High prevalence of colistin heteroresistance in specific species and lineages of Enterobacter cloacae complex derived from human clinical specimens

Molecular epidemiology and β-lactam resistance mechanisms of Enterobacter cloacae complex isolates obtained from bloodstream infections, Kyoto, Japan

The Enterobacter cloacae complex (ECC) comprises multiple species that require genomic analysis for precise identification. They produce inducible AmpC β-lactamase and may carry acquired β-lactamases, which are responsible for cefotaxime and cefepime resistance. To determine the molecular epidemiology, antimicrobial resistance, and β-lactam resistance mechanisms of the ECC, we conducted whole-genome sequencing analysis, antimicrobial susceptibility testing, and mutation analysis on bloodstream ECC isolates from patients in Kyoto, Japan. In 194 ECC isolates, 13 species and six unnamed taxa were identified, with Enterobacter xiangfangensis (36%) being the most common. A total of 38% of the isolates were nonsusceptible to cefotaxime and presented relatively high nonsusceptibility rates to all antimicrobial agents tested. Among the different species, Enterobacter hoffmannii presented the highest nonsusceptibility rates to both β-lactams and non-β-lactams. Among the cefotaxime-nonsusceptible isolates, 16% harbored genes encoding extended-spectrum β-lactamases (ESBLs), carbapenemase, and/or plasmid-mediated AmpC, and ampC derepression was the predominant resistance mechanism in the remaining isolates. The prevalent sequence types (STs) in cefotaxime-susceptible and cefotaxime-nonsusceptible isolates were different, although some STs were shared by both groups. Cefepime nonsusceptibility was detected in 7% of the isolates and was associated with E. hoffmannii ST78 and E. xiangfangensis ST93, which carry ESBLs. Sixty-four mutants, experimentally obtained from eight cefotaxime-susceptible isolates, had various ampD mutations, and 42% and 99% of the mutants were nonsusceptible to cefepime and piperacillin/tazobactam, respectively, indicating the risks associated with the use of these antimicrobials. Continuous surveillance via genomic and phenotypic analyses is needed to combat antimicrobial resistance in the ECC.IMPORTANCEThe Enterobacter cloacae complex (ECC) is a group of pathogenic bacteria that cause nosocomial infections. The ECC produces chromosomal inducible AmpC β-lactamases, which is associated with treatment failure despite initial susceptibility to third-generation cephalosporins in selected ampC-derepressed mutants. The complex antimicrobial resistance mechanisms of the ECC and challenges in species identification have complicated our understanding of the ECC and the selection of appropriate treatment. In this study, we performed phenotypic, whole-genome sequencing, and mutation analyses among ECC isolates from patients with bloodstream infections to determine the precise molecular-based epidemiology, resistance mechanisms to third-/fourth-generation cephalosporins, specific species and clones that contribute to antimicrobial resistance, and acquisition rates of fourth-generation cephalosporin resistance in ampC-derepressed mutants. These data will help elucidate the local epidemiology and complex β-lactam resistance mechanisms in the ECC and guide appropriate antimicrobial therapy and infection control strategies for ECC-related infections.

First Report of mcr-10 in a Seafood-Borne ESBL-Producing Enterobacter xiangfangensis Strain

mcr-10 is among the growing families of newly identified plasmid-mediated mobile colistin-resistance genes. In this study, we identified an mcr-10 gene in a seafood-borne extended-spectrum β-lactamase (ESBL)-producing Enterobacter xiangfangensis. E. xiangfangensis strain, B12-S77, was subjected to whole genome sequencing using Illumina MiSeq and Oxford Nanopore Technologies. Bioinformatic analysis was performed using tools from the Center for Genomic Epidemiology. The minimum inhibitory concentration (MIC) of 19 antibiotics was determined by the broth microdilution method. Transferability of mcr-10-carrying plasmid was investigated by the conjugation experiment. The strain exhibited a multidrug-resistant (MDR) phenotype against more than three classes of antibiotics but remained susceptible to colistin and polymyxin B. mcr-10 was identified on a fused conjugative plasmid of the IncFIB (K):FII (Yp) backbone adjacent to the XerC-type tyrosine recombinase-gene. At least one insertion sequence (IS) was identified in both the downstream and upstream regions of the xerC-mcr-10 conserved region, indicating that this region may contribute to mcr-10 mobilization or integration into the bacterial genome. The strain belonged to sequence type (ST) 143 and carried the nlpI and mrkA virulence genes, which promote fimbrial adhesion or biofilm formation in enteric bacteria. This report provides novel insights into the emergence of mcr-10 in seafood-borne bacteria, and highlights the importance of surveillance in the seafood supply chain.

