Structure of Pseudomonas aeruginosa ribosomes from an aminoglycoside-resistant clinical isolate

Creation of a macrolide antibiotic against non-tuberculous Mycobacterium using late-stage boron-mediated aglycon delivery

Non-tuberculous mycobacteria (NTM) is gaining clinical recognition as a recently emerging pulmonary pathogen. Mycobacterium avium complex (MAC), the most common NTM, is the cause of pulmonary MAC disease. Currently, the macrolide azithromycin (AZM) is the standard first-line antibiotic for treatment of the disease. However, the rise of drug-resistant MAC necessitates the development of alternative therapeutics. Here, we present a late-stage boron-mediated aglycon delivery strategy for selective modification of AZM, generating a library of potential anti-MAC drugs designated KU01 to KU13. Screening of KU01 to KU13 revealed that KU13 exhibited enhanced antimicrobial activity against wild-type and macrolide-resistant MAC compared to AZM. Cryo-electron microscopy analysis indicated that the inserted tercyclic moiety of KU13 formed a robust anchor on the bacterial ribosome, creating a binding pocket with base flipping of U2847, potentially bypassing the standard mechanism of macrolide resistance. These results position KU13 as a promising lead for therapeutics against macrolide-resistant MAC.

Genomic transfer via membrane vesicle: a strategy of giant phage phiKZ for early infection

During infection, the giant phiKZ phage forms a specialized structure at the center of the host cell called the phage nucleus. This structure is crucial for safeguarding viral DNA against bacterial nucleases and for segregating the transcriptional activities of late genes. Here, we describe a morphological entity, the early phage infection (EPI) vesicle, which appears to be responsible for earlier gene segregation at the beginning of the infection process. Using cryo-electron microscopy, electron tomography (ET), and fluorescence microscopy with membrane-specific dyes, we demonstrated that the EPI vesicle is enclosed in a lipid bilayer originating, apparently, from the inner membrane of the bacterial cell. Our investigations further disclose that the phiKZ EPI vesicle contains both viral DNA and viral RNA polymerase (vRNAP). We have observed that the EPI vesicle migrates from the cell pole to the center of the bacterial cell together with ChmA, the primary protein of the phage nucleus. The phage DNA is transported into the phage nucleus after phage maturation, but the EPI vesicle remains outside. We hypothesized that the EPI vesicle acts as a membrane transport agent, efficiently delivering phage DNA to the phage nucleus while protecting it from the nucleases of the bacterium.

IMPORTANCE

Our study shed light on the processes of phage phiKZ early infection stage, expanding our understanding of possible strategies for the development of phage infection. We show that phiKZ virion content during injection is packed inside special membrane structures called early phage infection (EPI) membrane vesicles originating from the bacterial inner cell membrane. We demonstrated the EPI vesicle fulfilled the role of the safety transport unit for the phage genome to the phage nucleus, where the phage DNA would be replicated and protected from bacterial immune systems.

Structural and mechanistic insights into the function of Leishmania ribosome lacking a single pseudouridine modification

Leishmania is the causative agent of cutaneous and visceral diseases affecting millions of individuals worldwide. Pseudouridine (Ψ), the most abundant modification on rRNA, changes during the parasite life cycle. Alterations in the level of a specific Ψ in helix 69 (H69) affected ribosome function. To decipher the molecular mechanism of this phenotype, we determine the structure of ribosomes lacking the single Ψ and its parental strain at ∼2.4-3 Å resolution using cryo-EM. Our findings demonstrate the significance of a single Ψ on H69 to its structure and the importance for its interactions with helix 44 and specific tRNAs. Our study suggests that rRNA modification affects translation of mRNAs carrying codon bias due to selective accommodation of tRNAs by the ribosome. Based on the high-resolution structures, we propose a mechanism explaining how the ribosome selects specific tRNAs.

Phage proteins target and co-opt host ribosomes immediately upon infection

Bacteriophages must seize control of the host gene expression machinery to replicate. To bypass bacterial anti-phage defence systems, this host takeover occurs immediately upon infection. A general understanding of phage mechanisms for immediate targeting of host transcription and translation processes is lacking. Here we introduce an integrative high-throughput approach to uncover phage-encoded proteins that target the gene expression machinery of Pseudomonas aeruginosa immediately upon infection with the jumbo phage ΦKZ. By integrating biochemical, genetic and structural analyses, we identify an abundant and conserved phage factor ΦKZ014 that targets the large ribosomal subunit by binding the 5S ribosomal RNA, and rapidly promotes replication in several clinical isolates. ΦKZ014 is among the earliest ΦKZ proteins expressed after infection and remains bound to ribosomes during the entire translation cycle. Our study provides a strategy to decipher molecular components of phage-mediated host takeover and argues that phage genomes represent an untapped discovery space for proteins that modulate the host gene expression machinery.

