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Staphylococcus (S.) aureus is a leading cause of bacterial infection world-wide, and currently no vaccine is available for humans. Vaccine development relies heavily on clinically relevant infection models. However, the suitability of mice for S. aureus infection models has often been questioned, because experimental infection of mice with human-adapted S. aureus requires very high infection doses. Moreover, mice were not considered to be natural hosts of S. aureus. The latter has been disproven by our recent findings, showing that both laboratory mice, as well as wild small mammals including mice, voles, and shrews, are naturally colonized with S. aureus. Here, we investigated whether mouse-and vole-derived S. aureus strains show an enhanced virulence in mice as compared to the human-adapted strain Newman. Using a step-wise approach based on the bacterial genotype and in vitro assays for host adaptation, we selected the most promising candidates for murine infection models out of a total of 254 S. aureus isolates from laboratory mice as well as wild rodents and shrews. Four strains representing the clonal complexes (CC) 8, 49, and 88 (n = 2) were selected and compared to the human-adapted S. aureus strain Newman (CC8) in murine pneumonia and bacteremia models. Notably, a bank vole-derived CC49 strain, named DIP, was highly virulent in BALB/c mice in pneumonia and bacteremia models, whereas the other murine and vole strains showed virulence similar to or lower than that of Newman. At one tenth of the standard infection dose DIP induced disease severity, bacterial load and host cytokine and chemokine responses in the murine bacteremia model similar to that of Newman. In the pneumonia model, DIP was also more virulent than Newman but the effect was less pronounced. Whole genome sequencing data analysis identified a pore-forming toxin gene, lukF-PV(P83)/lukM, in DIP but not in the other tested S. aureus isolates. To conclude, the mouse-adapted S. aureus strain DIP allows a significant reduction of the inoculation dose in mice and is hence a promising tool to develop clinically more relevant infection models.

Allergies are complex diseases featuring local tissue inflammation, which is characterized by an exaggerated type 2 immune response to environmental compounds known as allergens. Pollens, environmental fungi, and house dust mites are examples of common allergens. Bacteria have a dual role in allergy. Usually, they are associated with protection, however, certain bacterial species promote the development and exacerbation of allergic inflammation. Notably, IgE antibodies specific for bacterial antigens are found in the sera of allergic individuals. This implies that some bacterial factors are allergens, eliciting a specific type 2 immune response. However, to date, only a few of these are molecularly defined. This review summarizes the current knowledge about known bacterial allergens, and it provides an overview of the available techniques for the discovery of new allergens as well as for measuring the immune responses directed against them.

Successful vaccination relies on immune memory, a core competence of the adaptive immune system. This review summarizes the current knowledge about the adaptive immune response to Staphylococcus aureus as well as the bacterial escape mechanisms. The Janus-faced bacteria, both life-threatening pathogens and peaceful cohabitants of their human host, have so far frustrated all attempts at vaccine development. This begs the question of whether the adaptive immune system is at all able to protect against S. aureus infection. In search of an answer the main functions of the adaptive immune system are probed for potential mechanisms of protection against S. aureus, which may be put to the test in future research.

Staphylococcus aureus is a major human pathogen. Superantigens (SAg) are important virulence factors in S. aureus, but the regulation of SAg gene expression is largely unknown. Using 2 sequenced S. aureus strains (COL and Newman) and 4 clinical isolates, regulation of gene expression was investigated in more detail for 12 SAgs. The SAg-encoding genes were expressed in a growth phase-dependent manner: while the egc operon was mainly transcribed at low optical densities, the transcription of seb was induced at high optical densities. The transcript levels of sea, sek, seq, sep, and tst-1 did not change significantly during growth. The T cell-mitogenic activity of supernatants correlated with the transcription data. SaeRS and σB strongly influenced SAg gene transcription. σB activated transcription of seh, tst-1, and of the egc operon. A possible σB-dependent promoter was identified in front of the egc operon. In contrast, a loss of σB enhanced the transcript level of seb, suggesting an indirect effect of the alternative sigma factor on the transcription of this gene. Transcriptional studies of an saeS mutant showed that the two-component system only activates transcription of seb. The influence of σB and SaeRS on the expression of SAg genes was validated by T cell proliferation assays. For sigB mutants in different strains, different effects on the T cell-mitogenic potential were observed depending on the SAg gene repertoire of the isolates.