The wax moth Galleria mellonella as a novel model system to study Enteroaggregative Escherichia coli pathogenesis

Lipopolysaccharide with long O-antigen is crucial for Salmonella Enteritidis to evade complement activity and to facilitate bacterial survival in vivo in the Galleria mellonella infection model

Bacterial resistance to serum is a key virulence factor for the development of systemic infections. The amount of lipopolysaccharide (LPS) and the O-antigen chain length distribution on the outer membrane, predispose Salmonella to escape complement-mediated killing. In Salmonella enterica serovar Enteritidis (S. Enteritidis) a modal distribution of the LPS O-antigen length can be observed. It is characterized by the presence of distinct fractions: low molecular weight LPS, long LPS and very long LPS. In the present work, we investigated the effect of the O-antigen modal length composition of LPS molecules on the surface of S. Enteritidis cells on its ability to evade host complement responses. Therefore, we examined systematically, by using specific deletion mutants, roles of different O-antigen fractions in complement evasion. We developed a method to analyze the average LPS lengths and investigated the interaction of the bacteria and isolated LPS molecules with complement components. Additionally, we assessed the aspect of LPS O-antigen chain length distribution in S. Enteritidis virulence in vivo in the Galleria mellonella infection model. The obtained results of the measurements of the average LPS length confirmed that the method is suitable for measuring the average LPS length in bacterial cells as well as isolated LPS molecules and allows the comparison between strains. In contrast to earlier studies we have used much more precise methodology to assess the LPS molecules average length and modal distribution, also conducted more subtle analysis of complement system activation by lipopolysaccharides of various molecular mass. Data obtained in the complement activation assays clearly demonstrated that S. Enteritidis bacteria require LPS with long O-antigen to resist the complement system and to survive in the G. mellonella infection model.

Multidrug resistance among uropathogenic clonal group A E. Coli isolates from Pakistani women with uncomplicated urinary tract infections

OBJECTIVE

Multi-drug resistance (MDR) has notably increased in community acquired uropathogens causing urinary tract infections (UTIs), predominantly Escherichia coli. Uropathogenic E. coli causes 80% of uncomplicated community acquired UTIs, particularly in pre-menopausal women. Considering this high prevalence and the potential to spread antimicrobial resistant genes, the current study was conducted to investigate the presence of clinically important strains of E. coli in Pakistani women having uncomplicated cystitis and pyelonephritis. Women belonging to low-income groups were exclusively included in the study. Seventy-four isolates from urine samples were processed, phylotyped, and screened for the presence of two Single Nucleotide Polymorphisms (SNPs) particularly associated with a clinically important clonal group A of E. coli (CgA) followed by antibiotic susceptibility testing and genome sequence analysis.

RESULTS

Phylogroup B2 was most prevalent in patients and 44% of isolates were positive for the presence of CgA specific SNPs in Fumarate hydratase and DNA gyrase subunit B genes. Antibiotic susceptibility testing showed widespread resistance to trimethoprim-sulfamethoxazole and extended-spectrum beta-lactamase production. The infection analysis revealed the phylogroup B2 to be more pathogenic as compared to the other groups. The genome sequence of E. coli strain U17 revealed genes encoding virulence, multidrug resistance, and host colonization mechanisms.

CONCLUSIONS

Our research findings not only validate the significant occurrence of multidrug-resistant clonal group A E. coli (CgA) in premenopausal Pakistani women suffering from cystitis and pyelonephritis but also reveal the presence of genes associated withvirulence, and drug efflux pumps. The detection of highly pathogenic, antimicrobial-resistant phylogroup B2 and CgA E. coli strains is likely to help in understanding the epidemiology of the pathogen and may ultimately help to reduce the impact of these strains on human health. Furthermore, the findings of this study will particularly help to reduce the prevalence of uncomplicated UTIs and the cost associated with their treatment in women belonging to low-income groups.

Global emergence of a hypervirulent carbapenem-resistant Escherichia coli ST410 clone

Carbapenem-resistant Escherichia coli (CREC) ST410 has recently emerged as a major global health problem. Here, we report a shift in CREC prevalence in Chinese hospitals between 2017 and 2021 with ST410 becoming the most commonly isolated sequence type. Genomic analysis identifies a hypervirulent CREC ST410 clone, B5/H24RxC, which caused two separate outbreaks in a children's hospital. It may have emerged from the previously characterised B4/H24RxC in 2006 and has been isolated in ten other countries from 2015 to 2021. Compared with B4/H24RxC, B5/H24RxC lacks the blaOXA-181-bearing X3 plasmid, but carries a F-type plasmid containing blaNDM-5. Most of B5/H24RxC also carry a high pathogenicity island and a novel O-antigen gene cluster. We find that B5/H24RxC grew faster in vitro and is more virulent in vivo. The identification of this newly emerged but already globally disseminated hypervirulent CREC clone, highlights the ongoing evolution of ST410 towards increased resistance and virulence.

