Fitness cost constrains the spectrum of marR mutations in ciprofloxacin-resistant Escherichia coli

Diversity of Transcriptional Regulatory Adaptation in E. coli

The transcriptional regulatory network (TRN) in bacteria is thought to rapidly evolve in response to selection pressures, modulating transcription factor (TF) activities and interactions. In order to probe the limits and mechanisms surrounding the short-term adaptability of the TRN, we generated, evolved, and characterized knockout (KO) strains in Escherichia coli for 11 regulators selected based on measured growth impact on glucose minimal media. All but one knockout strain (Δlrp) were able to recover growth and did so requiring few convergent mutations. We found that the TF knockout adaptations could be divided into four categories: (i) Strains (ΔargR, ΔbasR, Δlon, ΔzntR, and Δzur) that recovered growth without any regulator-specific adaptations, likely due to minimal activity of the regulator on the growth condition, (ii) Strains (ΔcytR, ΔmlrA, and ΔybaO) that recovered growth without TF-specific mutations but with differential expression of regulators with overlapping regulons to the KO'ed TF, (iii) Strains (Δcrp and Δfur) that recovered growth using convergent mutations within their regulatory networks, including regulated promoters and connected regulators, and (iv) Strains (Δlrp) that were unable to fully recover growth, seemingly due to the broad connectivity of the TF within the TRN. Analyzing growth capabilities in evolved and unevolved strains indicated that growth adaptation can restore fitness to diverse substrates often despite a lack of TF-specific mutations. This work reveals the breadth of TRN adaptive mechanisms and suggests these mechanisms can be anticipated based on the network and functional context of the perturbed TFs.

Clonal and resistance profiles of fluoroquinolone-resistant uropathogenic Escherichia coli in countries with different practices of antibiotic prescription

Background

Antibiotic prescription practices differ between countries, influencing regional antimicrobial resistance prevalence. However, comparisons of clonal diversity among resistant bacteria in countries with different prescribing practices are rare. The rise of fluoroquinolone-resistant Escherichia coli (FQREC), often multidrug-resistant, exacerbates global antibiotic resistance. Unlike in the USA, antibiotics are commonly dispensed in Iraq without prescriptions, leading to widespread overuse and misuse. This study aimed to assess the impact of varying antibiotic use practices on FQREC diversity.

Methods

We compared FQREC prevalence, multidrug resistance, and clonality of FQREC among E. coli isolated from urine submitted between 2017 and 2018 to three US hospitals and two Iraqi hospitals. All FQREC isolates were analyzed for QRDR mutations and the presence of PMQR genes. A subset of FQREC strains from the ST131-H30R/Rx subgroups underwent whole-genome sequencing (WGS) and phylogenetic analysis.

Results

E. coli from Iraq showed significantly higher resistance to all tested antibiotics compared to those from the USA, with 76.2% being FQREC versus 31.2% in the USA (p < 0.01). Iraqi FQREC strains were more frequently multidrug resistant. The predominant subgroup in both countries was ST131-H30, with the notable absence of ST1193 among Iraqi FQREC. Iraqi-origin ST131-H30 strains exhibited higher minimum inhibitory concentrations (MICs) for ciprofloxacin and greater resistance to third-generation cephalosporins (3GC), trimethoprim/sulfamethoxazole (TMP/STX), and imipenem (IMI) than those from the USA. Increased 3GC resistance in Iraqi strains was linked to a higher proportion of bla CTX-M-15-carrying H30Rx subclade isolates. Additionally, Iraqi H30 strains exhibited higher MICs for fluoroquinolones due to more frequent carriage of PMQR determinants compared to US strains. Whole-genome sequencing was performed on 46 Iraqi and 63 US H30 isolates. Phylogenetic analysis revealed two clades-H30R and H30Rx-present in both countries, with isolates from both regions distributed throughout, without the emergence of distinct new major subclones. However, Iraqi isolates tended to cluster in separate subclades, indicating endemic circulation of the strain groups.

Conclusion

In regions like Iraq, where antibiotics are overused and misused, resistance among uropathogenic E. coli to various antibiotics is significantly higher. Most Iraqi resistant strains belong to well-known international groups, and no new highly successful strains have emerged. The absence of ST1193 in Iraq may reflect regional, socioeconomic, demographic, or cultural factors that hinder the success of certain strain groups in the country.

Central role of the ramAR locus in the multidrug resistance in ESBL-Enterobacterales

The aim of this study was to evaluate the proportion of resistance to a temocillin, tigecycline, ciprofloxacin, and chloramphenicol phenotype called t2c2 that resulted from mutations within the ramAR locus among extended-spectrum β-lactamases-Enterobacterales (ESBL-E) isolated in three intensive care units for 3 years in a French university hospital. Two parallel approaches were performed on all 443 ESBL-E included: (i) the minimal inhibitory concentrations of temocillin, tigecycline, ciprofloxacin, and chloramphenicol were determined and (ii) the genomes obtained from the Illumina sequencing platform were analyzed to determine multilocus sequence types, resistomes, and diversity of several tetR-associated genes including ramAR operon. Among the 443 ESBL-E strains included, isolates of Escherichia coli (n = 194), Klebsiella pneumoniae (n = 122), and Enterobacter cloacae complex (Ecc) (n = 127) were found. Thirty-one ESBL-E strains (7%), 16 K. pneumoniae (13.1%), and 15 Ecc (11.8%) presented the t2c2 phenotype in addition to their ESBL profile, whereas no E. coli presented these resistances. The t2c2 phenotype was invariably reversible by the addition of Phe-Arg-β-naphthylamide, indicating a role of resistance-nodulation-division pumps in these observations. Mutations associated with the t2c2 phenotype were restricted to RamR, the ramAR intergenic region (IR), and AcrR. Mutations in RamR consisted of C- or N-terminal deletions and amino acid substitutions inside its DNA-binding domain or within key sites of protein-substrate interactions. The ramAR IR showed nucleotide substitutions involved in the RamR DNA-binding domain. This diversity of sequences suggested that RamR and the ramAR IR represent major genetic events for bacterial antimicrobial resistance.IMPORTANCEMorbimortality caused by infectious diseases is very high among patients hospitalized in intensive care units (ICUs). A part of these outcomes can be explained by antibiotic resistance, which delays the appropriate therapy. The transferable antibiotic resistance gene is a well-known mechanism to explain the high rate of multidrug resistance (MDR) bacteria in ICUs. This study describes the prevalence of chromosomal mutations, which led to additional antibiotic resistance among MDR bacteria. More than 12% of Klebsiella pneumoniae and Enterobacter cloacae complex strains presented mutations within the ramAR locus associated with a dysregulation of an efflux pump called AcrAB-TolC and a porin: OmpF. These dysregulations led to an increase in antibiotic output notably tigecycline, ciprofloxacin, and chloramphenicol associated with a decrease of input for beta-lactam, especially temocillin. Mutations within transcriptional regulators such as ramAR locus played a major role in antibiotic resistance dissemination and need to be further explored.