Finding priority bacterial ribosomes for future structural and antimicrobial research based upon global RNA and protein sequence analysis

Ribosome-targeting antibiotics comprise over half of antibiotics used in medicine, but our fundamental knowledge of their binding sites is derived primarily from ribosome structures of non-pathogenic species. These include Thermus thermophilus, Deinococcus radiodurans and the archaean Haloarcula marismortui, as well as the commensal and sometimes pathogenic organism, Escherichia coli. Advancements in electron cryomicroscopy have allowed for the determination of more ribosome structures from pathogenic bacteria, with each study highlighting species-specific differences that had not been observed in the non-pathogenic structures. These observed differences suggest that more novel ribosome structures, particularly from pathogens, are required for a more accurate understanding of the level of diversity of the entire bacterial ribosome, with the potential of leading to innovative advancements in antibiotic research. In this study, high accuracy covariance and hidden Markov models were used to annotate ribosomal RNA and protein sequences respectively from genomic sequence, allowing us to determine the underlying ribosomal sequence diversity using phylogenetic methods. This analysis provided evidence that the current non-pathogenic ribosome structures are not sufficient representatives of some pathogenic bacteria, such as Campylobacter pylori, or of whole phyla such as Bacteroidota (Bacteroidetes).

Macrolide therapy in Pseudomonas aeruginosa infections causes uL4 ribosomal protein mutations leading to high-level resistance

OBJECTIVES

Pseudomonas aeruginosa colonizes the cystic fibrosis (CF) airways causing chronic bacterial lung infections. CF patients are routinely treated with macrolides, however, P. aeruginosa is considered insusceptible as consequence of inadequate susceptibility testing leaving resistance mechanism completely overlooked. Here, we investigated a new mechanism of macrolide resistance caused by ribosomal protein mutations.

METHODS

Investigating a longitudinal collection of 529 isolates from CF patients and analysing 5758 protein sequences from different sources, mutations in P. aeruginosa's ribosomal proteins connected to macrolide resistance were identified. Using a modified susceptibility testing protocol, isolates harbouring a mutated uL4 ribosomal protein were tested for resistance against macrolide antibiotics and macrolide-induced quorum sensing modulation. Proteome and ribosome profiling were applied to assess the impact of the mutations on the bacterial physiology.

RESULTS

Five uL4 mutations were identified in isolates from different CF patients. Most mapped to the conserved loop region of uL4 and resulted in increased macrolide tolerance (>10-fold relative to wt strains). Greater concentrations (>10-fold) of macrolide antibiotic were needed to inhibit the growth, reduce swimming motility, and induce redox sensitivity of the uL4 mutants. 16 proteins involved in ribosome adaptation displayed altered expression possibly to compensate for the uL4 mutations, which changed the ribosome stoichiometry without negatively affecting bacterial physiology.

CONCLUSIONS

Macrolide antibiotics should, therefore, be considered as active antimicrobial agents against P. aeruginosa and resistance development should be contemplated when patients are treated with prolonged courses of macrolides. Importantly, improved macrolide susceptibility testing is necessary for the detection of resistant bacteria.

Compact IF2 allows initiator tRNA accommodation into the P site and gates the ribosome to elongation

During translation initiation, initiation factor 2 (IF2) holds initiator transfer RNA (fMet-tRNAifMet) in a specific orientation in the peptidyl (P) site of the ribosome. Upon subunit joining IF2 hydrolyzes GTP and, concomitant with inorganic phosphate (Pi) release, changes conformation facilitating fMet-tRNAifMet accommodation into the P site and transition of the 70 S ribosome initiation complex (70S-IC) to an elongation-competent ribosome. The mechanism by which IF2 separates from initiator tRNA at the end of translation initiation remains elusive. Here, we report cryo-electron microscopy (cryo-EM) structures of the 70S-IC from Pseudomonas aeruginosa bound to compact IF2-GDP and initiator tRNA. Relative to GTP-bound IF2, rotation of the switch 2 α-helix in the G-domain bound to GDP unlocks a cascade of large-domain movements in IF2 that propagate to the distal tRNA-binding domain C2. The C2-domain relocates 35 angstroms away from tRNA, explaining how IF2 makes way for fMet-tRNAifMet accommodation into the P site. Our findings provide the basis by which IF2 gates the ribosome to the elongation phase.

