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Breaking the mould

Sometimes our cheddar cheese develops a bluish-green circular mould. My wife maintains the cheese is safe to eat, especially if you cut the mould off. I say you shouldn’t eat it. Which one of us is correct and why?

• Cheeses can easily be contaminated with moulds. They can develop during the ripening of the cheeses or infect the product when it is handled in the supermarket or at home. Because of this, some cheeses are treated with a coating containing preservatives that eliminate mould growth. However, one green Penicillium species is resistant to the cheese preservatives and may still be found as a contaminant. This mould is toxic and should not be eaten. Mould spores can provoke allergy and those who are sensitive should avoid exposure to any mould-contaminated food product.

Many common food-borne moulds develop at low temperatures, such as in the refrigerator. Some can produce toxic compounds (mycotoxins) that might penetrate the cheese. Depending on the size of the growth you can cut the mould off, but it can be difficult to determine how deep you need to cut. If the cheese only has a small green spot then a large piece of approximately 4 centimetres square can be removed. If the spot is larger, or if you have any doubts, the cheese should not be eaten.

Some moulds, of course, are part of cheese production and are not contaminants. For the production of the soft white cheeses such as camembert and brie, or blue cheeses such as stilton and gorgonzola, a starter culture of Penicillium mould is necessary for the fermentation that gives the cheeses their typical taste and texture. These moulds, Penicillium roqueforti and P. camemberti have the potential to produce toxic compounds, but they will never do so in these particular cheeses.

Robert Samson

Royal Netherlands Academy of Arts and Sciences

Utrecht, The Netherlands

More information on food and fungi can be found in Robert Samson’s book Fungi in Indoor Environments (Elsevier Science, 1994) – Ed

The life and the light

Some deep-ocean fish have luminescent lures projecting from their heads to attract prey. This luminescence is created by bacteria, but how do the baby fish acquire these bacteria? Do they come from the environment around them, and if so how are they subsequently concentrated in the lures? Or do they come from the mother, in which case how does she transfer them successfully to the young?

• There is a short and simple answer to this question – which is that nobody knows exactly. We tracked down the world’s leading expert on relationships between anglerfish and bacteria. She is Margo Haygood, a marine biologist at the University of California, San Diego, and here she explains the ongoing mystery – Ed.

• Anglerfish are loners. If they are lucky enough to find a mate, they usually become physically attached for life so they don’t lose each other. Only the females have luminescent lures, and anglerfish move deeper as they mature, with the reproductive females living at depths of 1000 metres or more.

However, when producing offspring, the females release buoyant egg masses that float to the surface. The larvae hatch without a light organ and live in the surface plankton. As they metamorphose and mature, they migrate deeper. This means that it is impossible for the young to get the light bacteria from their parents, unless they are already contained in the eggs. And if they are in the eggs, where do they stay until the light organ develops?

In other species with light organs, colonisation by symbiotic bacteria from seawater triggers light-organ development. Yet in deep-sea anglerfish the light organ develops fully before bacteria can be seen inside. Indeed, the light organ is sealed off from the outside until fully mature.

We don’t know a lot about the bacteria themselves. Logically one would assume that they belong to the one species of luminous bacteria that is found throughout the upper water column, and is a promiscuous symbiont, inhabiting the light organs of many different fish. However, of the species of deep-sea anglerfish that we have studied, it turns out that each has its own specific symbiont, and these are species of bacteria that have never been detected anywhere else before. There is also no obvious environmental reservoir of symbionts for the young to capture.

So, it seems unlikely that the fish get the symbionts from their mother, and equally unlikely that they recruit them directly from seawater. Perhaps they could get them from a common food item, but given that the adult fish live so deep, it seems likely that these bacteria would be at least moderately piezophilic – growing best in high-pressure environments – and therefore unlikely to be common in the shallower waters that are inhabited by the juveniles.

Margo Haygood

Scripps Institute of Oceanography,

San Diego, California

This week’s question

Mountain headgear

I’ve seen mountain-top clouds similar to the one in this astounding image of a sombrero-like cloud atop Mount Fuji. What causes them? Do they only occur above volcanoes or do they occur above any mountain of suitable height?

Kevin Enright

Ironbridge, Shropshire, UK