A single regulatory gene is sufficient to alter Vibrio aestuarianus pathogenicity in oysters

Abstract

Oyster diseases caused by pathogenic vibrios pose a major challenge to the sustainability of oyster farming. In France, since 2012 a disease affecting specifically adult oysters has been associated with the presence of Vibrio aestuarianus. Here, by combining genome comparison, phylogenetic analyses and high-throughput infections of strains isolated before or during the recent outbreaks, we show that virulent strains cluster into two V. aestuarianus lineages independently of the sampling dates. The bacterial lethal dose was not different between strains isolated before or after 2012. Hence, the emergence of a new highly virulent clonal strain is unlikely. Each lineage comprises nearly identical strains, the majority of them being virulent, suggesting that within these phylogenetically coherent virulent lineages a few strains have lost their pathogenicity. Comparative genomics allowed the identification of a single frameshift in a non-virulent strain. This mutation affects the varS gene that codes for a signal transduction histidine-protein kinase. Genetic analyses confirmed that varS is necessary for infection of oysters and for a secreted metalloprotease expression. For the first time in a Vibrio species, we show here that VarS is a key factor of pathogenicity.

Antimicrobial peptides in marine invertebrate health and disease

“…Vibrios also cause diseases at other oyster developmental stages. Vibrio aestuarianus is responsible for mortality of adult oysters [23], whereas V. tubiashi causes a necrotic disease in hatcheries [24].…”

Oysters and Vibrios as a Model for Disease Dynamics in Wild Animals

“…This mutation induces a frame shift in the varS gene that codes for the transduction histidine-protein kinase of the sensory system VarS/VarA [51]. The varS gene was intact in the other strains, and genetic analyses confirmed that it is necessary for infection of oysters and for expression of a secreted metalloprotease [46].The pathogenic clonal lineages of V. aestuarianus appear phylogenetically distinct from their closest conspecifics, suggesting that a recent clonal sweep or speciation event has differentiated them from their parental population ( Figure 4B,C) [46]. However, the population structure remains poorly resolved, since sampling has to date been highly biased towards moribund animals.…”

“…Once disease has been detected, live animals can be used in laboratory experiments to investigate the diversity and dynamics of microbes during disease progression. Moreover, these SPF oysters can be used for high-throughput experimental infections with hundreds of bacterial isolates [7,21,46]. Hence, standardized SPF oysters constitute an animal model to (i) sample naturally colonizing vibrios from the environment, (ii) allow natural progression of infection, and (iii) determine virulence mechanisms across populations or strains of infecting vibrios.…”

Oyster mortality

“…The species V. aestuarianus was described as being similar to Vibrio anguillarum, Vibrio ordalii and Vibrio pelagius (Pillidge et al, 1987;Goudenège et al, 2015) and was included in the Anguillarum clade on the basis of multilocus sequence analysis (MLSA) (Sawabe et al, 2007;Goudenège et al, 2015).…”

“…Some strains induce high mortality (> 50 % mortality) in C. gigas whereas others seem avirulent (< 15 % mortality). Goudenège et al (2015) defined three categories to classify V. aestuarianus strains: (1) highly virulent (> 50% mortality at 10 2 CFU/animal), (2) non-virulent (< 50% mortality at 10 7 CFU/animal) and (3) These virulence variations could be explained by the acquisition of virulence genes by lateral transfer inducing a different virulent lineage (Le Roux et al, 2011). All the virulence mechanisms of V. aestuarianus are not known but the virulence factor metalloprotease is involved in the virulence mechanisms of V. aestuarianus (Labreuche et al, 2010;Saulnier et al, 2010;Goudenège et al, 2015).…”