SARS-CoV-2 Membrane Protein: From Genomic Data to Structural New Insights

Severe Acute Respiratory Syndrome CoronaVirus-2 (SARS-CoV-2) is composed of four structural proteins and several accessory non-structural proteins. SARS-CoV-2's most abundant structural protein, Membrane (M) protein, has a pivotal role both during viral infection cycle and host interferon antagonism. This is a highly conserved viral protein, thus an interesting and suitable target for drug discovery. In this paper, we explain the structural nature of M protein homodimer. To do so, we developed and applied a detailed and robust in silico workflow to predict M protein dimeric structure, membrane orientation, and interface characterization. Single Nucleotide Polymorphisms (SNPs) in M protein were retrieved from over 1.2 M SARS-CoV-2 genomes and proteins from the Global Initiative on Sharing All Influenza Data (GISAID) database, 91 of which were located at the predicted dimer interface. Among those, we identified SNPs in Variants of Concern (VOC) and Variants of Interest (VOI). Binding free energy differences were evaluated for dimer interfacial SNPs to infer mutant protein stabilities. A few high-prevalent mutated residues were found to be especially relevant in VOC and VOI. This realization may be a game-changer to structure-driven formulation of new therapeutics for SARS-CoV-2.

Antibiotic Combination Therapy: A Strategy to Overcome Bacterial Resistance to Aminoglycoside Antibiotics

After the first aminoglycoside antibiotic streptomycin being applied in clinical practice in the mid-1940s, aminoglycoside antibiotics (AGAs) are widely used to treat clinical bacterial infections and bacterial resistance to AGAs is increasing. The bacterial resistance to AGAs is owed to aminoglycoside modifying enzyme modification, active efflux pump gene overexpression and 16S rRNA ribosomal subunit methylation, leading to modification of AGAs' structures and decreased concentration of drugs within bacteria. As AGAs's side effects and bacterial resistance, the development of AGAs is time-consuming and difficult. Because bacterial resistance may occur in a short time after application in clinical practice, it was found that the antibacterial effect of the combination was not only better than that of AGAs alone but also reduce the dosage of antibiotics, thereby reducing the occurrence of side effects. This article reviews the clinical use of AGAs, the antibacterial mechanisms, the molecular mechanisms of bacterial resistance, and especially focuses a recent development of the combination of AGAs with other drugs to exert a synergistic antibacterial effect to provide a new strategy to overcome bacterial resistance to AGAs.

The many antibiotic resistance and tolerance strategies of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a bacterial pathogen associated with a wide range of infections and utilizes several strategies to establish and maintain infection including biofilm production, multidrug resistance, and antibiotic tolerance. Multidrug resistance in P. aeruginosa, as well as in all other bacterial pathogens, is a growing concern. Aminoglycoside resistance, in particular, is a major concern in P. aeruginosa infections and must be better understood in order to maintain effective clinical treatment. In this review, the various antibiotic resistance and tolerance mechanisms of Pseudomonas are explored including: classic mutation driven resistance, adaptive resistance, persister cells, small colony variants, phoenix colonies, and biofilms. It is important to further characterize each of these phenotypes and continue to evaluate antibiotic surviving isolates for novel driving mechanisms, so that we are better prepared to combat the rising number of recurrent and recalcitrant infections.

Maturation of 23S rRNA includes removal of helix H1 in many bacteria

In most bacteria, the three ribosomal RNAs (rRNAs) are encoded together in each of several near-identical operons. As soon as the nascent precursor rRNA emerges from RNA polymerase, ribosome assembly begins. This process entails ribosomal protein binding, rRNA folding, rRNA modification, and rRNA processing. In the model organisms Escherichia coli and Bacillus subtilis, rRNA processing results in similar mature rRNAs, despite substantial differences in the cohort of RNAses involved. A recent study of Flavobacterium johnsoniae, a member of the phylum Bacteroidota (formerly Bacteroidetes), revealed that helix H1 of 23S rRNA is absent from ribosomes, apparently a consequence of rRNA maturation. In this work, we mined RNA-seq data from 19 individual organisms and ocean metatranscriptomic samples to compare rRNA processing across diverse bacterial lineages. We found that mature ribosomes from multiple clades lack H1, and typically these ribosomes also lack an encoded H98. For all groups analysed, H1 is predicted to form in precursor rRNA as part of a longer leader-trailer helix. Hence, we infer that evolutionary loss of H98 sets the stage for H1 removal during 50S subunit maturation.