Multi-drug resistance pattern and genome-wide SNP detection in levofloxacin-resistant uropathogenic Escherichia coli strains

OBJECTIVES

Antibiotic treatment is extremely stressful for bacteria and has profound effects on their viability. Such administration induces physiological changes in bacterial cells, with considerable impact on their genome structure that induces mutations throughout the entire genome. This study investigated drug resistance profiles and structural changes in the entire genome of uropathogenic Escherichia coli (UPEC) strains isolated from six adapted clones that had evolved under laboratory conditions.

METHODS

Eight UPEC strains, including two parental strains and six adapted clones, with different fluoroquinolone resistance levels originally isolated from two patients were used. The minimum inhibitory concentration (MIC) of 28 different antibiotics including levofloxacin was determined for each of the eight strains. In addition, the effects of mutations acquired with increased drug resistance in the levofloxacin-resistant strains on expression of genes implicated to be involved in drug resistance were examined.

RESULTS

Of the eight UPEC strains used to test the MIC of 28 different antibiotics, two highly fluoroquinolone-resistant strains showed increased MIC in association with many of the antibiotics. As drug resistance increased, some genes acquired mutations, including the transcriptional regulator acrR and DNA-binding transcriptional repressor marR. Two strain groups with genetically different backgrounds (GUC9 and GFCS1) commonly acquired mutations in acrR and marR. Notably, acquired mutations related to efflux pump upregulation also contributed to increases in MIC for various antibiotics other than fluoroquinolone.

CONCLUSIONS

The present results obtained using strains with artificially acquired drug resistance clarify the underlying mechanism of resistance to fluoroquinolones and other types of antibiotics.

The induced and intrinsic resistance of Escherichia coli to sanguinarine is mediated by AcrB efflux pump

IMPORTANCE

The use of plant extracts is increasing as an alternative to synthetic compounds, especially antibiotics. However, there is no sufficient knowledge on the mechanisms and potential risks of antibiotic resistance induced by these phytochemicals. In the present study, we found that stable drug resistant mutants of E. coli emerged after repetitive exposure to sanguinarine and demonstrated that the AcrB efflux pump contributed to the emerging of induced and intrinsic resistance of E. coli to this phytochemical. Our results offered some insights into comprehending and preventing the onset of drug-resistant strains when utilizing products containing sanguinarine.

QUIRMIA-A Phenotype-Based Algorithm for the Inference of Quinolone Resistance Mechanisms in Escherichia coli

OBJECTIVES

Quinolone resistance in Escherichia coli occurs mainly as a result of mutations in the quinolone-resistance-determining regions of gyrA and parC, which encode the drugs' primary targets. Mutational alterations affecting drug permeability or efflux as well as plasmid-based resistance mechanisms can also contribute to resistance, albeit to a lesser extent. Simplifying and generalizing complex evolutionary trajectories, low-level resistance towards fluoroquinolones arises from a single mutation in gyrA, while clinical high-level resistance is associated with two mutations in gyrA plus one mutation in parC. Both low- and high-level resistance can be detected phenotypically using nalidixic acid and fluoroquinolones such as ciprofloxacin, respectively. The aim of this study was to develop a decision tree based on disc diffusion data and to define epidemiological cut-offs to infer resistance mechanisms and to predict clinical resistance in E. coli. This diagnostic algorithm should provide a coherent genotype/phenotype classification, which separates the wildtype from any non-wildtype and further differentiates within the non-wildtype.

METHODS

Phenotypic susceptibility of 553 clinical E. coli isolates towards nalidixic acid, ciprofloxacin, norfloxacin and levofloxacin was determined by disc diffusion, and the genomes were sequenced. Based on epidemiological cut-offs, we developed a QUInolone Resistance Mechanisms Inference Algorithm (QUIRMIA) to infer the underlying resistance mechanisms responsible for the corresponding phenotypes, resulting in the categorization as "susceptible" (wildtype), "low-level resistance" (non-wildtype) and "high-level resistance" (non-wildtype). The congruence of phenotypes and whole genome sequencing (WGS)-derived genotypes was then assigned using QUIRMIA- and EUCAST-based AST interpretation.

RESULTS

QUIRMIA-based inference of resistance mechanisms and sequencing data were highly congruent (542/553, 98%). In contrast, EUCAST-based classification with its binary classification into "susceptible" and "resistant" isolates failed to recognize and properly categorize low-level resistant isolates.

CONCLUSIONS

QUIRMIA provides a coherent genotype/phenotype categorization and may be integrated in the EUCAST expert rule set, thereby enabling reliable detection of low-level resistant isolates, which may help to better predict outcome and to prevent the emergence of clinical resistance.

Contribution of efflux and mutations in fluoroquinolone susceptibility in MDR enterobacterial isolates: a quantitative and molecular study

OBJECTIVES

The emergence of MDR strains is a public health problem in the management of associated infections. Several resistance mechanisms are present, and antibiotic efflux is often found at the same time as enzyme resistance and/or target mutations. However, in the laboratory routinely, only the latter two are identified and the prevalence of antibiotic expulsion is underestimated, causing a misinterpretation of the bacterial resistance phenotype. The development of a diagnostic system to quantify the efflux routinely would thus improve the management of patients.

