Trade-offs constrain adaptive pathways to the type VI secretion system survival

Area of Science:

• Microbiology
• Evolutionary Biology
• Bacterial Interactions

Background:

• The Type VI Secretion System (T6SS) is a bacterial weapon for inter-bacterial competition.
• Mechanisms of T6SS toxicity are known, but bacterial defense strategies against T6SS attack are less understood, particularly in T6SS-deficient species.

Purpose of the Study:

• To investigate the evolutionary pathways of bacterial defense against T6SS attack.
• To identify genetic mechanisms enabling survival against T6SS-mediated toxicity in *Escherichia coli*.

Main Methods:

• Subjecting eight replicate populations of *Escherichia coli* to continuous T6SS attack by *Vibrio cholerae* over approximately 500 generations.
• Analyzing evolved *E. coli* isolates to identify genetic mutations conferring resistance using genomic sequencing.

Main Results:

• *E. coli* populations evolved an average of 27-fold increased survival against T6SS attack.
• Two convergent evolutionary pathways were identified: mutations in *apaH* (in 6/8 populations) and mutations in both *yejM* and *yjeP* (in 2/8 populations).
• Identified mutations were pleiotropic, causing reduced growth rates and increased susceptibility to antibiotics and high pH.

Conclusions:

• Bacterial populations can rapidly evolve resistance to T6SS attack through specific genetic mutations.
• Convergent evolution played a role in the development of T6SS defense mechanisms.
• The evolution of T6SS resistance involves significant trade-offs, impacting bacterial fitness and environmental tolerance.