Safe and easy in vitro evaluation of tmRNA-SmpB-mediated trans-translation from ESKAPE pathogenic bacteria

In bacteria, trans-translation is the major quality control system for rescuing stalled ribosomes. It is mediated by tmRNA, a hybrid RNA with properties of both a tRNA and a mRNA, and the small protein SmpB. Because trans-translation is absent in eukaryotes but necessary for bacterial fitness or survival, it is a promising target for the development of novel antibiotics. To facilitate screening of chemical libraries, various reliable in vitro and in vivo systems have been created for assessing trans-translational activity. However, the aim of the current work was to permit the safe and easy in vitro evaluation of trans-translation from pathogenic bacteria, which are obviously the ones we should be targeting. Based on green fluorescent protein (GFP) reassembly during active trans-translation, we have created a cell-free assay adapted to the rapid evaluation of trans-translation in ESKAPE bacteria, with 24 different possible combinations. It can be used for easy high-throughput screening of chemical compounds as well as for exploring the mechanism of trans-translation in these pathogens.

SEVAtile: a standardised DNA assembly method optimised for Pseudomonas

To meet the needs of synthetic biologists, DNA assembly methods have transformed from simple 'cut-and-paste' procedures to highly advanced, standardised assembly techniques. Implementing these standardised DNA assembly methods in biotechnological research conducted in non-model hosts, including Pseudomonas putida and Pseudomonas aeruginosa, could greatly benefit reproducibility and predictability of experimental results. SEVAtile is a Type IIs-based assembly approach, which enables the rapid and standardised assembly of genetic parts - or tiles - to create genetic circuits in the established SEVA-vector backbone. Contrary to existing DNA assembly methods, SEVAtile is an easy and straightforward method, which is compatible with any vector, both SEVA- and non-SEVA. To prove the efficiency of the SEVAtile method, a three-vector system was successfully generated to independently co-express three different proteins in P. putida and P. aeruginosa. More specifically, one of the vectors, pBGDes, enables genomic integration of assembled circuits in the Tn7 landing site, while self-replicatory vectors pSTDesX and pSTDesR enable inducible expression from the XylS/Pm and RhaRS/PrhaB expression systems, respectively. Together, we hope these vector systems will support research in both the microbial SynBio and Pseudomonas field.

Compensatory evolution of Pseudomonas aeruginosa's slow growth phenotype suggests mechanisms of adaptation in cystic fibrosis

Long-term infection of the airways of cystic fibrosis patients with Pseudomonas aeruginosa is often accompanied by a reduction in bacterial growth rate. This reduction has been hypothesised to increase within-patient fitness and overall persistence of the pathogen. Here, we apply adaptive laboratory evolution to revert the slow growth phenotype of P. aeruginosa clinical strains back to a high growth rate. We identify several evolutionary trajectories and mechanisms leading to fast growth caused by transcriptional and mutational changes, which depend on the stage of adaptation of the strain. Return to high growth rate increases antibiotic susceptibility, which is only partially dependent on reversion of mutations or changes in the transcriptional profile of genes known to be linked to antibiotic resistance. We propose that similar mechanisms and evolutionary trajectories, in reverse direction, may be involved in pathogen adaptation and the establishment of chronic infections in the antibiotic-treated airways of cystic fibrosis patients.

Pseudomonas aeruginosa adaptation and evolution in patients with cystic fibrosis

Intense genome sequencing of Pseudomonas aeruginosa isolates from cystic fibrosis (CF) airways has shown inefficient eradication of the infecting bacteria, as well as previously undocumented patient-to-patient transmission of adapted clones. However, genome sequencing has limited potential as a predictor of chronic infection and of the adaptive state during infection, and thus there is increasing interest in linking phenotypic traits to the genome sequences. Phenotypic information ranges from genome-wide transcriptomic analysis of patient samples to determination of more specific traits associated with metabolic changes, stress responses, antibiotic resistance and tolerance, biofilm formation and slow growth. Environmental conditions in the CF lung shape both genetic and phenotypic changes of P. aeruginosa during infection. In this Review, we discuss the adaptive and evolutionary trajectories that lead to early diversification and late convergence, which enable P. aeruginosa to succeed in this niche, and we point out how knowledge of these biological features may be used to guide diagnosis and therapy.

