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Peptidoglycan remodeling and conversion of an inner membrane into an outer membrane during sporulation

I just finished listening to the TWiM 35 on LPS in Vibrio (among other topics). Dr. Elio Schaechter mentioned a field in Microbiology that I think is of great interest to the scientific community and should definitely be covered in a podcast. The topic is: Outer Membrane Biogenesis in Gram-negative bacteria. For decades people have been hypothesizing and trying to find an evolutionary link that would answer the following questions: how did a second membrane in bacteria come about and was the last universal common ancestor (LUCA) a single or double-membraned organism. These questions are extremely difficult to address and rely heavily on bioinformatics and phylogenetic analysis.

I'm the first author of the paper that I'm suggesting here as a resource for a potential podcast. I've read extensive amounts of scientific literature before the paper was finalized and published in Cell to realize that currently, there are no favored hypotheses and the very few hypotheses that exist are highly controversial. In our paper we structurally characterize (using electron cryo-tomography) the process of endospore formation. The process is typically thought of as exclusive to Gram-positive bacteria members of the phylum Firmicutes, however, we imaged a Gram-negative organism (Acetonema longum) that is also able to sporulate! Through our structural studies, phylogenetic profiling and biochemical analysis we showed that A. longum possesses a true outer membrane. Not only that, after sporulation, the spore is surrounded by two membranes both of which originated from the inner membrane of the mother cell. Upon outgrowth, the second membrane of the spore becomes the outer membrane of the bacterium and therefore is remodeled from and inner into and outer membrane. These are fascinating new results that may provide us with a missing link between Gram-positive and Gram-negative bacteria and give insights into how an outer membrane may have evolved.

 
 

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