A while ago, I wrote about how Helicobacter pylori, the bacteria that cause stomach ulcers and are implicated in certain stomach cancers, cause the cells of the stomach wall to die. H. pylori kills cells very sneakily, by releasing a chemical that causes them to commit suicide. It turns out that this is not the only sneaky trick H. pylori has, it can also hide from the immune system by changing its outer cell membrane.
The immune system protects your body from any invading elements, and because of this it needs a way to distinguish body cells from invading cells. The cells of the immune system do this by recognising bits of bacteria as foreign invaders, and of course the first bit of the bacteria they see is the outer cell membrane. H. pylori has a way of making parts of its outer cell membrane look very similar to human blood group antigens, so the immune cell doesn't recognise it as an invader.
The bacterial cell membrane is made up of lipopolysaccharides (i.e lipids, or fats, connected to polysaccharides - sugars). H. pylori has ways to modify the biosynthesis pathway of these lipopolysaccharides to produce different structures. In particular to lipid A, which is part of the structure on the surface of H. pylori.
The bacteria have several different ways that they modify the lipid A. Firstly, they can reduce the lipids overall negative charge, either by adding positively charged substrates, or by removing negatively charged phosphate groups. This makes the bacteria more resilient to certain antimicrobial peptides that bind to negative charges. They can also add extra bits to the lipid A (in particular acyl groups) which make the surface of the bacteria harder to for immune cells to recognise. This lipid A modification pathway is highly ordered and efficient. Rather than producing all different kinds of lipid A at the surface, each bacterium will be covered in one type of modified lipid A.
The reason that H. pylori has this one specific modification pathway is likely to be due to the fact that it only has one host. H. pylori lives in humans and nowhere else. There is only one type of immune system it has to evade. Once the lipid A resembles human blood antigens, any changes or alterations would be strongly selected against in order to keep the bacteria well hidden.
For a more in-depth biochemical analysis of exactly how these modifications happen, take a look at the paper below!
Credit for image 2
Ref 1: Cullen TW, Giles DK, Wolf LN, Ecobichon C, Boneca IG, & Trent MS (2011). Helicobacter pylori versus the host: remodeling of the bacterial outer membrane is required for survival in the gastric mucosa. PLoS pathogens, 7 (12) PMID: 22216004