Resistance to oxyimino beta-lactams due to a mutation of chromosomal beta-lactamase in Citrobacter freundii

Assessing the effects of aquaculture on tidal flat ecological status using multi-metrics interaction-based index of biotic integrity (Mt-IBI)

Given tidal flat special environmental conditions and the degree of pollution caused by human activities, there is an urgent need to quantitatively assess their ecological status. Bioindication has become an indispensable part of environmental quality monitoring on account of its sensitivity to environmental disturbance. Thus, this study used bio-indicators to establish a multi-metrics-based index of biotic integrity (Mt-IBI) to evaluate the ecological status of the tidal flats with/without aquaculture through metagenomic sequencing. Four core indexes that were significantly correlated to other indexes with redundancy (p < 0.05), including Escherichia, beta-lactam antibiotic resistance genes, cellulase and xyloglucanases and the keystone species with 21° in the network, were selected after the screening processes. By implementing Mt-IBI in the tidal flats, the ecological health of the sampling sites was categorized into three levels, with Mt-IBI values of 2.01-2.63 (severe level), 2.81-2.93 (moderate level) and 3.23-4.18 (mild level), respectively. Through SEM analysis, water chemical oxygen demand and antibiotics were determined to be the primary controlling factors of the ecological status of tidal flat regions influenced by aquaculture, followed by salinity and total nitrogen. It is worth noting that the alteration of microbial communities impacted ecological status through the mediation of antibiotics. It is hoped that the results of our study will provide a theoretical basis for coastal environment restoration and that the use of Mt-IBI to assess ecosystem status in different aquatic environments will be further popularized in the future.

Species-specific mutation rates for ampC derepression in Enterobacterales with chromosomally encoded inducible AmpC β-lactamase

Background

AmpC β-lactamases are encoded on the chromosomes of certain Enterobacterales and lead to clinical resistance to various β-lactams in case of high-level expression. In WT bacteria with inducible AmpC, the expression is low, but selection of stably ampC-derepressed mutants may occur during β-lactam therapy. Thus, for Enterobacter spp., Citrobacter freundii complex, Serratia spp. and Morganella morganii that test susceptible in vitro to oxyimino-cephalosporins, the EUCAST expert rules recommend suppressing susceptibility testing results for these agents or noting that their use in monotherapy should be discouraged, owing to the risk of selecting resistance. However, clinical observations suggest that emergence of resistance is not equally common in all species with inducible AmpC.

Objectives

To determine species-specific mutation rates, which are more accurate and reproducible than previously described mutant frequencies, for ampC derepression in Enterobacterales with inducible AmpC.

Methods

Mutation rates were determined using a protocol based on Luria-Delbrück fluctuation analyses. Overall, 237 isolates were analysed.

Results

Mutation rates were high in Enterobacter cloacae complex, Enterobacter aerogenes, C. freundii complex and Hafnia alvei isolates, with a mean mutation rate of 3 × 10-8. In contrast, mean mutation rates were considerably lower in Providencia spp., Serratia spp. and especially M. morganii isolates. Furthermore, we observed species-specific variations in the resistance patterns of ampC-derepressed mutants.

Conclusions

Our data might help to predict the risk of treatment failure with oxyimino-cephalosporins in infections by different Enterobacterales with inducible AmpC. Moreover, we make a proposal for optimization of the current EUCAST expert rule.

Characterization of an extended-spectrum class C beta-lactamase of Citrobacter freundii

Citrobacter freundii GC3 is a clinical isolate which showed moderate resistance to oxyimino beta-lactams such as ceftazidime and aztreonam. This drug resistance was due to an extended-spectrum class C beta-lactamase encoded by chromosomal gene(s). The GC3 beta-lactamase showed high amino acid sequence homology to a known C. freundii beta-lactamase, i.e., 346 of 361 amino acids were identical with those of C. freundii GN346 beta-lactamase (Tsukamoto, K. et al, Eur. J. Biochem. 188, 15-22, 1990). Asp198 was the only dissimilar amino acid found in the omega loop region, known as the hot spot for extended-spectrum resistance in class C beta-lactamases (Haruta, S. et al, Microbiol. Immunol. 42, 165-169, 1998). However, Asp198 was eliminated as a cause of the extended-spectrum resistance by the substitution of Asn for Asp198. Subsequent investigation suggested that the moderate resistance to oxyimino beta-lactams is attributable to the replacement of amino acids on the enzyme's surface area, far from the active-site. Some or all of the replacements are assumed to delicately modify the active-site configuration. The GC3 beta-lactamase is the first example of an extended-spectrum class C beta-lactamase in which mutations are independent of the omega loop.

