss of ethanolamine utilization in Enterococcus faecalisincreases gastrointestinal tract colonization


The study examined the ability of the gut bacterium E. faecalis to use ethanolamine as a nutrient source. Ethanolamine, which can provide both carbon and nitrogen, is found in high concentrations throughout the gut, and promotes growth of foodborne pathogens such as Salmonella. First author Karan Kaval and senior scientist Danielle Garsin worked with a scientific team to generate E. faecalis mutants that can’t detect or digest ethanolamine. They hypothesized that these strains would be less fit and therefore less able to grow in the gastrointestinal tracts of mice.


CFU plots of wild-type or ethanolamine non using E. faecalis that shows a larger proportion of mutants colonize the mouse gutCompetitive colonization of the gastrointestinal tract by the E. faecalis wild type and mutant strains following combined inoculation. Percentages of CFU of E. faecalis strains from the initial inoculum mix and from stool samples collected 4 h and 1, 2, and 3 days after mixed inoculation of wild-type E. faecalis and the ΔeutVW mutant (AR2) that doesn't detect ethanolamine. Source.

Surprisingly, the E. faecalis that couldn’t digest ethanolamine were able to grow better in mice than those that could digest it. Both ethanolamine-digesting and -nondigesting strains were orally inoculated into mice, where they passed into the gut for colonization. Colonization was measured by plating bacteria from mouse fecal pellets and analyzing the number of colony forming units of each strain at 4 hours, 1 day, 2 days, and 3 days after feeding. Inoculating both strains simultaneously forces them to compete for the limited space available in the mouse gut; in all experiments, the scientists observed a higher proportion of the E. faecalis strain that didn’t use ethanolamine (see figure, right), suggesting this strain was better able to colonize and replicate in the gut.


The reason for the surprise is because ethanoloamine use by other organisms, including SalmonellaEscherichia coli, and Clostridiodes difficile, confers either a fitness advantage or equal fitness, which means mutants colonize at lower numbers than wild-type strains. The difference may be due to lifestyle, since the bacteria that benefit from ethanolamine use are pathogens and E. faecalis is a commensal (though it can be an opportunistic pathogen). The research team plans to investigate the underlying reason for this difference in their future experiments. 


E. faecalis SEM image


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Last modified on Thursday, 10 May 2018 15:57
Julie Wolf

Julie Wolf is the ASM Science Communications Specialist. She contributes to the ASM social media and blog network and hosts the Meet the Microbiologist podcast. She also runs workshops at ASM conferences to help scientists improve their own communication skills. Follow Julie on Twitter for more ASM and microbiology highlights at @JulieMarieWolf.

Julie earned her Ph.D. from the University of Minnesota, focusing on medical mycology and infectious disease. Outside of her work at ASM, she maintains a strong commitment to scientific education and teaches molecular biology at the community biolab, Genspace. She lives in beautiful New York City.