Replication of Dengue Virus 4 in Plodia interpunctella (Lepidoptera: Pyralidae) Larvae Under Laboratory Conditions

Background: Developing methods for the isolation and replication of dengue virus (DENV), based on nonhematophagous insect models to assess virus-host interaction, would contribute, for instance, to the creation of drugs or vaccines and eventually to the control of the disease. In this regard, nonhematophagous mosquitoes have been used as biological hosts for the isolation of DENV because they are specific and sensitive to a low viral load and viral particles with low infectivity. However, implementation of these models is mainly affected by the complexity of the establishment of the entomological colonies. Materials and Methods: In this study, the susceptibility of DENV-4 infection in Plodia interpunctella larvae was evaluated. Ten larvae, previously inoculated with supernatant from DENV-4-infected C6/36 cells, were processed to determine viral replication by the optical density and 2-ΔΔCt methods at different time intervals (1 and 7 days postinoculation). Results: A prospective increase in viral replication was observed, which did not influence the survival and development of P. interpunctella. Conclusion: These results demonstrate the infectivity of DENV-4 in P. interpunctella, thus becoming an option as a biological model for the study of this etiological agent.

Biofilm Inhibition of Inula viscosa (L.) Aiton and Globularia alypum L. Extracts Against Candida Infectious Pathogens and In Vivo Action on Galleria mellonella Model

The increasing importance of fungal infections has fueled the search for new beneficial alternatives substance from plant extracts. The current study investigates the antifungal and antibiofilm activity of Inula viscosa (L.) Aiton and Globularia alypum (L.) leaves extracts against Candida both in vitro and in vivo. The inhibition of planktonic and sessile Candida albicans and Candida glabrata growth using both leaf extracts are evaluated. Moreover; an in vivo infection model using Galleria mellonella larvae; infected and treated with the extracts are performed. All extracts show fungicidal activity; with a minimum fungicidal concentration (MFC) ranging from 128 to 512 µg mL-1 against the two selected strains of Candida. In particular, the best results are obtained with methanolic extract of I. viscosa and G. alypum with an MFC value of 128 µg mL-1 . The extracts are capable to prevent 90% of biofilm development at minor concentrations ranging from 100.71 ± 2.49 µg mL-1 to 380.4 ± 0.92 µg mL-1 . In vivo, tests on Galleria mellonella larvae show that the extracts increase the survival of the larvae infected with Candida. The attained results reveal that I. viscosa and G. alypum extracts may be considered as new antifungal agents and biofilm inhibiting agents for the pharmaceutical and agro-food field.

Characterization of an Escherichia coli Isolate Coharboring the Virulence Gene astA and Tigecycline Resistance Gene tet(X4) from a Dead Piglet

tet(X4) is the critical resistance gene for tigecycline degradation that has been continually reported in recent years. In particular, pathogenic bacteria carrying tet(X4) are a severe threat to human health. However, information describing Escherichia coli coharboring tet(X4) with virulence genes is limited. Here, we isolated an E. coli strain coharboring tet(X4) and the heat-stable toxin gene astA from a dead piglet. The strain named 812A1-131 belongs to ST10. The genome was sequenced using the Nanopore and Illumina platforms. The virulence genes astA and tet(X4) are located on the chromosome and in the IncHI1-type plasmid p812A1-tetX4-193K, respectively. The plasmid could be conjugatively transferred to recipient E. coli J53 with high frequency. In vivo experiments showed that strain 812A1-131 is pathogenic to Galleria mellonella and could colonize the intestines of mice. In summary, pathogenic E. coli could receive a plasmid harboring the tet(X4) gene, which can increase the difficulty of treatment. The prevalence and transmission mechanisms of pathogenic bacteria coharboring the tet(X4) gene need more attention.

Transposon mutagenesis of atypical enteroaggregative Escherichia coli reveals a hemagglutinin-associated protein that mediates cell adhesion and contributes to the Galleria mellonella virulence

Twenty-two atypical enteroaggregative Escherichia coli isolates from a previous epidemiological study harboring EAEC virulence genes were examined for their adhesion properties. Nine strains showed a typical aggregative adherence (AA) pattern, while 13 strains showed variant AA, such as AA with lined up cells characteristic of the chain-like adhesion (CLA) and AA mainly to HeLa cells characteristic of the diffuse adherence (DA). The aggregative forming pilus (AFP) genes afpA2 and afpR were detected only in strain Q015B, which exhibited an AA/DA pattern. Using Tn5-based transposon mutagenesis on Q015B strain, we identified a 5517-bp open reading frame (ORF) encoding a predicted 1838-amino-acid polypeptide that is genetically related to a putative filamentous hemagglutinin identified in E. coli strain 7-233-03_S3_C2. Therefore, the ORF was named orfHA. The regions flanking orfHA were sequenced and two ORFs were found; upstream, an ORF that encodes a 603-amino-acid polypeptide with 99% identity to hemolysin secretion/activation proteins of the ShlB/FhaC/HecB family, and downstream, another ORF, which encodes a 632-amino-acid polypeptide with 72% identity to the glycosyltransferase EtpC. An orfHA mutant (Q015BΔorfHA) was constructed from strain Q015B. Q015BΔorfHA strain did not adhere to HeLa cells, whereas Q015BΔ orfHA transformed with a pACYC184 plasmid carrying orfHA restored the AA/DA phenotype of strain Q015B. Furthermore, the Q015ΔorfHA mutant had a marked effect on the ability of strain Q015B to kill the larvae of Galleria mellonella. Our results suggest that the AA/DA pattern of strain Q015B is mediated by a hemagglutinin-associated protein which also contributes to its virulence in the G. mellonella model.