Dissemination of CTX-M-3 and CMY-2 beta-lactamases among clinical isolates of Escherichia coli in southern Taiwan

A total of 1,210 clinical isolates of Escherichia coli collected from a university hospital in southern Taiwan were screened for production of extended-spectrum beta-lactamases (ESBLs). Expression of classical ESBLs (resistant to extended-spectrum beta-lactam agents and susceptible to beta-lactam inhibitors) was inferred in 18 isolates by the phenotypic confirmatory test. These included 10 isolates producing CTX-M-3, 2 strains carrying SHV-12, 1 strain harboring SHV-5, 1 strain expressing TEM-10, and 4 strains producing unidentifiable ESBLs with a pI of 8.05, 8.0, or 7.4. Eighteen isolates that showed decreased susceptibilities to ceftazidime and/or cefotaxime, negative results for the confirmatory test, and high-level resistance to cefoxitin (MICs of >/=128 microg/ml) were also investigated. Five isolates were found to produce CMY-2 AmpC enzymes, one isolate carried both CTX-M-3 and CMY-2, and the remaining three and nine isolates expressed putative AmpC beta-lactamases with pIs of >9.0 and 8.9, respectively. Thus, together with the isolate producing CTX-M-3 and CMY-2, 19 (1.6%) isolates produced classical ESBLs. Pulsed-field gel electrophoresis revealed that all isolates carrying CTX-M-3 and/or CMY-2 were genetically unrelated, indicating that dissemination of resistance plasmids was responsible for the spread of these two enzymes among E. coli in this area. Among the 16 isolates expressing CTX-M-3 and/or CMY-2, 5 might have colonized outside the hospital environment. Our data indicate that CTX-M-3 and CMY-2, two beta-lactamases initially identified in Europe, have been disseminated to and are prevalent in Taiwan.

Heterogeneity of AmpC cephalosporinases of Hafnia alvei clinical isolates expressing inducible or constitutive ceftazidime resistance phenotypes

Ten unrelated Hafnia alvei clinical isolates were grouped according to either their low-level and inducible cephalosporinase production or their high-level and constitutive cephalosporinase production phenotype. Their AmpC sequences shared 85 to 100% amino acid identity. The immediate genetic environment of ampC genes was conserved in H. alvei isolates but was different from that found in other ampC-possessing enterobacterial species.

Biochemical-genetic characterization and regulation of expression of an ACC-1-like chromosome-borne cephalosporinase from Hafnia alvei

A naturally occurring AmpC beta-lactamase (cephalosporinase) gene was cloned from the Hafnia alvei 1 clinical isolate and expressed in Escherichia coli. The deduced AmpC beta-lactamase (ACC-2) had a pI of 8 and a relative molecular mass of 37 kDa and showed 50 and 47% amino acid identity with the chromosome-encoded AmpCs from Serratia marcescens and Providentia stuartii, respectively. It had 94% amino acid identity with the recently described plasmid-borne cephalosporinase ACC-1 from Klebsiella pneumoniae, suggesting the chromosomal origin of ACC-1. The hydrolysis constants (k(cat) and K(m)) showed that ACC-2 was a peculiar cephalosporinase, since it significantly hydrolyzed cefpirome. Once its gene was cloned and expressed in E. coli (pDEL-1), ACC-2 conferred resistance to ceftazidime and cefotaxime but also an uncommon reduced susceptibility to cefpirome. A divergently transcribed ampR gene with an overlapping promoter compared with ampC (bla(ACC-2)) was identified in H. alvei 1, encoding an AmpR protein that shared 64% amino acid identity with the closest AmpR protein from P. stuartii. beta-Lactamase induction experiments showed that the ampC gene was repressed in the absence of ampR and was activated when cefoxitin or imipenem was added as an inducer. From H. alvei 1 cultures that expressed an inducible-cephalosporinase phenotype, several ceftazidime- and cefpirome-cross-resistant H. alvei 1 mutants were obtained upon selection on cefpirome- or ceftazidime-containing plates, and H. alvei 1 DER, a ceftazidime-resistant mutant, stably overproduced cephalosporinase. Transformation of H. alvei 1 DER or E. coli JRG582 (ampDE mutant) harboring ampC and ampR from H. alvei 1 with a recombinant plasmid containing ampD from E. coli resulted in a decrease in the MIC of beta-lactam and recovery of an inducible phenotype for H. alvei 1 DER. Thus, AmpR and AmpD proteins may regulate biosynthesis of the H. alvei cephalosporinase similarly to other enterobacterial cephalosporinases.