Ribosome-targeting antibacterial agents: Advances, challenges, and opportunities

Ribosomes, which synthesize proteins, are critical organelles for the survival and growth of bacteria. About 60% of approved antibiotics discovered so far combat pathogenic bacteria by targeting ribosomes. However, several issues, such as drug resistance and toxicity, have impeded the clinical use of ribosome-targeting antibiotics. Moreover, the complexity of the bacteria ribosome structure has retarded the discovery of new ribosome-targeting agents that are considered as the key to the drug-resistance and toxicity. To deal with these challenges, efforts such as medicinal chemistry optimization, combination treatment, and new drug delivery system have been developed. But not enough, the development of structural biology and new screening methods bring powerful tools, such as cryo-electron microscopy technology, advanced computer-aided drug design, and cell-free in vitro transcription/translation systems, for the discovery of novel ribosome-targeting antibiotics. Thus, in this paper, we overview the research on different aspects of bacterial ribosomes, especially focus on discussing the challenges in the discovery of ribosome-targeting antibacterial drugs and advances made to address issues such as drug-resistance and selectivity, which, we believe, provide perspectives for the discovery of novel antibiotics.

Structural basis of sequestration of the anti-Shine-Dalgarno sequence in the Bacteroidetes ribosome

Genomic studies have indicated that certain bacterial lineages such as the Bacteroidetes lack Shine-Dalgarno (SD) sequences, and yet with few exceptions ribosomes of these organisms carry the canonical anti-SD (ASD) sequence. Here, we show that ribosomes purified from Flavobacterium johnsoniae, a representative of the Bacteroidetes, fail to recognize the SD sequence of mRNA in vitro. A cryo-electron microscopy structure of the complete 70S ribosome from F. johnsoniae at 2.8 Å resolution reveals that the ASD is sequestered by ribosomal proteins bS21, bS18 and bS6, explaining the basis of ASD inhibition. The structure also uncovers a novel ribosomal protein—bL38. Remarkably, in F. johnsoniae and many other Flavobacteriia, the gene encoding bS21 contains a strong SD, unlike virtually all other genes. A subset of Flavobacteriia have an alternative ASD, and in these organisms the fully complementary sequence lies upstream of the bS21 gene, indicative of natural covariation. In other Bacteroidetes classes, strong SDs are frequently found upstream of the genes for bS21 and/or bS18. We propose that these SDs are used as regulatory elements, enabling bS21 and bS18 to translationally control their own production.

Aminoglycoside Resistance: Updates with a Focus on Acquired 16S Ribosomal RNA Methyltransferases

The clinical usefulness of aminoglycosides has been revisited as an effective choice against β-lactam-resistant and fluoroquinolone-resistant gram-negative bacterial infections. Plazomicin, a next-generation aminoglycoside, was introduced for the treatment of complicated urinary tract infections and acute pyelonephritis. In contrast, bacteria have resisted aminoglycosides, including plazomicin, by producing 16S ribosomal RNA (rRNA) methyltransferases (MTases) that confer high-level and broad-range aminoglycoside resistance. Aminoglycoside-resistant 16S rRNA MTase-producing gram-negative pathogens are widespread in various settings and are becoming a grave concern. This article provides up-to-date information with a focus on aminoglycoside-resistant 16S rRNA MTases.

Structural Heterogeneities of the Ribosome: New Frontiers and Opportunities for Cryo-EM

The extent of ribosomal heterogeneity has caught increasing interest over the past few years, as recent studies have highlighted the presence of structural variations of the ribosome. More precisely, the heterogeneity of the ribosome covers multiple scales, including the dynamical aspects of ribosomal motion at the single particle level, specialization at the cellular and subcellular scale, or evolutionary differences across species. Upon solving the ribosome atomic structure at medium to high resolution, cryogenic electron microscopy (cryo-EM) has enabled investigating all these forms of heterogeneity. In this review, we present some recent advances in quantifying ribosome heterogeneity, with a focus on the conformational and evolutionary variations of the ribosome and their functional implications. These efforts highlight the need for new computational methods and comparative tools, to comprehensively model the continuous conformational transition pathways of the ribosome, as well as its evolution. While developing these methods presents some important challenges, it also provides an opportunity to extend our interpretation and usage of cryo-EM data, which would more generally benefit the study of molecular dynamics and evolution of proteins and other complexes.

