Understanding the role of Type 6 secretion system-5 in spread of Burkholderia thailandensis between host cells

Bacterial interactions with membranous barriers is a process that needs to be tightly regulated and executed in a timely manner. Burkholderia deploys several secretion systems to manipulate either prokaryotic or eukaryotic membranes. Two of these secretion systems will be discussed here, type 6 secretion system-1 (T6SS-1) and type 6 secretion system-5 (T6SS-5). To deliver effectors into the membranous barriers of other bacteria, T6SS-1 is used by Burkholderia. T6SS-1 was shown to be antibacterial and allows the elimination of competing bacteria. Precise targeting for this machinery is important to allow efficient delivery of T6SS-1 effectors. We found that the T6SS-1 machinery is precisely aimed to cell-cell contact sites with other bacteria by the post-translational regulator TagM1. We further show that this mechanism of sensing cell-cell contact is also conserved in other bacteria like Acinetobacter baylyi.
When Burkholderia is taken up by a eukaryotic organism it faces a multitude of membranous barriers. To survive in a host cell Burkholderia has to passthese membranous barrier without triggering the host cells immunity. After it’s initially uptake Burkholderia ends up in the primary vacuole, which it lyses with the help of a type 3 secretion system. This allows Burkholderia to enter the host cells cytoplasm where it has to find a way to pass through two membranous barriers to enter a new host cell. Inside the cytoplasm, Burkholderia forms actin-tails to generate membrane protrusions into neighboring host cells and use Type VI secretion system-5 (T6SS-5) to induce host cell-cell fusions. The regulation of T6SS-5 and its interaction with the host cells immune system when breaking through the eukaryotic membranes is poorly understood.
That T6SS-5 is expressed in all intracellular B. thailandensis but that it predominantly assembled when bacteria where inside of membrane protrusions. T6SS-5 assemblies were dependent on the expression of Tag proteins and assemblies could be triggered with pressure in vitro or inside of host cells. Once the bacterium inside the protrusion runs out of actin to polymerize, the membranous protrusion collapses. We found that these collapsed protrusions often showed dynamin-2 recruitment, which was followed by T6SS-5 assemblies. Assemblies of T6SS-5 lead to the influx of actin into the protrusions allowing renewed actin polymerization, protrusion lysis and dynamin-2 localization to membrane fragments. These T6SS-5 dependent protrusion lysis events were previously shown to cause host cellcell fusions. Interestingly, we observed that T6SS-5 protrusion lysis events only rarely lead to cell-cell fusion events. Damage of host membranes can trigger galectin-3 recruitment which is able to induce either the host cell immune system via LC3 leading to autophagy or the host cell repair mechanism via ESCRT-III. Surprisingly, most T6SS-5 dependent protrusion lysis events avoided the localization of galectin-3 to host membranes and thus the subsequent downstream factors LC3 and ESCRT-III. In contrast to this, T6SS-5 inactive bacteria were still able to spread from cell to cell however, their protrusion lysis events were recognized by the hosts immune system (galectin-3 and LC3).
Our results show that the bacterial T6SS of Burkholderia are tightly regulated, respond to external stimuli and that T6SS-5 is a unique secretion system that allows undetected and thus unrepairable membrane alterations to facilitate bacterial spread inside of a host cell.