Beyond the A‐layer: adsorption of lipopolysaccharides and characterization of bacteriophage‐insensitive mutants of Aeromonas salmonicida subsp. salmonicida

Abstract

helped to identify mutations in genes involved in the biogenesis of lipopolysaccharides (LPS) and on an uncharacterized gene (ASA_1998 ). The characterization of the LPS profile and gene complementation assays identified LPS as a phage receptor and confirmed the involvement of the uncharacterized protein ASA_1998 in phage infection. In addition, we confirmed that the presence of an A-layer at the bacterial surface could act as protection against phages. This study brings new elements into our understanding of the phage adsorption to A. salmonicida subsp. salmonicida cells.

Improvements of virulence factor phenotypic tests for Aeromonas salmonicida subsp. salmonicida, a major fish pathogen

“…A‐layer–negative strains can show two types of staining: the pellet might be completely white (Figure 1, E. coli B/R) or very faintly bluish (Figure 1, HER1110). This unspecific staining may be caused by the saline washes not being completely effective or more likely by the partial expression of the modified surface protein (Paquet et al., 2019). The result is not considered to be positive for the A‐layer unless the pellet is completely bright blue as the result for 01‐B526 (Figure 1).…”

“…The three tests developed during this study were then validated by testing them with a series of 23 strains previously tested with the old agar‐based methods and whose results have been published in the past (Daher et al., 2011; Emond‐Rheault et al., 2015; Paquet et al., 2019). Among these strains, 8 presented a negative result for the A‐layer and 5 did not produce proteases with the old tests (Table 2).…”

“…It can be difficult to differentiate positive A‐layer results from negative ones when using the classic agar‐based test, since the blue colour is barely noticeable on the blue agar medium. Using an agar plate for intermediate staining can also make it difficult to conclusively determine the colour (see as example Figure 1c from Paquet et al., 2019). The variation can be caused by a fluctuation of VapA protein surface coverage of the outer membrane of A. salmonicida subsp.…”

“…Intermediate results can also be caused by strains that may have genetic misexpression of the vapA gene or defective transport or anchoring of the protein to the cell surface. In this case, further investigation is required like a Sanger sequencing of the vapA gene (Paquet et al., 2019).…”

“…in the past (Daher et al, 2011;Emond-Rheault et al, 2015;Paquet et al, 2019). Among these strains, 8 presented a negative result for the A-layer and 5 did not produce proteases with the old tests (Table 2).…”