Microphysiological organ-on-chip models offer the potential to improve the prediction of drug safety and efficacy through recapitulation of human physiological responses. The importance of including multiple cell types within tissue models has been well documented. However, the study of cell interactions in vitro can be limited by complexity of the tissue model and throughput of current culture systems. Here, we describe the development of a co-culture microvascular model and relevant assays in a high-throughput thermoplastic organ-on-chip platform, PREDICT96. The system consists of 96 arrayed bilayer microfluidic devices containing retinal microvascular endothelial cells and pericytes cultured on opposing sides of a microporous membrane. Compatibility of the PREDICT96 platform with a variety of quantifiable and scalable assays, including macromolecular permeability, image-based screening, Luminex, and qPCR, is demonstrated. In addition, the bilayer design of the devices allows for channel- or cell type-specific readouts, such as cytokine profiles and gene expression. The microvascular model was responsive to perturbations including barrier disruption, inflammatory stimulation, and fluid shear stress, and our results corroborated the improved robustness of co-culture over endothelial mono-cultures. We anticipate the PREDICT96 platform and adapted assays will be suitable for other complex tissues, including applications to disease models and drug discovery.

CRISPR-Cas systems, which obstruct both viral infection and incorporation of mobile genetic elements by horizontal transfer, are a specific immune response common to prokaryotes. Antiviral protection by CRISPR-Cas comes at a cost, as horizontally-acquired genes may increase fitness and provide rapid adaptation to habitat change. To date, investigations into the prevalence of CRISPR have primarily focused on pathogenic and clinical bacteria, while less is known about CRISPR dynamics in commensal and environmental species. We designed PCR primers and coupled these with DNA sequencing of products to detect and characterize the presence of cas1, a universal CRISPR-associated gene and proxy for the Type II CRISPR1-Cas system, in environmental and non-clinical Enterococcus isolates. CRISPR1-cas1 was detected in approximately 33% of the 275 strains examined, and differences in CRISPR1 carriage between species was significant. Incidence of cas1 in E. hirae was 73%, nearly three times that of E. faecalis (23.6%) and 10 times more frequent than in E. durans (7.1%). Also, this is the first report of CRISPR1 presence in E. durans, as well as in the plant-associated species E. casseliflavus and E. sulfureus. Significant differences in CRISPR1-cas1 incidence among Enterococcus species support the hypothesis that there is a tradeoff between protection and adaptability. The differences in the habitats of enterococcal species may exert varying selective pressure that results in a species-dependent distribution of CRISPR-Cas systems.

Phage have gained renewed interest as an adjunctive treatment for life-threatening infections with the resistant nosocomial pathogen Acinetobacter baumannii. Our understanding of how A. baumannii defends against phage remains limited, although this information could lead to improved antimicrobial therapies. To address this problem, we identified genome-wide determinants of phage susceptibility in A. baumannii using Tn-seq. These studies focused on the lytic phage Loki, which targets Acinetobacter by unknown mechanisms. We identified 41 candidate loci that increase susceptibility to Loki when disrupted, and 10 that decrease susceptibility. Combined with spontaneous resistance mapping, our results support the model that Loki uses the K3 capsule as an essential receptor, and that capsule modulation provides A. baumannii with strategies to control vulnerability to phage. A key center of this control is transcriptional regulation of capsule synthesis and phage virulence by the global regulator BfmRS. Mutations hyperactivating BfmRS simultaneously increase capsule levels, Loki adsorption, Loki replication, and host killing, while BfmRS-inactivating mutations have the opposite effect, reducing capsule and blocking Loki infection. We identified novel BfmRS-activating mutations, including knockouts of a T2 RNase protein and the disulfide formation enzyme DsbA, that hypersensitize bacteria to phage challenge. We further found that mutation of a glycosyltransferase known to alter capsule structure and bacterial virulence can also cause complete phage resistance. Finally, additional factors including lipooligosaccharide and Lon protease act independently of capsule modulation to interfere with Loki infection. This work demonstrates that regulatory and structural modulation of capsule, known to alter A. baumannii virulence, is also a major determinant of susceptibility to phage.

Acinetobacter baumannii is a gram-negative bacterial pathogen that causes challenging nosocomial infections. β-lactam targeting of penicillin-binding protein (PBP)–mediated cell wall peptidoglycan (PG) formation is a well-established antimicrobial strategy. Exposure to carbapenems or zinc (Zn)-deprived growth conditions leads to a rod-to-sphere morphological transition in A. baumannii , an effect resembling that caused by deficiency in the RodA–PBP2 PG synthesis complex required for cell wall elongation. While it is recognized that carbapenems preferentially acylate PBP2 in A. baumannii and therefore block the transpeptidase function of the RodA–PBP2 system, the molecular details underpinning cell wall elongation inhibition upon Zn starvation remain undefined. Here, we report the X-ray crystal structure of A. baumannii PBP2, revealing an unexpected Zn coordination site in the transpeptidase domain required for protein stability. Mutations in the Zn-binding site of PBP2 cause a loss of bacterial rod shape and increase susceptibility to β-lactams, therefore providing a direct rationale for cell wall shape maintenance and Zn homeostasis in A. baumannii . Furthermore, the Zn-coordinating residues are conserved in various β- and γ-proteobacterial PBP2 orthologs, consistent with a widespread Zn-binding requirement for function that has been previously unknown. Due to the emergence of resistance to virtually all marketed antibiotic classes, alternative or complementary antimicrobial strategies need to be explored. These findings offer a perspective for dual inhibition of Zn-dependent PG synthases and metallo-β-lactamases by metal chelating agents, considered the most sought-after adjuvants to restore β-lactam potency against gram-negative bacteria.

