AcrAB multidrug efflux pump is associated with reduced levels of susceptibility to tigecycline (GAR-936) in Proteus mirabilis

Extending the Potency and Lifespan of Antibiotics: Inhibitors of Gram-Negative Bacterial Efflux Pumps

Efflux is a natural process found in all prokaryotic and eukaryotic cells that removes a diverse range of substrates from inside to outside. Many antibiotics are substrates of bacterial efflux pumps, and modifications to the structure or overexpression of efflux pumps are an important resistance mechanism utilized by many multidrug-resistant bacteria. Therefore, chemical inhibition of bacterial efflux to revitalize existing antibiotics has been considered a promising approach for antimicrobial chemotherapy over two decades, and various strategies have been employed. In this review, we provide an overview of bacterial multidrug resistance (MDR) efflux pumps, of which the resistance nodulation division (RND) efflux pumps are considered the most clinically relevant in Gram-negative bacteria, and describe over 50 efflux inhibitors that target such systems. Although numerous efflux inhibitors have been identified to date, none have progressed into clinical use because of formulation, toxicity, and pharmacokinetic issues or a narrow spectrum of inhibition. For these reasons, the development of efflux inhibitors has been considered a difficult and complex area of research, and few active preclinical studies on efflux inhibitors are in progress. However, recently developed tools, including but not limited to computational tools including molecular docking models, offer hope that further research on efflux inhibitors can be a platform for research and development of new bacterial efflux inhibitors.

Case report: A successfully treated case of community-acquired urinary tract infection due to Klebsiella aerogenes in Bangladesh

Klebsiella aerogenes, a nosocomial pathogen, is increasingly associated with extensive drug resistance and virulence profiles. It is responsible for high morbidity and mortality. This report describes the first successfully treated case of community-acquired urinary tract infection (UTI) caused by Klebsiella aerogenes in an elderly housewife with Type-2 diabetes (T2D) from Dhaka, Bangladesh. The patient was empirically treated with intravenous ceftriaxone (500 mg/8 h). However, she did not respond to the treatment. The urine culture and sensitivity tests, coupled with bacterial whole-genome sequencing (WGS) and analysis, revealed the bacteria to be K. aerogenes which was extensively drug-resistant but was susceptible to carbapenems and polymyxins. Based on these findings, meropenem (500 mg/8 h) was administered to the patient, who then responded to the treatment and recovered successfully without having a relapse. This case raises awareness of the importance of diagnosis of not-so-common etiological agents, correct identification of the pathogens, and targeted antibiotic therapy. In conclusion, correctly identifying etiological agents of UTI using WGS approaches that are otherwise difficult to diagnose could help improve the identification of infectious agents and improve the management of infectious diseases.

In vitro susceptibility of common Enterobacterales to eravacycline in Taiwan

BACKGROUND

New tetracycline derivatives exhibit broad-spectrum antimicrobial activities. This study aimed to assess the in vitro activity of eravacycline against common Enterobacterales.

METHODS

Clinical Enterobacterales isolates were collected between 2017 and 2021. The minimum inhibitory concentration (MIC) was determined using a broth microdilution test.

RESULTS

We identified Klebsiella pneumoniae (n = 300), Escherichia coli (n = 300), Klebsiella oxytoca (n = 100), Enterobacter cloacae complex (n = 100), Citrobacter freundii (n = 100), and Proteus mirabilis (n = 100). All P. mirabilis strains were resistant to eravacycline. Excluding P. mirabilis, the susceptibility rates to eravacycline, omadacycline, and tigecycline were 75.2%, 66.9%, and 73%, respectively. The MIC₅₀ and MIC₉₀ (mg/L) of eravacycline were 0.5 and 4 for K. pneumoniae, 0.5 and 1 for E. coli, 0.5 and 1 for K. oxytoca, 0.5 and 2 for E. cloacae complex, and 0.25 and 1 for C. freundii. In cefotaxime non-susceptible and meropenem susceptible Enterobacterales, excluding P. mirabilis, the susceptibility rates of eravacycline, omadacycline, and tigecycline were 69.7%, 57.1%, and 66.2%. We found decreased susceptibility rates of three new tetracycline derivatives against meropenem non-susceptible Enterobacterales (eravacycline: 47.1%, omadacycline: 39.4%, and tigecycline: 39.4%). Eravacycline showed a high susceptibility rate against cefotaxime non-susceptible and meropenem susceptible K. oxytoca (100%), C. freundii (93.2%), E. coli (85.9%), and meropenem non-susceptible E. coli (100%).

CONCLUSION

This study provides the MIC and susceptibility rate of eravacycline for common Enterobacterales. Eravacycline could be a therapeutic choice for cefotaxime non-susceptible or meropenem non-susceptible Enterobacterales, especially K. oxytoca, C. freundii, and E. coli.

Tripartite efflux pumps of the RND superfamily: what did we learn from computational studies?

Bacterial resistance to antibiotics has been long recognized as a priority to address for human health. Among all micro-organisms, the so-called multi-drug resistant (MDR) bacteria, which are resistant to most, if not all drugs in our current arsenal, are particularly worrisome. The World Health Organization has prioritized the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter species) pathogens, which include four Gram-negative bacterial species. In these bacteria, active extrusion of antimicrobial compounds out of the cell by means of 'molecular guns' known as efflux pumps is a main determinant of MDR phenotypes. The resistance-nodulation-cell division (RND) superfamily of efflux pumps connecting the inner and outer membrane in Gram-negative bacteria is crucial to the onset of MDR and virulence, as well as biofilm formation. Thus, understanding the molecular basis of the interaction of antibiotics and inhibitors with these pumps is key to the design of more effective therapeutics. With the aim to contribute to this challenge, and complement and inspire experimental research, in silico studies on RND efflux pumps have flourished in recent decades. Here, we review a selection of such investigations addressing the main determinants behind the polyspecificity of these pumps, the mechanisms of substrate recognition, transport and inhibition, as well as the relevance of their assembly for proper functioning, and the role of protein-lipid interactions. The journey will end with a perspective on the role of computer simulations in addressing the challenges posed by these beautifully complex machineries and in supporting the fight against the spread of MDR bacteria.

