Acinetobacter baumannii has emerged as an important nosocomial pathogen, particularly for patients in intensive care units and with invasive indwelling devices. The most recent clinical isolates are resistant to several classes of clinically important antibiotics, greatly restricting the ability to effectively treat critically ill patients. The bacterial envelope is an important driver of A. baumannii disease, both at the level of battling against antibiotic therapy and at the level of protecting from host innate immune function. This review provides a comprehensive overview of key features of the envelope that interface with both the host and antimicrobial therapies. Carbohydrate structures that contribute to protecting from the host are detailed, and mutations that alter these structures, resulting in increased antimicrobial resistance, are explored. In addition, protein complexes involved in both intermicrobial and host-microbe interactions are described. Finally we discuss regulatory mechanisms that control the nature of the cell envelope and its impact on host innate immune function.

Acinetobacter baumannii is increasingly refractory to antibiotic treatment in healthcare settings. As is true of most human pathogens, the genetic path to antimicrobial resistance (AMR) and the role that the immune system plays in modulating AMR during disease are poorly understood. Here we reproduced several routes to fluoroquinolone resistance, performing evolution experiments using sequential lung infections in mice that are replete with or depleted of neutrophils, providing two key insights into the evolution of drug resistance. First, neutropenic hosts acted as reservoirs for the accumulation of drug resistance during drug treatment. Selection for variants with altered drug sensitivity profiles arose readily in the absence of neutrophils, while immunocompetent animals restricted the appearance of these variants. Secondly, antibiotic treatment failure in the immunocompromised host was shown to occur without clinically defined resistance, an unexpected result that provides a model for how antibiotic failure occurs clinically in the absence of AMR. The genetic mechanism underlying both these results is initiated by mutations activating the drug egress pump regulator AdeL, which drives persistence in the presence of antibiotic. Therefore, antibiotic persistence mutations present a two-pronged risk during disease, causing drug treatment failure in the immunocompromised host while simultaneously increasing the emergence of high-level AMR.

Acinetobacter baumannii is increasingly refractory to antibiotic treatment in healthcare settings. As is true of most human pathogens, the genetic path to antimicrobial resistance (AMR) and the role that the immune system plays in modulating AMR during disease are poorly understood. Here we reproduced several routes to fluoroquinolone resistance, performing evolution experiments using sequential lung infections in mice that are replete or depleted of neutrophils, providing two key insights into the evolution of drug resistance. First, neutropenic hosts were demonstrated to act as reservoirs for the accumulation of drug resistance. Selection for variants with altered drug sensitivity profiles arose readily in the absence of neutrophils, while immunocompetent animals restricted the appearance of these variants. Secondly, antibiotic treatment failure was shown to occur without clinically defined resistance, an unexpected result that provides a model for how antibiotic failure occurs clinically in the absence of AMR. The genetic mechanism underlying both these results is initiated by mutations activating the drug egress pump regulator AdeL, which drives persistence in the presence of the antibiotic. Therefore, antibiotic persistence mutations are demonstrated to present a two-pronged risk during disease, causing drug treatment failure in the immunocompromised host while simultaneously increasing the likelihood of high-level AMR acquisition.

Endogenous retroviruses (ERVs), comprising a substantial portion of the vertebrate genome, are remnants of ancient genetic invaders. Although multiple layers of cell-intrinsic control are utilized to prevent retroviral reactivation, ERVs with intact coding potential were found to reactivate in immunodeficient mice. While previous studies have indicated that B cells are indispensable for preventing ERV reactivation, it is not yet clear which B cell population mediates the blockade of ERV emergence to prevent subsequent damage in the host. Here, we employed direct labeling of B cells reactive with emerged ERV particles to characterize the B cell population and clonal repertoire responsible for recognition of ERV, and to study the mechanism by which B cells provide protection against ERV emergence. We found that ERV-reactive B cells are enriched in innate-like B-1 cell compartment that predominantly reside in peritoneal and pleural cavities. We identified ERV-reactive antibodies in unimmunized mice, the level of which further increases upon innate sensor stimulation. B cell receptor repertoire profiling of ERV-reactive B-1 cells revealed increased usage of Igh VH genes that give rise to glycan-specific antibodies, which were further determined to target terminal N-Acetylglucosamine moieties exhibited by endogenous and exogenous viral antigens. We demonstrated that these glycan-specific natural antibodies engage complement pathway to facilitate clearance of reactivated ERV particles. In conclusion, we elucidated the role of glycan-specific B-1 cells and secreted natural antibodies in mediating blockade of ERV emergence through recognition of conserved glycan epitopes. Supported by grants from Howard Hughes Medical Institute