III.11.1 Some fluctuation tests demonstrate that mutations can occur even in a non-growing culture.
In the 1980’s, the British geneticist J. Cairns and others published a series of papers that threw doubt on the conclusions about the randomness of mutations (Cairns, Overbaugh, & Miller 1988; Cairns & Foster 1991).The relevant experiments took place as more or less classical fluctuation tests and differed only in that the selection factor did not kill the bacteria on the Petri dishes, but only prevented their multiplication.For example, auxotrophic mutant bacteria were used for the experiment, i.e. bacteria that, through mutation entailing loss of a certain biochemical pathway, are not capable of multiplying on mineral medium where, e.g., a certain amino acid is absent.Following seeding on a dish with mineral nutrient medium, the number of revertant colonies was counted, i.e. bacteria capable of growing on this mineral medium.After some time, approximately the same number of colonies grew on the individual dishes, which the authors of the experiments frequently interpreted in that the mutations occurred through the effect of particular selection pressure.
At the present time, it is thought that this interpretation of the obtained results is probably incorrect.Primarily, it is necessary to point out that colonies of mutated bacteria did not grow immediately in these experiments, but rather appeared after a certain time, gradually.This shows that the mutants were not already present in the test tubes from which the individual dishes were inoculated and that the mutations occurred only in the dishes, however, it says nothing about the effect of selection pressure in inducing mutations.The most important conclusion that we can draw from these experiments is that mutations can also occur in a non-growing culture.If the mutation did not in any way affect the ability of the bacteria to overcome the obstacle to growth, in this case neutralize the effect of the auxotrophic mutation, then we would not know about it at all.However, if a mutation allows this obstacle to be eliminated or avoided, for example, due to a reverse mutation, i.e. a mutation that renews the original information in the DNA prior to the formation of the auxotrophic mutation, or a suppressor mutation, i.e. a mutation that, while it does not renew the original state, renews, sometimes only partially, the original biological function, then this will be reflected in the results in that the mutated bacteria begin to multiply and lead to the formation of macroscopic colonies.
It is probable that the selection factor can affect the frequency of formation of mutations (Arber, Naas, & Blot 1994; Foster & Trimarchi 1994; Foster 1999).A hungry and not reproducing cell “knows” that something is wrong and simultaneously has available various SOS-mechanisms, through which it can try to overcome this unfavourable state of affairs.In fact, some of these mechanisms could be specific to a certain degree and could, for example, preferentially generate mutations in genes that are transcribed with maximum velocity (see III.12).However, there is no evidence that a cell would generate only or preferentially mutations that would assist it in overcoming a specific negative factor in the environment, that it would be capable environmentally directed mutation.