Bacteria are safeguarded against osmotic stress by a protein complex.

Everyone could use a little stress relief, even bacteria. Now, researchers from Japan have found that a bacterial nanomachine with an unusual cellular location can protect cells from stressful environments.

In a study recently published in mSphereresearchers from the University of Tsukuba revealed that a protein complex associated with the tail-like secretion systems of phages is expressed intracellularly in a Gram-positive organism and protects it from osmotic stress.

Many bacteria contain genes that encode phage tail-like nanomachines called contractile injection systems (CISs). These systems are basically small syringes that bacteria produce and release into their environment to communicate with other cells and inject their contents.

“Cis-related gene clusters are highly conserved in Gram-positive actinomycetes such as Streptomyces,” says lead author of the study Professor Toshiki Nagakubo. “Some strains encode CISs known as Streptomyces phage-like tailories (SLPs) that appear to provide an advantage under competitive growth conditions, but how they do so remains unclear.”

To determine how these SLPs function, the researchers investigated where SLP proteins are expressed in Streptomyces lividans, a model organism of Gram-positive filamentous bacteria with highly conserved gene clusters associated with cis. They also took a comprehensive look at the proteins that SLPs interact with.

“The results were very unexpected,” explains Professor Nagakubo. “Unlike standard cis, which are released by cells into the extracellular environment, S. lividans SLP is exclusively expressed inside the cell.”

Deletion of genes encoding SLP increased susceptibility to osmotic stress, which is an imbalance of intra- and extracellular electrolyte concentrations. In addition, SLP proteins appear to interact with cellular systems involved in cell wall synthesis and protein translation.

“Our findings indicate that SLPs are directly or indirectly linked to a protein interaction network within the S. lividans cytoplasm, and that loss of SLP ultimately affects the susceptibility of bacteria to specific stress conditions,” says Professor Nagakubo.

Given the unique intracellular localization of SLPs, they appear to represent a new class of phage tail-like nanostructures produced by Gram-positive bacteria and are distinct from known bacterial cis species. These SLPs appear to confer resistance against osmotic stress, which could increase the survival of S. lividans and improve its ability to survive in densely populated microbial communities in natural environments such as soil.