1) Surface and secretion systems
We are investigating the cell surface components of A. baumannii (Ab), with an emphasis in the glycoconjugates, such as capsule and glycoproteins and protein secretion systems (Fig 1.) Our group has identified and characterized Type 1 secretion system (T1SS), T2SS, T5SS and T6SS.
Figure 1: Cell surface components and secretion systems identified in Acinetobacter spp.
1.1) Type VI Secretion System (T6SS)
We focus on T6SS, an antibacterial apparatus of Gram-negative bacteria used to kill competitors (Fig. 2). The T6SS is composed of a cytoplasmic structure resembling the tail of contractile bacteriophages which is anchored to the cell envelope through a membrane complex. To export effectors, the T6SS machine needs to cross the PG layer. We recently described that transit through the PG in Acinetobacter occurs via the activity of a novel L,D-endopeptidase enzyme (named TagX) that is encoded in the T6SS gene cluster and is essential for T6SS assembly. Our investigations target insightful fundamental questions regarding T6SS and its regulation.
Figure 2. The type VI secretion system (T6SS) is used for interspecies competition and contains a novel L,D‑endopeptidase, TagX, which is required for transit of the machinery through the peptidoglycan layer. A large, conjugative plasmid, which also contains drug-resistance genes, regulates the expression of T6SS in some clinical isolates.
1.2) Other Type of Secretion System (TSS)
In the lab, we are also investigating T1SS and T2SS and their role in virulence. We are focusing in the regulation of these TSS and the characterization of their effectors (Fig. 3).
Figure 3. Schematic representation of secretory systems type I (T1SS), and type II (T2SS).
2) Acinetobacter pathogenesis
2.1) Ab as uropathogenic bacteria:
Acinetobacter baumannii is largely associated with healthcare-acquired infections, namely pneumonia and septicemia. However, the ability of Ab to cause urinary tract infections (UTI), including catheter-associated UTI, is underappreciated. Our work reveals that Ab is not a homogenous group of pathogens with a stagnant battery of virulence factors; instead, strains appear to acquire unique traits that better equip them to cause disease in specific host niches. Therefore, we are investigating the bacterial factors that allow Ab isolates successfully colonize specific host niches. Regarding to this, we established the first Ab catheter-associated urinary tract infection (CAUTI) murine model using UPAB1, a recent MDR urinary isolate (Fig. 4). UPAB1 carries the plasmid pAB5, a member of the family of large conjugative plasmids that represses T6SS in multiple Acinetobacter strains. pAB5 confers niche specificity, as its carriage improves UPAB1 survival in a CAUTI model and decreases virulence in a pneumonia model. Our results demonstrate that plasmids can impact bacterial infections by controlling the expression of chromosomal genes.
Figure 4. Proposed model of Ab uro-pathogenesis during early CAUTI. Adhesion to the catheter and bladder epithelial cells is a critical first step for an infection. Survival in the hostile bladder environment requires nutrient acquisition strategies and the evasion of innate immunity. pAB5 plays an important role in UPAB1 uro-pathogenesis, but plasmid-associated benefits remain unclear.
2.2) Antibiotic resistance dissemination
Multidrug resistant (MDR) infections caused by the bacterial pathogen Acinetobacter baumannii (Ab) are increasing at alarming rates. Today the MDR frequencies among Ab clinical isolates are higher than any other Gram-negative bacterium. Plasmids serve as vehicles for the spread of MDR among Ab clinical isolates (Fig. 5). We previously demonstrated that some Ab plasmids have the unique ability to abrogate the expression of the T6SS encoded in the host chromosome (Figure 2).
Figure 5. A. baumannii plasmids share common structural features, highlighting common conjugation machinery (red), antibiotic resistance (green), and T6SS regulation (blue) loci. Supporting their role in the emergence of multi-drug resistance (MDR), plasmids from recent strains isolated a few decades ago encoded a single antibiotic resistance gene.
3) OMVs in Bacteroides spp.
Bacteroides spp. are prominent components of the human gut microbiota and are well known to produce large amounts of uniformly sized OMVs, involved in the commensal-host relationship. We confirmed that OMVs produced by these organisms contain large quantities of glycosidases and proteases, with most of them being lipoproteins. Also, we found that all OMV-enriched lipoproteins possess a lipoprotein export sequence (LES), and we show that this signal mediates translocation of these proteins from the periplasmic face of the OM toward the extracellular milieu (Fig. 6). These experiments link, for the first time, surface exposure to recruitment of proteins into OMVs. Our results support the role of OMVs as “public goods” that can be utilized by other organisms with different metabolic capabilities.
Figure 6. A model for SusG LES-mediated surface exposure and packing into OMV. The SusG lipoprotein is probably transported to the OM by a machinery homologous to the LOL system. The presence of a LES sequence mediates the surface exposure of SusG and its incorporation into OMVs. Surface-exposed SusG in OMVs as well as in the OM hydrolyzes starch molecules into oligosaccharides that can be imported by TonB-dependent receptors by the OMV-producing cell, as well as other commensal and pathogen organisms.