Isolation and Characterization of Lytic Proteus Virus 309

Proteus mirabilis is frequently associated with complicated urinary tract infections (UTIs) and is the main cause of catheter-associated urinary tract infections (CAUTIs). Treatment of such infections is complicated and challenging due to the biofilm forming abilities of P. mirabilis. If neglected or mistreated, infections may lead to life-threating conditions such as cystitis, pyelonephritis, kidney failure, and bacteremia that may progress to urosepsis. Treatment with antibiotics, especially in cases of recurring and persistent infections, leads to the development of resistant strains. Recent insights into phage therapy and using phages to coat catheters have been evaluated with many studies showing promising results. Here, we describe a highly lytic bacteriophage, Proteus_virus_309 (41,740 bp), isolated from a wastewater treatment facility in Cape Town, South Africa. According to guidelines of the International Committee on Taxonomy of Viruses (ICTV), bacteriophage 309 is a species within the genus Novosibovirus. Similar to most members of the genus, bacteriophage 309 is strain-specific and lyse P. mirabilis in less than 20 min.

The Development of Third-Generation Tetracycline Antibiotics and New Perspectives

The tetracycline antibiotic class has acquired new valuable members due to the optimisation of the chemical structure. The first modern tetracycline introduced into therapy was tigecycline, followed by omadacycline, eravacycline, and sarecycline (the third generation). Structural and physicochemical key elements which led to the discovery of modern tetracyclines are approached. Thus, several chemical subgroups are distinguished, such as glycylcyclines, aminomethylcyclines, and fluorocyclines, which have excellent development potential. The antibacterial spectrum comprises several resistant bacteria, including those resistant to old tetracyclines. Sarecycline, a narrow-spectrum tetracycline, is notable for being very effective against Cutinebacterium acnes. The mechanism of antibacterial action from the perspective of the new compound is approached. Several severe bacterial infections are treated with tigecycline, omadacycline, and eravacycline (with parenteral or oral formulations). In addition, sarecycline is very useful in treating acne vulgaris. Tetracyclines also have other non-antibiotic properties that require in-depth studies, such as the anti-inflammatory effect effect of sarecycline. The main side effects of modern tetracyclines are described in accordance with published clinical studies. Undoubtedly, this class of antibiotics continues to arouse the interest of researchers. As a result, new derivatives are developed and studied primarily for the antibiotic effect and other biological effects.

Efficacy of Tigecycline and Linezolid Against Pan-Drug-Resistant Bacteria Isolated From Companion Dogs in South Korea

The emergence of multidrug-resistant bacteria in companion animals is an increasing concern in view of the concept of One Health. The antimicrobials linezolid (LZD) and tigecycline (TGC) are effective against multidrug-resistant bacteria isolated from humans; however, thus far, no previous study has evaluated the efficacy of these drugs against bacteria isolated from companion animals. This study aimed to evaluate the efficacy of LZD and TGC against bacteria that were isolated from companion dogs and showed resistance to all classes of antimicrobial agents. Clinical samples (auditory channel, eye, skin, and urine) were collected from dogs that visited the Veterinary Medical Teaching Hospital of Konkuk University (Seoul, South Korea) from October 2017 to September 2020. In total, 392 bacterial isolates were obtained, of which 85 were resistant to all classes of antimicrobial agents tested and were, therefore, considered potentially pan-drug resistant (PDR). The susceptibility of isolates to LZD and TGC was determined by the disk diffusion method and interpreted using the Clinical Laboratory Standards Institute guidelines. In total, 95.6% (43/45) and 97.8% (44/45) of gram-positive isolates were susceptible to LZD and TGC, respectively, whereas 82.5% (33/40) of gram-negative isolates were sensitive to TGC. In conclusion, both agents showed favorable efficacy, with the susceptibility rates for all potential PDR bacteria, except Pseudomonas spp., ranging from 72.7 to 100%. Thus, these drugs may serve as excellent antimicrobial options for veterinary medicine in the future.

Structure, Assembly, and Function of Tripartite Efflux and Type 1 Secretion Systems in Gram-Negative Bacteria

Tripartite efflux pumps and the related type 1 secretion systems (T1SSs) in Gram-negative organisms are diverse in function, energization, and structural organization. They form continuous conduits spanning both the inner and the outer membrane and are composed of three principal components-the energized inner membrane transporters (belonging to ABC, RND, and MFS families), the outer membrane factor channel-like proteins, and linking the two, the periplasmic adaptor proteins (PAPs), also known as the membrane fusion proteins (MFPs). In this review we summarize the recent advances in understanding of structural biology, function, and regulation of these systems, highlighting the previously undescribed role of PAPs in providing a common architectural scaffold across diverse families of transporters. Despite being built from a limited number of basic structural domains, these complexes present a staggering variety of architectures. While key insights have been derived from the RND transporter systems, a closer inspection of the operation and structural organization of different tripartite systems reveals unexpected analogies between them, including those formed around MFS- and ATP-driven transporters, suggesting that they operate around basic common principles. Based on that we are proposing a new integrated model of PAP-mediated communication within the conformational cycling of tripartite systems, which could be expanded to other types of assemblies.

Role of OB-Fold Protein YdeI in Stress Response and Virulence of Salmonella enterica Serovar Enteritidis

An essential feature of the pathogenesis of the Salmonella enterica serovar Enteritidis wild type (WT) is its ability to survive under diverse microenvironmental stress conditions, such as encountering antimicrobial peptides (AMPs) or glucose and micronutrient starvation. These stress factors trigger virulence genes carried on Salmonella pathogenicity islands (SPIs) and determine the efficiency of enteric infection. Although the oligosaccharide/oligonucleotide binding-fold (OB-fold) family of proteins has been identified as an important stress response and virulence determinant, functional information on members of this family is currently limited. In this study, we decipher the role of YdeI, which belongs to OB-fold family of proteins, in stress response and virulence of S Enteritidis. When ydeI was deleted, the ΔydeI mutant showed reduced survival during exposure to AMPs or glucose and Mg²⁺ starvation stress compared to the WT. Green fluorescent protein (GFP) reporter and quantitative real-time PCR (qRT-PCR) assays showed ydeI was transcriptionally regulated by PhoP, which is a major regulator of stress and virulence. Furthermore, the ΔydeI mutant displayed ∼89% reduced invasion into HCT116 cells, ∼15-fold-reduced intramacrophage survival, and downregulation of several SPI-1 and SPI-2 genes encoding the type 3 secretion system apparatus and effector proteins. The mutant showed attenuated virulence compared to the WT, confirmed by its reduced bacterial counts in feces, mesenteric lymph node (mLN), spleen, and liver of C57BL/6 mice. qRT-PCR analyses of the ΔydeI mutant displayed differential expression of 45 PhoP-regulated genes, which were majorly involved in metabolism, transport, membrane remodeling, and drug resistance under different stress conditions. YdeI is, therefore, an important protein that modulates S Enteritidis virulence and adaptation to stress during infection.IMPORTANCE S Enteritidis during its life cycle encounters diverse stress factors inside the host. These intracellular conditions allow activation of specialized secretion systems to cause infection. We report a conserved membrane protein, YdeI, and elucidate its role in protection against various intracellular stress conditions. A key aspect of the study of a pathogen's stress response mechanism is its clinical relevance during host-pathogen interaction. Bacterial adaptation to stress plays a vital role in evolution of a pathogen's resistance to therapeutic agents. Therefore, investigation of the role of YdeI is vital for understanding the molecular basis of regulation of Salmonella pathogenesis. In conclusion, our findings may contribute to finding potential targets to develop new intervention strategies for treatment and prevention of enteric diseases.

