From:
“-Second, and more limiting, the development of resistance. Even when the appropriate bacteriophage is applied to the appropriate host, the host bacterium has the ability to mutate its receptor and enlist other defenses to evade the phage. This problem can ben circumvented by using multiple phages, identifying new phages or trying to mutate the original treating phage to recognize the changed host bacterium.”
This point may cause misunderstandings about the efficacy of the Phage therapy:
the host bacterium has the ability to mutate its Phage receptor (example:the receptor for one single phage) but the genetic mutation for changing this receptor present on the bacterial cell wall (about 200 copies for one phage) requires a long time and several replications.
This effect is an example of Phenotypical Delay.
Mutation Rate or Mutation Frequency
A mutation rate is an estimation of the probability of a mutation occurring per cell division and corresponds to the probability of a mutation occurring in the lifetime of a bacterial cell.
A mutation frequency is simply the proportion of mutant bacteria present in a culture.
These terms are often used interchangeably, causing confusion.
The relationship between mutation frequency and the rate at which mutations occur is uncertain.
If a mutation arises early in the culture period, then a large number of mutant progeny occur and this would be represented by a high frequency.
This phenomenon is known as a “jackpot culture” and was first described in 1943 by Luria and Delbrück during their seminal set of experiments investigating the mutation of Escherichia coli from bacteriophage T1 sensitivity to resistance.
Fluctuation Test of Luria and Delbrück
Luria and Delbrück demonstrated that bacteriophage-resistant mutant colonies arise from a sensitive culture of E. coli if bacteriophage T1 is present in excess. Resistant colonies appeared from sensitive cultures, i.e., in which there was clearing,within 12 to 16 h. These bacteria were resistant to bacteriophage T1 but sensitive to other viruses capable of causing lysis in that strain of E. coli.
Luria and Delbrück showed that reversion to sensitivity was a rare event and that, in a growing culture, the proportion of resistant bacteria increased with time. They argued that if the presence of the phage was needed to trigger the change to resistance, then the distribution of mutant colonies should demonstrate a Poisson distribution.
The high variance in the numbers of mutants in the culture,however, led Luria and Delbrück to conclude that resistant mutants were present in the culture before bacteriophage exposure and that the bacteriophage resistance mutation arose independently. The Luria-Delbrück distribution is different from the Poisson distribution in that its variance is greater than 1.
Luria and Delbrück assumed that for a bacterium there was a small fixed chance that a resistance-conferring mutation could occur per unit of time if the bacteria are “in an identical state.” The number of mutated cells in a culture depends on how early the mutation occurred during the growth of the bacterial population. If mutation occurs early in the culture, the number of mutated cells will be higher than if it occurs later.
By the Fluctuation Test is possible to calculate the Mutation Rates and to determine the rate of mutation to Resistance to phage.
