Tigecycline: a new glycylcycline for treatment of serious infections

Drugs for treating infections caused by non-tubercular mycobacteria: a narrative review from the study group on mycobacteria of the Italian Society of Infectious Diseases and Tropical Medicine

BACKGROUND

Non-tuberculous mycobacteria (NTM) are generally free-living organism, widely distributed in the environment, with sporadic potential to infect. In recent years, there has been a significant increase in the global incidence of NTM-related disease, spanning across all continents and an increased mortality after the diagnosis has been reported. The decisions on whether to treat or not and which drugs to use are complex and require a multidisciplinary approach as well as patients' involvement in the decision process.

METHODS AND RESULTS

This review aims at describing the drugs used for treating NTM-associated diseases emphasizing the efficacy, tolerability, optimization strategies as well as possible drugs that might be used in case of intolerance or resistance. We also reviewed data on newer compounds highlighting the lack of randomised clinical trials for many drugs but also encouraging preliminary data for others. We also focused on non-pharmacological interventions that need to be adopted during care of individuals with NTM-associated diseases CONCLUSIONS: Despite insufficient efficacy and poor tolerability this review emphasizes the improvement in patients' care and the needs for future studies in the field of anti-NTM treatments.

Characterisation of a Novel Tigecycline Resistance Gene tet(X22) and its Coexistence with blaNDM-1 in a Pseudomonas caeni Isolate

OBJECTIVES

The emergence of pathogens that are resistant to both tigecycline and carbapenem poses a threat to public health globally. Continuous emergence of novel tet(X) variants accelerates the tigecycline resistance crisis. This study aimed to characterise the novel tigecycline resistance gene tet(X22) and its coexistence with carbapenem resistance gene blaNDM-1 in Pseudomonas caeni.

METHODS

This P. caeni isolate co-harbouring tet(X22) and blaNDM-1 was systematically investigated using antimicrobial susceptibility testing, conjugation assays, genome sequencing, bioinformatic analyses, cloning of tet(X22) and functional analysis, and protein structure prediction.

RESULTS

The carbapenem-resistant and tigecycline-resistant P. caeni isolate CE14 was obtained from chicken faeces in 2022. CE14 carried multiple antibiotic resistance genes, including the novel tet(X22) and blaNDM-1. Tet(X22) exhibited 64.72-90.48% amino acid identity with other variants [Tet(X) to Tet(X21)]. Cloning of the gene tet(X22) and protein structure prediction revealed that Tet(X22) confers resistance to tetracyclines, including tigecycline. tet(X22) and blaNDM-1 were located in two multidrug-resistant regions of the chromosome.

CONCLUSIONS

The occurrence of the novel ISCR2-flanked tet(X22) in P. caeni suggests that the tet(X) variant has adapted to new hosts and may widely spread to further expand the host range. The future global spread of such pathogens co-harbouring tet(X) and blaNDM variants needs to be continuously monitored according to the One Health approach.

In vitro antimicrobial activity and resistance mechanisms of the new generation tetracycline agents, eravacycline, omadacycline, and tigecycline against clinical Staphylococcus aureus isolates

In this study, we investigated the in vitro activity and resistance mechanisms of the new generation tetracycline agents, namely eravacycline, omadacycline, and tigecycline, against Staphylococcus aureus isolates. A total of 1,017 non-duplicate S. aureus isolates were collected and subjected to susceptibility testing against eravacycline, omadacycline, and tigecycline using the broth microdilution method. Tetracyclines-resistant (eravacycline/omadacycline/tigecycline-resistant) isolates were selected to elucidate the resistance mechanisms using polymerase chain reaction (PCR), cloning experiment, efflux pump inhibition, and quantitative real-time PCR. The results of the antibacterial susceptibility testing showed that compared with omadacycline, eravacycline and tigecycline had superior antibacterial activity against S. aureus isolates. Among 1,017 S. aureus, 41 tetracyclines-resistant isolates were identified. These resistant isolates possessed at least one tetracycline resistance gene and genetic mutation in the MepRAB efflux pump and 30S ribosome units. A frameshift mutation in mepB was detected in most tetracyclines-resistant strains (except for JP3349) compared with tetracyclines-susceptible (eravacycline/omadacycline/tigecycline-susceptible) strains. This was first shown to decrease susceptibility to omadacycline, but not to eravacycline and tigecycline. After treatment with eravacycline, omadacycline or tigecycline, overexpression of mepA, tet38, tet(K) and tet(L) was detected. Moreover, multi-locus sequence typing showed a major clonal dissemination type, ST5, and its variant ST764 were seen in most tetracyclines-resistant strains. To conclude, eravacycline and tigecycline exhibited better activity against S. aureus including tetracycline-resistant isolates than omadacycline. The resistance to these new generation tetracyclines due to an accumulation of many resistance mechanisms.

In vitro activity of tigecycline and proteomic analysis of tigecycline adaptation strategies in clinical Enterococcus faecalis isolates from China

OBJECTIVES

This study aimed to investigate the in vitro activities of tigecycline (TGC) and the underlying molecular mechanisms of TGC stress response and resistance in clinical Enterococcus faecalis isolates from China.

METHODS

Antimicrobial susceptibility and antibiofilm activities of TGC in 399 E. faecalis isolates were evaluated. Heteroresistance was evaluated by population analysis profiling. Resistance and heteroresistance mechanisms were investigated by identifying genetic mutations in tetracycline (tet) target sites and through analysis of efflux protein inhibitors (EPIs). Furthermore, quantitative proteomics was used to investigate the global proteomic response of E. faecalis to TGC stress, as well as the resistance mechanisms of TGC within in vitro induced resistant isolate.

RESULTS

TGC minimum inhibitory concentrations (MICs) against clinical E. faecalis isolates were ≤0.5 mg/L. TGC displayed remarkable inhibitory activity against biofilm formation. The occurrence rate of TGC heteroresistance was 1.75% (7/399), and the increased TGC MIC values of heteroresistance-derived clones could be reversed by EPI. TGC resistance was associated with mutations in the 16S rRNA site or 30S ribosomal protein S10. A total of 105 and 356 differentially expressed proteins was identified after being exposed to 1/2× MIC concentrations of TGC, while 356 differentially expressed proteins was identified in TGC-resistant isolate. The differentially expressed proteins were enriched in the translation and DNA replication process. In addition, multiple adenosine triphosphate (ATP)-binding cassette (ABC) transporters were upregulated.

CONCLUSIONS

TGC exhibited excellent activity against a substantial proportion of clinical isolates from China. However, E. faecalis exhibited a strong adaptation mechanism during TGC exposure: mutation of TGC target sites and elevated expression of efflux pumps under TGC selection, resulting in TGC resistance.

Functional and phylogenetic analysis of TetX variants to design a new classification system

Recently, many TetX variants such as Tet(X3~14) were reported to confer resistance to tigecycline which is a last-resort antibiotic used to treat infections caused by multidrug-resistant bacteria. In this study, we identified essential residues including 329, 339, 340, 350, and 351 in TetX variants that mediated the evolution of the tigecycline-inactive Tet(X2) enzyme to the active forms of Tet(X3) and Tet(X4). Based on their amino acid sequences and functional features, we classified TetX variants into TetX-A class, TetX-B class and TetX-C class. We further found that TetX-A class variants originated from Bacteroidetes, with some variants further evolving to TetX-C class and acquired by Enterobacteriaceae. On the other hand, our data showed that some variants genes belonging to TetX-A class evolved directly to TetX-B class, which was further transmitted to Acinetobacter spp. This new classification system may facilitate better clinical management of patients infected by TetX-producing strains.

