Recent research, by scientists in Madrid and New York, has found an astonishingly clever ‘weapon’ adopted by a deadly fungus - it morphs into a gigantic version, up to 900 times as big as ‘standard’ fungus cells. This renders the sufferer’s immune defences totally inadequate, as they are simply not designed to cope with germs of such proportions.
A key aim when studying disease is learning how a germ manages to escape immune defences of its victim. Humans have incredibly complex and fine-tuned immune systems – so why are we still defeated by some bugs? The reason is that deadly pathogens have evolved to be even smarter than our comprehensive immunity.
The meningitis-causing fungus, Cryptococcus neoformans is a catastrophic problem in countries plagued with AIDS, as patients suffer lowered immunity and succumb relatively easily to disease. The fungus resides in soil and its tiny cells hitch a ride on the air, and are then inhaled by people. The infection is uniformly fatal if it goes untreated (which it often does in poorer societies).
The researchers infected mice with fungus and measured fungal cell size three weeks later. Mice are often used in the laboratory to investigate human disease, because many elements of the immune system are shared across mammals. Fungus grown inside a mouse experienced a real-life infection scenario as it had to encounter retaliation from the animal. The dimensions of these fungal cells were compared to those grown artificially in the lab. The fungus that had to fight against the animals underwent the transformation to gigantism, whereas that grown outside of the mice did not. The scientists concluded that morphing into a giant was a strategy for the fungus to be more effective in tackling the mouse.
An important immune method used by many living creatures is that cells ‘eat up’ invading germs, thereby preventing illness. Some mouse cells were discovered stuck to the colossal fungal cells, indicating they had attempted to eat them but simply couldn’t manage it. An advantage to escaping the victim’s immune system in this way is that the fungus can live for long periods of time inside them, waiting until their natural defences become aged or faulty (as occurs when a person develops AIDS). Although the mammoth cells have this advantage, they are too big to spread rapidly throughout the body and reach the brain to cause meningitis. An important feature enabling them to achieve their devastating ultimate goal is that a giant parent cell can wait for opportune moment (such as AIDS acquisition) and then produce many normal-sized offspring that can proliferate through the victim’s weakened body.
A fungus that alters its morphology to increase disease-causing ferocity is not a new discovery: many are capable of shape-shifting. The researchers suggest that swelling to gigantic proportions may occur in other fungi too. Therefore a valuable line of study could be to develop ways of identifying huge cells in patients that may be missed by traditional tests, so any sleeping giants can be nipped in the bud.
This article is based on the journal article O. Zaragoza et al., PloS Pathogens, June 2010