Phage have gained renewed interest as an adjunctive treatment for life-threatening infections with the drug-resistant nosocomial pathogen Acinetobacter baumannii. Our understanding of the mechanisms used by A. baumannii to defend against phage remains limited, although this information could lead to improved antimicrobial therapies. To address this problem, we used Tn-seq to identify determinants of phage susceptibility in A. baumannii on a genome-wide scale. These studies focused on the lytic phage Loki, which uses unknown mechanisms to target Acinetobacter. We identified 41 candidate loci that increase susceptibility to Loki when disrupted, and 10 that decrease susceptibility. Combined with spontaneous resistance mapping, our results support the model that Loki uses the bacterial capsule as an essential receptor, and that modulation of capsule provides A. baumannii with key strategies to control vulnerability to phage. The center of this control is transcriptional capsule regulation by the two-component system BfmRS. Mutations hyperactivating BfmRS simultaneously increase capsule levels and Loki infectivity, while the opposite is true with BfmRS-inactivating mutations. We identified novel BfmRS-activating mutations, including knockouts of an RNase T2 homolog (RnaA) and the disulfide isomerase DsbA, that determine the outcome of a phage encounter. We further found that mutation of a glycosyltransferase (Gtr6) known to modify capsule structure and virulence in clinical strains can also confer complete phage resistance. Finally, additional factors including the outer core of lipooligosaccharide act independently of capsule modulation to interfere with Loki infection. This work demonstrates that regulatory and structural modulation of capsule, known to alter A. baumannii virulence, is also a major determinant of susceptibility to phage.

Acinetobacter baumannii is a bacterium the CDC categorizes as an urgent threat due to its distinctive level of antibiotic resistance. In A. baumannii, BfmRS is a global regulatory system that controls much of this resistance through regulating the cell envelope of the bacterium. However, the mechanisms with which BfmRS controls envelope synthesis and antibiotic resistance are unclear. Whether BfmRS regulates certain outer membrane proteins to control antibiotic resistance was studied. To test this hypothesis, reporters of gene expression of two outer membrane proteins, Omp25 and CarO, were constructed and then using the fluorescent protein GFP, the effect of BfmRS activating or inactivating mutations on the strength of gene expression was measured. The two genes were also deleted from the bacterial chromosome, and the resulting change in resistance to colistin, an important “last line of defense” drug against A. baumannii, were determined. From these experiments, the outer membrane proteins were indeed found to be regulated by BfmRS and to contribute to intrinsic colistin resistance. These findings suggest that Omp25 and CarO are important targets of regulation by BfmRS related to antibiotic resistance. Potential binding motifs in their promoter regions that may determine this regulation were also identified. For next steps, we will examine the role of these motifs on the strength of BfmRS‐dependent gene expression by testing truncated promoters with fewer predicted BfmRS target sites. The binding motifs will also be modified through substitution mutations and then effects on gene expression levels will be measured. The effect of Omp25 and CarO on permeability to additional antibiotics and molecules will be tested. Altogether, this work offers insight into the mechanisms of BfmRS control of the cell envelope and drug resistance, which can help us use the system as a potential target for new treatments.

There is an evolving threat to our healthcare system—the rise of antibiotic‐resistant bacteria, such as Acinetobacter baumannii, and the depletion of our few useful antibiotics. These bacteria have developed an ability to fight off increasing numbers of antibiotics used to treat hospitalized patients. To combat these infections, new solutions must be discovered to target the bacteria’s ability to resist drugs and restore the efficacy of our arsenal of antibiotics. One possible therapeutic target is the BfmRS two‐protein signaling system, which activates many genes that determine antibiotic resistance and virulence in A. baumannii. The transcription factor, BfmR, is key to this gene regulation pathway. How the activity of BfmR is controlled is unknown. We hypothesize that a phosphorylation signal activates BfmR, allowing it to form an oligomeric state that binds to and enhances expression of target genes. To test this hypothesis, we developed in vitrophosphorylation conditions using purified BfmR and phosphoramidate, a small molecule phosphodonor. We used phosphate‐affinity polyacrylamide gel electrophoresis to determine that phosphorylation of the purified protein was highly efficient. Size exclusion chromatography analysis of BfmR using high‐performance liquid chromatography (HPLC) was consistent with dimerization of the protein upon phosphorylation. Next steps will include evaluating the binding affinity of the phosphorylated protein to a series of target DNA sequences associated with resistance and virulence genes, chosen from chromatin immunoprecipitation sequencing (ChiP‐Seq) data showing conserved response elements. We will employ electrophoretic mobility shift assays (EMSAs) to analyze how phosphorylation affects BfmR binding to these sequences. The information we uncover on the regulatory mechanisms of BfmR can be used to guide the development of inhibitors that counteract A. baumannii’s resistance to essential antibiotics.