Biology subjects

Boll, J. M.

Publications and source records attributed to Boll, J. M..

13 recordsLinked to original sources

bioRxiv2026

Phosphate-responsive ornithine lipid biosynthesis remodels outer membrane composition and contributes to polymyxin tolerance

Bacteria frequently encounter environments in which essential nutrients are limiting. Phosphate limitation constrains phospholipid synthesis and challenges maintenance of the cell envelope. While many Gram-negative bacteria respond by producing phosphorous-free membrane lipids, the mechanisms and consequences of this remodeling remain poorly understood in Enterobacterales. Here, we characterize the genetic and physiological basis of ornithine lipid (OL) biosynthesis in the opportunistic pathogens Enterobacter cloacae and Klebsiella pneumoniae. We show that OL biosynthesis depends on OlsF, a bifunctional acyltransferase that catalyzes both acylation steps required for OL synthesis. Phosphate limitation promotes OlsF-dependent OL accumulation through the PhoBR two-component system. Membrane fractionation revealed that OlsF-derived OLs are present in both membrane compartments but are enriched in the outer membrane. OlsF-dependent OL production enhances polymyxin tolerance during phosphate limitation, linking nutrient availability to changes in antimicrobial susceptibility. Together, these findings identify OlsF-dependent OL synthesis as a phosphate-responsive membrane remodeling pathway that reshapes outer membrane composition and influences the physiological properties of the Gram-negative cell envelope.

microbiology↗

bioRxiv2026

DISRUPTION OF A DNA REPAIR PROTEIN PROMOTES ANTIBIOTIC RESISTANCE IN ACINETOBACTER BAUMANNII

Acinetobacter baumannii is a high priority Gram negative opportunistic pathogen known for its high rates of multidrug resistance (MDR). Minocycline (MIN), a tetracycline class antibiotic, is one of the most effective antibiotics for treating A. baumannii infections in patients. Unfortunately, MIN resistance is spreading internationally and has begun to emerge in the United States. While efflux pumps are correlated with MIN resistant A. baumannii, clinical data suggests alternative mechanisms of MIN resistance. To explore the genetic basis for MIN resistance in A. baumannii we employed a machine learning model to predict genetic resistance correlates from clinical isolates. Mutations in ruvB, a DNA repair protein, were strongly correlated with MIN resistant clinical strains of A. baumannii .Consistent with the prediction, tn26 insertion in ruvB in A. baumannii strain AB5075, and deletion of ruvB in strain ATCC 19606, increased MIN minimum inhibitory concentrations to a level that exceeds the MIN resistance breakpoint. RuvB complexes with RuvA and RuvC to resolve Holliday junctions during recombination. However, only ruvB mutants showed the resistance phenotype; neither ruvA nor ruvC mutants were MIN resistant, suggesting loss of the activity of the complex was not the basis for resistance. We observed ruvB mutants produced increased biomass during planktonic growth relative to the other two ruv mutants. Upon examination, the ruvB::tn26 mutant had a 451% increase in biomass and 360% thicker biofilms relative to wildtype. We determined the disruption of ruvB lead to thicker biofilms and enriched in extracellular DNA (eDNA), and DNase I treatment collapsed the enhanced biofilm phenotype and markedly reduced tetracycline class MICs. FLAG-RuvA accumulated within the biofilm matrix in the absence of RuvB, supporting a model in which RuvA contributes to stabilization of eDNA-rich structures. In a murine pneumonia model, ruvB disruption did not significantly alter survival or pulmonary burden in untreated infection but reduced bacterial dissemination and increased minocycline resistance. Together, these findings reveal an unexpected connection between Holliday junction processing, eDNA-rich biofilm architecture, and antibiotic resistance in A. baumannii.

microbiology↗

bioRxiv2026

Environmental context reveals a conditional role of the Tol-Pal system in envelope organization in Acinetobacter baumannii

