Competition for nutritional resources masks the true frequency of bacterial mutants

Table 2 Inferring the frequency of spontaneous PCMs in a mixed culture

5 plates each containing 10⁹	Total PCMs	Δpst::Cm	Spontaneous	Inhibition factor	Expected no. of
wt cells + the following no.	at day 7	colonies	PCM colonies	\(\left (\frac {\Delta pst\textrm {::Cm}\ \text {plated}}{\Delta pst\textrm {::Cm}\ \text {observed}}\right) \)	spontaneous PCMs/ 10⁹
of Δpst::Cm cells					cells
(5×) 10³	267	7	260	714	3.7×10⁴
(5×) 3×10³	269	20	249	750	1.9×10⁴
(5×) 4×10³	364	18	346	1111	7.7×10⁴
(5×) 5×10³	345	26	319	961	6.1×10⁴
(5×) 10⁴	209	31	178	1612	5.7×10⁴
(5×) 2.5×10⁴	254	114	140	658	1.8×10⁴
(5×) 5×10⁴	977	391	586	639	7.5×10⁴
(5×) 10⁵	1098	686	412	729	6.0×10⁴
Means				897	5.0×10⁴

Five cultures each containing 10⁹ wild-type bacteria were mixed with increasing numbers of Δpst::Cm mutants and immediately plated on TG2PP. After 7 days of incubation, the total number of colonies was counted and tested for chloramphenicol resistance. The ratio [ Δpst plated]/[ Δpst colonies observed] gives the inhibition factor of Δpst. The expected no. of PCM/ 10⁹ cells was calculated by multiplying the number of spontaneous PCMs on the 5 plates by the inhibition factor divided by 5. The values in columns 2, 3, and 4 correspond to the sum of mutants found in the five plates

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