Multiple acquisitions of CTX-M plasmids in the rare D2 genotype of Escherichia coli provide evidence for convergent evolution.

Over the last decade, CTX-M enzymes have become the most prevalent extended-spectrum beta-lactamases (ESBLs) worldwide, mostly in Escherichia coli, causing a major health problem. An epidemiological relationship has been established between a rare genotype of E. coli, the D(2) genotype, and the presence of CTX-M genes. We investigated this striking association by exploring the genetic backgrounds of 18 D(2) genotype CTX-M-producing strains and of the plasmids encoding CTX-M enzymes. The 18 strains had different genetic backgrounds, as assessed by multilocus sequence and O typing, and were associated with various plasmids bearing diverse CTX-M genes. The region encompassing the genetic marker of the D(2) genotype (TSPE4.C2) was not correlated with the presence of CTX-M genes. CTX-M-producing D(2) strains had far fewer virulence factors than a control group of 8 non-ESBL-producing D(2) strains, and an inverse relationship was found between the number of co-resistances associated with the CTX-M gene and the number of virulence factors found in the strain. These findings provide evidence for multiple acquisitions of plasmids carrying CTX-M genes in different D(2) genotype strains. They strongly suggest that convergent evolution has occurred, and indicate that there has been selection for the association of a specific genetic background of the strain and the CTX-M gene. This fine-tuning of the relationship between the D(2) genotype and CTX-M genes presumably increases the fitness of the strain, indicating a role for the host cell in the acquisition and dissemination of CTX-M genes.

Replicon typing of plasmids in Escherichia coli producing extended-spectrum beta-lactamases.

OBJECTIVES: Escherichia coli producing CTX-M-15 and CTX-M-14 extended-spectrum beta-lactamases (ESBLs) are spreading worldwide. The aim of this work was to investigate the replicons involved in the emergence and spread of ESBLs in relation to ESBL type. METHODS: A collection of 125 TEM, SHV and CTX-M ESBL-producing E. coli strains was analysed. The replicons carrying the ESBLs and the total plasmid content of the strains have been characterized by PCR replicon typing in relation to the type of ESBL. The ESBL replicons were then compared with the replicon content of E. coli strains carrying TEM-1 or inhibitor-resistant TEM (IRT) beta-lactamases. RESULTS: IncF plasmids were the most frequently carried replicons in our collection, but none carried TEM ESBL. Of TEM ESBLs, 67% were carried on IncA/C replicons except for TEM-52 genes, which were carried preferentially on IncI1 replicons. Although CTX-M enzymes can be carried by various replicons, the great majority of genes encoding CTX-M-14 and CTX-M-15 ESBLs were carried by IncF replicons, as were TEM-1 and IRT beta-lactamases. CONCLUSIONS: Resistance genes borne by the narrow host-range IncF replicon spread readily as this replicon is well adapted to E. coli. This is observed for blaTEM-1 and blaCTX-M-15 and, to a lesser extent, for blaCTX-M-14. Transposition immunity seems to play an important role in the diffusion process.

The CTX-M-15-producing Escherichia coli diffusing clone belongs to a highly virulent B2 phylogenetic subgroup.

OBJECTIVES: A clone of CTX-M-15-producing Escherichia coli has recently been reported to be spreading through Europe and Africa. The aim of this work was to thoroughly characterize this clone. MATERIALS AND METHODS: Representative isolates of this clone were subjected to multilocus sequence typing, O typing, virulence gene detection, adhesion assay on human cells, biofilm production assay and mouse lethality assay. RESULTS: The clone: (i) belongs to a unique B2 phylogenetic subgroup encompassing the pyelonephritogenic diffusely adhering EC7372 strain; (ii) exhibits a specific O25b molecular subtype; (iii) is identical to the E. coli clone O25:H4-ST131 producing CTX-M-15; (iv) produces biofilm; and (v) is highly virulent in mice despite lacking classical extraintestinal pathogenicity islands (except for high pathogenicity island) and the afa/dra gene. CONCLUSIONS: The CTX-M-15-producing E. coli diffusing clone is associated with a high level of antibiotic resistance and with high virulence, showing that, under certain selective pressures, the previously observed trade-off between resistance and virulence may not apply.

Intermediate mutation frequencies favor evolution of multidrug resistance in Escherichia coli.

In studying the interplay between mutation frequencies and antibiotic resistance among Escherichia coli natural isolates, we observed that modest modifications of mutation frequency may significantly influence the evolution of antibiotic resistance. The strains having intermediate mutation frequencies have significantly more antibiotic resistances than strains having low and high mutation frequencies.

Genetic relationship between methicillin-sensitive and methicillin-resistant Staphylococcus aureus strains from France and from international sources: delineation of genomic groups.

Cluster analysis of the SmaI patterns, generated by pulsed-field gel electrophoresis, of 44 methicillin-resistant (MRSA) and 118 methicillin-sensitive (MSSA) Staphylococcus aureus strains isolated in various French hospitals and 61 MRSA and 48 MSSA strains from 20 other countries revealed 20 genomic groups distributed into four distantly related phylogenic branches. Eighty-three of the 105 MRSA strains (79%) were clustered in the six genomic groups of phylogenic branch I; and 154 of the 166 MSSA strains (92.8%) were clustered in the 14 genomic groups of phylogenic branches II, III, and IV. Agreement between genomic group and two other markers, esterase type and phage group, was obtained, emphasizing the clonal structure of the population. The genomic groups were delineated by esterase type. The distribution of the strains within the genomic groups was independent of their geographical origin; French strains were clustered with strains from other countries. The three types of the staphylococcal cassette chromosome mec (SCCmec) complex were distributed according to genomic groups. Most of the time, type I and type II SCCmec complexes were found in the MRSA strains belonging to the same genomic groups. In contrast, the type III SCCmec complex was specific to the MRSA strains belonging to the three genomic groups characterized by a common esterase type.