The Transcriptomic Signature of Tigecycline in Acinetobacter baumannii

Tigecycline, a protein translation inhibitor, is a treatment of last resort for infections caused by the opportunistic multidrug resistance human pathogen Acinetobacter baumannii. However, strains resistant to tigecycline were reported not long after its clinical introduction. Translation inhibitor antibiotics perturb ribosome function and induce the reduction of (p)ppGpp, an alarmone involved in the stringent response that negatively modulates ribosome production. Through RNA sequencing, this study revealed a significant reduction in the transcription of genes in citric acid cycle and cell respiration, suggesting tigecycline inhibits or slows down bacterial growth. Our results indicated that the drug-induced reduction of (p)ppGpp level promoted the production but diminished the degradation of ribosomes, which mitigates the translational inhibition effect by tigecycline. The reduction of (p)ppGpp also led to a decrease of transcription coupled nucleotide excision repair which likely increases the chances of development of tigecycline resistant mutants. Increased expression of genes linked to horizontal gene transfer were also observed. The most upregulated gene, rtcB, involving in RNA repair, is either a direct tigecycline stress response or is in response to the transcription de-repression of a toxin-antitoxin system. The most down-regulated genes encode two β-lactamases, which is a possible by-product of tigecycline-induced reduction in transcription of genes associated with peptidoglycan biogenesis. This transcriptomics study provides a global genetic view of why A. baumannii is able to rapidly develop tigecycline resistance.

Gene interaction network approach to elucidate the multidrug resistance mechanisms in the pathogenic bacterial strain Proteus mirabilis

Proteus mirabilis is one among the most frequently identified pathogen in patients with the urinary tract infection. The multidrug resistance exhibited by P. mirabilis renders the treatment ineffective, and new progressive strategies are needed to overcome the antibiotic resistance (AR). We have analyzed the evolutionary relationship of 29 P. mirabilis strains available in the National Center for Biotechnology Information-Genome database. The antimicrobial resistance genes of P. mirabilis along with the enriched pathways and the Gene Ontology terms are analyzed using gene networks to understand the molecular basis of AR. The genes rpoB, tufB, rpsl, fusA, and rpoA could be exploited as potential drug targets as they are involved in regulating the vital functions within the bacterium. The drug targets reported in the present study will aid researchers in developing new strategies to combat multidrug-resistant P. mirabilis.

Developmental dynamics of antibiotic resistome in aerobic biofilm microbiota treating wastewater under stepwise increasing tigecycline concentrations

This study aimed to investigate the impact of tigecycline, the third generation tetracycline, on the antibiotic resistance development in environmental microbiota. Two biological contact oxidation reactors containing aerobic biofilm microbiota were constructed, one of which was constantly fed with synthetic wastewater spiked with increasing concentrations of tigecycline (0 to 25 mg/L) under a hydrolytic retention time of 24 h. Over a period of 636 days, chemical oxygen demand removal over 90% and complete nitrification were achieved for both the control and tigecycline-exposed reactors, and effluent tigecycline concentrations in the tigecycline-exposed system were always <0.051 mg/L. Significant increases (p < 0.01) in resistome abundance and resistant bacteria ratio were detected at a tigecycline dose of 10 and 25 mg/L, respectively, revealed by metagenomic sequencing and culture-based method. The increase of resistome in the tigecycline system was mainly attributed to the enrichment of tetX, one cooperative tetracycline degrading gene. Partial canonical correspondence analysis showed that the change of resistome was mainly driven by bacterial community shift (vertical pathway). Network and genome binning analyses further suggested that the proliferation of Flavobacterium harboring tetX contributed to a relatively low community-wide resistance development in the aerobic biofilm microbiota under tigecycline selection by reducing the antibiotic concentration. This work provides scientific bases for the management and evaluation of the resistance risk induced by this novel antibiotic.

Interaction of ArmZ with the DNA-binding domain of MexZ induces expression of mexXY multidrug efflux pump genes and antimicrobial resistance in Pseudomonas aeruginosa

Multidrug efflux pumps play an important role in antibiotic resistance in bacteria. In Pseudomonas aeruginosa, MexXY pump provides intrinsic resistance to many antimicrobials, including aminoglycosides. The expression of mexXY operon is negatively regulated by MexZ repressor. The repression is alleviated in response to the antibiotic-induced ribosome stress, which results in increased synthesis of anti-repressor ArmZ, interacting with MexZ. The molecular mechanism of MexZ inactivation by ArmZ is not known. Here, we showed that the N-terminal part of MexZ, encompassing the DNA-binding domain, is required for interaction with ArmZ. Using the bacterial two hybrid system based mutant screening and pull-down analyses we identified substitutions in MexZ that diminished (R3S, K6E, R13H) or completely impaired (K53E) the interaction with ArmZ without blocking MexZ activity as a transcriptional repressor. Introduction of corresponding mexZ missense mutations into P aeruginosa PAO1161 chromosome impaired (mexZ K6E, mexZ R13H) or blocked (mexZ K53E) tetracycline mediated induction of mexY expression. Concomitantly, PAO1161 mexZ K53E strain was more susceptible to aminoglycosides. The identified residues are highly conserved in MexZ-like transcriptional regulators found in bacterial genomes encoding both MexX/MexY/MexZ and ArmZ/PA5470 orthologs, suggesting that a similar mechanism may contribute to induction of efflux mediated resistance in other bacterial species. Overall, our data shed light on the molecular mechanism of ArmZ mediated induction of intrinsic antimicrobial resistance in P. aeruginosa.