Lepidopteran insects: emerging model organisms to study infection by enteropathogens

The in vivo analysis of a pathogen is a critical step in gaining greater knowledge of pathogen biology and host-pathogen interactions. In the last two decades, there has been a notable rise in the number of studies on developing insects as a model for studying pathogens, which provides various benefits, such as ethical acceptability, relatively short life cycle, and cost-effective care and maintenance relative to routinely used rodent infection models. Furthermore, lepidopteran insects provide many advantages, such as easy handling and tissue extraction due to their large size relative to other invertebrate models, like Caenorhabditis elegans. Additionally, insects have an innate immune system that is highly analogous to vertebrates. In the present review, we discuss the components of the insect's larval immune system, which strengthens its usage as an alternative host, and present an updated overview of the research findings involving lepidopteran insects (Galleria mellonella, Manduca sexta, Bombyx mori, and Helicoverpa armigera) as infection models to study the virulence by enteropathogens due to the homology between insect and vertebrate gut.

Antimicrobial Efficacy of Green Synthesized Nanosilver with Entrapped Cinnamaldehyde against Multi-Drug-Resistant Enteroaggregative Escherichia coli in Galleria mellonella

The global emergence of antimicrobial resistance (AMR) needs no emphasis. In this study, the in vitro stability, safety, and antimicrobial efficacy of nanosilver-entrapped cinnamaldehyde (AgC) against multi-drug-resistant (MDR) strains of enteroaggregative Escherichia coli (EAEC) were investigated. Further, the in vivo antibacterial efficacy of AgC against MDR-EAEC was also assessed in Galleria mellonella larval model. In brief, UV-Vis and Fourier transform infrared (FTIR) spectroscopy confirmed effective entrapment of cinnamaldehyde with nanosilver, and the loading efficiency was estimated to be 29.50 ± 0.56%. The AgC was of crystalline form as determined by the X-ray diffractogram with a mono-dispersed spherical morphology of 9.243 ± 1.83 nm in electron microscopy. AgC exhibited a minimum inhibitory concentration (MIC) of 0.008−0.016 mg/mL and a minimum bactericidal concentration (MBC) of 0.008−0.032 mg/mL against MDR- EAEC strains. Furthermore, AgC was stable (high-end temperatures, proteases, cationic salts, pH, and host sera) and tested safe for sheep erythrocytes as well as secondary cell lines (RAW 264.7 and HEp-2) with no negative effects on the commensal gut lactobacilli. in vitro, time-kill assays revealed that MBC levels of AgC could eliminate MDR-EAEC infection in 120 min. In G. mellonella larvae, AgC (MBC values) increased survival, decreased MDR-EAEC counts (p < 0.001), had an enhanced immunomodulatory effect, and was tested safe to the host. These findings infer that entrapment enhanced the efficacy of cinnamaldehyde and AgNPs, overcoming their limitations when used individually, indicating AgC as a promising alternative antimicrobial candidate. However, further investigation in appropriate animal models is required to declare its application against MDR pathogens.

The Role of graRS in Regulating Virulence and Antimicrobial Resistance in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a common cause of both community- and hospital-associated infections. The antibiotic resistance and virulence characteristics of MRSA are largely regulated by two-component signal transduction systems (TCS) including the graRS TCS. To make a relatively comprehensive insight into graRS TCS in MRSA, the bioinformatics analysis of dataset GSE26016 (a S. aureus HG001 WT strain vs. the ΔgraRS mutant) from Gene Expression Omnibus (GEO) database was performed, and a total of 563 differentially expressed genes (DEGs) were identified. GO analysis revealed that the DEGs were mainly enriched in the “de novo” IMP biosynthetic process, lysine biosynthetic process via diaminopimelate, and pathogenesis; and they were mainly enriched in purine metabolism, lysine biosynthesis, and monobactam biosynthesis in KEGG analysis. WGCNA suggested that the turquoise module was related to the blue module, and the genes in these two modules were associated with S. aureus virulence and infection. To investigate the role of graRS in bacterial virulence, a graRS knockout mutant (ΔgraRS) was constructed using MRSA USA500 2,395 strain as a parent strain. Compared to the wild-type strain, the USA500ΔgraRS showed reduced staphyloxanthin production, retarded coagulation, weaker hemolysis on blood agar plates, and a decreased biofilm formation. These altered phenotypes were restored by the complementation of a plasmid-expressed graRS. Meanwhile, an expression of the virulence-associated genes (coa, hla, hlb, agrA, and mgrA) was downregulated in the ΔgraRS mutant. Consistently, the A549 epithelial cells invasion of the ΔgraRS mutant was 4-fold lower than that of the USA500 wild-type strain. Moreover, on the Galleria mellonella infection model, the survival rate at day 5 post infection in the USA500ΔgraRS group (55%) was obviously higher than that in the USA500 group (20%), indicating graRS knockout leads to a decreased virulence in vivo. In addition, the deletion of the graRS in the MRSA USA500 strain resulted in its increased susceptibilities to ampicillin, oxacillin, vancomycin, and gentamicin. Our work suggests that the graRS TCS plays an important role in regulating S. aureus virulence in vitro and in vivo and modulate bacterial resistance to various antibiotics.

