A Lactococcus lactis expression vector set with multiple affinity tags to facilitate isolation and direct labeling of heterologous secreted proteins

Lactiplantibacillus plantarum: a new example of inclusion body producing bacteria

BACKGROUND

Lactic Acid Bacteria such as Lactococcus lactis, Latilactobacillus sakei (basonym: Lactobacillus sakei) and Lactiplantibacillus plantarum (basonym: Lactobacillus plantarum) have gained importance as recombinant cell factories. Although it was believed that proteins produced in these lipopolysaccharides (LPS)-free microorganisms do not aggregate, it has been shown that L. lactis produce inclusion bodies (IBs) during the recombinant production process. These protein aggregates contain biologically active protein, which is slowly released, being a biomaterial with a broad range of applications including the obtainment of soluble protein. However, the aggregation phenomenon has not been characterized so far in L. plantarum. Thus, the current study aims to determine the formation of protein aggregates in L. plantarum and evaluate their possible applications.

RESULTS

To evaluate the formation of IBs in L. plantarum, the catalytic domain of bovine metalloproteinase 9 (MMP-9cat) protein has been used as model protein, being a prone-to-aggregate (PTA) protein. The electron microscopy micrographs showed the presence of electron-dense structures in L. plantarum cytoplasm, which were further purified and analyzed. The ultrastructure of the isolated protein aggregates, which were smooth, round and with an average size of 250-300 nm, proved that L. plantarum also forms IBs under recombinant production processes of PTA proteins. Besides, the protein embedded in these aggregates was fully active and had the potential to be used as a source of soluble protein or as active nanoparticles. The activity determination of the soluble protein solubilized from these IBs using non-denaturing protocols proved that fully active protein could be obtained from these protein aggregates.

CONCLUSIONS

These results proved that L. plantarum forms aggregates under recombinant production conditions. These aggregates showed the same properties as IBs formed in other expression systems such as Escherichia coli or L. lactis. Thus, this places this LPS-free microorganism as an interesting alternative to produce proteins of interest for the biopharmaceutical industry, which are obtained from the IBs in an important number of cases.

Staphylococcus aureus cell wall maintenance - the multifaceted roles of peptidoglycan hydrolases in bacterial growth, fitness, and virulence

Staphylococcus aureus is an important human and livestock pathogen that is well-protected against environmental insults by a thick cell wall. Accordingly, the wall is a major target of present-day antimicrobial therapy. Unfortunately, S. aureus has mastered the art of antimicrobial resistance, as underscored by the global spread of methicillin-resistant S. aureus (MRSA). The major cell wall component is peptidoglycan. Importantly, the peptidoglycan network is not only vital for cell wall function, but it also represents a bacterial Achilles' heel. In particular, this network is continuously opened by no less than 18 different peptidoglycan hydrolases (PGHs) encoded by the S. aureus core genome, which facilitate bacterial growth and division. This focuses attention on the specific functions executed by these enzymes, their subcellular localization, their control at the transcriptional and post-transcriptional levels, their contributions to staphylococcal virulence and their overall importance in bacterial homeostasis. As highlighted in the present review, our understanding of the different aspects of PGH function in S. aureus has been substantially increased over recent years. This is important because it opens up new possibilities to exploit PGHs as innovative targets for next-generation antimicrobials, passive or active immunization strategies, or even to engineer them into effective antimicrobial agents.

Recombinant protein secretion by Bacillus subtilis and Lactococcus lactis: pathways, applications, and innovation potential

Secreted recombinant proteins are of great significance for industry, healthcare and a sustainable bio-based economy. Consequently, there is an ever-increasing need for efficient production platforms to deliver such proteins in high amounts and high quality. Gram-positive bacteria, particularly bacilli such as Bacillus subtilis, are favored for the production of secreted industrial enzymes. Nevertheless, recombinant protein production in the B. subtilis cell factory can be very challenging due to bottlenecks in the general (Sec) secretion pathway as well as this bacterium’s intrinsic capability to secrete a cocktail of highly potent proteases. This has placed another Gram-positive bacterium, Lactococcus lactis, in the focus of attention as an alternative, non-proteolytic, cell factory for secreted proteins. Here we review our current understanding of the secretion pathways exploited in B. subtilis and L. lactis to deliver proteins from their site of synthesis, the cytoplasm, into the fermentation broth. An advantage of this cell factory comparison is that it identifies opportunities for protein secretion pathway engineering to remove or bypass current production bottlenecks. Noteworthy new developments in cell factory engineering are the mini-Bacillus concept, highlighting potential advantages of massive genome minimization, and the application of thus far untapped ‘non-classical’ protein secretion routes. Altogether, it is foreseen that engineered lactococci will find future applications in the production of high-quality proteins at the relatively small pilot scale, while engineered bacilli will remain a favored choice for protein production in bulk.