Biology of MarR family transcription factors and implications for targets of antibiotics against tuberculosis

The emergence of multidrug resistant (MDR) Mycobacterium tuberculosis strains and increased incidence of HIV coinfection fueled the difficulty in controlling tuberculosis (TB). MarR (multiple antibiotic resistance regulator) family transcription factors can regulate marRAB operon and are involved in resistance to multiple environmental stresses. We have summarized the structure, function, distribution, and regulation of the MarR family proteins, as well as their implications for novel targets for antibiotics, especially for tuberculosis.

Treatment of infections caused by multidrug-resistant Gram-negative bacteria: report of the British Society for Antimicrobial Chemotherapy/Healthcare Infection Society/British Infection Association Joint Working Party

The Working Party makes more than 100 tabulated recommendations in antimicrobial prescribing for the treatment of infections caused by multidrug-resistant (MDR) Gram-negative bacteria (GNB) and suggest further research, and algorithms for hospital and community antimicrobial usage in urinary infection. The international definition of MDR is complex, unsatisfactory and hinders the setting and monitoring of improvement programmes. We give a new definition of multiresistance. The background information on the mechanisms, global spread and UK prevalence of antibiotic prescribing and resistance has been systematically reviewed. The treatment options available in hospitals using intravenous antibiotics and in primary care using oral agents have been reviewed, ending with a consideration of antibiotic stewardship and recommendations. The guidance has been derived from current peer-reviewed publications and expert opinion with open consultation. Methods for systematic review were NICE compliant and in accordance with the SIGN 50 Handbook; critical appraisal was applied using AGREE II. Published guidelines were used as part of the evidence base and to support expert consensus. The guidance includes recommendations for stakeholders (including prescribers) and antibiotic-specific recommendations. The clinical efficacy of different agents is critically reviewed. We found there are very few good-quality comparative randomized clinical trials to support treatment regimens, particularly for licensed older agents. Susceptibility testing of MDR GNB causing infection to guide treatment needs critical enhancements. Meropenem- or imipenem-resistant Enterobacteriaceae should have their carbapenem MICs tested urgently, and any carbapenemase class should be identified: mandatory reporting of these isolates from all anatomical sites and specimens would improve risk assessments. Broth microdilution methods should be adopted for colistin susceptibility testing. Antimicrobial stewardship programmes should be instituted in all care settings, based on resistance rates and audit of compliance with guidelines, but should be augmented by improved surveillance of outcome in Gram-negative bacteraemia, and feedback to prescribers. Local and national surveillance of antibiotic use, resistance and outcomes should be supported and antibiotic prescribing guidelines should be informed by these data. The diagnosis and treatment of both presumptive and confirmed cases of infection by GNB should be improved. This guidance, with infection control to arrest increases in MDR, should be used to improve the outcome of infections with such strains. Anticipated users include medical, scientific, nursing, antimicrobial pharmacy and paramedical staff where they can be adapted for local use.

Myroides odoratimimus Forms Structurally Complex and Inherently Antibiotic-Resistant Biofilm in a Wound-Like in vitro Model

Myroides odoratimimus is an aerobic, non-fermenting Gram-negative multidrug-resistant bacterium widely distributed in nature that rarely causes infections in immunocompromised patients. We recently described in a diabetic patient a case of recurrent calcaneal ulcer infection caused by a M. odoratimimus strain showing potential for biofilm formation. For the first time, we therefore evaluated the ability of M. odoratimimus to form biofilm under different pH values and glucose concentrations using an in vitro “skin-like” model, and its susceptibility to levofloxacin, meropenem, and tigecycline. The expression of some antibiotic-resistance related genes was also monitored by RT-PCR during planktonic-to-biofilm transition. Our results indicated that M. odoratimimus can produce relevant amounts of biofilm biomass, in a time-dependent manner, especially at acidic pH and regardless of glucose concentration tested. The comparative analysis of MIC and MBC values between planktonic and sessile cells showed that resistance to antibiotics increased during the planktonic-to-biofilm transition. Viable cell count indicated that none of the tested antibiotics were able to completely eradicate preformed biofilms, although meropenem and levofloxacin were the most active causing a significant, dose-independent, reduction of biofilm's viability, as also confirmed by microscopic analysis. RT-PCR showed that antibiotic-resistance related gyrA and acrB genes are over-expressed during the transition from planktonic to sessile (biofilm) lifestyle. Overall, our findings showed that M. odoratimimus can form relevant amounts of inherently antibiotic-resistant biofilm under conditions relevant to wound site, therefore suggesting a role in the pathogenesis of chronic ulcer infections.

Riemerella anatipestifer M949_0459 gene is responsible for the bacterial resistance to tigecycline

Based on its important role in last-line therapeutics against multidrug-resistant bacteria, tigecycline has been increasingly important in treating infections. However, mounting reports on tigecycline-resistant bacterial strains isolated from different sources are of concern, and molecular mechanisms regarding tigecycline resistance are poorly understood. Riemerella anatipestifer is a Gram-negative, non-motile, non-spore-forming, rod-shaped bacterium, which causes fibrinous pericarditis, perihepatitis, and meningitis in infected ducks. We previously constructed a random transposon mutant library using Riemerella anatipestifer strain CH3, in present study, we described that Riemerella anatipestifer M949_0459 gene is responsible for the bacterial resistance to tigecycline. Using the minimum inhibitory concentration assay, a mutant strain showed significantly increased (about six-fold) tigecycline susceptibility. Subsequently, the knocked-down gene was identified as M949_0459, a putative flavin adenine dinucleotide-dependent oxidoreductase. To confirm the resistance function, M949_0459 gene was overexpressed in Escherichia coli strain BL21, and the minimum inhibitory concentration analysis showed that the gene product conferred resistance to tigecycline. Additionally, expression of the M949_0459 gene under treatment with tigecycline was measured with quantitative real-time PCR. Results showed that the mRNA expression of M949_0459 gene was elevated under tigecycline treatment with dose range of 1-10 mg/L, and peaked at 4 mg/L. Moreover, two kinds of efflux pump inhibitors, carbonyl cyanide m-chlorophenyl hydrazone and phenylalanine arginyl β-naphthylamide were tested, which showed no function on tigecycline resistance in the strain CH3. Our results may provide insights into molecular mechanisms for chemotherapy in combating Riemerella anatipestifer infections.