Genetic Determinants of Tigecycline Resistance in Mycobacteroides abscessus

Mycobacteroides abscessus (formerly Mycobacterium abscessus) is a clinically important, rapid-growing non-tuberculous mycobacterium notoriously known for its multidrug-resistance phenotype. The intrinsic resistance of M. abscessus towards first- and second-generation tetracyclines is mainly due to the over-expression of a tetracycline-degrading enzyme known as MabTetX (MAB_1496c). Tigecycline, a third-generation tetracycline, is a poor substrate for the MabTetX and does not induce the expression of this enzyme. Although tigecycline-resistant strains of M. abscessus have been documented in different parts of the world, their resistance determinants remain largely elusive. Recent work on tigecycline resistance or reduced susceptibility in M. abscessus revealed the involvement of the gene MAB_3508c which encodes the transcriptional activator WhiB7, as well as mutations in the sigH-rshA genes which control heat shock and oxidative-stress responses. The deletion of whiB7 has been observed to cause a 4-fold decrease in the minimum inhibitory concentration of tigecycline. In the absence of environmental stress, the SigH sigma factor (MAB_3543c) interacts with and is inhibited by the anti-sigma factor RshA (MAB_3542c). The disruption of the SigH-RshA interaction resulting from mutations and the subsequent up-regulation of SigH have been hypothesized to lead to tigecycline resistance in M. abscessus. In this review, the evidence for different genetic determinants reported to be linked to tigecycline resistance in M. abscessus was examined and discussed.

Isolation and Characterization of a Novel Autographiviridae Phage and Its Combined Effect with Tigecycline in Controlling Multidrug-Resistant Acinetobacter baumannii-Associated Skin and Soft Tissue Infections

Multidrug-resistant Acinetobacter baumannii (MDR A. baumannii) is one of the ESKAPE pathogens that restricts available treatment options. MDR A. baumannii is responsible for a dramatic increase in case numbers of a wide variety of infections, including skin and soft tissue infections (SSTIs), resulting in pyoderma, surgical debridement, and necrotizing fasciitis. To investigate an alternative medical treatment for SSTIs, a broad range lytic Acinetobacter phage, vB _AbP_ABWU2101 (phage vABWU2101), for lysing MDR A. baumannii in associated SSTIs was isolated and the biological aspects of this phage were investigated. Morphological characterization and genomic analysis revealed that phage vABWU2101 was a new species in the Friunavirus, Beijerinckvirinae, family Autographiviridae, and order Caudovirales. Antibiofilm activity of phage vABWU2101 demonstrated good activity against both preformed biofilms and biofilm formation. The combination of phage vABWU2101 and tigecycline showed synergistic antimicrobial activities against planktonic and biofilm cells. Scanning electron microscopy confirmed that the antibacterial efficacy of the combination of phage vABWU2101 and tigecycline was more effective than the phage or antibiotic alone. Hence, our findings could potentially be used to develop a therapeutic option for the treatment of SSTIs caused by MDR A. baumannii.

The long non-coding RNA SAMMSON is essential for uveal melanoma cell survival

Long non-coding RNAs (lncRNAs) can exhibit cell-type and cancer-type specific expression profiles, making them highly attractive as therapeutic targets. Pan-cancer RNA sequencing data revealed broad expression of the SAMMSON lncRNA in uveal melanoma (UM), the most common primary intraocular malignancy in adults. Currently, there are no effective treatments for UM patients with metastatic disease, resulting in a median survival time of 6-12 months. We aimed to investigate the therapeutic potential of SAMMSON inhibition in UM. Antisense oligonucleotide (ASO)-mediated SAMMSON inhibition impaired the growth and viability of a genetically diverse panel of uveal melanoma cell lines. These effects were accompanied by an induction of apoptosis and were recapitulated in two uveal melanoma patient derived xenograft (PDX) models through subcutaneous ASO delivery. SAMMSON pulldown revealed several candidate interaction partners, including various proteins involved in mitochondrial translation. Consequently, inhibition of SAMMSON impaired global, mitochondrial and cytosolic protein translation levels and mitochondrial function in uveal melanoma cells. The present study demonstrates that SAMMSON expression is essential for uveal melanoma cell survival. ASO-mediated silencing of SAMMSON may provide an effective treatment strategy to treat primary and metastatic uveal melanoma patients.

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.

An Outbreak of tet(X6)-Carrying Tigecycline-Resistant Acinetobacter baumannii Isolates with a New Capsular Type at a Hospital in Taiwan

Dissemination of multidrug-resistant, particularly tigecycline-resistant, Acinetobacter baumannii is of critical importance, as tigecycline is considered a last-line antibiotic. Acquisition of tet(X), a tigecycline-inactivating enzyme mostly found in strains of animal origin, imparts tigecycline resistance to A. baumannii. Herein, we investigated the presence of tet(X) variants among 228 tigecycline-non-susceptible A. baumannii isolates from patients at a Taiwanese hospital via polymerase chain reaction using a newly designed universal primer pair. Seven strains (3%) carrying tet(X)-like genes were subjected to whole genome sequencing, revealing high DNA identity. Phylogenetic analysis based on the PFGE profile clustered the seven strains in a clade, which were thus considered outbreak strains. These strains, which were found to co-harbor the chromosome-encoded tet(X6) and the plasmid-encoded bla_OXA-72 genes, showed a distinct genotype with an uncommon sequence type (Oxford ST793/Pasteur ST723) and a new capsular type (KL129). In conclusion, we identified an outbreak clone co-carrying tet(X6) and bla_OXA-72 among a group of clinical A. baumannii isolates in Taiwan. To the best of our knowledge, this is the first description of tet(X6) in humans and the first report of a tet(X)-like gene in Taiwan. These findings identify the risk for the spread of tet(X6)-carrying tigecycline- and carbapenem-resistant A. baumannii in human healthcare settings.

Functional Nanomaterials and 3D-Printable Nanocomposite Hydrogels for Enhanced Cell Proliferation and for the Reduction of Bacterial Biofilm Formation

Biomaterial-associated infections are a major cause of biomaterial implant failure. To prevent the initial attachment of bacteria to the implant surface, researchers have investigated various surface modification methods. However, most of these approaches also prevent the attachment, spread, and growth of mammalian cells, resulting in tissue integration failure. Therefore, the success of biomaterial implants requires an optimal balance between tissue integration (cell adhesion to biomaterial implants) and inhibition of bacterial colonization. In this regard, we synthesize bifunctional nanomaterials by functionalizing the pores and outer surfaces of periodic mesoporous organosilica (PMO) with antibacterial tetracycline (Tet) and antibacterial and cell-adhesive bipolymer poly-d-lysine (PDL), respectively. Then, the fabricated ^TetPMO-PDL nanomaterials are incorporated into alginate-based hydrogels to create injectable and 3D-printable nanocomposite (NC) hydrogels (AlgL-^TetPMO-PDL). These bifunctional nanomaterial and 3D-printable NC hydrogel show pH-dependent release of Tet over 7 days. They also enhance the proliferation of eukaryotic cells (fibroblasts). ^TetPMO-PDL is inactive in reducing Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis biofilms. However, AlgL-^TetPMO-PDL shows significant antibiofilm activity against P. aeruginosa. These results suggest that the incorporation of ^TetPMO-PDL into AlgL may have a synergistic effect on the inhibition of the Gram-negative bacterial (P. aeruginosa) biofilm, while this has no effect on the reduction of the Gram-positive bacterial (S. aureus and E. faecalis) biofilm.