Structure of the 70S Ribosome from the Human Pathogen Acinetobacter baumannii in Complex with Clinically Relevant Antibiotics

Acinetobacter baumannii is a Gram-negative bacterium primarily associated with hospital-acquired, often multidrug-resistant (MDR) infections. The ribosome-targeting antibiotics amikacin and tigecycline are among the limited arsenal of drugs available for treatment of such infections. We present high-resolution structures of the 70S ribosome from A. baumannii in complex with these antibiotics, as determined by cryoelectron microscopy. Comparison with the ribosomes of other bacteria reveals several unique structural features at functionally important sites, including around the exit of the polypeptide tunnel and the periphery of the subunit interface. The structures also reveal the mode and site of interaction of these drugs with the ribosome. This work paves the way for the design of new inhibitors of translation to address infections caused by MDR A. baumannii.

•

Cryo-EM structures of the ribosome from the pathogenic bacteria A. baumannii

•

Unique structural features compared with other bacterial ribosomes

•

The site and mode of binding of amikacin and tigecycline to this ribosome

•

A putative alternative tigecycline-binding site at the 50S central protuberance

Current Knowledge and Future Directions in Developing Strategies to Combat Pseudomonas aeruginosa Infection

In the face of growing antimicrobial resistance, there is an urgent need for the development of effective strategies to target Pseudomonas aeruginosa. This metabolically versatile bacterium can cause a wide range of severe opportunistic infections in patients with serious underlying medical conditions, such as those with burns, surgical wounds or people with cystic fibrosis. Many of the key adaptations that arise in this organism during infection are centered on core metabolism and virulence factor synthesis. Interfering with these processes may provide a new strategy to combat infection which could be combined with conventional antibiotics. This review will provide an overview of the most recent work that has advanced our understanding of P. aeruginosa infection. Strategies that exploit this recent knowledge to combat infection will be highlighted alongside potential alternative therapeutic options and their limitations.

Prospects for antimicrobial development in the cryo-EM era – a focus on the ribosome

Resistance to antimicrobial drugs used to treat bacterial, viral, fungal and parasitic infections is a major health concern requiring a coordinated response across the globe. An important aspect in the fight against antimicrobial resistance is the development of novel drugs that are effective against resistant pathogens. Drug development is a complex trans-disciplinary endeavor, in which structural biology plays a major role by providing detailed functional and mechanistic information on an antimicrobial target and its interactions with small molecule inhibitors. Although X-ray crystallography and nuclear magnetic resonance have until now been the methods of choice to characterize microbial targets and drive structure-based drug development, cryo-electron microscopy is rapidly gaining ground in these areas. In this perspective, we will discuss how cryo-electron microscopy is changing our understanding of an established antimicrobial target, the ribosome, and how methodological developments could help this technique become an integral part of the antimicrobial drug discovery pipeline.

Cryo-electron Microscopy Structure of the Acinetobacter baumannii 70S Ribosome and Implications for New Antibiotic Development

Antimicrobial resistance is a major health threat as it limits treatment options for infection. At the forefront of this serious issue is Acinetobacter baumannii, a Gram-negative opportunistic pathogen that exhibits the remarkable ability to resist antibiotics through multiple mechanisms. As bacterial ribosomes represent a target for multiple distinct classes of existing antimicrobial agents, we here use single-particle cryo-electron microscopy (cryo-EM) to elucidate five different structural states of the A. baumannii ribosome, including the 70S, 50S, and 30S forms. We also determined interparticle motions of the 70S ribosome in different tRNA bound states using three-dimensional (3D) variability analysis. Together, our structural data further our understanding of the ribosome from A. baumannii and other Gram-negative pathogens and will enable structure-based drug discovery to combat antibiotic-resistant bacterial infections.IMPORTANCEAcinetobacter baumannii is a severe nosocomial threat largely due to its intrinsic antibiotic resistance and remarkable ability to acquire new resistance determinants. The bacterial ribosome serves as a major target for modern antibiotics and the design of new therapeutics. Here, we present cryo-EM structures of the A. baumannii 70S ribosome, revealing several unique species-specific structural features that may facilitate future drug development to combat this recalcitrant bacterial pathogen.