Tigecycline

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Mechanisms of Antibiotic Resistance

Emergence of resistance among the most important bacterial pathogens is recognized as a major public health threat affecting humans worldwide. Multidrug-resistant organisms have not only emerged in the hospital environment but are now often identified in community settings, suggesting that reservoirs of antibiotic-resistant bacteria are present outside the hospital. The bacterial response to the antibiotic "attack" is the prime example of bacterial adaptation and the pinnacle of evolution. "Survival of the fittest" is a consequence of an immense genetic plasticity of bacterial pathogens that trigger specific responses that result in mutational adaptations, acquisition of genetic material, or alteration of gene expression producing resistance to virtually all antibiotics currently available in clinical practice. Therefore, understanding the biochemical and genetic basis of resistance is of paramount importance to design strategies to curtail the emergence and spread of resistance and to devise innovative therapeutic approaches against multidrug-resistant organisms. In this chapter, we will describe in detail the major mechanisms of antibiotic resistance encountered in clinical practice, providing specific examples in relevant bacterial pathogens.

Adaptation of Salmonella enterica Serovar Senftenberg to Linalool and Its Association with Antibiotic Resistance and Environmental Persistence

A clinical isolate of Salmonella enterica serovar Senftenberg, isolated from an outbreak linked to the herb Ocimum basilicum L. (basil), has been shown to be resistant to basil oil and to the terpene alcohol linalool. To better understand how human pathogens might develop resistance to linalool and to investigate the association of this resistance with resistance to different antimicrobial agents, selective pressure was applied to the wild-type strain by sequential exposure to increasing concentrations of linalool. The results demonstrated that S Senftenberg adapted to linalool with a MIC increment of at least 8-fold, which also resulted in better resistance to basil oil and better survival on harvested basil leaves. Adaptation to linalool was shown to confer cross protection against the antibiotics trimethoprim, sulfamethoxazole, piperacillin, chloramphenicol, and tetracycline, increasing their MICs by 2- to 32-fold. The improved resistance was shown to correlate with multiple phenotypes that included changes in membrane fatty acid composition, induced efflux, reduced influx, controlled motility, and the ability to form larger aggregates in the presence of linalool. The adaptation to linalool obtained in vitro did not affect survival on the basil phyllosphere in planta and even diminished survival in soil, suggesting that development of extreme resistance to linalool may be accompanied by a loss of fitness. Altogether, this report notes the concern regarding the ability of human pathogens to develop resistance to commercial essential oils, a resistance that is also associated with cross-resistance to antibiotics and may endanger public health.IMPORTANCE Greater consumer awareness and concern regarding synthetic chemical additives have led producers to control microbial spoilage and hazards by the use of natural preservatives, such as plant essential oils with antimicrobial activity. This report establishes, however, that these compounds may provoke the emergence of resistant human pathogens. Herein, we demonstrate the acquisition of resistance to basil oil by Salmonella Senftenberg. Exposure to linalool, a component of basil oil, resulted in adaptation to the basil oil mixture, as well as cross protection against several antibiotics and better survival on harvested basil leaves. Collectively, this work highlights the hazard to public health while using plant essential oils without sufficient knowledge about their influence on pathogens at subinhibitory concentrations.

Temporal trends of the in vitro activity of tigecycline and comparator antibiotics against clinical aerobic bacterial isolates collected in Germany, 2006-2014: results of the Tigecycline Evaluation and Surveillance Trial (TEST)

Given the rapidly changing landscape of antimicrobial resistance, continuous monitoring of antimicrobial susceptibility in clinically relevant bacterial isolates plays an important role in the management of infectious diseases. The Tigecycline Evaluation and Surveillance Trial (TEST) is an ongoing worldwide surveillance programme monitoring the in vitro activity of tigecycline and a panel of representative comparator antibiotics. We report longitudinal susceptibility data on a large set of isolates (n=36,044) from clinically significant bacterial species collected in 25 microbiological laboratories from 2006 to 2014. Trends include a strong increase of carbapenem and levofloxacin resistance in Acinetobacter spp., and smaller increasing rates of ESBL-producing Escherichia coli and vancomycin-resistant enterococci. Across the reporting period, the tigecycline minimum inhibitory concentrations (MICs) at which 50% and 90% of isolates were inhibited remained stable and susceptibility rates were consistently high (93–100%) for all bacterial species.

Colistin and tigecycline resistance in carbapenemase-producing Gram-negative bacteria: emerging resistance mechanisms and detection methods

A literature review was undertaken to ascertain the molecular basis for tigecycline and colistin resistance mechanisms and the experimental basis for the detection and delineation of this resistance particularly in carbapenemase-producing Gram-negative bacteria. Pubmed, Google Scholar and Science Direct were searched with the keywords colistin, tigecycline, resistance mechanisms and detection methods. Trans-complementation and comparative MIC studies, mass spectrometry, chromatography, spectrofluorometry, PCR, qRT-PCR and whole genome sequencing (WGS) were commonly used to determine tigecycline and colistin resistance mechanisms, specifically modifications in the structural and regulatory efflux (acrAB, OqxAB, kpgABC adeABC-FGH-IJK, mexAB-XY-oprJM and soxS, rarA robA, ramRAB marRABC, adeLRS, mexRZ and nfxb) and lipid A (pmrHFIJFKLM, lpxA, lpxC lpxD and mgrB, pmrAB, phoPQ,) genes respectively. Mutations in the ribosomal 16S rRNA operon rrnBC, also yielded resistance to tigecycline through target site modifications. The mcr-1 gene conferring resistance to colistin was identified via WGS, trans-complementation and a murine thigh infection model studies. Common detection methods are mainly antibiotic sensitivity testing with broth microdilution while molecular identification tools are mostly PCR and WGS. Spectrofluorometry, MALDI-TOF MS, micro-array and real-time multiplex PCR hold much promise for the future as new detection tools.

In Vitro Activity of Eravacycline against Carbapenem-Resistant Enterobacteriaceae and Acinetobacter baumannii

Eravacycline and comparators were tested against carbapenem- and tigecycline-resistant Enterobacteriaceae and Acinetobacter isolates received at the United Kingdom's national reference laboratory. Eravacycline MICs correlated closely with those of tigecycline but mostly were around 2-fold lower; both molecules retained full activity against isolates with high-level tetracycline and minocycline resistance. MIC90s of eravacycline and tigecycline were raised ca. 2-fold for carbapenem-resistant Enterobacteriaceae compared with carbapenem-susceptible controls, probably reflecting subsets of isolates with increased efflux.

