Special Issue on ‘Microbe–host interactions’

Mucosal surfaces are colonized by both non-pathogenic bacteria and pathobionts, namely resident bacteria possessing in general a high virulence potential. The interplay of pathobionts with soluble or cellular factors of the host immune system and intermicrobial interactions are decisive for the survival of the individual species within a microbiome. Under conditions of imbalanced mucosal immunity, pathobionts cause severe and life-threatening infections. Moreover, fluctuations in the composition of the microbiota due to external stimuli interfere with the homeostasis of a certain host niche and may have dramatic implications in the colonization by pathobionts or obligate pathogenic bacteria [1]. Consequently, local, systemic or chronic diseases are more likely to occur [2,3]. The microbiota of the upper and lower respiratory tract of humans and animals is, in contrast to the gut microbiota, less well studied, and its relevance in pulmonary health and disease is still a matter of debate. However, carriage and infection studies demonstrated that the conversion of e.g. pneumococci (Streptococcus pneumoniae) from commensal to virulent germs depends on the respiratory microbiota [4,5]. Interestingly, the success of pneumococcal colonization is diminished by Haemophilus influenzae and Corynebacterium accolens, whereas pneumococci prevail over Staphylococcus aureus [6–9]. In addition, the fitness of bacteria and their repertoire of virulence factors, surface-exposed or secreted, to facilitate adherence to host tissue or to manipulate the host immune system are critical elements of commensalism or pathogenesis. This Special Issue on ‘Microbe–host interactions’ focuses mainly on respiratory extracellular and intracellular bacterial species that reside, often as commensals, in the upper respiratory tract (URT). The Review articles within this issue provide an overview of the current understanding of the complex microbe– microbe interactions, as well as the biogeography of the microbiota in the URT [10], but also point to the dynamics of lung and gut microbiota given the systemic mutuality that exists between the two communities [11]. Pathometabolic traits and the impact of environmental changes on pulmonary infections are discussed for intracellular pathogens such as Chlamydia pneumoniae and Legionella pneumophila as well as for extracellular streptococci. The various strategies of colonization, which depend on multifactorial pathogen–host interactions and the encountered host niche, as well as immune evasion mechanisms, are representatively summarized for Chlamydia spp. , nontypeable H. influenzae, meningococci, Mycobacterium tuberculosis, Pseudomonas aeruginosa, S. aureus and several streptococcal species including pneumococci, pyogenic streptococci and Streptococcus suis [12–14]. Moreover, the role of lipoproteins in physiological fitness, colonization and resistance against oxidative stress is discussed for pneumococci and M. tuberculosis, demonstrating the multifaceted involvement of this class of surface proteins [15,16]. Overall, the diverse Review articles that have been collected under this Special Issue illustrate how fluctuations in microbiota composition that are dictated by environmental changes, intermicrobial communication and the concerted action of virulence determinants of commensals and pathobionts may determine the potential of a pathobiont for establishing local or pulmonary infections.