It is a known fact that bacteria develop immunity to antibiotics. Why then did not all bacteria develop immunity to natural antibiotics like penicillin in the fungi?

It is for sure that fungi did not care to continue killing all bacteria so that them won't be able to develop immunity (as we're adviced today to use antibiotics carefully, in order not to develop a superinfection)..

Looking at all the time span that bacteria and fungi exist, it seems unlikely that there'd be any bacteria left not yet immune to penicillin at all. However, that's not the case, the antibiotic is still a rather successful one.

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    $\begingroup$ As far as my limited genetics background allows me to assess this question, it is population genetics. Let's not close this well-researched question based on a philosophical community question.. $\endgroup$
    – AliceD
    Nov 3, 2015 at 13:25
  • $\begingroup$ @AliceD hi! I am sorry, I think I don't get your second sentence... could you please say it differently? :) $\endgroup$
    – noncom
    Nov 3, 2015 at 18:30
  • $\begingroup$ @noncom - that second sentence was addressed to the community at large. Don't worry about it. $\endgroup$
    – AliceD
    Nov 3, 2015 at 23:01
  • $\begingroup$ @AliceD however well researched question might be, if it is a case of the CW post, then there is no point repeating the same stuff again and again. Newton's law does not change if you replace a "ball" with a "car". Why did all bacteria not evolve antibiotic resistance is clearly an example of the CW question. Just replace "organism" with "bacteria" and "trait" with "antibiotic resistance". Can you explain why this post is different? Also, that post is not philosophical. We created the post such that all possible relevant scientific points are cited to address the evolution problem. $\endgroup$
    Nov 4, 2015 at 5:38
  • $\begingroup$ @noncom many bacteria previously sensitive to the antibiotics are becoming resistant. $\endgroup$
    Nov 4, 2015 at 5:42

1 Answer 1


For a natural immunity to evolve, two things have to happen:

  • The immunity has to actually occur. That means all the necessary mutations will need to be there to grant this bacterium resistance to penicillin. For penicillin, for example, bacteria that evolved such a resistance evolved to produce the enzyme beta-lactamase.

  • The immunity has to grant an advantage. Penicillin is produced by Penicillium, which is a mold found in environments with lots of oxygen. Lots of bacterial species have never encountered penicillin and the vast majority (for example anaerobic bacteria that don't infect humans) never will. In the absence of this pressure, a resistance to penicillin, even if it evolves, will likely deteriorate within a few generations because the individuals with the resistance do not have an advantage in survival. Producing an enzyme such as beta-lactamase is not without its costs (protein synthesis).

While bacteria van exchange genetic material in the form of plasmids between different species, so not all species would need to evolve beta-lactamase de novo, this is also only "helpful" if the bacteria a) come in contact with resistant bacteria species, and b) if this resistance confers an advantage for the receiving species.

Origins and Evolution of Antibiotic Resistance

Mechanisms of Bacterial Resistance to Antibiotics

  • $\begingroup$ Penicillin is made by Penicillium, not Aspergillus. $\endgroup$
    Nov 4, 2015 at 5:32

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