Galleria mellonella as an infection model: an in-depth look at why it works and practical considerations for successful application

The larva of the greater wax moth Galleria mellonella is an increasingly popular model for assessing the virulence of bacterial pathogens and the effectiveness of antimicrobial agents. In this review, we discuss details of the components of the G. mellonella larval immune system that underpin its use as an alternative infection model, and provide an updated overview of the state of the art of research with G. mellonella infection models to study bacterial virulence, and in the evaluation of antimicrobial efficacy. Emphasis is given to virulence studies with relevant human and veterinary pathogens, especially Escherichia coli and bacteria of the ESKAPE group. In addition, we make practical recommendations for larval rearing and testing, and overcoming potential limitations of the use of the model, which facilitate intra- and interlaboratory reproducibility.

Exploring Galleria mellonella larval model to evaluate antibacterial efficacy of Cecropin A (1-7)-Melittin against multi-drug resistant enteroaggregative Escherichia coli

High throughput in vivo laboratory models is need for screening and identification of effective therapeutic agents to overcome microbial drug-resistance. This study was undertaken to evaluate in vivo antimicrobial efficacy of short-chain antimicrobial peptide- Cecropin A (1-7)-Melittin (CAMA) against three multi-drug resistant enteroaggregative Escherichia coli (MDR-EAEC) field isolates in a Galleria mellonella larval model. The minimum inhibitory concentration (MIC; 2.0 mg/L) and minimum bactericidal concentration (MBC; 4.0 mg/L) of CAMA were determined by microdilution assay. CAMA was found to be stable at high temperatures, physiological concentration of cationic salts and proteases; safe with sheep erythrocytes, secondary cell lines and commensal lactobacilli at lower MICs; and exhibited membrane permeabilization. In vitro time-kill assay revealed concentration- and time-dependent clearance of MDR-EAEC in CAMA-treated groups at 30 min. CAMA- treated G. mellonella larvae exhibited an increased survival rate, reduced MDR-EAEC counts, immunomodulatory effect and proved non-toxic which concurred with histopathological findings. CAMA exhibited either an equal or better efficacy than the tested antibiotic control, meropenem. This study highlights the possibility of G. mellonella larvae as an excellent in vivo model for investigating the host-pathogen interaction, including the efficacy of antimicrobials against MDR-EAEC strains.

Galleria mellonella larvae exhibit a weight-dependent lethal median dose when infected with methicillin-resistant Staphylococcus aureus

Galleria mellonella is a recognised model to study antimicrobial efficacy; however, standardisation across the scientific field and investigations of methodological components are needed. Here, we investigate the impact of weight on mortality following infection with Methicillin-resistant Staphylococcus aureus (MRSA). Larvae were separated into six weight groups (180-300 mg at 20 mg intervals) and infected with a range of doses of MRSA to determine the 50% lethal dose (LD50), and the 'lipid weight' of larvae post-infection was quantified. A model of LD50 values correlated with weight was developed. The LD50 values, as estimated by our model, were further tested in vivo to prove our model. We establish a weight-dependent LD50 in larvae against MRSA and demonstrate that G. mellonella is a stable model within 180-260 mg. We present multiple linear models correlating weight with: LD50, lipid weight, and larval length. We demonstrate that the lipid weight is reduced as a result of MRSA infection, identifying a potentially new measure in which to understand the immune response. Finally, we demonstrate that larval length can be a reasonable proxy for weight. Refining the methodologies in which to handle and design experiments involving G. mellonella, we can improve the reliability of this powerful model.

Ornithine Lipids in Burkholderia spp. Pathogenicity

The genus Burkholderia sensu lato is composed of a diverse and metabolically versatile group of bacterial species. One characteristic thought to be unique for the genus Burkholderia is the presence of two forms each (with and without 2-hydroxylation) of the membrane lipids phosphatidylethanolamine (PE) and ornithine lipids (OLs). Here, we show that only Burkholderia sensu stricto strains constitutively form OLs, whereas all other analyzed strains belonging to the Burkholderia sensu lato group constitutively form the two forms of PE, but no OLs. We selected two model bacteria to study the function of OL in Burkholderia sensu lato: (1) Burkholderia cenocepacia wild-type which constitutively forms OLs and its mutant deficient in the formation of OLs and (2) Robbsia andropogonis (formerly Burkholderia andropogonis) which does not form OL constitutively, and a derived strain constitutively forming OLs. Both were characterized under free-living conditions and during pathogenic interactions with their respective hosts. The absence of OLs in B. cenocepacia slightly affected bacterial growth under specific abiotic stress conditions such as high temperature and low pH. B. cenocepacia lacking OLs caused lower mortality in Galleria mellonella larvae while R. andropogonis constitutively forming OLs triggers an increased formation of reactive oxygen species immediately after infection of maize leaves, suggesting that OLs can have an important role during the activation of the innate immune response of eukaryotes.

