Galleria mellonella as a model system to assess the efficacy of antimicrobial agents against Klebsiella pneumoniae infection

Transferable AmpCs in Klebsiella pneumoniae: interplay with peptidoglycan recycling, mechanisms of hyperproduction, and virulence implications

Chromosomal and transferable AmpC β-lactamases represent top resistance mechanisms in different gram-negatives, but knowledge regarding the latter, mostly concerning regulation and virulence-related implications, is far from being complete. To fill this gap, we used Klebsiella pneumoniae (KP) and two different plasmid-encoded AmpCs [DHA-1 (AmpR regulator linked, inducible) and CMY-2 (constitutive)] as models to perform a study in which we show that blockade of peptidoglycan recycling through AmpG permease inactivation abolished DHA-1 inducibility but did not affect CMY-2 production and neither did it alter KP pathogenic behavior. Moreover, whereas regular production of both AmpC-type enzymes did not attenuate KP virulence, when blaDHA-1 was expressed in an ampG-defective mutant, Galleria mellonella killing was significantly (but not drastically) attenuated. Spontaneous DHA-1 hyperproducer mutants were readily obtained in vitro, showing slight or insignificant virulence attenuations together with high-level resistance to β-lactams only mildly affected by basal production (e.g., ceftazidime, ceftolozane/tazobactam). By analyzing diverse DHA-1-harboring clinical KP strains, we demonstrate that the natural selection of these hyperproducers is not exceptional (>10% of the collection), whereas mutational inactivation of the typical AmpC hyperproduction-related gene mpl was the most frequent underlying mechanism. The potential silent dissemination of this kind of strains, for which an important fitness cost-related contention barrier does not seem to exist, is envisaged as a neglected threat for most β-lactams effectiveness, including recently introduced combinations. Analyzing whether this phenomenon is applicable to other transferable β-lactamases and species as well as determining the levels of conferred resistance poses an essential topic to be addressed.IMPORTANCEAlthough there is solid knowledge about the regulation of transferable and especially chromosomal AmpC β-lactamases in Enterobacterales, there are still gaps to fill, mainly related to regulatory mechanisms and virulence interplays of the former. This work addresses them using Klebsiella pneumoniae as model, delving into a barely explored conception: the acquisition of a plasmid-encoded inducible AmpC-type enzyme whose production can be increased through selection of chromosomal mutations, entailing dramatically increased resistance compared to basal expression but minor associated virulence costs. Accordingly, we demonstrate that clinical K. pneumoniae DHA-1 hyperproducer strains are not exceptional. Through this study, we warn for the first time that this phenomenon may be a neglected new threat for β-lactams effectiveness (including some recently introduced ones) silently spreading in the clinical context, not only in K. pneumoniae but potentially also in other pathogens. These facts must be carefully considered in order to design future resistance-preventive strategies.

Identification of three capsule depolymerases in a bacteriophage infecting Klebsiella pneumoniae capsular types K7, K20, and K27 and therapeutic application

BACKGROUND

Klebsiella pneumoniae capsular types K1, K2, K5, K20, K54, and K57 are prevalent hypervirulent types associated with community infections, and worrisomely, hypervirulent strains that acquired drug resistance have been found. In the search for alternative therapeutics, studies have been conducted on phages that infect K. pneumoniae K1, K2, K5, and K57-type strains and their phage-encoded depolymerases. However, phages targeting K. pneumoniae K20-type strains and capsule depolymerases capable of digesting K20-type capsules have rarely been reported. In this study, we characterized a phage that can infect K. pneumoniae K20-type strains, phage vB_KpnM-20.

METHODS

A phage was isolated from sewage water in Taipei, Taiwan, its genome was analyzed, and its predicted capsule depolymerases were expressed and purified. The host specificity and capsule-digesting activity of the capsule depolymerases were determined. The therapeutic effect of the depolymerase targeting K. pneumoniae K20-type strains was analyzed in a mouse infection model.

RESULTS

The isolated Klebsiella phage, vB_KpnM-20, infects K. pneumoniae K7, K20, and K27-type strains. Three capsule depolymerases, K7dep, K20dep, and K27dep, encoded by the phage were specific to K7, K20, and K27-type capsules, respectively. K20dep also recognized Escherichia coli K30-type capsule, which is highly similar to K. pneumoniae K20-type. The survival of K. pneumoniae K20-type-infected mice was increased following administration of K20dep.

