Aerococcus viridans var. homari: The presence of capsule and the relationship to virulence in American lobster (Homarus americanus)

•Aerococcus viridans capsule was preserved by lysine-ruthenium red (LRR) fixation.•ATCC 10400 capsule thickness decreased during in vitro culture in tryptic soy broth.•ATCC 10400 virulence and capsule were not restored by culture in lobster serum.•Rabin’s strain virulence and capsule were maintained under all growth conditions.•Strain specific differences in capsule impact virulence.

The relationship between virulence and encapsulation of Aerococcus viridans var. homari was evaluated by growing virulent (Rabin’s) and avirulent (ATCC 10400) strains under varying culture conditions, and during challenge trials. Changes in capsule thickness were monitored using a modified lysine-ruthenium red (LRR) fixation method and transmission electron microscopy. The virulent Rabin’s strain possessed a prominent capsule of 0.252 μm ± 0.061 μm that was diminished by in vitro growth conditions to 0.206 μm ± 0.076 μm. The ATCC 10400 strain capsule thickness decreased from 0.157 μm ± 0.043 μm to 0.117 μm ± 0.043 μm after 10 in vitro passages. The virulent Rabin’s strain capsule was significantly thicker than the avirulent ATCC 10400 strain under all growth conditions. Rabin’s strain, regardless of pre-challenge growth conditions or dose (high dose 107 or low dose 102), was able to kill lobsters in 7 days at 15 °C. ATCC 10400 strain, regardless of pre-challenge growth conditions, killed lobster only at high doses (107) with varying median time to death of ∼15 days, while at low doses (102) all lobsters survived and no bacteria were present after 42 days. This work demonstrates the importance of the thickness of the A. viridans capsule to virulence in the American lobster.

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