METHODS

A quantitative technique based on detection of clinically used fluoroquinolones was investigated in Enterobacteriaceae clinical strains with a high or basal efflux activity. The detail of efflux involvement was studied from MIC determination and antibiotic accumulation inside bacteria. WGS was carried out on selected strains to determine the genetic background associated with efflux expression.

RESULTS

Only 1 Klebsiella pneumoniae isolate exhibited a lack of efflux whereas 13 isolates had a basal efflux and 8 presented efflux pump overexpression. The antibiotic accumulation evidenced the efficacy of the efflux mechanism in strains, and the contribution of dynamic expulsion versus target mutations in fluoroquinolone susceptibility.

CONCLUSIONS

We confirmed that phenylalanine arginine β-naphthylamide is not a reliable marker of efflux due to the affinity of the AcrB efflux pump for different substrates. We have developed an accumulation test that can be used efficiently on clinical isolates collected by the biological laboratory. The experimental conditions and protocols ensure a robust assay that with improvements in practice, expertise and equipment could be transferred to the hospital laboratory to diagnose the contribution of efflux in Gram-negative bacteria.

TCA and SSRI Antidepressants Exert Selection Pressure for Efflux-Dependent Antibiotic Resistance Mechanisms in Escherichia coli

Microbial diversity is reduced in the gut microbiota of animals and humans treated with selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs). The mechanisms driving the changes in microbial composition, while largely unknown, is critical to understand considering that the gut microbiota plays important roles in drug metabolism and brain function. Using Escherichia coli, we show that the SSRI fluoxetine and the TCA amitriptyline exert strong selection pressure for enhanced efflux activity of the AcrAB-TolC pump, a member of the resistance-nodulation-cell division (RND) superfamily of transporters. Sequencing spontaneous fluoxetine- and amitriptyline-resistant mutants revealed mutations in marR and lon, negative regulators of AcrAB-TolC expression. In line with the broad specificity of AcrAB-TolC pumps these mutants conferred resistance to several classes of antibiotics. We show that the converse also occurs, as spontaneous chloramphenicol-resistant mutants displayed cross-resistance to SSRIs and TCAs. Chemical-genomic screens identified deletions in marR and lon, confirming the results observed for the spontaneous resistant mutants. In addition, deletions in 35 genes with no known role in drug resistance were identified that conferred cross-resistance to antibiotics and several displayed enhanced efflux activities. These results indicate that combinations of specific antidepressants and antibiotics may have important effects when both are used simultaneously or successively as they can impose selection for common mechanisms of resistance. Our work suggests that selection for enhanced efflux activities is an important factor to consider in understanding the microbial diversity changes associated with antidepressant treatments. IMPORTANCE Antidepressants are prescribed broadly for psychiatric conditions to alter neuronal levels of synaptic neurotransmitters such as serotonin and norepinephrine. Two categories of antidepressants are selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs); both are among the most prescribed drugs in the United States. While it is well-established that antidepressants inhibit reuptake of neurotransmitters there is evidence that they also impact microbial diversity in the gastrointestinal tract. However, the mechanisms and therefore biological and clinical effects remain obscure. We demonstrate antidepressants may influence microbial diversity through strong selection for mutant bacteria with increased AcrAB-TolC activity, an efflux pump that removes antibiotics from cells. Furthermore, we identify a new group of genes that contribute to cross-resistance between antidepressants and antibiotics, several act by regulating efflux activity, underscoring overlapping mechanisms. Overall, this work provides new insights into bacterial responses to antidepressants important for understanding antidepressant treatment effects.

Genomic features of in vitro selected mutants of Escherichia coli with decreased susceptibility to tigecycline

OBJECTIVES

The increase in multidrug-resistant bacteria has reached an alarming rate globally, making it necessary to understand the underlying mechanisms mediating resistance in order to discover new therapeutics. Tigecycline (TGC) is a last-resort antimicrobial agent for the treatment of serious infections caused by extensively drug-resistant Enterobacteriaceae.

METHODS

The TGC-resistant Escherichia coli mutants were obtained by exposing three different TGC-susceptible isolates belonging to ST131 (n = 2) and ST405 (n = 1) to increasing concentrations of TGC. The genetic alterations associated with reduced susceptibility to TGC were identified using whole genome sequencing. The fitness cost of TGC resistance acquisition, as well as incidence of cross-resistance, was also investigated.

RESULTS

The TGC minimum inhibitory concentrations (MICs) of in vitro selected mutants were elevated 8 to 32 times compared with ancestral strains. Inactivating mutations (frameshift and nonsense) or amino acid substitutions were identified in genes encoding proteins with diverse functions, including AcrAB efflux pump or its regulators (lon and marR), Lipopolysaccharides (LPS) inner core biosynthesis enzymes (waaQ and eptB), ribosomal S9 protein (rpsI), and RNA polymerase β subunit. In most cases (but not all), acquisition of TGC resistance was associated with a fitness cost. While TGC resistance development was associated with cross-resistance to other members of the tetracycline family and chloramphenicol, hypersensitivity to nitrofurantoin was identified among heptose III-less LPS mutants.

CONCLUSION

TGC resistance among the studied mutants was found to be multifactorial with extrusion by efflux transports being the most common mechanism. The LPS inner core biosynthesis pathway, as well as ribosomal S9 protein, could be additional targets for TGC resistance.

The Extent of Antimicrobial Resistance Due to Efflux Pump Regulation

A key mechanism driving antimicrobial resistance (AMR) stems from the ability of bacteria to up-regulate efflux pumps upon exposure to drugs. The resistance gained by this up-regulation is pliable because of the tight regulation of efflux pump levels. This leads to temporary enhancement in survivability of bacteria due to higher efflux pump levels in the presence of antibiotics, which can be reversed when the cells are no longer exposed to the drug. Knowledge of the extent of resistance thus gained would inform intervention strategies aimed at mitigating AMR. Here, we combine mathematical modeling and experiments to quantify the maximum extent of resistance that efflux pump up-regulation can confer via phenotypic induction in the presence of drugs and genotypic abrogation of regulation. Our model describes the dynamics of drug transport in and out of cells coupled with the associated regulation of efflux pump levels and predicts the increase in the minimum inhibitory concentration (MIC) of drugs due to such regulation. To test the model, we measured the uptake and efflux as well as the MIC of the compound ethidium bromide (EtBr), a substrate of the efflux pump LfrA, in wild-type Mycobacterium smegmatis mc²155, as well as in two laboratory-generated strains. Our model captured the observed EtBr levels and MIC fold-changes quantitatively. Further, the model identified key parameters associated with the resulting resistance, variations in which could underlie the extent to which such resistance arises across different drug-bacteria combinations, potentially offering tunable handles to optimize interventions aimed at minimizing AMR.

