the observations made by Darwin were: "Observation #1: Members of a population often vary in their inherited traits. Observation #2: All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce. " From the observation we can say that the bacteria that can adapt themselves with the adverse environment, that can survive and next, these survived species will produce such species. So, population also increases. Part 2: As per Darwin, some bacteria has mutant genetic material which ca change time to time via mutation when they come close to antibodies. hence, some of the bacteria can survive by mutation but some which can not do any mutation, they are killed completely.