Exploiting Lactoferricin (17-30) as a Potential Antimicrobial and Antibiofilm Candidate Against Multi-Drug-Resistant Enteroaggregative Escherichia coli

The study evaluated the in vitro antimicrobial and antibiofilm efficacy of an antimicrobial peptide (AMP), lactoferricin (17–30) [Lfcin (17–30)], against biofilm-forming multi-drug-resistant (MDR) strains of enteroaggregative Escherichia coli (EAEC), and subsequently, the in vivo antimicrobial efficacy was assessed in a Galleria mellonella larval model. Initially, minimum inhibitory concentration (MIC; 32 μM), minimum bactericidal concentration (MBC; 32 μM), and minimum biofilm eradication concentration (MBEC; 32 μM) of Lfcin (17–30) were determined against MDR-EAEC field isolates (n = 3). Lfcin (17–30) was tested stable against high-end temperatures (70 and 90°C), physiological concentration of cationic salts (150 mM NaCl and 2 mM MgCl₂), and proteases (proteinase-K and lysozyme). Further, at lower MIC, Lfcin (17–30) proved to be safe for sheep RBCs, secondary cell lines (HEp-2 and RAW 264.7), and beneficial gut lactobacilli. In the in vitro time-kill assay, Lfcin (17–30) inhibited the MDR-EAEC strains 3 h post-incubation, and the antibacterial effect was due to membrane permeation of Lfcin (17–30) in the inner and outer membranes of MDR-EAEC. Furthermore, in the in vivo experiments, G. mellonella larvae treated with Lfcin (17–30) exhibited an increased survival rate, lower MDR-EAEC counts (P < 0.001), mild to moderate histopathological changes, and enhanced immunomodulatory effect and were safe to larval cells when compared with infection control. Besides, Lfcin (17–30) proved to be an effective antibiofilm agent, as it inhibited and eradicated the preformed biofilm formed by MDR-EAEC strains in a significant (P < 0.05) manner both by microtiter plate assay and live/dead bacterial quantification-based confocal microscopy. We recommend further investigation of Lfcin (17–30) in an appropriate animal model before its application in target host against MDR-EAEC strains.

Virulence Potential of a Multidrug-Resistant Escherichia coli Strain Belonging to the Emerging Clonal Group ST101-B1 Isolated from Bloodstream Infection

Escherichia coli EC121 is a multidrug-resistant (MDR) strain isolated from a bloodstream infection of an inpatient with persistent gastroenteritis and T-zone lymphoma that died due to septic shock. Despite causing an extraintestinal infection, previous studies showed that it did not have the usual characteristics of an extraintestinal pathogenic E. coli. Instead, it belonged to phylogenetic group B1 and harbored few known virulence genes. To evaluate the pathogenic potential of strain EC121, an extensive genome sequencing and in vitro characterization of various pathogenicity-associated properties were performed. The genomic analysis showed that strain EC121 harbors more than 50 complete virulence genetic clusters. It also displays the capacity to adhere to a variety of epithelial cell lineages and invade T24 bladder cells, as well as the ability to form biofilms on abiotic surfaces, and survive the bactericidal serum complement activity. Additionally, EC121 was shown to be virulent in the Galleria mellonella model. Furthermore, EC121 is an MDR strain harboring 14 antimicrobial resistance genes, including bla_CTX-M-2. Completing the scenario, it belongs to serotype O154:H25 and to sequence type 101-B1, which has been epidemiologically linked to extraintestinal infections as well as to antimicrobial resistance spread. This study with E. coli strain EC121 shows that clinical isolates considered opportunistic might be true pathogens that go underestimated.

Both clinical and environmental Caulobacter species are virulent in the Galleria mellonella infection model

The Caulobacter genus, including the widely-studied model organism Caulobacter crescentus, has been thought to be non-pathogenic and thus proposed as a bioengineering vector for various environmental remediation and medical purposes. However, Caulobacter species have been implicated as the causative agents of several hospital-acquired infections, raising the question of whether these clinical isolates represent an emerging pathogenic species or whether Caulobacters on whole possess previously-unappreciated virulence capability. Given the proposed environmental and medical applications for C. crescentus, understanding the potential pathogenicity of this bacterium is crucial. Consequently, we sequenced a clinical Caulobacter isolate to determine if it has acquired novel virulence determinants. We found that the clinical isolate represents a new species, Caulobacter mirare that, unlike C. crescentus, grows well in standard clinical culture conditions. C. mirare phylogenetically resembles both C. crescentus and the related C. segnis, which was also thought to be non-pathogenic. The similarity to other Caulobacters and lack of obvious pathogenesis markers suggested that C. mirare is not unique amongst Caulobacters and that consequently other Caulobacters may also have the potential to be virulent. We tested this hypothesis by characterizing the ability of Caulobacters to infect the model animal host Galleria mellonella. In this context, two different lab strains of C. crescentus proved to be as pathogenic as C. mirare, while lab strains of E. coli were non-pathogenic. Further characterization showed that Caulobacter pathogenesis in the Galleria model is mediated by lipopolysaccharide (LPS), and that differences in LPS chemical composition across species could explain their differential toxicity. Taken together, our findings suggest that many Caulobacter species can be virulent in specific contexts and highlight the importance of broadening our methods for identifying and characterizing potential pathogens.

