Functional properties of the major outer membrane protein in Stenotrophomonas maltophilia

σP-NagA-L1/L2 Regulatory Circuit Involved in ΔompA299-356-Mediated Increase in β-Lactam Susceptibility in Stenotrophomonas maltophilia

OmpA, the most abundant porin in Stenotrophomonas maltophilia KJ, exists as a two-domain structure with an N-terminal domain of β-barrel structure embedded in the outer membrane and a C-terminal domain collocated in the periplasm. KJΔOmpA299-356, an ompA mutant of S. maltophilia KJ with a truncated OmpA devoid of 299 to 356 amino acids (aa), was able to stably embed in the outer membrane. KJΔOmpA299-356 was more susceptible to β-lactams than wild-type KJ. We aimed to elucidate the mechanism underlying the ΔompA299-356-mediated increase in β-lactam susceptibility (abbreviated as "ΔOmpA299-356 phenotype"). KJΔOmpA299-356 displayed a lower ceftazidime (CAZ)-induced β-lactamase activity than KJ. Furthermore, KJ2, a L1/L2 β-lactamases-null mutant, and KJ2ΔOmpA299-356, a KJ2 mutant with truncated OmpA devoid of299 to 356 aa, had comparable β-lactam susceptibility. Both lines of evidence indicate that decreased β-lactamase activity contributes to the ΔOmpA299-356 phenotype. We analyzed the transcriptome results of KJ and KJΔOmpA299-356, focusing on PG homeostasis-associated genes. Among the 36 genes analyzed, the nagA gene was upregulated 4.65-fold in KJΔOmpA299-356. Deletion of the nagA gene from the chromosome of KJΔOmpA299-356 restored β-lactam susceptibility and CAZ-induced β-lactamase activity to wild-type levels, verifying that nagA-upregulation in KJΔOmpA299-356 contributes to the ΔOmpA299-356 phenotype. Furthermore, transcriptome analysis revealed that rpoE (Smlt3555) and rpoP (Smlt3514) were significantly upregulated in KJΔOmpA299-356. The deletion mutant construction, β-lactam susceptibility, and β-lactamase activity analysis demonstrated that σP, but not σE, was involved in the ΔOmpA299-356 phenotype. A real-time quantitative (qRT-PCR) assay confirmed that nagA is a member of the σP regulon. The involvement of the σP-NagA-L1/L2 regulatory circuit in the ΔOmpA299-356 phenotype was manifested. IMPORTANCE Porins of Gram-negative bacteria generally act as channels that allow the entry or extrusion of molecules. Moreover, the structural role of porins in stabilizing the outer membrane by interacting with peptidoglycan (PG) and the outer membrane has been proposed. The linkage between porin deficiency and antibiotic resistance increase has been reported widely, with a rationale for blocking antibiotic influx. In this study, a link between porin defects and β-lactam susceptibility increase was demonstrated. The underlying mechanism revealed that a novel σP-NagA-L1/L2 regulatory circuit is triggered due to the loss of the OmpA-PG interaction. This study extends the understanding on the porin defect and antibiotic susceptibility. Porin defects may cause opposite impacts on antibiotic susceptibility, which is dependent on the involvement of the defect. Blocking the porin channel role can increase antibiotic resistance; in contrast, the loss of porin structure role may increase antibiotic susceptibility.

Recombinant Ax21 protein is a promising subunit vaccine candidate against Stenotrophomonas maltophilia in a murine infection model

Stenotrophomonas maltophilia is an emerging pathogen that can cause several disease manifestations such as bacteremia, meningitis, respiratory tract infections and others. More seriously, this pathogen has a highly evolving antibiotic resistance profile. Antibiotic misuse is further aggravating the situation by inducing the development of multi- and even pan-resistance. Thus, employing diverse strategies to overcome this increasing antibiotic resistance is of paramount importance. In general, vaccination is one of these strategies that prevents the onset of infection, provides long term protection against infection, and most importantly diminishes the antibiotic consumption, thus, resulting in controlling resistance. Unfortunately, vaccine research concerning S. maltophilia is very scarce in the literature. Ax21 protein is an outer membrane protein implicated in several virulence mechanisms of S. maltophilia such as quorum sensing, biofilm formation, and antibiotic resistance. Our computational analysis of Ax21 revealed its potential immunogenicity. In the current study, Ax21 protein of S. maltophilia was cloned and heterologously expressed in Escherichia coli. Mice were immunized with the purified recombinant antigen using Bacillus Calmette-Guérin(BCG) and incomplete Freund's adjuvant (IFA) as immune-adjuvants. Enzyme-linked immunosorbent assay (ELISA) revealed significant antigen-specific IgG1, IgG2a and total IgG levels in immunized mice which reflected successful immune stimulation. Immunized mice that were challenged with S. maltophilia showed a substantialreduction in bacterial bioburden in lungs, liver, kidneys, and heart. In addition, liver histological examination demonstrated a remarkable decrease in pathological signs such as necrosis, vacuolation, bile duct fibrosis and necrosis, infiltration of inflammatory cells, and hemorrhage. Whole cell ELISA and opsonophagocytic assay confirmed the ability of serum antibodies from immunized mice to bind and facilitate phagocytosis of S. maltophilia, respectively. To our knowledge, this is the first report to demonstrate the vaccine protective efficacy of Ax21 outer membrane protein against S. maltophilia infection.