The Atypical Antipsychotic Quetiapine Promotes Multiple Antibiotic Resistance in Escherichia coli

Atypical antipsychotic (AAP) medication is a critical tool for treating symptoms of psychiatric disorders. While AAPs primarily target dopamine (D2) and serotonin (5HT2A and 5HT1A) receptors, they also exhibit intrinsic antimicrobial activity as an off-target effect. Because AAPs are often prescribed to patients for many years, a potential risk associated with long-term AAP use is the unintended emergence of bacteria with antimicrobial resistance (AMR). Here, we show that exposure to the AAP quetiapine at estimated gut concentrations promotes AMR in Escherichia coli after 6 weeks. Quetiapine-exposed isolates exhibited an increase in MICs for ampicillin, tetracycline, ceftriaxone, and levofloxacin. By whole-genome sequencing analysis, we identified mutations in genes that confer AMR, including the repressor for the multiple antibiotic resistance mar operon (marR), and real-time reverse transcription-quantitative PCR (RT-qPCR) analysis showed increased levels of marA, acrA, and tolC mRNAs and reduced levels of ompF mRNA in the isolates carrying marR mutations. To determine the contribution of each marR mutation to AMR, we constructed isogenic strains carrying individual mutant marR alleles in the parent background and reevaluated their resistance phenotypes using MIC and RT-qPCR assays. While marR mutations induced robust activity of the mar operon, they resulted in only modest increases in MICs. Interestingly, although these marR mutations did not fully recapitulate the AMR phenotype of the quetiapine-exposed isolates, we show that marR mutations promote growth fitness in the presence of quetiapine. Our findings revealed an important link between the use of AAPs and AMR development in E. coli. IMPORTANCE AAP medication is a cornerstone in the treatment of serious psychiatric disease. The AAPs are known to exhibit antimicrobial activity; therefore, a potential unintended risk of long-term AAP use may be the emergence of AMR, although such risk has received little attention. In this study, we describe the development of multidrug antibiotic resistance in Escherichia coli after 6 weeks of exposure to the AAP quetiapine. Investigation of mutations in the marR gene, which encodes a repressor for the multiple antibiotic resistance (mar) operon, reveals a potential mechanism that increases the fitness of E. coli in the presence of quetiapine. Our findings establish a link between the use of AAPs and AMR development in bacteria.

The role of chemotaxis and efflux pumps on nitrate reduction in the toxic regions of a ciprofloxacin concentration gradient

Spatial concentration gradients of antibiotics are prevalent in the natural environment. Yet, the microbial response in these heterogeneous systems remains poorly understood. We used a microfluidic reactor to create an artificial microscopic ecosystem that generates diffusive gradients of solutes across interconnected microenvironments. With this reactor, we showed that chemotaxis toward a soluble electron acceptor (nitrate) allowed Shewanella oneidensis MR-1 to inhabit and sustain metabolic activity in highly toxic regions of the antibiotic ciprofloxacin (>80× minimum inhibitory concentration, MIC). Acquired antibiotic resistance was not observed for cells extracted from the reactor, so we explored the role of transient adaptive resistance by probing multidrug resistance (MDR) efflux pumps, ancient elements that are important for bacterial physiology and virulence. Accordingly, we constructed an efflux pump deficient mutant (∆mexF) and used resistance-nodulation-division (RND) efflux pump inhibitors (EPIs). While batch results showed the importance of RND efflux pumps for microbial survival, microfluidic studies indicated that these pumps were not necessary for survival in antibiotic gradients. Our work contributes to an emerging body of knowledge deciphering the effects of antibiotic spatial heterogeneity on microorganisms and highlights differences of microbial response in these systems versus well-mixed batch conditions.

Mutant RNA polymerase can reduce susceptibility to antibiotics via ppGpp-independent induction of a stringent-like response

Background

Mutations in RNA polymerase (RNAP) can reduce susceptibility to ciprofloxacin in Escherichia coli, but the mechanism of transcriptional reprogramming responsible is unknown. Strains carrying ciprofloxacin-resistant (Cip^R) rpoB mutations have reduced growth fitness and their impact on clinical resistance development is unclear.

Objectives

To assess the potential for Cip^RrpoB mutations to contribute to resistance development by estimating the number of distinct alleles. To identify fitness-compensatory mutations that ameliorate the fitness costs of Cip^RrpoB mutations. To understand how Cip^RrpoB mutations reprogramme RNAP.

Methods

E. coli strains carrying five different Cip^RrpoB alleles were evolved with selection for improved fitness and characterized for acquired mutations, relative fitness and MIC_Cip. The effects of dksA mutations and a ppGpp⁰ background on growth and susceptibility phenotypes associated with Cip^RrpoB alleles were determined.

Results

The number of distinct Cip^RrpoB mutations was estimated to be >100. Mutations in RNAP genes and in dksA can compensate for the fitness cost of Cip^RrpoB mutations. Deletion of dksA reduced the MIC_Cip for strains carrying Cip^RrpoB alleles. A ppGpp⁰ phenotype had no effect on drug susceptibility.

Conclusions

Cip^RrpoB mutations induce an ppGpp-independent stringent-like response. Approximately half of the reduction in ciprofloxacin susceptibility is caused by an increased affinity of RNAP to DksA while the other half is independent of DksA. Stringent-like response activating mutations might be the most diverse class of mutations reducing susceptibility to antibiotics.