Tetracycline Antibiotics and Resistance

Tetracyclines possess many properties considered ideal for antibiotic drugs, including activity against Gram-positive and -negative pathogens, proven clinical safety, acceptable tolerability, and the availability of intravenous (IV) and oral formulations for most members of the class. As with all antibiotic classes, the antimicrobial activities of tetracyclines are subject to both class-specific and intrinsic antibiotic-resistance mechanisms. Since the discovery of the first tetracyclines more than 60 years ago, ongoing optimization of the core scaffold has produced tetracyclines in clinical use and development that are capable of thwarting many of these resistance mechanisms. New chemistry approaches have enabled the creation of synthetic derivatives with improved in vitro potency and in vivo efficacy, ensuring that the full potential of the class can be explored for use against current and emerging multidrug-resistant (MDR) pathogens, including carbapenem-resistant Enterobacteriaceae, MDR Acinetobacter species, and Pseudomonas aeruginosa.

Anaerobic choline metabolism in microcompartments promotes growth and swarming of Proteus mirabilis

Gammaproteobacteria are important gut microbes but only persist at low levels in the healthy gut. The ecology of Gammaproteobacteria in the gut environment is poorly understood. Here, we demonstrate that choline is an important growth substrate for representatives of Gammaproteobacteria. Using Proteus mirabilis as a model, we investigate the role of choline metabolism and demonstrate that the cutC gene, encoding a choline‐trimethylamine lyase, is essential for choline degradation to trimethylamine by targeted mutagenesis of cutC and subsequent complementation experiments. Proteus mirabilis can rapidly utilize choline to enhance growth rate and cell yield in broth culture. Importantly, choline also enhances swarming‐associated colony expansion of P. mirabilis under anaerobic conditions on a solid surface. Comparative transcriptomics demonstrated that choline not only induces choline‐trimethylamine lyase but also genes encoding shell proteins for the formation of bacterial microcompartments. Subsequent analyses by transmission electron microscopy confirmed the presence of such novel microcompartments in cells cultivated in liquid broth and hyper‐flagellated swarmer cells from solid medium. Together, our study reveals choline metabolism as an adaptation strategy for P. mirabilis and contributes to better understand the ecology of this bacterium in health and disease.

Interaction of antibacterial compounds with RND efflux pumps in Pseudomonas aeruginosa

Pseudomonas aeruginosa infections are becoming increasingly difficult to treat due to intrinsic antibiotic resistance and the propensity of this pathogen to accumulate diverse resistance mechanisms. Hyperexpression of efflux pumps of the Resistance-Nodulation-Cell Division (RND)-type multidrug efflux pumps (e.g., MexAB-OprM), chromosomally encoded by mexAB-oprM, mexCD-oprJ, mexEF-oprN, and mexXY (-oprA) is often detected in clinical isolates and contributes to worrying multi-drug resistance phenotypes. Not all antibiotics are affected to the same extent by the aforementioned RND efflux pumps. The impact of efflux on antibiotic activity varies not only between different classes of antibiotics but also between members of the same family of antibiotics. Subtle differences in physicochemical features of compound-pump and compound-solvent interactions largely determine how compounds are affected by efflux activity. The combination of different high-resolution techniques helps to gain insight into the functioning of these molecular machineries. This review discusses substrate recognition patterns based on experimental evidence and computer simulations with a focus on MexB, the pump subunit of the main RND transporter in P. aeruginosa.

Case-control study of the risk factors for acquisition of Pseudomonas and Proteus species during tigecycline therapy

Tigecycline is an important agent in clinical practice because of its broad-spectrum activity. However, it has no activity against Pseudomonas or Proteus species. We conducted a case-control study to analyze risk factors for the acquisition of Pseudomonas or Proteus spp. during tigecycline therapy. Placement of suction drainage at infected wound sites, ICU stay, and neurologic disease were identified as independent risk factors for the acquisition of Pseudomonas and Proteus spp.

The challenge of efflux-mediated antibiotic resistance in Gram-negative bacteria

The global emergence of multidrug-resistant Gram-negative bacteria is a growing threat to antibiotic therapy. The chromosomally encoded drug efflux mechanisms that are ubiquitous in these bacteria greatly contribute to antibiotic resistance and present a major challenge for antibiotic development. Multidrug pumps, particularly those represented by the clinically relevant AcrAB-TolC and Mex pumps of the resistance-nodulation-division (RND) superfamily, not only mediate intrinsic and acquired multidrug resistance (MDR) but also are involved in other functions, including the bacterial stress response and pathogenicity. Additionally, efflux pumps interact synergistically with other resistance mechanisms (e.g., with the outer membrane permeability barrier) to increase resistance levels. Since the discovery of RND pumps in the early 1990s, remarkable scientific and technological advances have allowed for an in-depth understanding of the structural and biochemical basis, substrate profiles, molecular regulation, and inhibition of MDR pumps. However, the development of clinically useful efflux pump inhibitors and/or new antibiotics that can bypass pump effects continues to be a challenge. Plasmid-borne efflux pump genes (including those for RND pumps) have increasingly been identified. This article highlights the recent progress obtained for organisms of clinical significance, together with methodological considerations for the characterization of MDR pumps.

Differential contribution of AcrAB and OqxAB efflux pumps to multidrug resistance and virulence in Klebsiella pneumoniae

OBJECTIVES

In Klebsiella pneumoniae, overexpression of the AcrAB efflux pump and the more recently described OqxAB efflux pump has been linked to an antibiotic cross-resistance phenotype, but the mechanisms of regulation are largely unknown. Moreover, while AcrAB has been shown to participate in K. pneumoniae virulence, the contribution of OqxAB has not yet been assessed.

METHODS

In the present study we investigated a K. pneumoniae clinical isolate (KPBj1 E+), displaying cross-resistance to quinolones, chloramphenicol and cefoxitin, and its phenotypic revertant (KPBj1 Rev, susceptible to antibiotics) by using whole-genome sequencing, RT-PCR, complementation and a Caenorhabditis elegans virulence model.

RESULTS

We detected a point mutation in the oqxR repressor gene of KPBj1 E+, which overexpressed genes rarA, encoding a transcriptional regulator, and oqxB, but not acrB. Complementation with wild-type oqxR restored antibiotic susceptibility and normalized rarA and oqxB expression levels. Whole-genome sequencing showed that KPBj1 Rev had lost the entire rarA-oqxABR locus, situated close to an integration hot spot of phage P4. This large deletion seemed responsible for the significantly lower virulence potential of strain KPBj1 Rev compared with KPBj1 E+. Moreover, we found that KPBj1 E+ ΔacrB was significantly less virulent than its parental strain.

CONCLUSIONS

This work demonstrates the role of the overexpression of efflux pump OqxAB, due to a mutation in gene oqxR, in the antibiotic resistance phenotype of a clinical isolate, and suggests that the presence of AcrAB, associated with overexpression of OqxAB, is required for high virulence potential.