Clinical characteristics, molecular epidemiology and mechanisms of colistin heteroresistance in Enterobacter cloacae complex

Introduction

Colistin has emerged as the last resort for treating multidrug-resistant Enterobacter cloacae complex (ECC) infections. The primary purposes of this study were to demonstrate the presence of colistin heteroresistance in ECC and to further investigate their clinical characteristics, molecular epidemiology and mechanisms.

Methods

Population analysis profiles (PAP) were performed to confirm the heteroresistance phenotype. Average nucleotide identity (ANI) was determined to classify ECC species. Phylogenetic analysis based on core genome single nucleotide polymorphisms (cg-SNPs), multilocus sequence typing (MLST) and core genome MLST (cg-MLST). Risk factors and clinical outcomes of infections were analyzed through a retrospective case-control study. Potential mechanisms of colistin heteroresistance were evaluated using polymerase chain reaction (PCR), efflux pump inhibition assays and reverse transcription quantitative PCR (RT-qPCR).

Results

A high proportion (24.4%) of the non-resistant strains were colistin-heteroresistant isolates. Among the several ECC species, Enterobacter kobei had the largest percentage (29.4%) of colistin-heteroresistant isolates, followed by Enterobacter hormaechei (20.5%) and Enterobacter bugandensis (20.0%). Notably, only one strain (0.8%; 1/132) of Enterobacter hormaechei was fully resistant to colistin. Different ECC species showed varying heteroresistance levels: Enterobacter roggenkampii, Enterobacter kobei, Enterobacter asburiae and Enterobacter bugandensis displayed high heteroresistance levels  (MIC ≥ 128 mg/L). 75% of all ST116 and ST56 strains were heteroresistant to colistin. The infection of ST116 and ST56 strains as well as exposure to cephalosporin antibiotics were independent risk factors for colistin-heteroresistant ECC infections. Mechanistic analysis revealed that heteroresistance strongly correlated with the overexpression of arnA, regulated by the PhoPQ two-component system (TCS). Notably, mgrB had minimal impact. AcrAB-TolC efflux pump genes showed unsynchronized expression; High acrB expression was strongly associated with colistin heteroresistance, while acrA and tolC were not.

Discussion

Colistin heteroresistance showed species-dependent variations in levels and prevalence rates. The colistin-heteroresistant mechanisms were complex, involving coordinated regulation of multiple genes. These results highlighted the need for tailored antimicrobial stewardship. In addition, the development of direct, reliable and rapid clinical methods for detecting heteroresistance is essential for improving infection management and prevention.

Insight into the Mechanisms and Clinical Relevance of Antifungal Heteroresistance

Antifungal resistance poses a critical global health threat, particularly in immuno-compromised patients. Beyond the traditional resistance mechanisms rooted in heritable and stable mutations, a distinct phenomenon known as heteroresistance has been identified, wherein a minority of resistant fungal cells coexist within a predominantly susceptible population. Heteroresistance may be induced by pharmacological factors or non-pharmacological agents. The reversible nature of it presents significant clinical challenges, as it can lead to undetected resistance during standard susceptibility testing. As heteroresistance allows fungal pathogens to survive antifungal treatment, this adaptive strategy often leads to treatment failure and recurring infection. Though extensively studied in bacteria, limited research has explored its occurrence in fungi. This review summarizes the current findings on antifungal heteroresistance mechanisms, highlighting the clinical implications of fungal heteroresistance and the pressing need for deeper mechanism insights. We aim to bring together the latest research advances in the field of antifungal heteroresistance, summarizing in detail its known characteristics, inducing factors, molecular mechanisms, and clinical significance, and describing the similarities and differences between heteroresistance, tolerance and persistence. Further research is needed to understand this phenomenon and develop more effective antifungal therapies to combat fungal infections.

Genomic analysis of Enterobacter cloacae complex from Southern Thailand reveals insights into multidrug resistance genotypes and genetic diversity