Resistance/fitness trade-off is a barrier to the evolution of MarR inactivation mutants in Escherichia coli

Background

Mutations that inactivate MarR reduce susceptibility to ciprofloxacin and competitive growth fitness in Escherichia coli. Both phenotypes are caused by overexpression of the MarA regulon, which includes the AcrAB-TolC drug efflux pump.

Objectives

We asked whether compensatory evolution could reduce the fitness cost of MarR-inactivating mutations without affecting resistance to ciprofloxacin.

Methods

The cost of overexpressing the AcrAB-TolC efflux pump was measured independently of MarA overexpression. Experimental evolution of MarR-inactive strains was used to select mutants with increased fitness. The acquired mutations were identified and their effects on drug susceptibility were measured.

Results

Overexpression of the AcrAB-TolC efflux pump was found not to contribute to the fitness cost of MarA regulon overexpression. Fitness-compensatory mutations were selected in marA and lon. The mutations reduced the level of MarA protein thus reducing expression of the MarA regulon. They restored growth fitness but also reduced resistance to ciprofloxacin.

Conclusions

The fitness cost caused by overexpression of the MarA regulon has multiple contributing factors. Experimental evolution did not identify any single pump-independent cost factor. Instead, efficient fitness compensation occurred only by mechanisms that reduce MarA concentration, which simultaneously reduce the drug resistance phenotype. This resistance/fitness trade-off is a barrier to the successful spread of MarR inactivation mutations in clinical isolates where growth fitness is essential.

Shared and Unique Evolutionary Trajectories to Ciprofloxacin Resistance in Gram-Negative Bacterial Pathogens

Resistance to the broad-spectrum antibiotic ciprofloxacin is detected at high rates for a wide range of bacterial pathogens. To investigate the dynamics of ciprofloxacin resistance development, we applied a comparative resistomics workflow for three clinically relevant species of Gram-negative bacteria: Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa. We combined experimental evolution in a morbidostat with deep sequencing of evolving bacterial populations in time series to reveal both shared and unique aspects of evolutionary trajectories. Representative clone characterization by sequencing and MIC measurements enabled direct assessment of the impact of mutations on the extent of acquired drug resistance. In all three species, we observed a two-stage evolution: (i) early ciprofloxacin resistance reaching 4- to 16-fold the MIC for the wild type, commonly as a result of single mutations in DNA gyrase target genes (gyrA or gyrB), and (ii) additional genetic alterations affecting the transcriptional control of the drug efflux machinery or secondary target genes (DNA topoisomerase parC or parE).

Expression of the qepA1 gene is induced under antibiotic exposure

Background

The qepA1 gene encodes an efflux pump that reduces susceptibility to ciprofloxacin. Little is known about the regulation of qepA1 expression.

Objectives

To assess the potential role of ciprofloxacin and other antibiotics in the regulation of qepA1 gene expression. To identify the promoter that drives qepA1 expression and other factors involved in expression regulation. To assess whether the identified features are universal among qepA alleles.

Methods

A translational qepA1-yfp fusion under the control of the qepA1 upstream region was cloned into the Escherichia coli chromosome. Expression of the fusion protein was measured in the presence of various antibiotics. Deletions within the upstream region were introduced to identify regions involved in gene expression and regulation. The qepA1 coding sequence and upstream region were compared with all available qepA sequences.

Results

Cellular stress caused by the presence of various antibiotics can induce qepA1 expression. The qepA1 gene is fused to a class I integron and gene expression is driven by the Pc promoter within the integrase gene. A segment within the integron belonging to a truncated dfrB4 gene is essential for the regulation of qepA1 expression. This genetic context is universal among all sequenced qepA alleles.

Conclusions

The fusion of the qepA1 gene to a class I integron has created a novel regulatory unit that enables qepA1 expression to be under the control of antibiotic exposure. This setup mitigates potential negative effects of QepA1 production on bacterial fitness by restricting high-level expression to environmental conditions in which QepA1 is beneficial.

Balancing Positive and Negative Selection: In Vivo Evolution of Candida lusitaniae MRR1

The evolution of pathogens in response to selective pressures present during chronic infections can influence their persistence and virulence and the outcomes of antimicrobial therapy. Because subpopulations within an infection can be spatially separated and the host environment can fluctuate, an appreciation of the pathways under selection may be most easily revealed through the analysis of numerous isolates from single infections. Here, we continued our analysis of a set of clonally derived Clavispora (Candida) lusitaniae isolates from a single chronic lung infection with a striking enrichment in the number of alleles of MRR1 Genetic and genomic analyses found evidence for repeated acquisition of gain-of-function mutations that conferred constitutive Mrr1 activity. In the same population, there were multiple alleles with both gain-of-function mutations and secondary suppressor mutations that either attenuated or abolished the constitutive activity, suggesting the presence of counteracting selective pressures. Our studies demonstrated trade-offs between high Mrr1 activity, which confers resistance to the antifungal fluconazole, host factors, and bacterial products through its regulation of MDR1, and resistance to hydrogen peroxide, a reactive oxygen species produced in the neutrophilic environment associated with this infection. This inverse correlation between high Mrr1 activity and hydrogen peroxide resistance was observed in multiple Candida species and in serially collected populations from this individual over 3 years. These data lead us to propose that dynamic or variable selective pressures can be reflected in population genomics and that these dynamics can complicate the drug resistance profile of the population.IMPORTANCE Understanding microbial evolution within patients is critical for managing chronic infections and understanding host-pathogen interactions. Here, our analysis of multiple MRR1 alleles in isolates from a single Clavispora (Candida) lusitaniae infection revealed the selection for both high and low Mrr1 activity. Our studies reveal trade-offs between high Mrr1 activity, which confers resistance to the commonly used antifungal fluconazole, host antimicrobial peptides, and bacterial products, and resistance to hydrogen peroxide. This work suggests that spatial or temporal differences within chronic infections can support a large amount of dynamic and parallel evolution and that Mrr1 activity is under both positive and negative selective pressure to balance different traits that are important for microbial survival.

