The odour and taste of microorganisms depends on various factors. For example, the type and number of microorganisms and what they feed on are important. When they break down their own food, many microbes produce metabolic products that we can detect with our nose and mouth. These metabolites are chemical molecules that can be described as savoury, sour, rancid, alcoholic or even floral.

 

You can easily detect these tastes and odours on your own body: Sweaty feet, for example, only typically smell "cheesy" when bacteria living on the skin decompose the sweat. However, as the bacteria live permanently on our skin, this also means that they are initially rather odourless until they have the opportunity to feast on our sweat.

 

The stale taste in the mouth every morning is also due to the fact that bacteria have multiplied in the mouth overnight and broken down food residues or saliva into volatile sulphur compounds and short-chain fatty acids. The number of microbes is also decisive as to whether we can perceive them by smell or taste. As they are very small, they only produce small quantities of metabolic molecules. This means that after brushing your teeth in the evening, some bacteria must first grow back so that the amount of odorous substances is sufficient to be able to perceive them.

Our own digestion shows how the different foods of the bacteria affect their odour and taste: the gases that escape from our intestines smell particularly unpleasant if, for example, Brussels sprouts were previously on the lunch table. This is because they contain sulphur-containing glucosinolate, an ingredient that our intestinal bacteria break down into foul-smelling hydrogen sulphide and methanethiol. The same applies here as with the example of cheese feet: bacteria are always present in our intestines, but our nose only really becomes aware of them when they ingest certain foods.

 

Microorganisms also characterise the typical smell of human faeces. Escherichia coli is the culprit here, breaking down the amino acid tryptophan into indole. As E. coli is a common experimental bacterium, microbiologists often "enjoy" this odour in the laboratory.

But microorganisms can also impress with pleasant odours! Bacteria of the genus Streptomyces produce geosmin, which is responsible for the typical earthy odour of forest soil. This molecule is also responsible for the wonderful odour of summer rain after prolonged drought.

 

Yeasts (mostly Saccharomyces) also enhance the heavenly odour of freshly baked bread in the dough. A number of aromatic compounds, such as acetaldehyde and ethyl acetate, are produced as metabolic by-products during the breakdown of sugars and amino acids, which give the bread its characteristic malty, yeasty odour.

 

Incidentally, the different flavour characteristics of microorganisms are also used commercially in the production of food: Certain bacteria, yeasts and moulds, for example, are responsible for the tart freshness of yoghurt, the strong taste of cheese or the intoxicating effect of wine and beer.