In this study, we conducted a comprehensive investigation into the Enterobacter cloacae complex (ECC), a group of notorious pathogens responsible for various hospital-acquired infections. We aimed to gain critical insights into antimicrobial resistance profiles and genomic diversity among 17 ECC isolates, which were previously collected as part of a short-term surveillance effort for 6 months in 2019. We identified two novel sequence types (ST-1936 in E. bugandensis PSU30 and ST-1937 in E. roggenkampii PSU45) among the 14 distinct STs identified in our ECC isolates. Furthermore, our expanded investigation revealed 296 novel STs within the NCBI Reference Sequence database. We identified six isolates carrying the mcr-9 gene, highlighting a significant concern in antimicrobial resistance (AMR). These genes confer a reduced susceptibility to colistin, a critical last-resort drug for the treatment of multidrug-resistant (MDR) infection. In addition to the AMR complexity, we found that three isolates carried the blaNDM gene on IncN2 plasmids, further emphasizing the urgency of monitoring and managing ECC-related infections. Our study provided evidence of intra-hospital transmission involving E. asburiae isolates PSU37, PSU39, and PSU40, all collected from the nasopharynx of three individuals in the intensive care unit (ICU) of the same hospital. These findings highlight the need for stringent infection control measures to prevent similar outbreaks and emphasize the importance of effective surveillance and management strategies to address ECC-related challenges.

Clinical and microbiological analyses of colistin-resistant strains among carbapenem-resistant Enterobacter cloacae complex clinical isolates

Carbapenem-resistant Enterobacter cloacae complex (CR-ECC), which is rapidly increasing as the cause of nosocomial infections, has limited treatment options. The aim of this study is to investigate the microbiological and clinical traits and molecular epidemiology of isolates of CR-ECC and provide guidance for antibiotic selection in clinical practice. Clinical CR-ECC isolates (ertapenem MIC ≥ 2 mg/L) were collected from 2021 to 2022. Species identification was performed using hsp60 gene analysis, and antibiotic susceptibility tests were conducted by broth microdilution. The clinical characteristics of patients with CR-ECC isolates were retrospectively analyzed. Among the 108 CR-ECC isolates, 25 (23.2%) were non-susceptible to colistin, with colistin susceptibility being higher in Enterobacter hormaechei compared to non-E. hormaechei isolates (P < 0.0001). Of the 108 CR-ECC isolates, 9 (8.3%) produced carbapenemases, and only 6 of the 22 colistin-resistant CR-ECC isolates (27.3%) harbored the mcr gene. A total of 73 sequence types (STs), including 28 newly identified STs, were detected, demonstrating significant clonal diversity. The most common ST was ST74, known for its high prevalence and association with carbapenem resistance, with 77.8% identified as E. hormaechei subsp. hoffmannii. E. hormaechei was more common in the colistin-susceptible group than in the non-susceptible group (88.0% vs 37.5%, P < 0.0001), and E. hormaechei was the only protective factor for colistin resistance (HR 0.089, CI 0.030-0.261, P < 0.001). Although colistin resistance of CR-ECC is high, colistin could be administered safely to E. hormaechei. It is imperative to maintain ongoing surveillance and to further research on CR-ECC.IMPORTANCEAlthough new antibiotics are being developed, there are still limited options for treating carbapenem-resistant Enterobacter cloacae complex (CR-ECC) in regions where their use is restricted. The resistance level to one of these options, colistin, was investigated using bacteria isolated from clinical samples. In clinical practice, colistin is frequently administered empirically without susceptibility testing. However, this study suggests that colistin can be safely administered to certain species such as Enterobacter hormaechei within the CR-ECC.

Study of the mechanisms of colistin heteroresistance in a strain of Enterobacter cloacae by random mutagenesis

OBJECTIVE

The Enterobacter cloacae complex belongs to a group in which colistin resistance is not well documented due to their frequent heteroresistance.

METHODS

We investigated the molecular basis of colistin heteroresistance using genome analysis and random mutagenesis in a strain of E. cloacae.

RESULTS

The arnA gene was truncated in the six colistin susceptible mutants we obtained. However, the mcr-9 gene was present in these mutants suggesting this gene is not responsible for colistin resistance.

CONCLUSIONS

Previous studies have highlighted the role of the two-component system PhoPQ and its negative regulator mgrB or the arnBCADTEF operon. Our results confirm the crucial role of this operon.

Colistin Resistance Mechanisms and Molecular Epidemiology of Enterobacter cloacae Complex Isolated from a Tertiary Hospital in Shandong, China

Background

Enterobacter cloacae complex (ECC), which includes major nosocomial pathogens, causes urinary, respiratory, and bloodstream infections in humans, for which colistin is one of the last-line drugs.

Objective

This study aimed to analyse the epidemiology and resistance mechanisms of colistin-resistant Enterobacter cloacae complex (ECC) strains isolated from Shandong, China.

Methods

Two hundred non-repetitive ECC strains were collected from a tertiary hospital in Shandong Province, China, from June 2020 to June 2022. Whole-genome sequencing and bioinformatics analyses were performed to understand the molecular epidemiology of the colistin-resistant ECC strains. The nucleotide sequences of heat shock protein (hsp60) were analyzed by using BLAST search to classify ECC. The gene expression levels of ramA, soxS, acrA, acrB, phoP, and phoQ were assessed using RT-qPCR. MALDI-TOF MS was used to analyse the modification of lipid A.

