Cell membrane of Bacteria
There are two ways cells can get hold of the cholesterol needed for the membranes, by using food sources containing low-density lipoproteins (LDL), or by synthesising it within the cell. Defects in the cholesterol synthesis pathway can increase the likelihood of the cell breaking down through apoptosis or due to oxidative stress. Around 20-25% of the cell membrane is made up of cholesterol in mammalian cells.
Despite the above diagram, the phosolipid molecules are not rigidly stuck in place within the cell membrane, as long as they keep the phosphate facing outwards and the tails inwards both they and the steroids can travel around the membrane. This means that some areas will gather clumps of cholesterol, known as lipid rafts, which play important roles in cell signalling, membrane shape, and of course, bacterial invasion. Many bacteria target these lipid rafts when looking for places to attach onto human cells, and they act as the first point of cellular invasion.
Researchers found that limiting the amount of cholesterol in the mammalian cell membrane (by blocking the internal cholesterol synthesis pathway) led to far less effective invasion of bacteria and bacterial toxins. The diagram below shows an electron micrograph of mouse tissue, in the one on the left the cells cannot make cholesterol and in the one on the right the cells have normal cholesterol-making activity. Little black arrows show where the toxins produced by the cholera bacteria have been taken up by the cells.
Only 9% of −cholesterol cells contained 10 or more toxin-containing vacuoles, compared to 80% of the +cholesterol cells.
Repeating the assay shown above with different bacterial strains revealed that the bacteria C. burnetii also require cholesterol to enter the cells, while Salmonella typhimurium and Chlamydia trachomatis enter both cholesterol and non-cholesterol containing cells at the same rate. While lipid rafts are required for cell entry by some bacteria, it seems that others do not seem to rely on them.
The researchers suggest that as well as affecting bacterial cell attachment to the cell surface, the cholesterol may also be vital for the uptake of certain bacteria and their internal transport. It may therefore be possible that the cholesterol is not only important for helping bacteria enter the cells, but also for their further growth and development inside the host cell.
The particularly interesting thing about this research was the method used to remove cholesterol from the cells. Because it is such an important membrane component, chemical methods tend to drastically alter the shape of the cells which causes more problems for bacteria trying to get in. For this paper, the researchers instead targeted the cholesterol synthesis pathway, removing the final enzyme. This system therefore allows a cholesterol-free environment to be explored without causing any significant changes to the cell membrane integrity.