Advances in the Microbiology of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is an opportunistic pathogen of significant concern to susceptible patient populations. This pathogen can cause nosocomial and community-acquired respiratory and bloodstream infections and various other infections in humans. Sources include water, plant rhizospheres, animals, and foods. Studies of the genetic heterogeneity of S. maltophilia strains have identified several new genogroups and suggested adaptation of this pathogen to its habitats. The mechanisms used by S. maltophilia during pathogenesis continue to be uncovered and explored. S. maltophilia virulence factors include use of motility, biofilm formation, iron acquisition mechanisms, outer membrane components, protein secretion systems, extracellular enzymes, and antimicrobial resistance mechanisms. S. maltophilia is intrinsically drug resistant to an array of different antibiotics and uses a broad arsenal to protect itself against antimicrobials. Surveillance studies have recorded increases in drug resistance for S. maltophilia, prompting new strategies to be developed against this opportunist. The interactions of this environmental bacterium with other microorganisms are being elucidated. S. maltophilia and its products have applications in biotechnology, including agriculture, biocontrol, and bioremediation.

Intranasal immunization with recombinant outer membrane protein A induces protective immune response against Stenotrophomonas maltophilia infection

Stenotrophomonas maltophilia (S. maltophilia), a multi-drug resistant opportunistic pathogen, is associated with nosocomial and community-acquired infections. Preventive and therapeutic strategies for such infections are greatly needed. In this study, sequence alignment analysis revealed that Outer membrane protein A (OmpA) was highly conserved among S. maltophilia strains but shared no significant similarity with human and mouse proteomes. In mice, intranasal immunization with S. maltophilia recombinant OmpA (rOmpA) without additional adjuvant induced sustained mucosal and systemic rOmpA-specific antibody responses. Treatment with rOmpA stimulated significantly higher levels of secretion of IFN-γ, IL-2, and IL-17A (All P<0.05) from the primary splenocytes isolated from rOmpA-immunized mice than from the primary splenocytes isolated from PBS-immunized mice. Furthermore, mice immunized with rOmpA showed significantly reduced bacterial burden in the lung and reduced levels of pro-inflammatory cytokines (TNF-α and IL-6) in bronchoalveolar lavage fluid (BALF) 24 hours after intranasal S. maltophilia infection, indicating that immunization with rOmpA may have protective effects against S. maltophilia challenge in mice. Our findings suggest that intranasal immunization with rOmpA may induce mucosal and systemic immune responses in mice, trigger Th1- and Th17-mediated cellular immune responses, and thus stimulate host immune defense against S. maltophilia infection. These results also demonstrate that intranasal vaccination may offer an alternative approach to current strategies since it induces a mucosal as well as a systemic immune response.

The Major Outer Membrane Protein MopB Is Required for Twitching Movement and Affects Biofilm Formation and Virulence in Two Xylella fastidiosa strains

MopB is a major outer membrane protein (OMP) in Xylella fastidiosa, a bacterial plant pathogen that causes losses on many economically important crops. Based on in silico analysis, the uncharacterized MopB protein of X. fastidiosa contains a β-barrel structure with an OmpA-like domain and a predicted calcium-binding motif. Here, MopB function was studied by mutational analysis taking advantage of the natural competence of X. fastidiosa. Mutants of mopB were constructed in two different X. fastidiosa strains, the type strain Temecula and the more virulent WM1-1. Deletion of the mopB gene impaired cell-to-cell aggregation, surface attachment, and biofilm formation in both strains. Interestingly, mopB deletion completely abolished twitching motility. Electron microscopy of the bacterial cell surface revealed that mopB deletion eliminated type IV and type I pili formation, potentially caused by destabilization of the outer membrane. Both mopB mutants showed reduced virulence using tobacco (Nicotiana tabacum) as a host under greenhouse conditions. These results suggest that MopB has pleiotropic functions in biofilm formation and twitching motility and is important for virulence of X. fastidiosa.