Gram negative outer membranes: a missing link found?
Both Gram positive and Gram negative bacteria possess peptidoglycan-based cell walls, but the Gram negative cell wall contains an additional outer membrane composed of phospholipids outside of a thinner layer of peptidoglycan. It is this outer membrane that confers upon Gr(-) bacteria the ability to withstand some antibiotics, as well as gives the bacteria a major virulence trait in the form of lipopolysaccharide (LPS). Where the outer membrane originally came from turns out possibly to be an interesting problem in topology.
Scientists at Caltech recently published a manuscript in the scientific journal Cell, which has been the topic on a number of news blogs including Science Daily. The scientists used a technique called electron cryotomography (ECT), an imaging technique that freezes biological samples to extremely low temperatures essentially instantaneously. This allows the sample to be captured in a native state in a layer of ice. The sample is then examined by imaging as it is rotated in three dimensions. A model of the sample can then be rebuilt by computer assisted image analysis.
The Caltech researchers were looking at the bacterium Acetonema longum, a bacterium closely related to Gram positive organisms originally isolated from the hindgut of termites. Acetonema longum is interesting from two standpoints: even though it is very related to Gram positive bacteria, it possesses a phospholipid layer surrounding its peptidoglycan cell wall, and it is able to sporulate. These researchers were interested in the cell biology of the sporulation process, which has been well studied in the common soil bacterium Bacillus subtilis. However, B. subtilis has a very thick peptidoglycan and is a relatively large bacterial cell, which makes high resolution imaging difficult to accomplish. A. longum circumvented these disadvantages as the cells are somewhat smaller, and the presence of the outer membrane offered a unique opportunity to study the biology of membrane generation during sporulation.
The process of forming a spore in prokaryotes is the culmination of a long series of events, where the cell is drastically remodeled. The cell ultimately becomes metabolically inactive and the cell wall changes character and becomes much thicker, producing a structure (spore coat) that is able to survive adverse environmental conditions. In the case of A. longum, this may happen fairly regularly as they become exposed to oxygen after being passed from the termite gut. Forming a spore will allow the organism to survive until it can again can be in an anaerobic environment.
So what happens when Acetonema begins to form an endospore? An event similar to cellular division begins to occur. Under normal conditions, the two daughter cells would dissociate. Normal cellular growth results in an exponential increase in cell number, as long as nutrients are not limiting. Under sporulation conditions, one of the two cells is ultimately lost (mother cell), and the other one forms the endospore (prespore). In this image from the Cell paper, outer membranes are indicated in blue, and inner membranes are indicated in red. In Acetonema, the mother cell engulfs the developing prespore, which covers the prespore with an extra layer of inner membrane. The prespore then has the following structure from inside to outside: cytoplasm -> its own inner membrane -> peptidoglycan/spore coat -> layer of inner membrane from mother cell. The mother cell then is lost, leaving the developed spore. When conditions improve, the outer acquired membrane (which came from inside the mother cell during the process of sporulation) changes character to become a true outer membrane.
This model does not explain the origin of all Gram negative outer membranes. There are many organisms that do not form endospores. The biosynthesis of the Gram negative outer membrane requires the complex interaction of many enzymes, and has been best studied in in the workhorse of prokaryotic cell biology, Escherichia coli (for a review, see here, or YCP link here). The inner and outer phospholipid layers in these organisms have very different properties and biochemical natures. Even with Acetonema, the nascent outer membrane formed during the sporulation process must be drastically altered by modifying the lipids found there, however this is the first description of how the scaffold for creating this structure can arise.
Posted on September 5, 2011, in Lecture, Microbes in the News and tagged Bacterial outer membrane, Cell wall, Gram-negative bacteria, Gram-positive bacteria. Bookmark the permalink. Comments Off on Gram negative outer membranes: a missing link found?.