Gram-negative bacteria must coordinate remodeling of the peptidoglycan cell wall with invagination of the outer membrane to preserve envelope integrity during growth and division. The conserved Tol-Pal system has been implicated in coordinating these processes, yet its physiological contribution to envelope organization remains unclear and may depend on environmental context. Here, we examined the role of Tol-Pal in coordinating envelope remodeling in Acinetobacter baumannii across distinct growth environments. Loss of Tol-Pal did not cause a major population growth defect, and septal peptidoglycan incorporation remained largely preserved under standard laboratory growth conditions. In contrast, under specific environmental conditions--including nutrient-rich media, altered osmotic conditions, and host-like environments--Tol-Pal deficiency disrupted the spatial organization of cell division and cell morphology. Tol-Pal mutants also exhibited modest but reproducible reductions in outer membrane barrier robustness and decreased fitness in environmental and host-associated contexts. Together, these findings demonstrate that Tol-Pal is not an essential component of the core division machinery but instead contributes to the coordinated organization of the Gram-negative envelope under conditions that impose additional physiological demands. More broadly, our results highlight how environmental context can reveal conditional roles for conserved envelope systems that are not apparent during standard laboratory growth. ImportanceThe Gram-negative envelope is a complex, multilayered structure that must remain intact as cells grow and divide across diverse and often challenging environments. Coordination between peptidoglycan remodeling and outer membrane invagination is therefore critical for maintaining envelope organization and cellular fitness. Here, we show that the conserved Tol-Pal system in Acinetobacter baumannii contributes to the spatial organization of cell division and outer membrane robustness under specific environmental conditions. Although Tol-Pal deficiency permits sustained population growth under standard laboratory conditions, its absence disrupts envelope organization and compromises bacterial fitness in environmental and host-associated contexts. These findings demonstrate how environmental conditions can expose conditional roles for conserved envelope systems and highlight the importance of physiological context in shaping bacterial cell envelope organization.

microbiology↗

bioRxiv2026

Outer membrane remodeling via lipid-peptidoglycan crosstalk enables lipooligosaccharide-deficient colistin resistance

Gram-negative bacteria rely on an asymmetric outer membrane (OM) for barrier integrity, with phospholipids confined to the inner leaflet and glycolipids such as lipopolysaccharide (LPS) or lipooligosaccharide (LOS) forming the outer leaflet. Although LPS/LOS was long considered essential, recent findings challenge this view, leaving the mechanistic basis and evolutionary flexibility unclear. Here, we identify lipid asymmetry as a structural checkpoint that governs access to LOS-independent survival. Using Acinetobacter baumannii as a model, we show that disrupting retrograde phospholipid transport and surface phospholipid degradation destabilizes OM lipid balance, creating a permissive state that enables emergence of LOS-deficient, colistin-resistant variants. Integrated lipidomic and transcriptomic analyses reveal a staged remodeling program that reinforces lipoprotein scaffolds, rewires peptidoglycan synthesis, and expands trafficking pathways to stabilize a glycolipid-free envelope. Critically, loss of LOS coincides with sharp repression of PBP1A, and maintaining its activity blocks adaptation, demonstrating interdependence between OM and peptidoglycan homeostasis. We propose a three-state model--basal, permissive, adapted--that explains how envelope architecture gates evolutionary trajectories to antibiotic resistance. SignificanceColistin is a last-resort antibiotic targeting lipopolysaccharide (LPS) or lipooligosaccharide (LOS) in Gram-negative pathogens, and many emerging antimicrobials aim to inhibit LPS/LOS biosynthesis and transport. Resistance usually arises via lipid A modification, which preserves LPS/LOS while reducing colistin binding. Resistance to colistin can also arise via complete loss of LOS, which occurs in some Acinetobacter baumannii strains but is constrained in others, such as strain ATCC 17978. Here, we demonstrate that LOS essentiality is not fixed but dictated by outer membrane architecture. Disrupting phospholipid homeostasis creates a permissive envelope that allows LOS-deficient, colistin-resistant variants to emerge, while reducing peptidoglycan synthesis further promotes this state. These findings identify lipid asymmetry as a structural checkpoint in resistance evolution and suggest that preserving envelope homeostasis could limit bacterial escape from colistin and guide strategies for next-generation antibiotic development.

microbiology↗

bioRxiv2026

PBP1A and LdtJ support cell envelope homeostasis and impact selection of Colistin-resistance in Acinetobacter baumannii

