Successful Host Adaptation of IncK2 Plasmids

Area of Science:

• Microbiology
• Genetics
• Molecular Biology

Background:

• The IncK plasmid group comprises IncK1 and IncK2 lineages.
• IncK2 plasmids are predominantly found in poultry, while IncK1 plasmids are found in mammals.
• The physiological reasons for this host specificity remain unclear.

Purpose of the Study:

• To investigate the fitness costs of IncK1 and IncK2 plasmids at mammalian (37°C) and avian (42°C) body temperatures.
• To analyze how plasmid characteristics like conjugation frequency and copy number differ between IncK1 and IncK2 at varying temperatures.
• To explore the impact of these plasmids on the host bacterium's stress response, specifically sigma-32 (σ-32) levels.

Main Methods:

• Comparative analysis of IncK1 and IncK2 plasmid fitness, conjugation frequency, and copy number in *Escherichia coli* at 37°C and 42°C.
• Measurement of σ-32 levels in *E. coli* strains harboring either wild-type or conjugation-deficient IncK plasmids.
• Assessment of plasmid fitness cost through direct competition experiments.

Main Results:

• IncK2 plasmids exhibited higher conjugation frequency and lower copy number at 42°C compared to IncK1.
• IncK2 plasmids imposed a higher overall fitness cost, which was temperature-independent, while IncK1's fitness cost increased at 42°C.
• Increased σ-32 expression was observed in *E. coli* carrying IncK2 plasmids, correlating with temperature and functional Type IV Secretion (T4S) systems, but not with conjugation-deficient plasmids.

Conclusions:

• The distinct prevalence of IncK2 in poultry may be linked to its higher conjugation efficiency and altered fitness cost at avian body temperatures.
• Increased σ-32 activation, potentially mediated by functional T4S assembly in IncK2 plasmids, may contribute to their adaptation in poultry.
• Temperature-dependent fitness costs and stress response modulation provide a physiological basis for the observed host specificity of IncK1 and IncK2 plasmids.