CONCLUSIONS

The potential of capsule depolymerase K20dep for the treatment of K. pneumoniae infections was revealed using an in vivo infection model. In addition, K7dep, K20dep, and K27dep capsule depolymerases could be used for K. pneumoniae capsular typing.

Genomic characterization of a new phage BUCT541 against Klebsiella pneumoniae K1-ST23 and efficacy assessment in mouse and Galleria mellonella larvae

Over the past decades, the spread of multi-drug-resistant Klebsiella pneumoniae (MDR-KP) is becoming a new threat and new effective therapies against this pathogen are needed. Bacteriophage (phage) therapy is considered to be a promising alternative treatment for MDR-KP infections compared with antibacterial drug usage. Here, we reported a new phage BUCT541 which can lyse MDR-KP ST23. The genome of BUCT541 is a double-stranded linear 46,100-bp long DNA molecule with 48% GC content through the Next generation sequencing (NGS) data. A total of 81 open reading frames and no virulence or antimicrobial resistance genes are annotated in the BUCT541 genome. BUCT541 was able to lyse 7 of the 30 tested MDR-KP according to the host range analysis. And the seven sensitive strains belonged to the K. pneumoniae K1-ST23. BUCT541 exhibited high thermal stability (4–70°C) and broad pH tolerance (pH 3-11) in the stability test. The in vivo results showed that BUCT541 (4 × 10⁵ plaque-forming units (PFU)/each) significantly increased the survival rate of K. pneumoniae infected Galleria mellonella from 5.3% to 83.3% within 48 h. Moreover, in the mouse lung infection model, high doses of BUCT541 (2 × 10⁷ PFU/each) cured 100% of BALB/c mice that were infected with K. pneumoniae. After 30 h of treatment with phage BUCT541 of the multiplicity of infection (MOI) = 10, the K. pneumoniae in the lungs of mice was lower than 10⁴ CFU/mL, compared to the control group 10⁹ CFU/mL. Together, these findings indicate that phage BUCT541 holds great promise as an alternative therapy with excellent stability and a wide lysis range for the treatment of MDR-KP ST23 infection.

UtilisingGalleria mellonella larvae for studying in vivo activity of conventional and novel antimicrobial agents

The immune response of insects displays many structural and functional similarities to the innate immune response of mammals. As a result of these conserved features, insects may be used for evaluating microbial virulence or for testing the in vivo efficacy and toxicity of antimicrobial compounds and results show strong similarities to those from mammals. Galleria mellonella larvae are widely used in this capacity and have the advantage of being easy to use, inexpensive to purchase and house, and being free from the ethical and legal restrictions that relate to the use of mammals in these tests. Galleria mellonella larvae may be used to assess the in vivo toxicity and efficacy of novel antimicrobial compounds. A wide range of antibacterial and antifungal therapies have been evaluated in G. mellonella larvae and results have informed subsequent experiments in mammals. While insect larvae are a convenient and reproducible model to use, care must be taken in their use to ensure accuracy of results. The objective of this review is to provide a comprehensive account of the use of G. mellonella larvae for assessing the in vivo toxicity and efficacy of a wide range of antibacterial and antifungal agents.

Genomic Features and Virulence Characteristics of a Community-Acquired Bloodstream Infection-Causing Hypervirulent Klebsiella pneumoniae ST86 Strain Harboring KPC-2-Encoding IncX6 Plasmid

The emergence and spread of carbapenem-resistant hypervirulent Klebsiella pneumoniae (CR-hvKP) is causing worldwide concern. Sequence type (ST) 86 K. pneumoniae, a major hvKP clone, is rarely resistant to carbapenem. In this study, we report the genomic features and virulence characteristics of a community-acquired bloodstream infection (CA-BSI)-causing CR-hvKP ST86 strain (KPN55602). This strain is resistant to carbapenem but sensitive to amikacin, gentamicin, tigecycline, and colistin. According to in vitro and in vivo virulence assessments, it was classified as hypervirulent. Whole-genome sequencing revealed that KPN55602 has a single 5.13 Mb chromosome and two plasmids. The chromosome of KPN55602 is phylogenetically similar to those of other sequenced ST86 strains. The incompatibility (Inc) group HI1B plasmid pK55602_1, harboring a set of virulence genes, was classified as a virulence plasmid. The IncX6 plasmid pK55602_2, carrying bla_KPC-2, was transferable through conjugation and is highly homologous to all five sequenced bla_KPC-bearing IncX6 plasmids. In conclusion, to our knowledge, this is the first report of a CA-BSI-causing CR-hvKP ST86 strain harboring an exogenous bla_KPC-2-bearing IncX6 plasmid, supplementing existing knowledge on the CR-hvKP evolutionary scenario. The IncX6 plasmid may be an important vehicle for bla_KPC, and its horizontal transfer may have led to CR-hvKP evolution in the community setting.