The multilayered cell envelope of Acinetobacter baumannii is an essential structure that maintains cellular integrity and protects the bacterial cell against external stresses and antibiotics. It consists of an inner membrane, a thin peptidoglycan (PG) layer and an asymmetric outer membrane (OM) enriched in lipooligosaccharide (LOS), whose lipid A moiety is the target of colistin, a last-resource antibiotic. Although lipid A is essential in most Gram-negatives, A. baumannii can survive without LOS through envelope remodeling, particularly in strains producing low levels of the bifunctional penicillin-binding protein PBP1A (encoded by mrcA) or in {Delta}mrcA mutants. Here, we identify a functional interplay between the LD-transpeptidase LdtJ, which generates 3-3 cross-links, and PBP1A, which catalyzes 4-3 transpeptidation during PG synthesis. We show that simultaneous inactivation of both enzymes severely affected growth, viability, morphology, and OM homeostasis. PG analyses revealed that the {Delta}ldtJ {Delta}mrcA mutants displays reduced overall cross-linkage and shorter glycan chains, producing a weakened sacculus. Co-immunoprecipitation demonstrated that PBP1A associates with LdtJ, supporting their coordinated activity at sites of PG synthesis. Notably, {Delta}ldtJ {Delta}mrcA mutants exhibited the highest recovery frequency of colistin-resistant, LOS-deficient variants compared with wild type or single mutants. Together, our findings demonstrate that coupling between 4-3 and 3-3 transpeptidation is critical for envelope stability in A. baumannii and highlight how disrupting this coordination favors the emergence of colistin resistance. This work identifies a conserved PG remodeling vulnerability that directly links PG integrity to the evolution of antibiotic resistance, offering a new conceptual framework for destabilizing the A. baumannii envelope. ImportanceThe global rise of multidrug-resistant Acinetobacter baumannii represents an urgent clinical threat, largely driven by its extraordinary capacity to withstand cell envelope damages and escape last-resort antibiotics such as colistin. Although peptidoglycan synthesis and remodeling are known to influence outer membrane stability, how these processes are coordinated in A. baumannii has remained unclear. Here, we identify a crucial interplay between two critical envelope biogenesis and remodeling activities, 4-3 and 3-3 transpeptidations, mediated by the bifunctional PBP1A and the LD-transpeptidase LdtJ, respectively. Disrupting this coordination weakens the peptidoglycan, destabilizes the outer membrane, and increases the emergence frequency of colistin-resistant, LOS-deficient variants. These findings highlight a previously unrecognized vulnerability in the envelope homeostasis of A. baumannii, suggesting that simultaneously targeting DD and LD-transpeptidation could potentiate therapeutic strategies aimed at limiting antibiotic resistance development.

microbiology↗

bioRxiv2025

Peptidoglycan DD-peptidases have distinct activities that impact fitness of Acinetobacter baumannii

The Gram-negative cell envelope is a vital interface between the bacterial cell and its environment. It acts as a selective barrier, blocking harmful agents while permitting nutrient uptake. Additionally, it enables environmental sensing and adaptive responses. Structurally, it is composed of the outer membrane, the cytoplasmic (inner) membrane, and the periplasm, which contains the peptidoglycan layer. Peptidoglycan is a conserved polymer that provides structural integrity, allowing the cell to withstand the internal turgor. It consists of glycan strands connected by short peptides, forming a mesh-like structure. In Gram-negative bacteria, the majority of the peptidoglycan subunits contain tetrapeptides. Tetrapeptides are generated through the action of DD-carboxypeptidases (DD-CPases), which cleave the terminal D-alanine from pentapeptides. Gram-negative bacteria encode multiple DD-CPases, but their precise role in maintaining cell shape and structural integrity remain poorly understood. The nosocomial pathogen Acinetobacter baumannii encodes three putative DD-CPases. To investigate the role of these enzymes, we generated single mutants, as well as double mutants in dacC, dacD, and pbpG, which encode the homologs of Escherichia coli DD-CPases PBP5, PBP6a, PBP6b, and the endopeptidase (DD-EPase) PBP7, respectively. We assessed the mutants for changes in cell morphology, growth dynamics, and stress tolerance. Additionally, we analyzed the composition of their peptidoglycan layers to determine the biochemical consequences of their inactivation. Each mutant exhibited distinct alterations in coccobacillary morphology and growth. Peptidoglycan analysis confirmed the enzymes possess DD-CPase activity, and PBP6b also demonstrated endopeptidase activity. Together, our results demonstrate that each peptidoglycan-modifying enzyme contributes uniquely to cell growth and morphology. These findings underscore their non-redundant functions and suggest their specific activities may serve as valuable targets for developing new antimicrobial therapies. ImportanceDD-peptidases, including carboxypeptidases and endopeptidases are crucial for maintaining cell envelope homeostasis, with distinct roles for each enzyme in cell wall biogenesis and structural integrity. The enzymatic characterization presented in this study not only advance our understanding of fundamental A. baumannii biology but also highlight these enzymatic activities as targets for development of innovative therapeutic strategies to combat infections caused by this multidrug-resistant microbe.