Limited Multidrug Resistance Efflux Pump Overexpression among Multidrug-Resistant Escherichia coli Strains of ST131

Gram-negative bacteria partly rely on efflux pumps to facilitate growth under stressful conditions and to increase resistance to a wide variety of commonly used drugs. In recent years, Escherichia coli sequence type 131 (ST131) has emerged as a major cause of extraintestinal infection frequently exhibiting a multidrug resistance (MDR) phenotype. The contribution of efflux to MDR in emerging E. coli MDR clones, however, is not well studied. We characterized strains from an international collection of clinical MDR E. coli isolates by MIC testing with and without the addition of the AcrAB-TolC efflux inhibitor 1-(1-naphthylmethyl)-piperazine (NMP). MIC data for 6 antimicrobial agents and their reversion by NMP were analyzed by principal-component analysis (PCA). PCA revealed a group of 17 MDR E. coli isolates (n = 34) exhibiting increased susceptibility to treatment with NMP, suggesting an enhanced contribution of efflux pumps to antimicrobial resistance in these strains (termed enhanced efflux phenotype [EEP] strains). Only 1/17 EEP strains versus 12/17 non-EEP MDR strains belonged to the ST131 clonal group. Whole-genome sequencing revealed marked differences in efflux-related genes between EEP and control strains, with the majority of notable amino acid substitutions occurring in AcrR, MarR, and SoxR. Quantitative reverse transcription-PCR (qRT-PCR) of multiple efflux-related genes showed significant overexpression of the AcrAB-TolC system in EEP strains, whereas in the remaining strains, we found enhanced expression of alternative efflux proteins. We conclude that a proportion of MDR E. coli strains exhibit an EEP, which is linked to an overexpression of the AcrAB-TolC efflux pump and a distinct array of genomic variations. Members of ST131, although highly successful, are less likely to exhibit the EEP.

Metabolic fitness landscapes predict the evolution of antibiotic resistance

Bacteria evolve resistance to antibiotics by a multitude of mechanisms. A central, yet unsolved question is how resistance evolution affects cell growth at different drug levels. Here, we develop a fitness model that predicts growth rates of common resistance mutants from their effects on cell metabolism. The model maps metabolic effects of resistance mutations in drug-free environments and under drug challenge; the resulting fitness trade-off defines a Pareto surface of resistance evolution. We predict evolutionary trajectories of growth rates and resistance levels, which characterize Pareto resistance mutations emerging at different drug dosages. We also predict the prevalent resistance mechanism depending on drug and nutrient levels: low-dosage drug defence is mounted by regulation, evolution of distinct metabolic sectors sets in at successive threshold dosages. Evolutionary resistance mechanisms include membrane permeability changes and drug target mutations. These predictions are confirmed by empirical growth inhibition curves and genomic data of Escherichia coli populations. Our results show that resistance evolution, by coupling major metabolic pathways, is strongly intertwined with systems biology and ecology of microbial populations.

Diffusible Signaling Factor, a Quorum-Sensing Molecule, Interferes with and Is Toxic Towards Bdellovibrio bacteriovorus 109J

Bdellovibrio bacteriovorus 109J is a predatory bacterium which lives by predating on other Gram-negative bacteria to obtain the nutrients it needs for replication and survival. Here, we evaluated the effects two classes of bacterial signaling molecules (acyl homoserine lactones (AHLs) and diffusible signaling factor (DSF)) have on B. bacteriovorus 109J behavior and viability. While AHLs had a non-significant impact on predation rates, DSF considerably delayed predation and bdelloplast lysis. Subsequent experiments showed that 50 μM DSF also reduced the motility of attack-phase B. bacteriovorus 109J cells by 50% (38.2 ± 14.9 vs. 17 ± 8.9 μm/s). Transcriptomic analyses found that DSF caused genome-wide changes in B. bacteriovorus 109J gene expression patterns during both the attack and intraperiplasmic phases, including the significant downregulation of the flagellum assembly genes and numerous serine protease genes. While the former accounts for the reduced speeds observed, the latter was confirmed experimentally with 50 μM DSF completely blocking protease secretion from attack-phase cells. Additional experiments found that 30% of the total cellular ATP was released into the supernatant when B. bacteriovorus 109J was exposed to 200 μM DSF, implying that this QS molecule negatively impacts membrane integrity.

Genome-Based Analyses of Fitness Effects and Compensatory Changes Associated with Acquisition of bla CMY-, bla CTX-M-, and bla OXA-48/VIM-1-Containing Plasmids in Escherichia coli

(1) Background: Resistance plasmids are under selective conditions beneficial for the bacterial host, but in the absence of selective pressure, this carriage may cause fitness costs. Compensation of this fitness burden is important to obtain competitive ability under antibiotic-free conditions. In this study, we investigated fitness effects after a conjugative transfer of plasmids containing various beta-lactamase genes transferred into Escherichia coli. (2) Methods: Fourteen beta-lactamase-encoding plasmids were transferred from clinical donor strains to E. coli J53. Growth rates were compared for all transconjugants and the recipient. Selected transconjugants were challenged in long-term growth experiments. Growth rates were assessed at different time points during growth for 500 generations. Whole-genome sequencing (WGS) of initial and evolved transconjugants was determined. Results: Most plasmid acquisitions resulted in growth differences, ranging from -4.5% to 7.2%. Transfer of a single bla _CMY-16-carrying plasmid resulted in a growth burden and a growth benefit in independent mating. Long-term growth led to a compensation of fitness burdens and benefits. Analyzing WGS revealed genomic changes caused by Single Nucleotide Polymorphisms (SNPs) and insertion sequences over time. Conclusions: Fitness effects associated with plasmid acquisitions were variable. Potential compensatory mutations identified in transconjugants' genomes after 500 generations give interesting insights into aspects of plasmid-host adaptations.