Differential binding of human and murine IgGs to catalytic and cell wall binding domains of Staphylococcus aureus peptidoglycan hydrolases

Staphylococcus aureus is an opportunistic pathogen causing high morbidity and mortality. Since multi-drug resistant S. aureus lineages are nowadays omnipresent, alternative tools for preventive or therapeutic interventions, like immunotherapy, are urgently needed. However, there are currently no vaccines against S. aureus. Surface-exposed and secreted proteins are regarded as potential targets for immunization against S. aureus infections. Yet, many potential staphylococcal antigens of this category do not elicit protective immune responses. To obtain a better understanding of this problem, we compared the binding of serum IgGs from healthy human volunteers, highly S. aureus-colonized patients with the genetic blistering disease epidermolysis bullosa (EB), or immunized mice to the purified S. aureus peptidoglycan hydrolases Sle1, Aly and LytM and their different domains. The results show that the most abundant serum IgGs from humans and immunized mice target the cell wall-binding domain of Sle1, and the catalytic domains of Aly and LytM. Interestingly, in a murine infection model, these particular IgGs were not protective against S. aureus bacteremia. In contrast, relatively less abundant IgGs against the catalytic domain of Sle1 and the N-terminal domains of Aly and LytM were almost exclusively detected in sera from EB patients and healthy volunteers. These latter IgGs may contribute to the protection against staphylococcal infections, as previous studies suggest that serum IgGs protect EB patients against severe S. aureus infection. Together, these observations focus attention on the use of particular protein domains for vaccination to direct potentially protective immune responses towards the most promising epitopes within staphylococcal antigens.

A new approach to obtain pure and active proteins from Lactococcus lactis protein aggregates

The production of pure and soluble proteins is a complex, protein-dependent and time-consuming process, in particular for those prone-to-aggregate and/or difficult-to-purify. Although Escherichia coli is widely used for protein production, recombinant products must be co-purified through costly processes to remove lipopolysaccharide (LPS) and minimize adverse effects in the target organism. Interestingly, Lactococcus lactis, which does not contain LPS, could be a promising alternative for the production of relevant proteins. However, to date, there is no universal strategy to produce and purify any recombinant protein, being still a protein-specific process. In this context and considering that L. lactis is also able to form functional protein aggregates under overproduction conditions, we explored the use of these aggregates as an alternative source of soluble proteins. In this study, we developed a widely applicable and economically affordable protocol to extract functional proteins from these nanoclusters. For that, two model proteins were used: mammary serum amyloid A3 (M-SAA3) and metalloproteinase 9 (MMP-9), a difficult-to-purify and a prone-to-aggregate protein, respectively. The results show that it is possible to obtain highly pure, soluble, LPS-free and active recombinant proteins from L. lactis aggregates through a cost-effective and simple protocol with special relevance for difficult-to-purify or highly aggregated proteins.

Human antibody responses against non-covalently cell wall-bound Staphylococcus aureus proteins

Human antibody responses to pathogens, like Staphylococcus aureus, are important indicators for in vivo expression and immunogenicity of particular bacterial components. Accordingly, comparing the antibody responses to S. aureus components may serve to predict their potential applicability as antigens for vaccination. The present study was aimed at assessing immunoglobulin G (IgG) responses elicited by non-covalently cell surface-bound proteins of S. aureus, which thus far received relatively little attention. To this end, we applied plasma samples from patients with the genetic blistering disease epidermolysis bullosa (EB) and healthy S. aureus carriers. Of note, wounds of EB patients are highly colonized with S. aureus and accordingly these patients are more seriously exposed to staphylococcal antigens than healthy individuals. Ten non-covalently cell surface-bound proteins of S. aureus, namely Atl, Eap, Efb, EMP, IsaA, LukG, LukH, SA0710, Sle1 and SsaA2, were selected by bioinformatics and biochemical approaches. These antigens were recombinantly expressed, purified and tested for specific IgG responses using human plasma. We show that high exposure of EB patients to S. aureus is mirrored by elevated IgG levels against all tested non-covalently cell wall-bound staphylococcal antigens. This implies that these S. aureus cell surface proteins are prime targets for the human immune system.