microbiology↗

bioRxiv2025

Molecular interplay between peptidoglycan integrity and outer membrane asymmetry in maintaining cell envelope homeostasis

The bacterial cell envelope is a critical interface with the environment, particularly in Gram-negative species where the outer membrane and peptidoglycan layers coordinate to maintain structural integrity and resist turgor. Although this coordination is essential for survival, the molecular mechanisms linking outer membrane and peptidoglycan homeostasis remain poorly understood. LD-transpeptidases (LDTs) are enzymes that crosslink peptides in peptidoglycan and incorporate D-amino acids, but their physiological roles are not fully defined. Here, we characterize the activity of the LDT enzyme LdtJ in Acinetobacter baumannii and investigate the consequences of its deletion. Loss of LdtJ disrupts cell morphology, downregulates peptidoglycan precursor genes (e.g., dadA, alr), and activates the stringent response, including elevated ppGpp levels and dksA upregulation. These defects are fully suppressed in a {Delta}ldtJ {Delta}mla double mutant, implicating the outer membrane lipid transport Mla pathway in compensatory regulation. RNA sequencing revealed that transcriptional changes in the {Delta}ldtJ mutant are reversed in the double mutant, highlighting a functional interplay between peptidoglycan remodeling and outer membrane lipid asymmetry. Our findings suggest that LdtJ contributes to envelope integrity not only through peptidoglycan modification but also by influencing broader regulatory and metabolic networks. IMPORTANCEAcinetobacter baumannii is a leading cause of hospital-acquired infections and is highly resistant to antibiotics. Its survival relies on the integrity of the cell envelope, composed of the peptidoglycan layer and outer membrane. While LD-transpeptidases (LDTs) are traditionally known for reinforcing peptidoglycan structure through non-canonical crosslinking, our findings reveal that the LdtJ enzyme also plays a critical role in regulating cellular metabolism and stress responses. Deletion of ldtJ results in pronounced growth defects and abnormal cell morphology - phenotypes that are fully suppressed by disrupting the outer membrane lipid asymmetry transport system, Mla. This genetic interaction uncovers a previously unrecognized link between peptidoglycan remodeling and outer membrane lipid homeostasis. These insights deepen our understanding of envelope coordination in Gram-negative bacteria and suggest that targeting interconnected stress response pathways could offer novel strategies to undermine bacterial resilience.

microbiology↗

bioRxiv2024

Aminolipid biosynthesis in response to phosphate limitation promotes colistin tolerance in Acinetobacter baumannii

The Gram-negative outer membrane protects bacterial cells from environmental toxins such as antibiotics. The outer membrane lipid bilayer is asymmetric; while glycerophospholipids compose the periplasmic facing leaflet, the surface layer is enriched with phosphate-containing lipopolysaccharides. The anionic phosphates that decorate the cell surface promote electrostatic interactions with cationic antimicrobial peptides such as colistin, allowing them to penetrate the bilayer, form pores, and lyse the cell. Colistin is prescribed as a last-line therapy to treat multidrug-resistant Gram-negative infections. Acinetobacter baumannii is an ESKAPE pathogen that rapidly develops resistance to antibiotics and persists for extended periods in the host or on abiotic surfaces. Survival in environmental stress such as phosphate scarcity, represents a clinically significant challenge for nosocomial pathogens. In the face of phosphate starvation, certain bacteria encode adaptive strategies, including the substitution of glycerophospholipids with phosphorus-free lipids. In bacteria, phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin are conserved glycerophospholipids that can form lipid bilayers, particularly in the presence of other lipids. Here, we demonstrate that in response to phosphate limitation, conserved regulatory mechanisms induce alternative lipid production in A. baumannii. Specifically, phosphate limitation induces formation of three lipids, including amine-containing ornithine and lysine aminolipids. Mutations that inactivate aminolipid biosynthesis exhibit fitness defects relative to wild type in colistin growth and killing assays. Furthermore, we show that other Gram-negative ESKAPE pathogens accumulate aminolipids under phosphate limiting growth conditions, suggesting aminolipid biosynthesis may represent a broad strategy to overcome cationic antimicrobial peptide-mediated killing. Author SummaryGram-negative ESKAPE pathogens, including Acinetobacter baumannii, are responsible for a dramatic increase in the morbidity and mortality of patients in healthcare settings over the past two decades. Infections are difficult to treat due to antibiotic resistance and tolerance; however, broadly conserved mechanisms that promote antibiotic treatment failure have not been extensively studied. Herein, we identify an alternative lipid biosynthesis pathway that is induced in phosphate starvation that enables Gram-negative ESKAPE pathogens, including A. baumannii, Klebsiella pneumoniae, and Enterobacter cloacae to build lipid bilayers in the absence of glycerophospholipids, which are the canonical bilayers lipid. Replacement of the anionic phosphate in the lipid headgroup with zwitterionic ornithine and lysine promote survival against colistin, a last resort antimicrobial used against Gram-negative infections. These studies suggest that ESKAPE pathogens can remodel their bilayers with phosphate free lipids to overcome colistin treatment and that aminolipid biosynthesis could be targeted to improve antimicrobial treatment.

