Budding microbiologists learn that the cell wall of bacteria and fungi plays an vital structural role. Indeed, spheroplasts (cells that have had their walls removed) are easily lysed with water, unable to maintain osmotic stability. Part of the cell wall duty is to remain rigid and provide the cell shape – this is most obvious when seeing the round spheroplasts from a rod-shaped cell (pictured).
Research published in mBio by Iuliana Ene, et. al. this week demonstrates the fungal cell wall isn’t as rigid as previously understood. In fact, the research suggests that the elastic nature of the cell wall is necessary to provide protection after hyperosmotic shock.
The fungus Candida albicans is part of the commensal flora of most individuals but can also cause opportunistic infections. Ene et. al. shocked C. albicans cells by exposing them to high solute (salt or sugar alcohol) concentrations. Ene et al observed C. albicans cells in lactate are more resistant to osmotic shock than cells grown in glucose, in which the well-characterized Hog1 pathway is activated with osmotic stress. The lack of Hog1 (and other) signal transduction cascades led the researchers to look at the physiology of the cells before and after stress.
In hyperosmotic stress, water leeches from the cell, trying to balance the solute concentration on each side of the cell membrane. If the cell can’t protect itself, so much water leaves the cell that the membrane collapses in on itself and the cell dies. (This is also why jelly is rarely contaminated, and why some veterinarians will dress wounds with sugar – sugar and salt mess up microbial electrolyte balances.) In glucose-based osmotic stress conditions, C. albicans Hog1 activation leads to glycerol production, which balances the concentrations of intracellular and extracellular solutions. How could cells survive without a change in osmolarity?
The C. albicans cells shrunk in size when they were put under high salt stress, as expected. The surprising result came from observing the expansion of the cell wall – in the stressed cells, a much greater proportion of the volume was made of the expanded cell wall. This expansion occurred within seconds of exposure, which is too short a time for signaling cascade activation of new genes, let alone generate high concentrations of glycerol – the question still remained: how?
The electron micrographs showed the semilayered property of the cell wall beautifully: a layer of chitin and beta-glucans close to the cell, decorated with an outer layer of mannans. In the lactate-grown cells, the immediate expansion of the cell wall chitin/beta-glucan layer is clear. Examining the sensitivity of cell-wall remodeling mutant strains revealed that cross-links between beta-glucans and chitin influenced the osmotic stress response in these cells, and that this response is modulated through the calcineurin pathway.
Why is this an important finding? First, the idea of a malleable cell wall is a major paradigm shift in fungal biology. The researchers themselves quote a 1979 report likening the fungal cell wall to concrete. Concrete is neither flexible nor responsive in the way these cell walls are. The researchers suggest a model in which the beta-glucans, rather than solely conferring rigidity, are able to expand and contract in a spring-like manner.
Also important is the uncovered role for the cell wall in protecting the cell from stress. We know beta-glucans are important enough to the cell wall to dedicate an entire class of antifungals against their synthesis. Learning more about how the cell wall works will reveal new antifungal targets; as a differential cell structure indispensible to the microbe, this organelle has great potential as a drug target.
The biomechanics of the Staphylococcus aureus cell wall are better characterized, in part through a second paper published in mBio this week by Wheeler et al. This research furthers the known phenomenon of cell wall expansion due to peptidoglycan hydrolase activity. The research shows that S. aureus cells lacking certain hydrolases increase their cell wall stiffness. The authors go on to show that modulation of cell wall plasticity plays an important regulatory role in bacterial cell enlargement.
This last point – that cell wall plasticity regulates other biological functions – is what may lay ahead for those studying C. albicans. Peptidoglycan’s flexibility has been understood for several decades, but the work by Ene et al lay the groundwork for similar studies in the fungal cell wall as well.
-- Julie Wolf