#FEMSmicroBlog: Sending out unique membrane packages

14-05-2021

Many Gram-positive and Gram-negative bacteria produce and send out membrane vesicles to facilitate communication, antimicrobial resistance or host immune evasion. In Enterococcus faecalis, membrane vesicle formation has not been previously observed. Now, the study “The composition and function of Enterococcus faecalis membrane vesicles” published in microLife, describes for the first time membrane vesicles produced by this opportunistic pathogen. Sarah Wettstadt explains for the #FEMSmicroBlog how the unique content of E. faecalis membrane vesicles could play a role in the virulence of this Gram-positive bacterium. #FascinatingMicrobes

 

Bacteria produce and send out membrane vesicles

Bacterial membrane vesicles represent a specialised secretion pathway since bacteria use them to transport cargo molecules like proteins, DNA, RNA or antibiotics. In Gram-negative bacteria, membrane vesicles derive from the outer membrane which is why they contain large amounts of outer membrane proteins and specific lipid compositions.

On the contrary, Gram-positive bacteria lack an outer membrane. Hence, some studies suggest that the thick peptidoglycan layer could impact membrane vesicle formation. While dying Gram-positive bacteria can form membrane vesicles, a conservative blebbing mechanism was also hypothesised.

Interestingly, Gram-positive bacteria like Streptococcus mutans, Clostridium perfringens and Bacillus subtilis fill their membrane vesicles with different cargoes. These can include extracellular DNA, toxins and immunity proteins, while their roles are not always clear.

 

The antibiotic resistance challenge of Enterococcus faecalis

For the Gram-positive Enterococcus faecalis membrane vesicle formation has not been described yet. The study “The composition and function of Enterococcus faecalis membrane vesicles” published in microLife embarked on this journey and observed for the first time membrane vesicles in this opportunistic pathogen.

E. faecalis lives harmlessly in the gastrointestinal tract of many people. However, it can spread to other parts of the body and cause life-threatening infections like bacteraemia and endocarditis. Especially in hospital settings, infections with this pathogen are increasingly difficult to cure since it produces biofilm and frequently acquires antibiotic resistance genes.

How this bacterium acquires new genes from the environment is still not well understood. But in Gram-negative bacteria, outer membrane vesicles often contain chromosomal, plasmid or viral DNA. Hence, this study hypothesised that membrane vesicles might be involved in transferring plasmids and transposons in E. faecalis.

While chromosomal DNA was indeed present within the membrane vesicles, plasmid DNA only co-purified with them. However, the bacteria did not transfer the plasmid DNA into neighbouring bacteria in this study.

 

Enterococcus faecalis produces unique membrane vesicles

Using a proteomic approach, the study found that E. faecalis enriches its membrane vesicles with unsaturated lipids and septal proteins. From these findings and by following live imaging, the study suggested that E. faecalis forms its membrane vesicles from the septum of the cell. Here, the cell wall is thinnest and more flexible facilitating the blebbing process.

Model for how Enterococcus faecalis forms membrane vesicles
Septal model for membrane vesicle formation in E. faecalis. From Afonina et al. (2021).

The study further aimed to elucidate the role of membrane vesicles in immune response modulation and showed that membrane vesicles from E. faecalis activate NF-κB signaling in macrophages. With additional transmission electron microscopy, the study identified bacteriophage tails within the membrane vesicle fraction that were absent in whole-cell samples. While other studies demonstrated that bacteriophages are involved in triggering an immune response, this was not the case in E. faecalis.

Hence, the study hypothesises that the content of the membrane vesicles – and not the phages – activates the host inflammatory response. Yet, it remains an interesting question what the role of the bacteriophage tails is within the membrane vesicles.

The content of Enterococcus faecalis membrane vesicles activates the host inflammatory response.

For the first time, this study shed light on how E. faecalis uses membrane vesicles to modulate virulence. As such, these results might pave the way towards novel treatment strategies for enterococcal infections.

Read the article “The composition and function of Enterococcus faecalis membrane vesicles” published in microLife by Afonina et al. (2021).

 

About the author of this blog

headshot of Dr Sarah Wettstadt: science writer and science communicatorDr Sarah Wettstadt is a microbiologist-turned science writer and communicator working on various outreach projects and helping researchers talk and write about their scientific results. Her overall vision is to empower through learning: she shares scientific knowledge with both scientists and non-scientists and coaches scientists in writing about their research. Sarah is blog commissioner for the FEMSmicroBlog and was a social media editor for FEMS for 1.5 years. Previous to her science communication career, she worked as a postdoc in Marían Llamas’ lab on Pseudomonas aeruginosa’s ability to use heterologous iron sources and completed her PhD with Alain Filloux investigating the type 6 secretion system in Pseudomonas aeruginosa.

About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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