microbiology↗

bioRxiv2022

PBP1A directly interacts with the divisome complex to promote septal peptidoglycan synthesis in Acinetobacter baumannii

The class A penicillin-binding proteins (aPBPs), PBP1A and PBP1B, are major peptidoglycan synthases that synthesize more than half of the peptidoglycan per generation in Escherichia coli. Whereas aPBPs have distinct roles in peptidoglycan biosynthesis during growth (i.e., elongation and division), they are semi-redundant; disruption of either is rescued by the other to maintain envelope homeostasis and promote proper growth. Acinetobacter baumannii is a nosocomial pathogen that demonstrated a high propensity to overcome antimicrobial treatment. A. baumannii encodes both PBP1A and PBP1B (encoded by mrcA and mrcB, respectively), but only mrcA deletion decreased fitness and contributed to colistin resistance through inactivation of lipooligosaccharide biosynthesis, indicating that PBP1B was not functionally redundant with PBP1A activity. While previous studies suggested a distinct role for PBP1A in division, it was unknown if its role in septal peptidoglycan biosynthesis was direct. Here, we show that A. baumannii PBP1A has a direct role in division through interactions with divisome components. PBP1A localizes to septal sites during growth, where it interacts with the transpeptidase, PBP3, an essential division component that regulates daughter cell formation. PBP3 overexpression was sufficient to rescue the division defect in {Delta}mrcA A. baumannii; however, PBP1A overexpression was not sufficient to rescue the septal defect when PBP3 was inhibited, suggesting their activity is not redundant. Overexpression of a major DD-carboxypeptidase, PBP5, also restored the canonical A. baumannii coccobacilli morphology in {Delta}mrcA cells. Together, these data support a direct role for PBP1A in A. baumannii division and highlights its role as a septal peptidoglycan synthase. ImportancePeptidoglycan biosynthesis is a validated target of {beta}-lactam antibiotics, and it is critical that we understand essential processes in multidrug resistant pathogens such as Acinetobacter baumannii. While model systems such as Escherichia coli have shown that PBP1A is associated with side wall peptidoglycan synthesis, we show herein that A. baumannii PBP1A directly interacts with the divisome component PBP3 to promote division, suggesting a unique role for the enzyme in the highly drug resistant nosocomial pathogen. A. baumannii demonstrated unanticipated resistance and tolerance to envelope-targeting antibiotics, which may be driven by rewired peptidoglycan machinery, and may underlie therapeutic failure during antibiotic treatment.

microbiology↗

bioRxiv2021

Peptidoglycan recycling contributes to outer membrane integrity and carbapenem tolerance in Acinetobacter baumannii