Characterization of extended-spectrum-β-lactamase producing Klebsiella pneumoniae phage KP1801 and evaluation of therapeutic efficacy in vitro and in vivo

Extended spectrum β lactamase-producing Klebsiella pneumoniae (ESBL-KP) is being reported with high morbidity and mortality rates and is considered as the highest priority for new antimicrobial strategies. To develop an alternative antimicrobial agent, phage KP1801 with broad lytic activity was isolated. The genome of phage KP1801 was double stranded DNA of 49,835 base pairs, with a GC content of 50.26%. There were 75 putative open reading frames. Phage KP1801 was classified as being in the order Caudovirales, belonging to the Siphoviridae family. About 323 proteins were detected by shotgun proteome analysis. The phage inhibited biofilm formation and reduced pre-formed biofilm in a dose dependent manner. Scanning electron microscopic studies demonstrated a membrane damage of bacterial cells treated with phage, resulting in cell death. Prophylactic and therapeutic efficacies of the phage were evaluated in Galleria mellonella. Administration of ESBL-KP infection with phage significantly improved the survival of G. mellonella. The number of intracellular bacteria in larvae showed a significant decrease compared with untreated control while the number of phage increased. These studies suggested that phage KP1801 has the potential for development as an alternative for antibiotics and biocontrol agents against ESBL-KP infection.

In vitro and in vivo activity of ciprofloxacin/fosfomycin combination therapy against ciprofloxacin-resistant Shigella flexneri isolates

Objective: Ciprofloxacin resistance (CIP^R) for Shigella isolates is becoming more prevalent. This study systematically investigated the antibacterial activity of ciprofloxacin (CIP)/fosfomycin (FOS) combination in vitro and in vivo against CIP^R S. flexneri isolates. Method: Eighty CIP^R S. flexneri isolates were selected for synergy studies by the microtiter plate checkerboard assay. Two S. flexneri isolates (GN120471, CIP^RFOS^R; GN120454, CIP^RFOS^S) were used to investigate the efficacy of the CIP/FOS combination by the time-kill methodology. Clinically relevant concentrations (CIP, 0.5, 1, or 2.5 μg/mL; FOS, 30, 150, or 300 μg/mL) were combined, and the colony counts were conducted at 3, 5, 8, and 24 hours. The in vivo activity of the CIP/FOS combination was assessed using a Galleria mellonella larvae model. Results: In checkerboard assays, 31 strains (38.75%) showed synergy for the CIP/FOS combination. For the isolate GN120471, monotherapy with CIP or FOS at all concentrations produced little or no bacterial killing, while the CIP/FOS combination produced enhanced bacterial killing with FOS concentrations of 150 and 300 μg/mL, especially when combined with CIP at 2.5 μg/mL. For the isolate GN120454, the CIP/FOS combination at all concentrations produced more rapid and extensive killing (up to 5log₁₀ colony forming units (CFU)/mL with many combinations) than with either antibiotic alone. Mortality at 96 hours was around 80% at approximately 10⁴ CFU/larva for GN120471 and GN120454. When CIP at 2.5 μg/mL was combined with FOS at 150 μg/mL for the bactericidal activity in vivo, the survival rates for CIP/FOS combination against GN120471-infected and GN120454-infected larvae were significantly higher than that of CIP (68.75% vs 25%, P=0.013; 81.25% vs 37.5%, P=0.012, respectively). Conclusion: Against CIP^R S. flexneri isolates, the CIP/FOS combination induced synergy, and increased bacterial killing in vitro and in a simple invertebrate model of infection.