Surface-enhanced Raman spectroscopy for the identification of tigecycline-resistant E. coli strains

Tigecycline (TGC) is recognised as last resort of drugs against several antibiotic-resistant bacteria. Bacterial resistance to tigecycline due to presence of plasmid-mediated mobile TGC resistance genes (tet X3/X4) has broken another defense line. Therefore, rapid and reproducible detection of tigecycline-resistant E. coli (TREC) is required. The current study is designed for the identification and differentiation of TREC from tigecycline-sensitive E. coli (TSEC) by employing SERS by using Ag NPs as a SERS substrate. The SERS spectral fingerprints of E. coli strains associated directly or indirectly with the development of resistance against tigecycline have been distinguished by comparing SERS spectral data of TSEC strains with each TREC strain. Moreover, the statistical analysis including Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA) and Partial Least Squares-Discriminant Analysis (PLS-DA) were employed to check the diagnostic potential of SERS for the differentiation among TREC and TSEC strains. The qualitative identification and differentiation between resistant and sensitive strains and among individual strains have been efficiently done by performing both PCA and HCA. The successful discrimination among TREC and TSEC at the strain level is performed by PLS-DA with 98% area under ROC curve, 100% sensitivity, 98.7% specificity and 100% accuracy.

The poultry pathogen Riemerella anatipestifer appears as a reservoir for Tet(X) tigecycline resistance

The transferability of bacterial resistance to tigecycline, the 'last-resort' antibiotic, is an emerging challenge of global health concern. The plasmid-borne tet(X) that encodes a flavin-dependent monooxygenase represents a new mechanism for tigecycline resistance. Natural source for an ongoing family of Tet(X) resistance determinants is poorly understood. Here, we report the discovery of 26 new variants [tet(X18) to tet(X44)] from the poultry pathogen Riemerella anatipestifer, which expands extensively the current Tet(X) family. R. anatipestifer appears as a natural reservoir for tet(X), of which the chromosome harbours varied copies of tet(X) progenitors. Despite that an inactive ancestor rarely occurs, the action and mechanism of Tet(X2/4)-P, a putative Tet(X) progenitor, was comprehensively characterized, giving an intermediate level of tigecycline resistance. The potential pattern of Tet(X) dissemination from ducks to other animals and humans was raised, in the viewpoint of ecological niches. Therefore, this finding defines a large pool of natural sources for Tet(X) tigecycline resistance, heightening the need of efficient approaches to manage the inter-species transmission of tet(X) resistance determinants.

Antimicrobial Effects of Minocycline, Tigecycline, Ciprofloxacin, and Levofloxacin against Elizabethkingia anophelis Using In Vitro Time-Kill Assays and In Vivo Zebrafish Animal Models

Elizabethkingia anophelis is a multidrug-resistant pathogen. This study evaluated the antimicrobial activity of minocycline, tigecycline, ciprofloxacin, and levofloxacin using in vitro time-kill assays and in vivo zebrafish animal models. The E. anophelis strain ED853-49 was arbitrarily selected from a bacterial collection which was concomitantly susceptible to minocycline, tigecycline, ciprofloxacin, and levofloxacin. The antibacterial activities of single agents at 0.5-4 × minimum inhibitory concentration (MIC) and dual-agent combinations at 2 × MIC using time-kill assays were investigated. The therapeutic effects of antibiotics in E. anophelis-infected zebrafish were examined. Both minocycline and tigecycline demonstrated bacteriostatic effects but no bactericidal effect. Minocycline at concentrations ≥2 × MIC and tigecycline at concentrations ≥3 × MIC exhibited a long-standing inhibitory effect for 48 h. Bactericidal effects were observed at ciprofloxacin and levofloxacin concentrations of ≥3 × MIC within 24 h of initial inoculation. Rapid regrowth of E. anophelis occurred after the initial killing phase when ciprofloxacin was used, regardless of the concentration. Levofloxacin treatment at the concentration of ≥2 × MIC consistently resulted in the long-lasting and sustainable inhibition of bacterial growth for 48 h. The addition of minocycline or tigecycline weakened the killing effect of fluoroquinolones during the first 10 h. The minocycline-ciprofloxacin or minocycline-levofloxacin combinations achieved the lowest colony-forming unit counts at 48 h. Zebrafish treated with minocycline or a combination of minocycline and levofloxacin had the highest survival rate (70%). The results of these in vitro and in vivo studies suggest that the combination of minocycline and levofloxacin is the most effective therapy approach for E. anophelis infection.

Co-production of Tet(X) and MCR-1, two resistance enzymes by a single plasmid

Tigecycline and colistin are few of 'last-resort' antibiotic defences used in anti-infection therapies against carbapenem-resistant bacterial pathogens. The successive emergence of plasmid-borne tet(X) tigecycline resistance mechanism and mobile colistin resistance (mcr) determinant, renders them clinically useless. Here, we report that co-carriage of tet(X6) and mcr-1 gives co-resistance to both classes of antibiotics by a single plasmid in Escherichia coli. Tet(X6), the new tigecycline resistance enzyme is functionally defined. Both Tet(X6) and MCR-1 robustly interfere accumulation of antibiotic-induced reactive oxygen species (ROS). Unlike that mcr-1 exerts fitness cost in E. coli, tet(X6) does not. In the tet(X6)-positive strain that co-harbors mcr-1, tigecycline resistance is independently of colistin resistance caused by MCR-1-mediated lipid A remodelling, and vice versa. In general consistency with that of MCR-1, Tet(X6) leads to the failure of tigecycline treatment in the infection model of G. mellonella. Taken together, the co-production of Tet(X) and MCR-1 appears as a major clinic/public health concern.

Tigecycline antibacterial activity, clinical effectiveness, and mechanisms and epidemiology of resistance: narrative review

Tigecycline is unique glycylcycline class of semisynthetic antimicrobial agents developed for the treatment of polymicrobial infections caused by multidrug-resistant Gram-positive and Gram-negative pathogens. Tigecycline evades the main tetracycline resistance genetic mechanisms, such as tetracycline-specific efflux pump acquisition and ribosomal protection, via the addition of a glycyclamide moiety to the 9-position of minocycline. The use of the parenteral form of tigecycline is approved for complicated skin and skin structure infections (excluding diabetes foot infection), complicated intra-abdominal infections, and community-acquired bacterial pneumonia in adults. New evidence also suggests the effectiveness of tigecycline for the treatment of severe Clostridioides difficile infections. Tigecycline showed in vitro susceptibility to Coxiella spp., Rickettsia spp., and multidrug-resistant Neisseria gonnorrhoeae strains which indicate the possible use of tigecycline in the treatment of infections caused by these pathogens. Except for intrinsic, or often reported resistance in some Gram-negatives, tigecycline is effective against a wide range of multidrug-resistant nosocomial pathogens. Herein, we summarize the currently available data on tigecycline pharmacokinetics and pharmacodynamics, its mechanism of action, the epidemiology of tigecycline resistance, and its clinical effectiveness.