The Gram-negative cell envelope is an essential structure that not only protects the cell against lysis from the internal turgor, but also forms a barrier to limit entry of antibiotics. Some of our most potent bactericidal antibiotics, the {beta}-lactams, exploit the essentiality of the cell envelope by inhibiting its biosynthesis, typically inducing lysis and rapid death. However, many Gram-negative bacteria exhibit "antibiotic tolerance", the ability to sustain viability in the presence of {beta}-lactams for extended time periods. Despite several studies showing that antibiotic tolerance contributes directly to treatment failure, and is a steppingstone in acquisition of true resistance, the molecular factors that promote intrinsic tolerance are not well-understood. Acinetobacter baumannii is a critical-threat nosocomial pathogen notorious for its ability to rapidly develop multidrug resistance. While typically reserved to combat multidrug resistant infections, carbapenem {beta}-lactam antibiotics (i.e., meropenem) are first-line prescriptions to treat A. baumannii infections. Meropenem tolerance in Gram-negative pathogens is characterized by morphologically distinct populations of spheroplasts, but the impact of spheroplast formation is not fully understood. Here, we show that susceptible A. baumannii clinical isolates demonstrate high intrinsic tolerance to meropenem, form spheroplasts with the antibiotic and revert to normal growth after antibiotic removal. Using transcriptomics and genetics screens, we characterized novel tolerance factors and found that outer membrane integrity maintenance, drug efflux and peptidoglycan homeostasis collectively contribute to meropenem tolerance in A. baumannii. Furthermore, outer membrane integrity and peptidoglycan recycling are tightly linked in their contribution to meropenem tolerance in A. baumannii. ImportanceCarbapenem treatment failure associated with "superbug" infections has rapidly increased in prevalence, highlighting an urgent need to develop new therapeutic strategies. Antibiotic tolerance can directly lead to treatment failure but has also been shown to promote acquisition of true resistance within a population. While some studies have addressed mechanisms that promote tolerance, factors that underlie Gram-negative bacterial survival during carbapenem treatment are not well-understood. Here, we characterized a role for peptidoglycan recycling in outer membrane integrity maintenance and carbapenem tolerance in A. baumannii. These studies suggest that the pathogen limits antibiotic concentrations in the periplasm and highlights physiological processes that could be targeted to improve antimicrobial treatment.

microbiology↗

bioRxiv2021

The PhoPQ two-component system promotes carbapenem tolerance in Enterobacterales.

Antibiotic tolerance is an understudied potential contributor to antibiotic treatment failure and the emergence of multidrug-resistant bacteria. The molecular mechanisms governing tolerance remain poorly understood. A prominent type of {beta}-lactam tolerance relies on the formation of cell wall-deficient spheroplasts, which maintain structural integrity via their outer membrane (OM), an asymmetric lipid bilayer consisting of phospholipids on the inner leaflet and a lipid-linked polysaccharide (lipopolysaccharide, LPS) enriched in the outer monolayer on the cell surface. How a membrane structure like LPS, with its reliance on mere electrostatic interactions to maintain stability, is capable of countering internal turgor pressure is unknown. Here, we have uncovered a novel role for the PhoPQ two-component system in tolerance to the {beta}-lactam antibiotic meropenem in Enterobacterales. We found that PhoPQ is induced by meropenem treatment and promotes an increase in 4-amino-4-deoxy-L-aminoarabinose [L-Ara4N] modification of lipid A, the membrane anchor of LPS. L-Ara4N modifications enhance structural integrity, and consequently tolerance to meropenem, in several Enterobacterales species. Importantly, mutational inactivation of the negative PhoPQ regulatory element mgrB (commonly selected for during clinical therapy with the last-resort antibiotic colistin, an antimicrobial peptide [AMP]) results in dramatically enhanced tolerance, suggesting that AMPs can collaterally select for meropenem tolerance via stable overactivation of PhoPQ. Lastly, we identify histidine kinase inhibitors (including an FDA-approved drug) that inhibit PhoPQ-dependent LPS modifications and consequently potentiate meropenem to enhance killing of tolerant cells. In summary, our results suggest that PhoPQ-mediated LPS modifications play a significant role in stabilizing the OM, promoting survival when the primary integrity maintenance structure, the cell wall, is removed. SignificanceTreating an infection with an antibiotic often fails, resulting in a tremendous public health burden. One understudied likely reason for treatment failure is the development of "antibiotic tolerance", the ability of bacteria to survive normally lethal exposure to an antibiotic. Here, we describe a molecular mechanism promoting tolerance. A bacterial stress sensor (PhoPQ) is activated in response to antibiotic (meropenem) treatment and consequently strengthens a bacterial protective "shell" to enhance survival. We also identify inhibitors of this mechanism, opening the door to developing compounds that help antibiotics work better against tolerant bacteria.