Profiling antibiotic resistance in Escherichia coli strains displaying differential antibiotic susceptibilities using Raman spectroscopy

The rapid identification of antibiotic resistant bacteria is important for public health. In the environment, bacteria are exposed to sub-inhibitory antibiotic concentrations which has implications in the generation of multi-drug resistant strains. To better understand these issues, Raman spectroscopy was employed coupled with partial least squares-discriminant analysis to profile Escherichia coli strains treated with sub-inhibitory concentrations of antibiotics. Clear differences were observed between cells treated with bacteriostatic (tetracycline and rifampicin) and bactericidal (ampicillin, ciprofloxacin, and ceftriaxone) antibiotics for 6 hr: First, atomic force microscopy revealed that bactericidal antibiotics cause extensive cell elongation whereas short filaments are observed with bacteriostatic antibiotics. Second, Raman spectral analysis revealed that bactericidal antibiotics lower nucleic acid to protein (I₈₁₂ /I₈₃₀ ) and nucleic acid to lipid ratios (I₁₄₈₃ /I₁₄₅₂ ) whereas the opposite is seen with bacteriostatic antibiotics. Third, the protein to lipid ratio (I₂₉₃₆ /I₂₈₈₅ and I₂₉₃₆ /I₂₈₅₀ ) is a Raman stress signature common to both the classes. These signatures were validated using two mutants, Δlon and ΔacrB, that exhibit relatively high and low resistance towards antibiotics, respectively. In addition, these spectral markers correlated with the emergence of phenotypic antibiotic resistance. Overall, this study demonstrates the efficacy of Raman spectroscopy to identify resistance in bacteria to sub-lethal concentrations of antibiotics.

Population Bottlenecks Strongly Influence the Evolutionary Trajectory to Fluoroquinolone Resistance in Escherichia coli

Experimental evolution is a powerful tool to study genetic trajectories to antibiotic resistance under selection. A confounding factor is that outcomes may be heavily influenced by the choice of experimental parameters. For practical purposes (minimizing culture volumes), most experimental evolution studies with bacteria use transmission bottleneck sizes of 5 × 106 cfu. We currently have a poor understanding of how the choice of transmission bottleneck size affects the accumulation of deleterious versus high-fitness mutations when resistance requires multiple mutations, and how this relates outcome to clinical resistance. We addressed this using experimental evolution of resistance to ciprofloxacin in Escherichia coli. Populations were passaged with three different transmission bottlenecks, including single cell (to maximize genetic drift) and bottlenecks spanning the reciprocal of the frequency of drug target mutations (108 and 1010). The 1010 bottlenecks selected overwhelmingly mutations in drug target genes, and the resulting genotypes corresponded closely to those found in resistant clinical isolates. In contrast, both the 108 and single-cell bottlenecks selected mutations in three different gene classes: 1) drug targets, 2) efflux pump repressors, and 3) transcription-translation genes, including many mutations with low fitness. Accordingly, bottlenecks smaller than the average nucleotide substitution rate significantly altered the experimental outcome away from genotypes observed in resistant clinical isolates. These data could be applied in designing experimental evolution studies to increase their predictive power and to explore the interplay between different environmental conditions, where transmission bottlenecks might vary, and resulting evolutionary trajectories.

Development and selection of low-level multi-drug resistance over an extended range of sub-inhibitory ciprofloxacin concentrations in Escherichia coli

To better combat bacterial antibiotic resistance, a growing global health threat, it is imperative to understand its drivers and underlying biological mechanisms. One potential driver of antibiotic resistance is exposure to sub-inhibitory concentrations of antibiotics. This occurs in both the environment and clinic, from agricultural contamination to incorrect dosing and usage of poor-quality medicines. To better understand this driver, we tested the effect of a broad range of ciprofloxacin concentrations on antibiotic resistance development in Escherichia coli. We observed the emergence of stable, low-level multi-drug resistance that was both time and concentration dependent. Furthermore, we identified a spectrum of single mutations in strains with resistant phenotypes, both previously described and novel. Low-level class-wide resistance, which often goes undetected in the clinic, may allow for bacterial survival and establishment of a reservoir for outbreaks of high-level antibiotic resistant infections.

Evolutionary landscapes of Pseudomonas aeruginosa towards ribosome-targeting antibiotic resistance depend on selection strength

It is generally accepted that antibiotic-resistant mutants are selected in a range of concentrations ranging from the minimum inhibitory concentration (MIC) to the mutant preventive concentration. More recently, it has been found that antibiotic-resistant mutants can also be selected at concentrations below MIC, which expands the conditions where this selection may occur. Using experimental evolution approaches followed by whole-genome sequencing, the current study compares the evolutionary trajectories of Pseudomonas aeruginosa in the presence of tobramycin or tigecycline at lethal and sublethal concentrations. Mutants were selected at sublethal concentrations of tigecycline (1/10 and 1/50 MIC), whereas no mutants were selected in the case of tobramycin, indicating that the width of sub-MIC selective windows is antibiotic-specific. In addition, the patterns of evolution towards tigecycline resistance depend on selection strength. Sublethal concentrations of tigecycline select mutants with lower tigecycline MICs and higher MICs to other antibiotics belonging to different structural families than mutants selected under lethal concentrations. This indicates that the strength of the cross-resistance phenotype associated with tigecycline resistance is decoupled from selection strength. Accurate information on the sublethal selection window for each antibiotic of clinical value, including the phenotypes of cross-resistance of mutants selected at each antibiotic concentration, is needed to understand the role of ecosystems polluted with different antibiotic concentrations in the selection of antibiotic resistance. Integration of this information into clinical and environmental safety controls may help to tackle the problem of antibiotic resistance.

The SNAP hypothesis: Chromosomal rearrangements could emerge from positive Selection during Niche Adaptation

The relative linear order of most genes on bacterial chromosomes is not conserved over evolutionary timescales. One explanation is that selection is weak, allowing recombination to randomize gene order by genetic drift. However, most chromosomal rearrangements are deleterious to fitness. In contrast, we propose the hypothesis that rearrangements in gene order are more likely the result of selection during niche adaptation (SNAP). Partial chromosomal duplications occur very frequently by recombination between direct repeat sequences. Duplicated regions may contain tens to hundreds of genes and segregate quickly unless maintained by selection. Bacteria exposed to non-lethal selections (for example, a requirement to grow on a poor nutrient) can adapt by maintaining a duplication that includes a gene that improves relative fitness. Further improvements in fitness result from the loss or inactivation of non-selected genes within each copy of the duplication. When genes that are essential in single copy are lost from different copies of the duplication, segregation is prevented even if the original selection is lifted. Functional gene loss continues until a new genetic equilibrium is reached. The outcome is a rearranged gene order. Mathematical modelling shows that this process of positive selection to adapt to a new niche can rapidly drive rearrangements in gene order to fixation. Signature features (duplication formation and divergence) of the SNAP model were identified in natural isolates from multiple species showing that the initial two steps in the SNAP process can occur with a remarkably high frequency. Further bioinformatic and experimental analyses are required to test if and to which extend the SNAP process acts on bacterial genomes.