Coexistence and characterization of Tet(X5) and NDM-3 in the MDR-Acinetobacter indicus of duck origin

Tigecycline and carbapenem are last-resort antibiotics for serious infections caused by pathogens with multi-drug resistance (MDR). Whereas, bacterial pathogens with co-resistance to tigecycline and carbapenem are poorly addressed. Here we report a tigecycline- and carbapenem-resistant Acinetobacter indicus strain HY20 of duck origin, which co-produces Tet(X5) and NDM-3. Tet(X5) is harbored by a novel plasmid pAI01 (116,992 bp long), which carries 10 antimicrobial resistance genes (AMRs), and heavy metal resistance system cobalt-zinc-cadmium (czc) gene cluster. Unlike that tet(X5) is located in the res-tet(X5)-xerD segment of plasmid, the chromosomal bla_NDM-3 is flanked by insertion ISAba125. Collectively, our result represents an example of co-carriage of tet(X5) and bla_NDM-3, heightening the importance of AMR surveillance needed in poultry production.

Harnessing efficient multiplex PCR methods to detect the expanding Tet(X) family of tigecycline resistance genes

A growing number of tet(X)-type tigecycline resistance determinants [tet(X1) to tet(X5)] constitutes an expanding family of tetracycline-inactivating enzymes, posing a potential risk to global public health. Here, we report the development of an efficient multiplex PCR method to detect the family of tet(X) variants. This method is successfully applied in the screen and validation of tet(X) genes in the field and clinic bacterial samples. In addition, we found that the formerly proposed tet(X1) is a premature truncated version by the inappropriate annotation, and fixed this error. Overall, it might be the first genetic tool for the detection of different Tet(X) members.

Omadacycline: A Novel Oral and Intravenous Aminomethylcycline Antibiotic Agent

Omadacycline is a novel aminomethylcycline antibiotic developed as a once-daily, intravenous and oral treatment for acute bacterial skin and skin structure infection (ABSSSI) and community-acquired bacterial pneumonia (CABP). Omadacycline, a derivative of minocycline, has a chemical structure similar to tigecycline with an alkylaminomethyl group replacing the glycylamido group at the C-9 position of the D-ring of the tetracycline core. Similar to other tetracyclines, omadacycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. Omadacycline possesses broad-spectrum antibacterial activity against Gram-positive and Gram-negative aerobic, anaerobic, and atypical bacteria. Omadacycline remains active against bacterial isolates possessing common tetracycline resistance mechanisms such as efflux pumps (e.g., TetK) and ribosomal protection proteins (e.g., TetM) as well as in the presence of resistance mechanisms to other antibiotic classes. The pharmacokinetics of omadacycline are best described by a linear, three-compartment model following a zero-order intravenous infusion or first-order oral administration with transit compartments to account for delayed absorption. Omadacycline has a volume of distribution (V_d) ranging from 190 to 204 L, a terminal elimination half-life (t_½) of 13.5-17.1 h, total clearance (CL_T) of 8.8-10.6 L/h, and protein binding of 21.3% in healthy subjects. Oral bioavailability of omadacycline is estimated to be 34.5%. A single oral dose of 300 mg (bioequivalent to 100 mg IV) of omadacycline administered to fasted subjects achieved a maximum plasma concentration (C_max) of 0.5-0.6 mg/L and an area under the plasma concentration-time curve from 0 to infinity (AUC_0-∞) of 9.6-11.9 mg h/L. The free plasma area under concentration-time curve divided by the minimum inhibitory concentration (i.e., fAUC_24h/MIC), has been established as the pharmacodynamic parameter predictive of omadacycline antibacterial efficacy. Several animal models including neutropenic murine lung infection, thigh infection, and intraperitoneal challenge model have documented the in vivo antibacterial efficacy of omadacycline. A phase II clinical trial on complicated skin and skin structure infection (cSSSI) and three phase III clinical trials on ABSSSI and CABP demonstrated the safety and efficacy of omadacycline. The phase III trials, OASIS-1 (ABSSSI), OASIS-2 (ABSSSI), and OPTIC (CABP), established non-inferiority of omadacycline to linezolid (OASIS-1, OASIS-2) and moxifloxacin (OPTIC), respectively. Omadacycline is currently approved by the FDA for use in treatment of ABSSSI and CABP. Phase II clinical trials involving patients with acute cystitis and acute pyelonephritis are in progress. Mild, transient gastrointestinal events are the predominant adverse effects associated with use of omadacycline. Based on clinical trial data to date, the adverse effect profile of omadacycline is similar to studied comparators, linezolid and moxifloxacin. Unlike tigecycline and eravacycline, omadacycline has an oral formulation that allows for step-down therapy from the intravenous formulation, potentially facilitating earlier hospital discharge, outpatient therapy, and cost savings. Omadacycline has a potential role as part of an antimicrobial stewardship program in the treatment of patients with infections caused by antibiotic-resistant and multidrug-resistant Gram-positive [including methicillin-resistant Staphylococcus aureus (MRSA)] and Gram-negative pathogens.

Coexistence of bla OXA-58 and tet(X) on a Novel Plasmid in Acinetobacter sp. From Pig in Shanghai, China

The purpose of this study was to characterize the complete sequence of a novel plasmid carrying tigecycline resistance gene tet(X) and carbapenemase gene bla_OXA-58 from a swine Acinetobacter sp. strain SH19PTT10. Minimal inhibitory concentration (MIC) was performed using microbroth dilution method. The isolate SH19PTT10 was highly resistant (16 mg/L) to tigecycline, and also exhibited resistance to ampicillin, streptomycin, tetracycline, chloramphenicol, florfenicol, ciprofloxacin, and sulfamethoxazole/trimethoprim. Although SH19PTT10 harbored bla_OXA-58, it was susceptible to cefotaxime and meropenem. The genome sequence of SH19PTT10 was determined using PacBio single-molecule real-time sequencing. Plasmid pYUSHP10-1 had a size of 174,032 bp and showed partial homology to several plasmids found in Acinetobacter isolates. It contained two repA genes, putative toxin-antitoxin systems (HipA/HipB, RelE/RelB, and BrnT/BrnA), partitioning genes (parA and parB), and heavy metal resistance-associated genes (copA/copB, nrp, and czcA/czcD) but the transfer region or proteins was not found. pYUSHP10-1 carried 16 resistance genes, mainly clustered in two mosaic multiresistance regions (MRRs). The first MRR contained sul3, qacI-aadA1-clmA1-aadA2-blaCARB-2-dfrA16 cassette, aac(3)-IId, and bla_OXA-58. The bla_OXA-58 gene was associated with ISAba3, as previously described. The second MRR is the tet(X) region (ISAcsp12-aph(3')-Ia-IS26-ΔxerD-tet(X)-res-ISCR2-sul2) related to the corresponding region in other tet(X)-bearing plasmids. The pdif sites, as well as mobile elements, play an important role in mobilization of DNA modules and plasmid evolution. Coexistence of numerous resistance genes on a single plasmid may contribute to the dissemination of these genes under pressure posed by different agents, which may explain the presence of clinically crucial resistance genes tet(X) and bla_OXA-58 in livestock. Thus, rational drug use and continued surveillance of tet(X) and bla_OXA-58 in livestock are warranted.

Mechanisms and phenotypic consequences of acquisition of tigecycline resistance by Stenotrophomonas maltophilia

OBJECTIVES

To elucidate the potential mutation-driven mechanisms involved in the acquisition of tigecycline resistance by the opportunistic pathogen Stenotrophomonas maltophilia. The mutational trajectories and their effects on bacterial fitness, as well as cross-resistance and/or collateral susceptibility to other antibiotics, were also addressed.