New Bacteriophages against Emerging Lineages ST23 and ST258 of Klebsiella pneumoniae and Efficacy Assessment in Galleria mellonella Larvae

Klebsiella pneumoniae is a bacterial pathogen of high public health importance. Its polysaccharide capsule is highly variable but only a few capsular types are associated with emerging pathogenic sublineages. The aim of this work is to isolate and characterize new lytic bacteriophages and assess their potential to control infections by the ST23 and ST258 K. pneumoniae sublineages using a Galleria mellonella larvae model. Three selected bacteriophages, targeting lineages ST258 (bacteriophages vB_KpnP_KL106-ULIP47 and vB_KpnP_KL106-ULIP54) and ST23 (bacteriophage vB_KpnP_K1-ULIP33), display specificity for capsular types KL106 and K1, respectively. These podoviruses belong to the Autographivirinae subfamily and their genomes are devoid of lysogeny or toxin-associated genes. In a G. mellonella larvae model, a mortality rate of 70% was observed upon infection by K. pneumoniae ST258 and ST23. This number was reduced to 20% upon treatment with bacteriophages at a multiplicity of infection of 10. This work increases the number of characterized bacteriophages infecting K. pneumoniae and provides information regarding genome sequence and efficacy during preclinical phage therapy against two prominent sublineages of this bacterial species.

Evaluating Different Virulence Traits of Klebsiella pneumoniae Using Dictyostelium discoideum and Zebrafish Larvae as Host Models

Multiresistant and invasive hypervirulent Klebsiella pneumoniae strains have become one of the most urgent bacterial pathogen threats. Recent analyses revealed a high genomic plasticity of this species, harboring a variety of mobile genetic elements associated with virulent strains, encoding proteins of unknown function whose possible role in pathogenesis have not been addressed. K. pneumoniae virulence has been studied mainly in animal models such as mice and pigs, however, practical, financial, ethical and methodological issues limit the use of mammal hosts. Consequently, the development of simple and cost-effective experimental approaches with alternative host models is needed. In this work we described the use of both, the social amoeba and professional phagocyte Dictyostelium discoideum and the fish Danio rerio (zebrafish) as surrogate host models to study K. pneumoniae virulence. We compared three K. pneumoniae clinical isolates evaluating their resistance to phagocytosis, intracellular survival, lethality, intestinal colonization, and innate immune cells recruitment. Optical transparency of both host models permitted studying the infective process in vivo, following the Klebsiella-host interactions through live-cell imaging. We demonstrated that K. pneumoniae RYC492, but not the multiresistant strains 700603 and BAA-1705, is virulent to both host models and elicits a strong immune response. Moreover, this strain showed a high resistance to phagocytosis by D. discoideum, an increased ability to form biofilms and a more prominent and irregular capsule. Besides, the strain 700603 showed the unique ability to replicate inside amoeba cells. Genomic comparison of the K. pneumoniae strains showed that the RYC492 strain has a higher overall content of virulence factors although no specific genes could be linked to its phagocytosis resistance, nor to the intracellular survival observed for the 700603 strain. Our results indicate that both zebrafish and D. discoideum are advantageous host models to study different traits of K. pneumoniae that are associated with virulence.

An Update on Candida tropicalis Based on Basic and Clinical Approaches

Candida tropicalis has emerged as one of the most important Candida species. It has been widely considered the second most virulent Candida species, only preceded by C. albicans. Besides, this species has been recognized as a very strong biofilm producer, surpassing C. albicans in most of the studies. In addition, it produces a wide range of other virulence factors, including: adhesion to buccal epithelial and endothelial cells; the secretion of lytic enzymes, such as proteinases, phospholipases, and hemolysins, bud-to-hyphae transition (also called morphogenesis) and the phenomenon called phenotypic switching. This is a species very closely related to C. albicans and has been easily identified with both phenotypic and molecular methods. In addition, no cryptic sibling species were yet described in the literature, what is contradictory to some other medically important Candida species. C. tropicalis is a clinically relevant species and may be the second or third etiological agent of candidemia, specifically in Latin American countries and Asia. Antifungal resistance to the azoles, polyenes, and echinocandins has already been described. Apart from all these characteristics, C. tropicalis has been considered an osmotolerant microorganism and this ability to survive to high salt concentration may be important for fungal persistence in saline environments. This physiological characteristic makes this species suitable for use in biotechnology processes. Here we describe an update of C. tropicalis, focusing on all these previously mentioned subjects.