Scientists, puzzled about the development of penicillin-resistant bacteria in patients who had not been taking penicillin, believe they have found an explanation. The relevant group of patients have dental fillings made of mercury-containing amalgam, and the bacteria the patients develop are immune to mercury poisoning. Scientists have concluded that the genes causing resistance to penicillin are closely bundled on the chromosomes of bacteria with the gene that produces immunity to mercury poisoning. Exposure to the mercury kills off bacteria that lack the relevant immunity gene, and leaves room for those that possess both the mercury-immunity gene and the penicillin-resistance gene to flourish.
The statement 'It is a generalization that, if true, rules out the possibility that some people who do not take penicillin develop bacteria resistant to it' most accurately characterizes the role played in the passage by the unstated assumption that some patients who take penicillin develop bacteria with an immunity to penicillin.