microbiology↗

bioRxiv2020

The stringent factor Rel from Mycobacterium abscessus regulates metabolism, but does not promote survival in stress or antibiotics

The stringent response is a broadly conserved stress response system that exhibits functional variability across bacterial clades. Here, we characterize the role of the stringent factor Rel in the non-tuberculous mycobacterial pathogen, Mycobacterium abscessus (Mab). We found that deletion of rel does not ablate (p)ppGpp synthesis, and that rel does not provide a survival advantage in several stress conditions, or in antibiotic treatment. Transcriptional data show that RelMab is involved in regulating expression of anabolism and growth genes in stationary phase. However, it does not activate transcription of stress response or antibiotic resistance genes, and actually represses transcription of many antibiotic resistance genes. This work shows that there is an unannotated (p)ppGpp synthetase in Mab. ImportanceIn this study, we examined the functional roles of the stringent factor Rel in Mycobacterium abscessus (Mab). In most species, stringent factors synthesize the alarmone (p)ppGpp, which globally alters transcription to promote growth arrest and survival under stress and in antibiotic treatment. Our work shows that in Mab, an emerging pathogen which is resistant to many antibiotics, the stringent factor Rel is not solely responsible for synthesizing (p)ppGpp. We find that RelMab downregulates many metabolic genes under stress, but does not upregulate stress response genes and does not promote antibiotic tolerance. This study implies that there is another critical but unannotated (p)ppGpp synthetase in Mab, and suggests that RelMab inhibitors are unlikely to sensitize Mab infections to antibiotic treatment.

microbiology↗

bioRxiv2019

Colistin heteroresistance in Enterobacter cloacae is mediated by PmrAB-independent 4-amino-4-deoxy-l-arabinose addition to lipid A

The Enterobacter cloacae complex (ECC) consists of closely-related, but genetically distinct bacteria commonly associated with the human microbiota. ECC have been increasingly isolated from healthcare-associated infections, demonstrating that these Enterobacteriaceae are emerging nosocomial pathogens. ECC strains can rapidly acquire multidrug resistance to conventional antibiotics. Cationic antimicrobial peptides (CAMPs) have served as therapeutic alternatives because they target the highly conserved lipid A component of the Gram-negative outer membrane to lyse the bacterial cell. Many Gram-negative Enterobacteriaceae fortify their outer membrane with cationic amine-containing moieties to protect from CAMP-inflicted lysis. The PmrAB two-component system (TCS) transcriptionally activates 4-amino-4-deoxy-O_SCPCAPLC_SCPCAP-arabinose (O_SCPCAPLC_SCPCAP-Ara4N) biosynthesis to result in amine moiety addition to lipid A in many Enterobacteriaceae such as E. coli and Salmonella. In contrast, PmrAB in E. cloacae is dispensable for CAMP resistance. Instead, fitness against CAMPs presents as heteroresistance, or a subpopulation of cells that exhibit clinically significant increases in resistance levels compared to the majority population. We demonstrate that E. cloacae lipid A is modified with O_SCPCAPLC_SCPCAP-Ara4N to induce CAMP heteroresistance and that the regulatory mechanism is independent of the PmrABEcl TCS. We show that the response regulator, PhoPEcl, directly binds to the arnBEcl promoter to induce expression of O_SCPCAPLC_SCPCAP-Ara4N biosynthesis and PmrAB-independent addition to the lipid A disaccharolipid. Therefore, we have identified a mechanism of ECC colistin heteroresistance that directly involves the PhoPQ system.\n\nImportanceMembers of the Enterobacter cloacae complex (ECC) are Gram-negative nosocomial pathogens that have emerged within healthcare facilities around the world. ECC infections are associated with immunocompromised patients and infections are often life threatening. The cationic antimicrobial peptide, colistin (polymyxin E), is a last-line treatment option to combat Gram-negative multidrug resistant infections. However, many ECC intrinsically encode a colistin heteroresistance mechanism. Our analysis to characterize colistin heteroresistance in E. cloacae revealed that 4-amino-4-deoxy-O_SCPCAPLC_SCPCAP-arabinose is conjugated to the lipid A disaccharolipid to protect from colistin-mediated lysis. Additionally, this mechanism is directly regulated by the PhoPQEcl two-component system. Elucidation of outer membrane antimicrobial resistance modifications and their regulatory pathways in E. cloacae isolates will advance our understanding of CAMP heteroresistance.

microbiology↗