METHODS

S. maltophilia populations were submitted to experimental evolution in the presence of increasing concentrations of tigecycline for 30 days. The genetic mechanisms involved in the acquisition of tigecycline resistance were determined by WGS. Resistance was evaluated by performing MIC assays. Fitness of the evolved populations and individual clones was assessed by measurement of the maximum growth rates.

RESULTS

All the tigecycline-evolved populations attained high-level resistance to tigecycline following different mutational trajectories, yet with some common elements. Among the mechanisms involved in low susceptibility to tigecycline, mutations in the SmeDEF efflux pump negative regulator smeT, changes in proteins involved in the biogenesis of the ribosome and modifications in the LPS biosynthesis pathway seem to play a major role. Besides tigecycline resistance, the evolved populations presented cross-resistance to other antibiotics, such as aztreonam and quinolones, and they were hypersusceptible to fosfomycin, suggesting a possible combination treatment. Further, we found that the selected resistance mechanisms impose a relevant fitness cost when bacteria grow in the absence of antibiotic.

CONCLUSIONS

Mutational resistance to tigecycline was easily selected during exposure to this antibiotic. However, the fitness cost may compromise the maintenance of S. maltophilia tigecycline-resistant populations in the absence of antibiotic.

The global prevalence of Daptomycin, Tigecycline, Quinupristin/Dalfopristin, and Linezolid-resistant Staphylococcus aureus and coagulase-negative staphylococci strains: a systematic review and meta-analysis

Objective

Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase-negative Staphylococcus (MRCoNS) are among the main causes of nosocomial infections, which have caused major problems in recent years due to continuously increasing spread of various antibiotic resistance features. Apparently, vancomycin is still an effective antibiotic for treatment of infections caused by these bacteria but in recent years, additional resistance phenotypes have led to the accelerated introduction of newer agents such as linezolid, tigecycline, daptomycin, and quinupristin/dalfopristin (Q/D). Due to limited data availability on the global rate of resistance to these antibiotics, in the present study, the resistance rates of S. aureus, Methicillin-resistant S. aureus (MRSA), and CoNS to these antibiotics were collected.

Method

Several databases including web of science, EMBASE, and Medline (via PubMed), were searched (September 2018) to identify those studies that address MRSA, and CONS resistance to linezolid, tigecycline, daptomycin, and Q/D around the world.

Result

Most studies that reported resistant staphylococci were from the United States, Canada, and the European continent, while African and Asian countries reported the least resistance to these antibiotics. Our results showed that linezolid had the best inhibitory effect on S. aureus. Although resistances to this antibiotic have been reported from different countries, however, due to the high volume of the samples and the low number of resistance, in terms of statistical analyzes, the resistance to this antibiotic is zero. Moreover, linezolid, daptomycin and tigecycline effectively (99.9%) inhibit MRSA. Studies have shown that CoNS with 0.3% show the lowest resistance to linezolid and daptomycin, while analyzes introduced tigecycline with 1.6% resistance as the least effective antibiotic for these bacteria. Finally, MRSA and CoNS had a greater resistance to Q/D with 0.7 and 0.6%, respectively and due to its significant side effects and drug-drug interactions; it appears that its use is subject to limitations.

Conclusion

The present study shows that resistance to new agents is low in staphylococci and these antibiotics can still be used for treatment of staphylococcal infections in the world.

Characterization of OXA-48-like carbapenemase producers in Canada, 2011-14

Objectives

Since the first identification of the OXA-48 carbapenemase in 2001, Enterobacteriaceae harbouring OXA-48-like enzymes have been reported globally. Here, we applied WGS to characterize the molecular epidemiology of these bacterial isolates.

Methods

Enterobacteriaceae non-susceptible to carbapenems isolated from patients between 2011 and 2014 were voluntarily submitted to the Canadian National Microbiology Laboratory where they were screened for carbapenemase genes. WGS was conducted on OXA-48-like producers using the Illumina MiSeq platform. WGS data were used for single nucleotide variant (SNV) analysis, MLST analysis, detection of resistance genes and partial plasmid characterization. Susceptibilities were determined using Vitek2 and Etest. Patient data provided from sites were reviewed.

Results

Sixty-seven non-duplicated cases were identified among Escherichia coli (n = 21) and Klebsiella pneumoniae (n = 46). Recent international travel was observed in 40.4% of cases. OXA-181 (52.2%) and OXA-48 (31.3%) were the most common variants, one E. coli OXA-48 producer was found to harbour the acquired colistin resistance gene mcr-1. The dominant STs were ST38 and ST410 in E. coli and ST14 in K. pneumoniae. Three common plasmid types were observed among isolates: IncL/M associated with OXA-48 producers, and ColKP3 and IncX3 associated with OXA-181/232 producers.

Conclusions

Enterobacteriaceae with OXA-48-like carbapenemases are emerging in Canada. This study highlights the complexity of OXA-48-types identified in Canada owing to travel and the successful clones and plasmids harbouring the OXA-48-like enzyme.

Klebsiella pneumoniae disassembles host microtubules in lung epithelial cells

Klebsiella pneumoniae raises significant concerns to the health care industry as these microbes are the source of widespread contamination of medical equipment, cause pneumonia as well as other multiorgan metastatic infections and have gained multidrug resistance. Despite soaring mortality rates, the host cell alterations occurring during these infections remain poorly understood. Here, we show that during in vitro and in vivo K. pneumoniae infections of lung epithelia, microtubules are severed and then eliminated. This destruction does not require direct association of K. pneumoniae with the host cells, as microtubules are disassembled in cells that are distant from the infecting bacteria. This microtubule dismantling is dependent on the K. pneumoniae (Kp) gene ytfL as non-pathogenic Escherichia coli expressing Kp ytfL disassemble microtubules in the absence of K. pneumoniae itself. Our data points to the host katanin catalytic subunit A like 1 protein (KATNAL1) and the katanin regulatory subunit B1 protein (KATNB1) as the gatekeepers to the microtubule severing event as both proteins localise specifically to microtubule cut sites. Infected cells that had either of these proteins knocked out maintained intact microtubules. Taken together, we have identified a novel mechanism that a bacterial pathogen has exploited to cause microtubule destruction within the host epithelia.

Surveillance of tigecycline activity tested against clinical isolates from a global (North America, Europe, Latin America and Asia-Pacific) collection (2016)

