Three small, cryptic plasmids from Aeromonas salmonicida subsp. salmonicida A449.

Area of Science:

• Microbiology
• Bacterial Genetics
• Plasmid Biology

Background:

• Aeromonas salmonicida is a significant fish pathogen.
• Plasmids play a crucial role in bacterial adaptation and virulence.
• Understanding plasmid diversity in A. salmonicida is essential for disease management.

Purpose of the Study:

• To characterize the nucleotide sequences of three small plasmids from Aeromonas salmonicida subsp. salmonicida A449.
• To investigate the replication mechanisms, gene content, and distribution of these plasmids within A. salmonicida strains.

Main Methods:

• Nucleotide sequencing of three plasmids (pAsa1, pAsa2, pAsa3).
• Bioinformatic analysis to identify replication mechanisms (ColE2-type, ColE1-type), mobilization genes (mobABCD), toxin-antitoxin systems, and open reading frames (ORFs).
• Plasmid profiling of 12 A. salmonicida strains to determine the prevalence of identified plasmids.

Main Results:

• Two plasmids (pAsa1, pAsa3) utilize a ColE2-type replication mechanism, while one (pAsa2) uses a ColE1-type replicon.
• All three plasmids contain mobilization genes (mobABCD) but lack transfer genes.
• pAsa1 and pAsa3 harbor toxin-antitoxin gene pairs for stability; orf1 is conserved across all three plasmids.
• pAsa1 and pAsa2 were found in all 12 strains, pAsa3 in 11 strains, and a fourth plasmid (pAsal1) in 7 strains.

Conclusions:

• The sequenced plasmids exhibit distinct but compatible replication strategies, contributing to their maintenance within A. salmonicida.
• The presence of mobilization genes suggests potential for horizontal gene transfer, while toxin-antitoxin systems enhance plasmid stability.
• The widespread distribution of pAsa1 and pAsa2 highlights their importance in the A. salmonicida population.