Halloween has finally arrived and everyone around you has been busy working on their costumes in anticipation for this glorious day of tricks and treats. Come Halloween night, you may not even recognize the faces behind the costumes. How would you tell apart friends with a sweet tooth from foes ready for some tricks?
For some bacteria, putting on costumes is an everyday event when it comes to tricking the immune system. The immune system distinguishes the body’s own cells from those of invading bacteria and viruses. Since the microbe’s outer membrane is usually the first thing the immune system sees, the body takes advantage of molecules found on the outer membrane to generate an immune response. This seems like a wise defense mechanism but microbes have several tricks up their sleeves; they disguise themselves from the immune response by changing molecules on their outer membrane (antigenic variation) or by turning on and off expression of genes encoding for surface molecules (phase variation).
Antigenic variation occurs when the bacterium modifies outer membrane structures so that they appear slightly different. Pretend you have been invaded by a bacterium covered in menacing grey pili (pili are hair-like structures found on bacterial surfaces). Your immune system encounters this, recognizes the grey pili as dangerous and mounts an immune response against them. Unfortunately, you are later infected by this same bacterium but it displays green pili instead after undergoing antigenic variation. The immune system cannot recognize these and treats the bacterium as a complete stranger. Unfortunately for the host, some bacteria and viruses have hundreds of antigenic variants that make it difficult for the immune system to recognize. There are over 150 antigenic variants of the human rhinovirus (viruses that cause the common cold) explaining why you get the common cold over and over again.
In contrast to antigenic variation, phase variation is the switching of phenotypes by modulating protein expression using an “on-off” switch. Let’s take a look again at the grey pili bacterium. To hide from the immune system, the bacterium switches off production of pili and evade detection by the host immune system.
To make matters even worse for the immune system, bacteria can trick the host by producing molecules that resemble host cells. The bacterium Helicobacter pylori can mimics human blood group antigens identical to those found in the gastric mucosa where it resides. H. pylori‘s cell membrane is composed in part of lipopolysaccharides (sugar molecules with lipids attached). H. pylori has the ability to modify these lipopolysaccharides and under selective pressure, those that have been modified to resemble the gastric mucosa will dominate the H. pylori population.
Costumes and disguises are celebrated by microbes everyday. Through selective pressure from the host immune system or other environmental changes, the microbes that devise the most creative strategies for survival prevail.