Tigecycline and comparators were tested by the reference broth microdilution method against 33 348 non-duplicate bacterial isolates collected prospectively in 2016 from medical centres in the Asia-Pacific (3443 isolates), Europe (13 530 isolates), Latin America (3327 isolates) and the USA (13 048 isolates). Among 7098 Staphylococcus aureus isolates tested, >99.9% were inhibited by ≤0.5 mg/L tigecycline (MIC_50/90, 0.06/0.12 mg/L), including >99.9% of methicillin-resistant S. aureus and 100.0% of methicillin-susceptible S. aureus. Tigecycline was slightly more active against Enterococcus faecium (MIC_50/90, 0.03/0.06 mg/L) compared with Enterococcus faecalis (MIC_50/90, 0.06/0.12 mg/L) and its activity was not adversely affected by vancomycin resistance when tested against these organisms. Tigecycline potency was comparable for Streptococcus pneumoniae (MIC_50/90, 0.03/0.06 mg/L), viridans group streptococci (MIC_50/90, 0.03/0.06 mg/L) and β-haemolytic streptococci (MIC_50/90, 0.06/0.06 mg/L) regardless of species and penicillin susceptibility. Tigecycline was active against Enterobacteriaceae (MIC_50/90, 0.25/1 mg/L; 97.8% inhibited at ≤2 mg/L) but was slightly less active against Enterobacteriaceae isolates expressing resistant phenotypes: carbapenem-resistant Enterobacteriaceae (MIC_50/90, 0.5/2 mg/L; 98.0% susceptible); multidrug-resistant (MIC_50/90, 0.5/2 mg/L; 93.1% susceptible); and extensively drug-resistant (MIC_50/90, 0.5/4 mg/L; 87.8% susceptible). Tigecycline inhibited 74.4% of 888 Acinetobacter baumannii isolates at ≤2 mg/L (MIC_50/90, 2/4 mg/L) and demonstrated good in vitro activity against Stenotrophomonas maltophilia (MIC_50/90, 1/2 mg/L; 90.6% inhibited at ≤2 mg/L) Tigecycline was active against Haemophilus influenzae (MIC_50/90, 0.12/0.25 mg/L) regardless of β-lactamase status. Tigecycline represents an important treatment option for resistant Gram-negative and Gram-positive bacterial infections.

Vancomycin-resistant enterococcus infection in the hematopoietic stem cell transplant recipient: an overview of epidemiology, management, and prevention

Vancomycin-resistant enterococcus (VRE) is now one of the leading causes of nosocomial infections in the United States. Hematopoietic stem cell transplantation (HSCT) recipients are at increased risk of VRE colonization and infection. VRE has emerged as a major cause of bacteremia in this population, raising important clinical questions regarding the role and impact of VRE colonization and infection in HSCT outcomes as well as the optimal means of prevention and treatment. We review here the published literature and scientific advances addressing these thorny issues and provide a rational framework for their approach.

Step-Wise Increase in Tigecycline Resistance in Klebsiella pneumoniae Associated with Mutations in ramR, lon and rpsJ

Klebsiella pneumoniae is a gram-negative bacterium that causes numerous diseases, including pneumonia and urinary tract infections. An increase in multidrug resistance has complicated the treatment of these bacterial infections, and although tigecycline shows activity against a broad spectrum of bacteria, resistant strains have emerged. In this study, the whole genomes of two clinical and six laboratory-evolved strains were sequenced to identify putative mutations related to tigecycline resistance. Of seven tigecycline-resistant strains, seven (100%) had ramR mutations, five (71.4%) had lon mutations, one (14.2%) had a ramA mutation, and one (14.2%) had an rpsJ mutation. A higher fitness cost was observed in the laboratory-evolved strains but not in the clinical strains. A transcriptome analysis demonstrated high expression of the ramR operon and acrA in all tigecycline-resistant strains. Genes involved in nitrogen metabolism were induced in the laboratory-evolved strains compared with the wild-type and clinical strains, and this difference in nitrogen metabolism reflected the variation between the laboratory-evolved and the clinical strains. Complementation experiments showed that both the wild-type ramR and the lon genes could partially restore the tigecycline sensitivity of K. pneumoniae. We believe that this manuscript describes the first construct of a lon mutant in K. pneumoniae, which allowed confirmation of its association with tigecycline resistance. Our findings illustrate the importance of the ramR operon and the lon and rpsJ genes in K. pneumoniae resistance to tigecycline.

In Vitro Activity of Tigecycline Against Orientia tsutsugamushi

Scrub typhus is a zoonosis caused by Orientia tsutsugamushi (O. tsutsugamushi) occurring mainly in autumn in Korea. The need of new antibiotics has arisen with a report on strains resistant to antibiotics and chronic infection. This study aims to identify susceptibility of tigecycline in-vitro as a new therapeutic option for O. tsutsugamushi. Antibacterial activity of tigecycline against the O. tsutsugamushi was compared with doxycycline using flow cytometry assay. The inhibitory concentration 50 (IC₅₀) was 3.59×10⁻³ μg/mL in doxycycline-treated group. Whereas in 0.71×10⁻³ μg/mL tigecycline-treated group. These findings indicate that tigecycline may be a therapeutic option for the treatment of scrub typhus.

Evaluation of Trimethoprim/Sulfamethoxazole (SXT), Minocycline, Tigecycline, Moxifloxacin, and Ceftazidime Alone and in Combinations for SXT-Susceptible and SXT-Resistant Stenotrophomonas maltophilia by In Vitro Time-Kill Experiments

Background

The optimal therapy for infections caused by Stenotrophomonas maltophilia (S. maltophilia) has not yet been established. The objective of our study was to evaluate the efficacy of trimethoprim/sulfamethoxazole (SXT), minocycline, tigecycline, moxifloxacin, levofloxacin, ticarcillin-clavulanate, polymyxin E, chloramphenicol, and ceftazidime against clinical isolated S. maltophilia strains by susceptibility testing and carried out time-kill experiments in potential antimicrobials.

Methods

The agar dilution method was used to test susceptibility of nine candidate antimicrobials, and time-killing experiments were carried out to evaluate the efficacy of SXT, minocycline, tigecycline, moxifloxacin, levofloxacin, and ceftazidime both alone and in combinations at clinically relevant antimicrobial concentrations.

Results

The susceptibility to SXT, minocycline, tigecycline, moxifloxacin, levofloxacin, ticarcillin-clavulanate, chloramphenicol, polymyxin E, and ceftazidime were 93.8%, 95.0%, 83.8%, 80.0%, 76.3%, 76.3%, 37.5%, 22.5%, and 20.0% against 80 clinical consecutively isolated strains, respectively. Minocycline and tigecycline showed consistent active against 22 SXT-resistant strains. However, resistance rates were high in the remaining antimicrobial agents against SXT-resistant strains. In time-kill experiments, there were no synergisms in most drug combinations in time-kill experiments. SXT plus moxifloxacin displayed synergism when strains with low moxifloxacin MICs. Moxifloxacin plus Minocycline and moxifloxacin plus tigecycline displayed synergism in few strains. No antagonisms were found in these combinations. Overall, compared with single drug, the drug combinations demonstrated lower bacterial concentrations. Some combinations showed bactericidal activity.

Conclusions

In S. maltophilia infections, susceptibility testing suggests that minocycline and SXT may be considered first-line therapeutic choices while tigecycline, moxifloxacin, levofloxacin, and ticarcillin-clavulanate may serve as second-line choices. Ceftazidime, colistin, and chloramphenicol show poor active against S. maltophilia. However, monotherapy is inadequate in infection management, especially in case of immunocompromised patients. Combination therapy, especially SXT plus moxifloxacin, may benefit than monotherapy in inhibiting or killing S. maltophilia.

KPC enzymes in the UK: an analysis of the first 160 cases outside the North-West region

OBJECTIVES

Klebsiella pneumoniae carbapenemases (KPCs) have been increasingly reported in the UK since 2003. We analysed patient and isolate data for KPC-positive bacteria confirmed by the national reference laboratory from UK laboratories from August 2003 to August 2014, excluding North-West England, where the epidemiology has previously been studied.

METHODS

MICs were determined by BSAC agar dilution. Carbapenem-resistant isolates lacking imipenem/EDTA synergy were tested by PCR for blaKPC. MLST and blaKPC sequencing were performed on a subset of isolates. Plasmid analysis was performed by transformation, PCR-based replicon typing and, in some cases, whole-plasmid sequencing. Patient data provided by the sending laboratories were reviewed.

