The cytoskeleton is the cellular umbrella of bacteria

Pseudomonas aeruginosa (P.
a.
) is resistant to most antibiotics and causes life-threatening wounds or lung infections
.
P.
a.
Bacterial species have a whole set of strategies to
evade the immune system and infected tissues.
The researchers, in collaboration with Prof.
Winfried Römer and Dr.
Carsten Schwan of the University of Freiburg and the excellent cluster of the CIBSS Center for Integrative Biosignaling, have discovered a previously unknown natural defense mechanism that protects cells from Pseudomonas infection
.
The study was published in
Cell Reports.

Pseudomonas aeruginosa also enters cells in a targeted manner

The inside of the cell is surrounded by nutrients and is not discovered by the immune system, which is an ideal place for
pathogenic bacteria to multiply.
Still, it has long been thought that most of them are found outside the
cell.
"At the same time, it became clear that the situation was more complicated, with bacteria targeting the cells they penetrated," explains Römer
.
With the help of a live-cell imaging microscope, this can be observed
in the laboratory.
The bacteria are in close contact with the cell, leaving dents in the cell membrane until it is completely engulfed
by the cell.

Pseudomonas causes depressions in the cell membrane by binding the virulence factor LecA to sugar molecules on the cell membrane surface
.
Proteins bound to sugars are common in nature and can also be observed
on other bacteria and viruses.
The strong binding of bacteria and host cells causes the cell membrane to close itself like a zipper around the bacterium – something Römer's group has been studying for quite some time
.

"Now we are intentionally focusing on the role of sepatin in Pseudomonas infection," Schwan said
in describing the goals of the current study.
"We know that other bacteria can manipulate the diaphragm of host cells and use them to ease permeation
.
At the same time, however, septin can also envelop bacteria that have penetrated and induce their clearance, so they have paradoxical functions in infection," Schwan continued
.
The diaphragm, part of the cytoskeleton, is a key structural element similar to modular scaffolds that can be flexibly assembled and disassembled
.
They also often play a role
when curved membranes are involved.

The binding of LecA is sufficient to induce septin accumulation

In their latest study, the researchers used human lung cells
infected with Pseudomonas aeruginosa in the laboratory.
They found that the attachment of bacteria caused the diaphragm to gather at the site
within seconds or minutes.
If the bacteria can't penetrate it, the accumulated bacteria will soon dissolve
again.
The researchers also observed this effect
when replacing bacteria with polymer pellets coated with LecA.
This suggests that septin accumulation
is induced by LecA binding.

Schwan added: "We expected that Pseudomonas wanted the accumulation of this septin, which helps them during infection, just like
other types of bacteria.
" Instead, they found that when the researchers removed some of the diaphragms, twice as
many bacteria could penetrate.
"At first we were surprised by the results, but now we don't think it's an isolated case
.
It could be that septin also plays a protective role against many other pathogens," he concluded
.

This is the basis for the development of drugs to treat drug-resistant bacteria

The researchers were able to measure the strength
of the diaphragm barrier in further experiments.
To do this, they attached the smallest bacterial model to an atomic force microscope (AFM) cantilever, which allows even the tiniest indentations and resistance on the surface to be measured as force
.
The researchers found that when polymer beads were packed with LecA and septin, the cell membrane was significantly harder locally
.

"That's a huge impact
when you consider that in the infection process, there are many different molecules involved.
For an intervention to have such a huge impact, you usually have to suppress several factors or there will be compensation," Römer says
.
He continues: "This is a strong indication that the nasal septum does play an important role
in fighting infection.
" The researchers now intend to further study lectins and septin
.
A better understanding of these proteins can help find the point of attack of drugs to prevent infection
by antibiotic-resistant bacteria.

Sahaja Aigal, Ramin Omidvar, Kai Stober, Jessica Ziegelbauer, Thorsten Eierhoff, Janik Niklas Schampera, Winfried Rö mer, Carsten Schwan.
Septin barriers protect mammalian host cells against Pseudomonas aeruginosa invasion.
Cell Reports, 2022; 41 (3): 111510