As penicillium mold spores drift about through the air, the fortunate ones land on a suitably moist food supply.
The spores almost need the moisture more than the food , as they can feed on almost any organic matter.
But when they get stressed, usually when the food supply runs low , they may respond by producing the antibiotics we rather loosely call by the single word 'penicillin' , to keep at bay bacterial competitors for that same food.
Note I said antibiotics in the plural, because, depending on the particular food source at hand, each penicillin produced might be in fact ever so slightly different.
Technically, the key part of the penicillin molecule - the famous strained beta lactam ring - must always be there , for it to work as a antibiotic .
But the extra bit on the side of that key component - the side chain - can vary widely and will tend to produce various kinds and strengths of antibiotics.
Many of them offer different - useful - qualities - from the original penicillin (type G).
Now, in Nature , all these marvellous variants are mere happy accidents induced by the chance of varying wind currents and drifting spores.
In the Lab, biologists and chemists feed the penicillium spores all sorts of extra food treats , (fancied up ,the technical word for these humble food supplements is precursors) hoping to see a useful variant emerge.
When the new improved variants do emerge , the chemists tend to glorify this 'brute force' approach to the discovery of new chemicals by daring to call the results "semi-synthetic" penicillin.
But this just hubris and hooey.
It is merely a way for human chemists - who tried and failed to make penicillin from scratch* like the tiny mold spores so readilyand so easily did - to regain some lost luster for their profession.
(*To chemists , this is their holy grail of "total synthesis".)
Because it is really no different than the ancient practise of farmers trying out plants on differing soil mixtures and cultivation techniques and running with the methods that gave the best yields.
The chemists, rather like the British , lost really big in WWII.
Partly to physics (the A-Bomb) and partly to biology (natural penicillin triumphing over the chemists' failure to make synthetic penicillin) .
And like the British, they've been trying to re-write the history of WWII ever since ....