RESULTS

Two hundred and ten isolates with KPC enzymes were submitted from 71 UK laboratories outside North-West England, representing 160 patients. All were Enterobacteriaceae, predominantly K. pneumoniae (82%; 173/210), and most (91%; 191/210) were from hospitalized patients. Analysis of 100 isolates identified blaKPC-2 (62%), blaKPC-3 (30%) and blaKPC-4 (8%). Clonal group (CG) 258 was dominant among K. pneumoniae (64%; 54/84), but 21 unrelated STs were also identified. Plasmid analysis identified a diverse range of plasmids representing >11 different replicon types and found in multiple STs and species. Most (34/35) plasmids with IncFIB/FIIK replicons exhibited >99% sequence identity to pKpQIL.

CONCLUSIONS

KPC enzymes are increasingly detected in Enterobacteriaceae in the UK, albeit without the major outbreaks seen in North-West England. K. pneumoniae CG258 are the dominant hosts, but plasmid spread plays a major role in KPC dissemination between other K. pneumoniae STs and enterobacterial species.

Antibiotic drug tigecycline reduces neuroblastoma cells proliferation by inhibiting Akt activation in vitro and in vivo

As the first member of glycylcycline bacteriostatic agents, tigecycline is approved as a novel expanded-spectrum antibiotic, which is clinically available. However, accumulating evidence indicated that tigecycline was provided with the potential application in cancer therapy. In this paper, tigecycline was shown to exert an anti-proliferative effect on neuroblastoma cell lines. Furthermore, it was found that tigecycline induced G1-phase cell cycle arrest instead of apoptosis by means of Akt pathway inhibition. In neuroblastoma cell lines, the Akt activator insulin-like growth factor-1 (hereafter referred to as IGF-1) reversed tigecycline-induced cell cycle arrest. Besides, tigecycline inhibited colony formation and suppressed neuroblastoma cells xenograft formation and growth. After tigecycline treatment in vivo, the Akt pathway inhibition was confirmed as well. Collectively, our data provided strong evidences that tigecycline inhibited neuroblastoma cells growth and proliferation through the Akt pathway inhibition in vitro and in vivo. In addition, these results were supported by previous studies concerning the application of tigecycline in human tumors treatment, suggesting that tigecycline might act as a potential candidate agent for neuroblastoma treatment.

Potential of Tetracycline Resistance Proteins To Evolve Tigecycline Resistance

Tigecycline is a glycylcycline antibiotic active against multidrug-resistant bacterial pathogens. The objectives of our study were to examine the potential of the Tet(A), Tet(K), Tet(M), and Tet(X) tetracycline resistance proteins to acquire mutations causing tigecycline resistance and to determine how this affects resistance to earlier classes of tetracyclines. Mutations in all four tet genes caused a significant increase in the tigecycline MIC in Escherichia coli, and strains expressing mutant Tet(A) and Tet(X) variants reached clinically relevant MICs (2 mg/liter and 3 mg/liter, respectively). Mutations predominantly accumulated in transmembrane domains of the efflux pumps, most likely increasing the accommodation of tigecycline as a substrate. All selected Tet(M) mutants contained at least one mutation in the functionally most important loop III of domain IV. Deletion of leucine 505 of this loop led to the highest increase of the tigecycline MIC (0.5 mg/liter) among Tet(M) mutants. It also caused collateral sensitivity to earlier classes of tetracyclines. A majority of the Tet(X) mutants showed increased activity against all three classes of tetracylines. All tested Tet proteins have the potential to acquire mutations leading to increased MICs of tigecycline. As tet genes are widely found in pathogenic bacteria and spread easily by horizontal gene transfer, resistance development by alteration of existing Tet proteins might compromise the future medical use of tigecycline. We predict that Tet(X) might become the most problematic future Tet determinant, since its weak intrinsic tigecycline activity can be mutationally improved to reach clinically relevant levels without collateral loss in activity to other tetracyclines.

The In Vitro Evaluation of Tigecycline and the In Vivo Evaluation of RPX-978 (0.5% Tigecycline) as an Ocular Antibiotic

PURPOSE

The goals of the current study were to determine the in vitro antibacterial activity of tigecycline against multiple clinically relevant ocular pathogens and to evaluate the in vivo ocular tolerability and efficacy of 0.5% tigecycline in a methicillin-resistant Staphylococcus aureus (MRSA) keratitis model.

METHODS

In vitro: Minimum inhibitory concentrations (MICs) were determined for 110 clinical conjunctivitis isolates, 26 keratitis isolates of Pseudomonas aeruginosa, and 10 endophthalmitis isolates each of MRSA, methicillin-susceptible S. aureus (MSSA), MR, and MS coagulase-negative Staphylococcus.

TOLERABILITY

Six uninfected rabbits were topically treated in both eyes with 0.5% tigecycline, vehicle, or saline every 15 min for 3 h.

EFFICACY

Thirty-two rabbits were intrastromally injected with 700 Colony Forming Units (CFU) of MRSA in both eyes and were separated into 4 groups (n = 8): tigecycline 0.5%; vancomycin 5%; saline; and no treatment (euthanized before treatment for baseline CFU). Four hours after MRSA challenge, topical treatment of 1 drop every 15 min for 5 h was initiated. One hour after treatment, the corneas were harvested for CFU. The data were analyzed nonparametrically.

RESULTS

In vitro: Tigecycline demonstrated lower MICs than the other tested antibiotics against gram-positive organisms, especially MRSA, while MICs against gram-negative pathogens, including fluoroquinolone-resistant P. aeruginosa, appeared to be in the treatable range with aggressive topical therapy.

TOLERABILITY

0.5% tigecycline was graded as minimally irritating.

EFFICACY

0.5% tigecycline and vancomycin produced similar reductions in CFU and were less than saline (P < 0.05). Tigecycline and vancomycin demonstrated 99.9% reductions compared with baseline CFU.

CONCLUSIONS

Tigecycline is a potential candidate for a topical ocular antibiotic.

Efficacy and safety of tigecycline for the treatment of severe infectious diseases: an updated meta-analysis of RCTs

OBJECTIVES

To assess the efficacy and safety of tigecycline in comparison with other antimicrobial treatments for infectious diseases.

DESIGN

Databases of PubMed, Embase and the Cochrane Library were searched through Feb. 2015. The reference lists of the initially identified articles and systemic review articles were manually searched. Randomized controlled trials assessing tigecycline and other antibiotics for infectious diseases in adult patients were included.

RESULTS

Fifteen RCTs including 7689 cases were identified. We found that tigecycline was not as effective as the comparator agents for clinical treatment success (for the clinically evaluable population, odds ratio [OR] = 0.83, 95% confidence interval [CI] = (0.73, 0.96), P=0.01; for the clinically modified intent-to-treat (mITT) population, OR = 0.81, 95% CI = (0.72, 0.92), P=0.001). There was no significant difference in microbiological treatment success with lower eradication rate in tigecycline versus comparators (for the microbiologically evaluable population, OR = 0.94, 95% CI = (0.77, 1.16), P=0.56; for the microbiological mITT populations, OR = 0.91, 95% CI = (0.74, 1.11), P=0.35). Adverse events and all-cause mortality were more common in the tigecycline group.

CONCLUSIONS

Tigecycline is not as effective as other antibiotics with relatively more frequency of adverse events and higher mortality rate.

