“Open and Closed”


Will Staphylococcus aureus ever become completely resistant to penicillin?

At present about 10% of Staphylococcus aureus isolates worldwide retain susceptibility to penicillin. It has been like this for many years. The reason for this is probably two-fold.

  1. Bacterial populations do not exist in a closed system. Therefore they can “escape” to places where there is no selection pressure.
  2. The acquisition of resistant determinants by a bacterium almost always (if not always) comes with a fitness cost. Thus in the absence of any selection pressure, the resistant strain is unlikely to become dominant in such a hypothetical setting.

I would be very worried if Staphylococcus aureus as a population became completely resistant to penicillin, but I don’t think it will happen, not in “real time” anyway. On a population basis, I am not aware of any bacterial species which has variable resistance to an antibiotic, which has then become completely resistant to an antibiotic due to the use of that antibiotic in the population.

People worry about individual bacterial isolates being resistant to all known antibiotic classes, and label it as “The end of the antibiotic era”, which of course it is in a sense, and obviously terrible for the individual patients involved. But as microbiologists, I believe we need to be thinking about resistance much more in terms of bacterial populations, because this is where the answers to future resistance rates and the evolution of resistance really lie….


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