We put 2 flasks inoculated with Bacillus cereus in 37⁰C: one with 100μg/ml penicillin + 50μg/ml chloramphenicol and the other without penicillin. We found that the OD is higher in the one without the penicillin. These strains are supposedly penicillin resistant as verified by growing them in a test-tube containing penicillin (5mg/ml) at 30⁰C. It contradicts my hypothesis that they should have the same growth rate because of the $\beta$-lactamase secretion that would destroy the penicillin. I can't find any explanation in the web. help me

  • $\begingroup$ You need to add a lot more detail for any answer to be possible. Which kind of bacteria were in there? Were they resistant to Penicillin. Did you work sterile? $\endgroup$ Jun 11, 2014 at 13:55
  • $\begingroup$ the bacteria was bacillus cereus. as far as i know weve worked sterile (they gave us the bacteria). i didnt mention two other things: 1. we onother erenmyer that contain penicillin (0.2 ml) and chloranphenicol (0.1 ml). 2. the real changes in the OD began after 90 min. $\endgroup$
    – ebeginner
    Jun 11, 2014 at 14:14
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    $\begingroup$ Can you please edit your question and add information about the bacteria, resistances and, media and all the other things that may be important. Additionally: This sounds a lot like a homework question, so please add some interpretation of yourself. $\endgroup$
    – Chris
    Jun 11, 2014 at 14:18
  • $\begingroup$ You could also show a diagram with OD measurements over time for both cultures. $\endgroup$
    – Chris
    Jun 11, 2014 at 14:25
  • $\begingroup$ Resistance to penicillin in B cereus is due to chromosomal genes, but not all B cereus strains are penicillin resistant. Do you know for certain that you used a resistant strain? $\endgroup$
    – Alan Boyd
    Jun 11, 2014 at 14:36

1 Answer 1


Your hypothesis is incorrect.

You must be assuming that lactamase destroys the antibiotic with perfect efficiency. This is incorrect.

You must be assuming that the population will remain genetically stable. This is doubly incorrect. Firstly the penicillin will select highly resistant bacteria which suffer a greater fitness burden from expressing the lactamase. Second, in absence of penicillin, spontaneously arising mutants which suppress the lactamase are free to proliferate.

You must be assuming that carrying capacity is identical for Pen+ and Pen- media. Trivially, you claim your experiment disproves this, but the assumption is also baseless since this need not be the case.

Furthermore, your results are inconclusive. You lack crucial controls, such as putting the bacteria from the Pen+ into Pen- and vice versa to see if the growth difference is intrinsic. You have not only failed to provide the error of your measurements, but have failed to provide the quantitative measurements themselves, so we do not even know if the difference is statistically significant.

Lastly, there are several basic things which you could rightfully expect us to assume, but have nevertheless failed to provide. For instance, you do not say if the populations are clonal.

  • $\begingroup$ Can you provide a ref. for "suffer a greater fitness burden from expressing the lactamase," and explain why it would slow growth, if that's what you are getting at? Thanks. $\endgroup$
    – daniel
    Jun 13, 2014 at 13:03
  • $\begingroup$ @daniel Assuming not every cell in the population expresses the exact same amount of lactamase, it stands to reason that some of them make more of it than others. It is well known that expressing arbitrary proteins, or antibiotic resistance, by itself constitutes an energy burden. I cannot provide references unless OP clarifies the experiment. $\endgroup$
    – Superbest
    Jun 13, 2014 at 20:18
  • $\begingroup$ Not questioning your assertion, just wanted to read further if you had a ref. The question is a bit vague and I think your second paragraph is correct. If the results are qualitatively accurate it's an interesting question. How is the difference in growth rates allocated between the 'fitness burden' of resistance and the ability of non-resistant mutations to proliferate in the pen.-free nutrient? That's why I was interested in a ref if you have one. +1 $\endgroup$
    – daniel
    Jun 13, 2014 at 20:30

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