Bacterial species have historically been classified as either capable of causing disease in an animal (pathogenic) or not. Caulobacter species represent a class of bacteria that were thought to be non-pathogenic. Caulobacters have been widely studied and proposed to be used for various industrial and medical applications due to their presumed safety. However, recent reports of human Caulobacter infections raised the question of whether disease-causing Caulobacters have acquired special factors that help them cause disease or whether the ability to infect is a more general feature of most Caulobacters. By combining genomic sequencing and animal infection studies we show that a clinical Caulobacter strain is similar to lab Caulobacters and that all Caulobacters studied can cause disease in a model host. We explore the mechanism of this infectivity and show that it is due to the production of a toxic factor that is made by all Caulobacter cells. We also provide a possible explanation for why Caulobacters have not traditionally been isolated from human patients, owing to their inability to tolerate the salt levels used in most medical culturing systems.

Growth Control of Adherent-Invasive Escherichia coli (AIEC) by the Predator Bacteria Bdellovibrio bacteriovorus: A New Therapeutic Approach for Crohn's Disease Patients

In Crohn’s disease (CD) patients, intestinal dysbiosis with an overgrowth of Proteobacteria, mainly Escherichia coli, has been reported. A new pathotype of E. coli, the adherent-invasive Escherichia coli strain (AIEC), has been isolated from the mucosae of CD patients. AIEC strains play an important role in CD pathogenesis, increasing intestinal mucosa damage and inflammation. Several studies have been undertaken to find possible strategies/treatments aimed at AIEC strain reduction/elimination from CD patients’ intestinal mucosae. To date, a truly effective strategy against AIEC overgrowth is not yet available, and as such, further investigations are warranted. Bdellovibrio bacteriovorus is a predator bacterium which lives by invading Gram-negative bacteria, and is usually present both in natural and human ecosystems. The aim of this study was to evaluate a novel possible strategy to treat CD patients’ mucosae when colonized by AIEC strains, based on the utilization of the Gram-negative predatory bacteria, B. bacteriovorus. The overall results indicate that B. bacteriovorus is able to interfere with important steps in the dynamics of pathogenicity of AIEC strains by its predatory activity. We indicate, for the first time, the possibility of counteracting AIEC strain overgrowth by exploiting what naturally occurs in microbial ecosystems (i.e., predation).

Schinus terebinthifolia leaf lectin (SteLL) has anti-infective action and modulates the response of Staphylococcus aureus-infected macrophages

Staphylococcus aureus is recognized as an important pathogen causing a wide spectrum of diseases. Here we examined the antimicrobial effects of the lectin isolated from leaves of Schinus terebinthifolia Raddi (SteLL) against S. aureus using in vitro assays and an infection model based on Galleria mellonella larvae. The actions of SteLL on mice macrophages and S. aureus-infected macrophages were also evaluated. SteLL at 16 µg/mL (8 × MIC) increased cell mass and DNA content of S. aureus in relation to untreated bacteria, suggesting that SteLL impairs cell division. Unlike ciprofloxacin, SteLL did not induce the expression of recA, crucial for DNA repair through SOS response. The antimicrobial action of SteLL was partially inhibited by 50 mM N-acetylglucosamine. SteLL reduced staphyloxathin production and increased ciprofloxacin activity towards S. aureus. This lectin also improved the survival of G. mellonella larvae infected with S. aureus. Furthermore, SteLL induced the release of cytokines (IL-6, IL-10, IL-17A, and TNF-α), nitric oxide and superoxide anion by macrophagens. The lectin improved the bactericidal action of macrophages towards S. aureus; while the expression of IL-17A and IFN-γ was downregulated in infected macrophages. These evidences suggest SteLL as important lead molecule in the development of anti-infective agents against S. aureus.

Antimicrobial Efficacy of Indolicidin Against Multi-Drug Resistant Enteroaggregative Escherichia coli in a Galleria mellonella Model

Antimicrobial resistance against enteroaggregative Escherichia coli (EAEC), an emerging food-borne pathogen, has been observed in an increasing trend recently. In the recent wake of antimicrobial resistance, alternate strategies especially, cationic antimicrobial peptides (AMPs) have attracted considerable attention to source antimicrobial technology solutions. This study evaluated the in vitro antimicrobial efficacy of Indolicidin against multi-drug resistant enteroaggregative Escherichia coli (MDR-EAEC) strains and further to assess its in vivo antimicrobial efficacy in Galleria mellonella larval model. The minimum inhibitory concentration (MIC; 32 μM) and minimum bactericidal concentration (MBC; 64 μM) of Indolicidin against MDR-EAEC was determined by micro broth dilution method. Indolicidin was also tested for its stability (high-end temperatures, physiological concentration of salts and proteases); safety (sheep RBCs; HEp-2 and RAW 264.7 cell lines); effect on beneficial microflora (Lactobacillus rhamnosus and Lactobacillus acidophilus) and its mode of action (flow cytometry; nitrocefin and ONPG uptake). In vitro time-kill kinetic assay of MDR-EAEC treated with Indolicidin was performed. Further, survival rate, MDR-EAEC count, melanization rate, hemocyte enumeration, cytotoxicity assay and histopathological examination were carried out in G. mellonella model to assess in vivo antimicrobial efficacy of Indolicidin against MDR-EAEC strains. Indolicidin was tested stable at high temperatures (70°C; 90°C), physiological concentration of cationic salts (NaCl; MgCl₂) and proteases, except for trypsin and tested safe with sheep RBCs and cell lines (RAW 264.7; HEp-2) at MIC (1X and 2X); the beneficial flora was not inhibited. Indolicidin exhibited outer membrane permeabilization in a concentration- and time-dependent manner. In vitro time-kill assay revealed concentration-cum-time dependent clearance of MDR-EAEC in Indolicidin-treated groups at 120 min, while, in G. mellonella, the infected group treated with Indolicidin revealed an increased survival rate, immunomodulatory effect, reduced MDR-EAEC counts and were tested safe to the larval cells which was concurred histopathologically. To conclude, the results suggests Indolicidin as an effective antimicrobial candidate against MDR-EAEC and we recommend its further investigation in appropriate animal models (mice/piglets) before its application in the target host.

