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The Ax21 protein influences virulence and biofilm formation in the nosocomial pathogen Stenotrophomonas maltophilia

Shi-qi An,Ji-Liang Tang

Published 2017 in Archives of Microbiology

ABSTRACT

Ax21 is an outer membrane protein that is extensively conserved in plant pathogenic Xanthomonas and the associated genera Xylella and Stenotrophomonas, including S. maltophilia, some strains of which are hospital-acquired human pathogens (Ryan et al. 2009). Studies on Ax21 in Xanthomonas oryzae pv. oryzae (Xoo) have shown that the protein can be sulphated and that it is secreted into the bacterial medium in association with the outer membrane vesicles (Han et al. 2012; Bahar et al. 2014). Although a sulphated peptide derived from the N-terminus of Ax21 was originally thought to be a specific trigger for XA21-dependent innate immunity in rice, subsequent work has shown that this is not the case. Nevertheless, this peptide does induce defence-related responses in plants (Danna et al. 2011). A second proposed role for Ax21 is as a diffusible signal that controls the gene expression in Xoo as a response to bacterial cell density (Bahar et al. 2014). These observations led us to examine the potential role(s) of Ax21 in S. maltophilia. In 2011, it was reported that the mutation of ax21 had effects on different phenotypes in S. maltophilia (McCarthy et al. 2011). However, this paper was recently retracted due to errors in data presentation of Fig. 2 (McCarthy et al. 2017). Here, we report on the outcomes of repeated key experiments that indicate the pleiotropic nature of ax21 mutation and the effects of addition of the Ax21 protein on the restoration of the wild-type phenotype. Here, our aim was to repeat key experiments that indicate the pleiotropic nature of ax21 mutation and the effects of the addition of the Ax21 protein on the restoration of the wild-type phenotype. Abstract Stenotrophomonas maltophilia is an antibioticresistant Gram-negative pathogen, which is associated with hospital-acquired infection. The genome encodes a protein highly related to the Ax21 protein of Xanthomonas oryzae that is implicated in interactions of this plant pathogen with rice. Here, we report on the pleiotropic nature of ax21 mutation in S. maltophilia and the effects of addition of the Ax21 protein on the restoration of the wild-type phenotype. We show that loss by mutation of Ax21 leads to reduced motility, reduced biofilm formation, reduced tolerance to the antibiotic tobramycin and reduced virulence to larvae of Galleria mellonella, as well as alteration in the expression of specific genes associated with virulence or antibiotic resistance. Addition of the Ax21protein restored motility and the level of gene expression towards wild type. These findings are consistent with the notion that the Ax21 protein is involved in intraspecies communication, although other interpretations cannot be discounted.

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