Results

Twenty-three colistin-resistant strains were detected among the 200 ECC clinical strains (11.5%). The hsp60 cluster analysis revealed that 20 of the 23 ECC strains belonged to heterogeneous resistance clusters. Variants of mgrB, phoPQ, and pmrAB, particularly phoQ and pmrB, were detected in the 23 ECC strains. The soxS and acrA genes were significantly overexpressed in all 23 colistin-resistant ECC strains (P < 0.05). Additionally, all 23 ECC strains contained modified lipid A related to colistin resistance, which showed five ion peaks at m/z 1876, 1920, 1955, 2114, and 2158. Among the 23 ECC strains, 6 strains possessed a phosphoethanolamine (pETN) moiety, 16 strains possessed a 4-amino-4-deoxy-L-arabinose (-L-Ara4N) moiety, and one strain had both pETN and -L-Ara4N moieties.

Conclusion

This study suggests that diverse colistin resistance existed in ECC, including unknown resistance mechanisms, exist in ECC. Mechanistic investigations of colistin resistance are warranted to optimise colistin use in clinical settings and minimise the emergence of resistance.

Microbial and Metabolic Gut Profiling across Seven Malignancies Identifies Fecal Faecalibacillus intestinalis and Formic Acid as Commonly Altered in Cancer Patients

The key association between gut dysbiosis and cancer is already known. Here, we used whole-genome shotgun sequencing (WGS) and gas chromatography/mass spectrometry (GC/MS) to conduct metagenomic and metabolomic analyses to identify common and distinct taxonomic configurations among 40, 45, 71, 34, 50, 60, and 40 patients with colorectal cancer, stomach cancer, breast cancer, lung cancer, melanoma, lymphoid neoplasms and acute myeloid leukemia (AML), respectively, and compared the data with those from sex- and age-matched healthy controls (HC). α-diversity differed only between the lymphoid neoplasm and AML groups and their respective HC, while β-diversity differed between all groups and their HC. Of 203 unique species, 179 and 24 were under- and over-represented, respectively, in the case groups compared with HC. Of these, Faecalibacillus intestinalis was under-represented in each of the seven groups studied, Anaerostipes hadrus was under-represented in all but the stomach cancer group, and 22 species were under-represented in the remaining five case groups. There was a marked reduction in the gut microbiome cancer index in all case groups except the AML group. Of the short-chain fatty acids and amino acids tested, the relative concentration of formic acid was significantly higher in each of the case groups than in HC, and the abundance of seven species of Faecalibacterium correlated negatively with most amino acids and formic acid, and positively with the levels of acetic, propanoic, and butanoic acid. We found more differences than similarities between the studied malignancy groups, with large variations in diversity, taxonomic/metabolomic profiles, and functional assignments. While the results obtained may demonstrate trends rather than objective differences that correlate with different types of malignancy, the newly developed gut microbiota cancer index did distinguish most of the cancer cases from HC. We believe that these data are a promising step forward in the search for new diagnostic and predictive tests to assess intestinal dysbiosis among cancer patients.

The Use of CHROMID® Colistin R for the Detection of Colistin-Resistant Gram-Negative Bacteria in Positive Blood Cultures

The aim of this study was to assess the utility of CHROMID® Colistin R for direct detection of colistin-resistant Gram-negative bacteria from positive blood cultures. A total of 390 blood cultures from hospitalised patients containing Gram-negative bacteria were included in this study. These blood cultures were referred to clinical laboratories in the United Kingdom and Türkiye. A further 16 simulated positive blood culture bottles were included that contained a range of colistin-resistant strains as well as susceptible control strains. Fluid from each positive blood culture was diluted 1/200 in saline and 10 µL aliquots cultured onto cystine-lactose-electrolyte-deficient agar and CHROMID® Colistin R. All recovered bacteria were identified, and for Gram-negative bacteria, their minimum inhibitory concentration of colistin was measured using the broth microdilution method. From a total of 443 Gram-negative isolates, 57 colistin-resistant isolates were recovered, of which 53 (93%) grew on CHROMID® Colistin R within 18 h. Of the 377 isolates determined to be colistin-susceptible, only 9 isolates were able to grow, including 6 isolates of Pseudomonas aeruginosa. For positive blood cultures that are shown to contain Gram-negative bacteria, culture on CHROMID® Colistin R is a useful diagnostic tool to detect susceptibility or resistance to colistin within 18 h.