Typical and Atypical Enteroaggregative Escherichia coli Are Both Virulent in the Galleria mellonella Model

Enteroaggregative Escherichia coli (EAEC) is an emerging pathotype responsible for acute and persistent diarrhea. It can be classified as typical and atypical strains, respectively, based on the presence or absence of the AggR regulon, suggesting a higher virulence for typical EAEC. This study aims to evaluate in the Galleria mellonella model if there are differences in the virulence profiles among clinical strains of typical and atypical EAEC, prototype strains EAEC C1096, 042 and its aggR mutant. The clinical EAEC strains (n = 20) were analyzed for the presence of 22 putative virulence factors of EAEC or extraintestinal E. coli by PCR, as well as phenotypic characteristics of virulence (enzymes, siderophore, and biofilm). The survival of the larvae was analyzed after inoculation of 10⁴-10⁷ CFU/larva; the monitoring of bacterial growth in vivo and hemocyte quantification was determined after inoculation of the prototype strains (10⁵ CFU/larva) at different periods after infection. The strains of typical and atypical EAEC presented the same virulence profile for the larva, regardless of the amount or type of genes and phenotypic aspects of virulence analyzed. In addition, the EAEC 042 aggR mutant strain showed a significant reduction in the mortality of the inoculated larvae compared to the wild-type strain. In conclusion, the results obtained herein demonstrate that the virulence of EAEC seems to be related to the AggR regulon, but not exclusively, and atypical EAEC strains may be as virulent as typical ones in vivo in the G. mellonella model.

New Perspectives on Galleria mellonella Larvae as a Host Model Using Riemerella anatipestifer as a Proof of Concept

Galleria mellonella larvae have been used as a host model to study interactions between pathogens and hosts for several years. However, whether the model is useful to interrogate Riemerella anatipestifer infection biology remained unknown. This study aimed to exploit the potential of G. mellonella larvae and reveal their limitations as a host model for R. anatipestifer infection. G. mellonella larvae were shown to be effective for virulence evaluations of different R. anatipestifer strains. Furthermore, the virulent strain R. anatipestifer CH-1 had a stronger ability to proliferate than the attenuated strain R. anatipestifer ATCC 11845 in both G. mellonella larvae and ducklings. Unconventionally it was shown that G. mellonella larvae cannot be used to evaluate the efficacy of antimicrobials and their combinations. Additionally, it was shown that certain virulence factors, such as OmpA (B739_0861), B739_1208, B739_1343, and Wza (B739_1124), were specific only for ducklings, suggesting that G. mellonella larvae must be cautiously used to identify virulence factors of R. anatipestifer Evaluation of heme uptake-related virulence genes, such as tonB1 and tonB2, required preincubating the strains with hemoglobin before infection of G. mellonella larvae since R. anatipestifer cannot obtain a heme source from G. mellonella larvae. In conclusion, this study revealed the applicability and limitations of G. mellonella as a model with which to study the pathogen-host interaction, particularly in the context of R. anatipestifer infection.

Enteroaggregative Escherichia coli subclinical and clinical infections

PURPOSE OF REVIEW

The current review is to update the results on epidemiology, pathobiology, and genes related to virulence, clinical presentation, molecular diagnosis, antimicrobial resistance, and extraintestinal infection of enteroaggregative Escherichia coli (EAEC).

RECENT FINDINGS

EAEC subclinical infection was significantly associated with reduced length at 2 years of age and EAEC and coinfections were associated with reduced delta weight-for-length and weight-for-age z-scores in the first 6 months of age in the MAL-ED birth cohort study. EAEC was associated with malnutrition in children 6-24 months of age in prospective case-control studies in Bangladesh and Brazil. Virulence gene-based studies have suggested aggregative fimbriae II may be a major contributor to disease, whereas AggR-activated regulator a marker of less severe disease. The high ability of EAEC colonization likely exacerbates effects of other microbial virulence strategies. Molecular diagnosis has been useful for understanding EAEC burden, although different criteria may relate to different pathogenic outcomes.

SUMMARY

EAEC gained special interest in the past few years, especially due to association with growth decrements in children with subclinical infections and its important role as a copathogen. Understanding of EAEC pathogenesis advanced but further research is needed for elucidating both microbial and host factors influencing infection outcomes.