Tigecycline: a novel glycylcycline antibiotic

Tigecycline, the first-in-class glycylcycline, was developed to recapture the broad spectrum of activity of the tetracycline class and to treat patients with difficult-to-treat bacterial infections. Tigecycline's in vitro spectrum of activity encompasses aerobic, facultative and anaerobic Gram-positive and -negative bacteria, including antimicrobial-resistant bacteria such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecalis and Enterococcus faecium, and extended-spectrum beta-lactamase-producing Enterobacteriaceae. Clinical trials involving patients with complicated skin and skin-structure infections and complicated intra-abdominal infections, including patients infected with methicillin-resistant S. aureus, demonstrated that tigecycline was bacteriologically and clinically effective with mild-to-moderate gastrointestinal adverse events (i.e., nausea, vomiting and diarrhea) the most commonly reported. Tigecycline is a promising new broad-spectrum parenteral monotherapy for the treatment of patients with Gram-positive and -negative bacterial infections.

Genomics of KPC-producing Klebsiella pneumoniae sequence type 512 clone highlights the role of RamR and ribosomal S10 protein mutations in conferring tigecycline resistance

Full genome sequences were determined for five Klebsiella pneumoniae strains belonging to the sequence type 512 (ST512) clone, producing KPC-3. Three strains were resistant to tigecycline, one showed an intermediate phenotype, and one was susceptible. Comparative analysis performed using the genome of the susceptible strain as a reference sequence identified genetic differences possibly associated with resistance to tigecycline. Results demonstrated that mutations in the ramR gene occurred in two of the three sequenced strains. Mutations in RamR were previously demonstrated to cause overexpression of the AcrAB-TolC efflux system and were implicated in tigecycline resistance in K. pneumoniae. The third strain showed a mutation located at the vertex of a very well conserved loop in the S10 ribosomal protein, which is located in close proximity to the tigecycline target site in the 30S ribosomal subunit. This mutation was previously shown to be associated with tetracycline resistance in Neisseria gonorrhoeae. A PCR-based approach was devised to amplify the potential resistance mechanisms identified by genomics and applied to two additional ST512 strains showing resistance to tigecycline, allowing us to identify mutations in the ramR gene.

Contribution of AcrAB efflux pump to ciprofloxacin resistance in Klebsiella pneumoniae isolated from burn patients

Resistance to fluoroquinolones has been recently increased among bacterial strains isolated from outpatients. Multidrug-resistant K. pneumoniae is one of the major organisms isolated from burn patients and the AcrAB efflux pump is the principal pump contributing to the intrinsic resistance in K. pneumoniae against multiple antimicrobial agents including ciprofloxacin and other fluoroquinolones.

Fifty-two K. pneumoniae isolated from burn patients in Shahid Motahari hospital and confirmed by conventional biochemical tests. Antimicrobial susceptibility testing was done according to CLSI 2011 guidelines, to determine the antimicrobial resistance pattern of isolates. AcrA gene was detected among ciprofloxacin-resistant isolates by PCR assay. MICs to ciprofloxacin were measured with and without carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Forty out of the 52 K. pneumoniae isolated from burn patients in Shahid Motahari hospital were resistant to ciprofloxacin according to breakpoint of CLSI guideline. PCR assay for acrA gene demonstrated that all ciprofloxacin-resistant isolates harbored acrA gene coding the membrane fusion protein AcrA and is a part of AcrAB efflux system. Among these isolates, 19 strains (47.5%) showed 2 to 32 fold reduction in MICs after using CCCP as an efflux pump inhibitor. The other 21 strains (52.5%) showed no disparity in MICs before and after using CCCP. In conclusion, the AcrAB efflux system is one of the principal mechanisms contribute in ciprofloxacin resistance among K. pneumoniae isolates but there are some other mechanisms interfere with ciprofloxacin resistance such as mutation in target proteins of DNA gyrase of topoisomerase IV enzymes.

Antimicrobial resistance and virulence: a successful or deleterious association in the bacterial world?

Hosts and bacteria have coevolved over millions of years, during which pathogenic bacteria have modified their virulence mechanisms to adapt to host defense systems. Although the spread of pathogens has been hindered by the discovery and widespread use of antimicrobial agents, antimicrobial resistance has increased globally. The emergence of resistant bacteria has accelerated in recent years, mainly as a result of increased selective pressure. However, although antimicrobial resistance and bacterial virulence have developed on different timescales, they share some common characteristics. This review considers how bacterial virulence and fitness are affected by antibiotic resistance and also how the relationship between virulence and resistance is affected by different genetic mechanisms (e.g., coselection and compensatory mutations) and by the most prevalent global responses. The interplay between these factors and the associated biological costs depend on four main factors: the bacterial species involved, virulence and resistance mechanisms, the ecological niche, and the host. The development of new strategies involving new antimicrobials or nonantimicrobial compounds and of novel diagnostic methods that focus on high-risk clones and rapid tests to detect virulence markers may help to resolve the increasing problem of the association between virulence and resistance, which is becoming more beneficial for pathogenic bacteria.

Tigecycline: an update

Tigecycline is a broad-spectrum antibiotic with activity against difficult-to-treat pathogens such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus spp., Acinetobacter baumannii, and Gram-negative bacterial strains that produce extended-spectrum β-lactamases. Minimal organ toxicity and lack of dosage adjustment in most patients are important considerations for tigecycline use. Tigecycline has been shown to be as effective and safe as standard antimicrobial therapy for treatment of adults with complicated intra-abdominal infections, complicated skin and skin structure infections, and community-acquired bacterial pneumonia. The clearest applications of tigecycline are for on-label indications. Whether tigecycline should be utilized as therapy for other infections including hospital-acquired infections with a high likelihood of multidrug-resistant pathogens is a complex issue that requires ongoing assessment. This article offers an updated overview of tigecycline clinical studies, current microbial resistance patterns, pharmacokinetic/pharmacodynamic investigations, and safety analyses.