All life on earth has evolved from a universal common ancestor with a specific order of genes on the chromosome. This order is not maintained in modern species and the standard hypothesis is that changes reflect a lack of strong selection on gene order. Here, we propose an alternative hypothesis, SNAP. The occupation of a novel environment by bacteria is generally a trade-off situation. For example, while the bacteria may not be adapted to grow well under the new conditions, they may benefit by not having to share available resources with other microorganisms. Bacterial populations frequently acquire duplications of chromosomal segments containing genes that can help them adapt to a new environment. Other genes that are also duplicated are not required in two copies so that over time a superfluous copy can be lost. Eventually, the process of duplication and gene loss can lead to the rearrangement of the gene order in the chromosomal segment. The major benefit of this model over the standard hypothesis is that the process is driven by positive selection and can reach fixation rapidly.

Rapid decline of bacterial drug-resistance in an antibiotic-free environment through phenotypic reversion

Antibiotic resistance typically induces a fitness cost that shapes the fate of antibiotic-resistant bacterial populations. However, the cost of resistance can be mitigated by compensatory mutations elsewhere in the genome, and therefore the loss of resistance may proceed too slowly to be of practical importance. We present our study on the efficacy and phenotypic impact of compensatory evolution in Escherichia coli strains carrying multiple resistance mutations. We have demonstrated that drug-resistance frequently declines within 480 generations during exposure to an antibiotic-free environment. The extent of resistance loss was found to be generally antibiotic-specific, driven by mutations that reduce both resistance level and fitness costs of antibiotic-resistance mutations. We conclude that phenotypic reversion to the antibiotic-sensitive state can be mediated by the acquisition of additional mutations, while maintaining the original resistance mutations. Our study indicates that restricting antimicrobial usage could be a useful policy, but for certain antibiotics only.

Evaluation of Acquired Antibiotic Resistance in Escherichia coli Exposed to Long-Term Low-Shear Modeled Microgravity and Background Antibiotic Exposure

Stress factors experienced during space include microgravity, sleep deprivation, radiation, isolation, and microbial contamination, all of which can promote immune suppression (1, 2). Under these conditions, the risk of infection from opportunistic pathogens increases significantly, particularly during long-term missions (3). If infection occurs, it is important that the infectious agent should not be antibiotic resistant. Minimizing the occurrence of antibiotic resistance is, therefore, highly desirable. To facilitate this, it is important to better understand the long-term response of bacteria to the microgravity environment. This study demonstrated that the use of antibiotics as a preventive measure could be counterproductive and would likely result in persistent resistance to that antibiotic. In addition, unintended resistance to other antimicrobials might also occur as well as permanent genome changes that might have other unanticipated and undesirable consequences.

The long-term response of microbial communities to the microgravity environment of space is not yet fully understood. Of special interest is the possibility that members of these communities may acquire antibiotic resistance. In this study, Escherichia coli cells were grown under low-shear modeled microgravity (LSMMG) conditions for over 1,000 generations (1000G) using chloramphenicol treatment between cycles to prevent contamination. The results were compared with data from an earlier control study done under identical conditions using steam sterilization between cycles rather than chloramphenicol. The sensitivity of the final 1000G-adapted strain to a variety of antibiotics was determined using Vitek analysis. In addition to resistance to chloramphenicol, the adapted strain acquired resistance to cefalotin, cefuroxime, cefuroxime axetil, cefoxitin, and tetracycline. In fact, the resistance to chloramphenicol and cefalotin persisted for over 110 generations despite the removal of both LSMMG conditions and trace antibiotic exposure. Genome sequencing of the adapted strain revealed 22 major changes, including 3 transposon-mediated rearrangements (TMRs). Two TMRs disrupted coding genes (involved in bacterial adhesion), while the third resulted in the deletion of an entire segment (14,314 bp) of the genome, which includes 14 genes involved with motility and chemotaxis. These results are in stark contrast with data from our earlier control study in which cells grown under the identical conditions without antibiotic exposure never acquired antibiotic resistance. Overall, LSMMG does not appear to alter the antibiotic stress resistance seen in microbial ecosystems not exposed to microgravity.

Effects of sub-minimum inhibitory concentrations of ciprofloxacin on biofilm formation and virulence factors of Escherichia coli

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A functional study of the global transcriptional regulator PadR from a strain Streptomyces fradiae-nitR+bld, resistant to nitrone-oligomycin

We describe Streptomyces fradiae mechanisms of sensitivity to nitrone-oligomycin A, a derivative of oligomycin A. We obtained S. fradiae-nitR⁺ bld, a nitrone-oligomycin A resistant mutant with a «bald» phenotype. Comparative genomic analysis of the wild-type S. fradiae ATCC19609 and S. fradiae-nitR⁺ bld revealed a mutation in padR - a gene encoding a multifunction transcription regulator, which resulted in the amino acid replacement in a highly conserved DNA-binding domain. Bioinformatics genome analysis of S. fradiae ATCC19609 discovered a PadR binding site 13 bp upstream the start codon of the marR transcription factor gene. Induction of S. fradiaenitR⁺ bld and w.t. strains with nitrone-oligomycin A lead to a significant increase in expression level of the marR gene in the w.t. strain, but no change observed in mutant strain. We identified differences between DNA-protein interactions of the mutant and native PadR proteins with its putative binding site in S. fradiae ATCC19609. This allowed us to suggest that the padR gene, that harbored a single nucleotide mutation in the S. fradiaenitR⁺ bld strain, might be involved in the mechanism of resistance to nitrone-oligomycin A. We assume the participation of the transcriptional factorpadR in the formation of the bald phenotype.