Environmental stresses can alleviate the average deleterious effect of mutations

Journal of Biology

Table 1 The stresses tested, and their influence on average relative mutation effects

Stress	Strength	ε	*		Lethal^†	Bias^‡	p ^§
Acidic stress	pH 5^¶	0.19	0.27	0.30	0.11	-0.18	< 0.01
Unsupplemented^¥	30% old supernatant	0.31	0.27	0.26	0	0.02	NS
High osmolarity	600 mM NaCl	0.43	0.28	0.25	0.05	0.05	NS
Dithiothreitol	1.6 mg/ml	0.30	0.29	0.21	0.05	0.26	< 0.01
Trimethoprim	0.4 μg/ml	0.53	0.28	0.10	0.05	0.33	< 0.0001
Chloramphenicol	1 μg/ml	0.43	0.28	0.15	0	0.30	< 0.001
Low temperature	17°C	1.77	0.27	0.15	0.05	0.07	< 0.03

η = log(ν_F / ν_S) representing the reduction of the parental strain's growth rate by the stress. The average relative mutation effects and are defined in Figure 1 and are calculated here as median values of the mutant library. *Measurements of mutant growth rates in the favorable environment were repeated in parallel with each of the stress measurements. ^†'Lethal' indicates the fraction of mutants showing growth in the favorable media but no growth under stress after one week. ^‡Bias ≡ (-)/ε represents a bias towards alleviation of the mutations' effects under the stress. ^§The p value is from a paired Student's t-test for the difference between mutation effects under stress and under favorable conditions; NS, not significant (p > 0.05). ^¶Acid stress is 0.25 mM sorbic acid and 16 mM citric acid. ^¥The standard favorable environment is defined as 'unsupplemented' stress and is compared to an even more favorable environment constructed by supplementing it with 30% supernatant of an old culture (see text for further details).