Adaptive and mutational resistance: role of porins and efflux pumps in drug resistance

The substantial use of antibiotics in the clinic, combined with a dearth of new antibiotic classes, has led to a gradual increase in the resistance of bacterial pathogens to these compounds. Among the various mechanisms by which bacteria endure the action of antibiotics, those affecting influx and efflux are of particular importance, as they limit the interaction of the drug with its intracellular targets and, consequently, its deleterious effects on the cell. This review evaluates the impact of porins and efflux pumps on two major types of resistance, namely, mutational and adaptive types of resistance, both of which are regarded as key phenomena in the global rise of antibiotic resistance among pathogenic microorganisms. In particular, we explain how adaptive and mutational events can dramatically influence the outcome of antibiotic therapy by altering the mechanisms of influx and efflux of antibiotics. The identification of porins and pumps as major resistance markers has opened new possibilities for the development of novel therapeutic strategies directed specifically against these mechanisms.

Tigecycline challenge triggers sRNA production in Salmonella enterica serovar Typhimurium

BACKGROUND

Bacteria employ complex transcriptional networks involving multiple genes in response to stress, which is not limited to gene and protein networks but now includes small RNAs (sRNAs). These regulatory RNA molecules are increasingly shown to be able to initiate regulatory cascades and modulate the expression of multiple genes that are involved in or required for survival under environmental challenge. Despite mounting evidence for the importance of sRNAs in stress response, their role upon antibiotic exposure remains unknown. In this study, we sought to determine firstly, whether differential expression of sRNAs occurs upon antibiotic exposure and secondly, whether these sRNAs could be attributed to microbial tolerance to antibiotics.

RESULTS

A small scale sRNA cloning strategy of Salmonella enterica serovar Typhimurium SL1344 challenged with half the minimal inhibitory concentration of tigecycline identified four sRNAs (sYJ5, sYJ20, sYJ75 and sYJ118) which were reproducibly upregulated in the presence of either tigecycline or tetracycline. The coding sequences of the four sRNAs were found to be conserved across a number of species. Genome analysis found that sYJ5 and sYJ118 mapped between the 16S and 23S rRNA encoding genes. sYJ20 (also known as SroA) is encoded upstream of the tbpAyabKyabJ operon and is classed as a riboswitch, whilst its role in antibiotic stress-response appears independent of its riboswitch function. sYJ75 is encoded between genes that are involved in enterobactin transport and metabolism. Additionally we find that the genetic deletion of sYJ20 rendered a reduced viability phenotype in the presence of tigecycline, which was recovered when complemented. The upregulation of some of these sRNAs were also observed when S. Typhimurium was challenged by ampicillin (sYJ5, 75 and 118); or when Klebsiella pneumoniae was challenged by tigecycline (sYJ20 and 118).

CONCLUSIONS

Small RNAs are overexpressed as a result of antibiotic exposure in S. Typhimurium where the same molecules are upregulated in a related species or after exposure to different antibiotics. sYJ20, a riboswitch, appears to possess a trans-regulatory sRNA role in antibiotic tolerance. These findings imply that the sRNA mediated response is a component of the bacterial response to antibiotic challenge.

Efflux: how bacteria use pumps to control their microenvironment

Efflux pumps are a potent and clinically important cause of antibiotic resistance. The particular focus of this chapter is on the efflux pump as a target for antimicrobial therapy and the development of new antibacterials to address the efflux problem.Tigecycline is an example of how old antibiotics, in this case tetracyclines, which have become substrates for efflux pumps, can be extensively modified to restore antimicrobial activity and clinical efficacy.

In vitro selection of ramR and soxR mutants overexpressing efflux systems by fluoroquinolones as well as cefoxitin in Klebsiella pneumoniae

The relationship between efflux system overexpression and cross-resistance to cefoxitin, quinolones, and chloramphenicol has recently been reported in Klebsiella pneumoniae. In 3 previously published clinical isolates and 17 in vitro mutants selected with cefoxitin or fluoroquinolones, mutations in the potential regulator genes of the AcrAB efflux pump (acrR, ramR, ramA, marR, marA, soxR, soxS, and rob) were searched, and their impacts on efflux-related antibiotic cross-resistance were assessed. All mutants but 1, and 2 clinical isolates, overexpressed acrB. No mutation was detected in the regulator genes studied among the clinical isolates and 8 of the mutants. For the 9 remaining mutants, a mutation was found in the ramR gene in 8 of them and in the soxR gene in the last one, resulting in overexpression of ramA and soxS, respectively. Transformation of the ramR mutants and the soxR mutant with the wild-type ramR and soxR genes, respectively, abolished overexpression of acrB and ramA in the ramR mutants and of soxS in the soxR mutant, as well as antibiotic cross-resistance. Resistance due to efflux system overexpression was demonstrated for 4 new antibiotics: cefuroxime, cefotaxime, ceftazidime, and ertapenem. This study shows that the ramR and soxR genes control the expression of efflux systems in K. pneumoniae and suggests the existence of efflux pumps other than AcrAB and of other loci involved in the regulation of AcrAB expression.

RT-PCR and statistical analyses of adeABC expression in clinical isolates of Acinetobacter calcoaceticus-Acinetobacter baumannii complex

The relationship between expression of adeABC and minimal inhibitory concentration (MIC) of tigecycline was investigated by RT-PCR and statistical analyses in a population of 106 clinical isolates (MIC range, 0.0313-16 microg/ml) of Acinetobacter calcoaceticus-Acinetobacter baumannii complex. There was a statistically significant linear relationship (p < 0.0001) between log-transformed expression values and log-transformed MIC values, indicating that overexpression of AdeABC efflux pump is a prevalent mechanism for decreased susceptibility to tigecycline in A. calcoaceticus-A. baumannii complex.

Antimicrobial activity against Streptococcus pneumoniae and Haemophilus influenzae collected globally between 2004 and 2008 as part of the Tigecycline Evaluation and Surveillance Trial

We report here on the in vitro activity of tigecycline and comparators against a global collection of Streptococcus pneumoniae and Haemophilus influenzae collected between 2004 and 2008 as part of the Tigecycline Evaluation and Surveillance Trial. A total of 6785 S. pneumoniae and 6642 H. influenzae isolates were collected, most from North America. The percentages of penicillin-intermediate resistance and penicillin resistance among S. pneumoniae in North America were 27.8% and 14.3%, respectively. Penicillin resistance ranged from 9.3% in Europe to 25.1% in the Asia-Pacific Rim. The rate of beta-lactamase-producing H. influenzae was 25.8% in North America, and among the other regions, it ranged from 8.7% in South Africa to 26.8% in the Asia-Pacific Rim. Tigecycline MIC(90)'s were 0.03 to 0.12 mg/L and 0.5 to 2 mg/L, depending on the region considered, against S. pneumoniae and H. influenzae, respectively. Tigecycline had low MIC(90)'s against S. pneumoniae and H. influenzae, irrespective of resistance to beta-lactams.