Genomic analysis of reduced susceptibility to tigecycline in Enterococcus faecium

Tigecycline (TIG) is approved for use for the treatment of complicated intra-abdominal infections, skin and skin structure infections, as well as pneumonia. Acquired resistance or reduced susceptibility to TIG has been observed in Gram-negative rods, has seldom been reported in Gram-positive organisms, and has not yet been reported in Enterococcus faecium. Using the serial passage method, in vitro mutant AusTig and in vitro mutants HMtig1 and HMtig2 with decreased TIG susceptibility (MICs, 0.25 μg/ml) were obtained from strains E. faecium Aus0004 and HM1070 (MICs, 0.03 μg/ml), respectively. In addition, two vancomycin-resistant E. faecium clinical isolates (EF16 and EF22) with reduced susceptibility to TIG (MICs, 0.5 and 0.25 μg/ml, respectively) were studied. Compared to the wild-type strains, the in vitro mutants also showed an increase in the MICs of other tetracyclines. An efflux mechanism did not seem to be involved in the reduced TIG susceptibility, since the presence of efflux pump inhibitors (reserpine or pantoprazole) did not affect the MICs of TIG. Whole-genome sequencing of AusTig was carried out, and genomic comparison with the Aus0004 genome was performed. Four modifications leading to an amino acid substitution were found. These mutations affected the rpsJ gene (efau004_00094, coding for the S10 protein of the 30S ribosomal subunit), efau004_01228 (encoding a cation transporter), efau004_01636 (coding for a hypothetical protein), and efau004_02455 (encoding the l-lactate oxidase). The four other strains exhibiting reduced TIG susceptibility were screened for the candidate mutations. This analysis revealed that three of them showed an amino acid substitution in the same region of the RpsJ protein. In this study, we characterized for the first time genetic determinants linked to reduced TIG susceptibility in enterococci.

Antimicrobial resistance: impact on clinical and economic outcomes and the need for new antimicrobials

INTRODUCTION

Antimicrobial resistance is a well-recognized global threat; thus, the development of strong infection control policies coupled with antimicrobial stewardship strategies and new therapies is required to reverse this process. In its 2013 report on antimicrobial resistance, the Centers for Disease Control and Prevention focused on this problem while presenting estimated annual rates of infections with antimicrobial-resistant organisms and their related mortality rates. Whereas some resistant pathogens were considered less threatening, others such as carbapenem-resistant Enterobacteriaceae were associated with higher mortality rates owing to limited treatment options.

AREAS COVERED

An overview of the most common antimicrobial-resistant pathogens, focusing on risk factors for acquisition, clinical and economic outcomes, as well as current treatment options. Strategies to optimize antimicrobial therapy with currently available agents, in addition to newly developed antimicrobials are also discussed.

EXPERT OPINION

The emergence of pathogens with a variety of resistance mechanisms has intensified the challenges associated with infection control and treatment strategies. Therefore, prudent use of currently available antimicrobial agents, as well as implementing measures to limit spread of resistance is paramount. Although several new antimicrobials have been recently approved or are in the pipeline showing promise in the battle against resistance, the appropriate use of these agents is required as the true benefits of these treatments are to be recognized in the clinical care setting.

Vancomycin-Resistant Enterococcal Bacteremia Pharmacotherapy

Objective: To review the literature on the pharmacotherapy of bloodstream infections (BSI) caused by vancomycin-resistant enterococci (VRE). Data Sources: A MEDLINE literature search was performed for the period 1946 to May 2014 using the search terms Enterococcus, enterococci, vancomycin-resistant, VRE, bacteremia, and bloodstream infection. References were also identified from selected review articles. Study Selection and Data Extraction: English-language case series, cohort studies, and meta-analyses assessing the options in the pharmacotherapy of VRE BSIs in adult patients were evaluated. Data Synthesis: Studies were identified that utilized linezolid, quinupristin/dalfopristin (Q/D), and daptomycin. In all, 8 comparative retrospective cohort studies, 2 meta-analyses of daptomycin and linezolid, and 3 retrospective comparisons of linezolid and Q/D were included for review. Mortality associated with VRE BSIs was high across studies, and the ability to determine differences in outcomes between agents was confounded by the complex nature of the patients included. Two meta-analyses comparing daptomycin with linezolid for VRE BSIs found modest advantages for linezolid, but these conclusions may be hampered by heterogeneity within the included studies. Conclusions: VRE BSIs remain a difficult-to-treat clinical situation. Differences in toxicity between the agents used to treat it are clear, but therapeutic differences are more difficult to discern. Meta-analyses suggest that a moderate advantage for linezolid over daptomycin may exist, but problems with the nature of studies that they included make definitive conclusions difficult.

IS5 element integration, a novel mechanism for rapid in vivo emergence of tigecycline nonsusceptibility in Klebsiella pneumoniae

Tigecycline nonsusceptibility is concerning because tigecycline is increasingly relied upon to treat carbapenem- or colistin-resistant organisms. In Enterobacteriaceae, tigecycline nonsusceptibility is mediated by the AcrAB-TolC efflux pump, among others, and pump activity is often a downstream effect of mutations in their transcriptional regulators, cognate repressor genes, or noncoding regions, as demonstrated in Enterobacteriaceae and Acinetobacter isolates. Here, we report the emergence of tigecycline nonsusceptibility in a longitudinal series of multidrug-resistant (MDR) and extensively drug-resistant (XDR) Klebsiella pneumoniae isolates collected during tigecycline therapy and the elucidation of its resistance mechanisms. Clinical isolates were recovered prior to and during tigecycline therapy of a 2.5-month-old Honduran neonate. Antimicrobial susceptibility tests to tigecycline determined that the MIC increased from 1 to 4 μg/ml prior to the completion of tigecycline therapy. Unlike other studies, we did not find increased expression of ramA, ramR, oqxA, acrB, marA, or rarA genes by reverse transcription-quantitative PCR (qRT-PCR). Whole-genome sequencing revealed an IS5 insertion element in nonsusceptible isolates 85 bp upstream of a putative efflux pump operon, here named kpgABC, previously unknown to be involved in resistance. Introduction of the kpgABC genes in a non-kpgABC background increased the MIC of tigecycline 4-fold and is independent of a functional AcrAB-TolC pump. This is the first report to propose a function for kpgABC and identify an insertion element whose presence correlated with the in vivo development of tigecycline nonsusceptibility in K. pneumoniae.

Tigecycline in treatment of multidrug-resistant Gram-negative bacillus urinary tract infections: a systematic review

OBJECTIVES

To review cases of multidrug-resistant (MDR) Gram-negative bacillus urinary tract infections (UTIs) treated with tigecycline and the literature related to this subject.

METHODS

We performed a systematic review of the literature identifying patients with MDR Gram-negative bacillus UTIs treated with tigecycline.

RESULTS

Fourteen cases describing treatment of UTIs caused by MDR Gram-negative bacilli with tigecycline are reviewed. Favourable clinical outcomes were noted in 11 of 14 cases. An initial favourable microbiological outcome was noted in 12 cases. Post-treatment cultures in two cases were positive for tigecycline-resistant organisms.

CONCLUSIONS

The clinical efficacy of tigecycline for treatment of UTIs has not been extensively evaluated. Based on the available literature, tigecycline appears to have efficacy in some patients with MDR Gram-negative bacillus UTIs. Further research in this area is needed to fully elucidate the role of tigecycline in treating such patients.