Galleria mellonella Infection Model Identifies Both High and Low Lethality of Clostridium perfringens Toxigenic Strains and Their Response to Antimicrobials

Research progress into mechanisms of the anaerobe Clostridium perfringens and associated diseases has been frustrated by the lack of reliable infection models. Wax moth larvae (Galleria mellonella) have emerged as a viable alternative to other models of infection since they are economic, survive at 37°C and require no specialist equipment. This study aims to establish to what extent G. mellonella larvae can be used to study the virulence of C. perfringens strains and its suitability for studying novel treatment strategies by an improved time-lapse approach to data collection. Mortality and morbidity rates of larvae challenged with 10⁵ CFU of C. perfringens isolates from various sources were observed over 72 h and dose response data obtained. Phenoloxidase enzyme activity was investigated as a marker for immune response and tissue burden assessed by histopathological techniques. Results demonstrate that C. perfringens is pathogenic toward G. mellonella although potency varies dramatically between C. perfringens isolates and the reference strain ATCC 13124 was shown to be avirulent. Infection with C. perfringens strains activated the melanisation pathway resulting in melanin deposition but no increase in enzyme activity was observed. Efficacy of antibiotic therapy (penicillin G, bacitracin, neomycin, and tetracycline) administered parenterally to some extent correlates with that of in vitro analysis. The findings suggest G. mellonella might be a useful in vivo model of infection and convenient as a pre-screening assay for virulence of C. perfringens strains or as a simple, cheap and rapid in vivo assay in the first stage development of novel therapeutics against anaerobes.

HIGHLIGHTS

-Potential novel in vivo model for the study of Clostridium perfringens infection.-Novel time-lapse approach to data collection.-First report of the pathogenicity of C. perfringens toward G. mellonella.-First report of the efficacy of antibiotic therapy in response to C. perfringens infection in G. mellonella.

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.

Assessing the Pathogenicity of Two Bacteria Isolated from the Entomopathogenic Nematode Heterorhabditis indica against Galleria mellonella and Some Pest Insects

The entomopathogenic nematodes Heterorhabditis are parasites of insects and are associated with mutualist symbiosis enterobacteria of the genus Photorhabdus; these bacteria are lethal to their host insects. Heterorhabditis indica MOR03 was isolated from sugarcane soil in Morelos state, Mexico. The molecular identification of the nematode was confirmed using sequences of the ITS1-5.8S-ITS2 region and the D2/D3 expansion segment of the 28S rRNA gene. In addition, two bacteria HIM3 and NA04 strains were isolated from the entomopathogenic nematode. The genomes of both bacteria were sequenced and assembled de novo. Phylogenetic analysis was confirmed by concatenated gene sequence datasets as Photorhabdus luminescens HIM3 (16S rRNA, 23S rRNA, dnaN, gyrA, and gyrB genes) and Pseudomonas aeruginosa NA04 (16S rRNA, 23S rRNA and gyrB genes). H. indica MOR03 infects Galleria mellonella, Tenebrio molitor, Heliothis subflexa, and Diatraea magnifactella larvae with LC50 values of 1.4, 23.5, 13.7, and 21.7 IJs/cm², respectively, at 48 h. These bacteria are pathogenic to various insects and have high injectable insecticide activity at 24 h.

Methods and Challenges of Using the Greater Wax Moth (Galleria mellonella) as a Model Organism in Antimicrobial Compound Discovery

Among non-mammalian infection model organisms, the larvae of the greater wax moth Galleria mellonella have seen increasing popularity in recent years. Unlike other invertebrate models, these larvae can be incubated at 37 °C and can be dosed relatively precisely. Despite the increasing number of publications describing the use of this model organism, there is a high variability with regard to how the model is produced in different laboratories, with respect to larva size, age, origin, storage, and rest periods, as well as dosing for infection and treatment. Here, we provide suggestions regarding how some of these factors can be approached, to facilitate the comparability of studies between different laboratories. We introduce a linear regression curve correlating the total larva weight to the liquid volume in order to estimate the in vivo concentration of pathogens and the administered drug concentration. Finally, we discuss several other aspects, including in vivo antibiotic stability in larvae, the infection doses for different pathogens and suggest guidelines for larvae selection.

Balancing mcr-1 expression and bacterial survival is a delicate equilibrium between essential cellular defence mechanisms

MCR-1 is a lipid A modifying enzyme that confers resistance to the antibiotic colistin. Here, we analyse the impact of MCR-1 expression on E. coli morphology, fitness, competitiveness, immune stimulation and virulence. Increased expression of mcr-1 results in decreased growth rate, cell viability, competitive ability and significant degradation in cell membrane and cytoplasmic structures, compared to expression of catalytically inactive MCR-1 (E246A) or MCR-1 soluble component. Lipopolysaccharide (LPS) extracted from mcr-1 strains induces lower production of IL-6 and TNF, when compared to control LPS. Compared to their parent strains, high-level colistin resistance mutants (HLCRMs) show reduced fitness (relative fitness is 0.41-0.78) and highly attenuated virulence in a Galleria mellonella infection model. Furthermore, HLCRMs are more susceptible to most antibiotics than their respective parent strains. Our results show that the bacterium is challenged to find a delicate equilibrium between expression of MCR-1-mediated colistin resistance and minimalizing toxicity and thus ensuring cell survival.