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Escherichia coli is one of the first causes of Gram-negative orthopedic implant infections (OII), but little is known about the pathogenicity of this species in such infections that are increasing due to the ageing of the population. We report how this pathogen interacts with human osteoblastic MG-63 cells in vitro, by comparing 20 OII E. coli strains to two Staphylococcus aureus and two Pseudomonas aeruginosa strains. LDH release assay revealed that 6/20 (30%) OII E. coli induced MG-63 cell lysis whereas none of the four control strains was cytotoxic after 4 h of coculture. This high cytotoxicity was associated with hemolytic properties and linked to hlyA gene expression. We further showed by gentamicin protection assay and confocal microscopy that the non-cytotoxic E. coli were not able to invade MG-63 cells unlike S. aureus strains (internalization rate <0.01% for the non-cytotoxic E. coli versus 8.88 ± 2.31% and 4.60 ± 0.42% for both S. aureus). The non-cytotoxic E. coli also demonstrated low adherence rates (<7%), the most adherent E. coli eliciting higher IL-6 and TNF-α mRNA expression in the osteoblastic cells. Either highly cytotoxic or slightly invasive OII E. coli do not show the same infection strategies as S. aureus towards osteoblasts.

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Extended-spectrum AmpC ␤-lactamase (ESAC) Escherichia coli producers were investigated over a 5-year period. Eleven isolates presenting a strong ampC promoter and different strategic AmpC mutations, including two newly described modifications (A292V and an L-A-A insertion at 295), were characterized. All the isolates belonged to phylogenetic group A and to the ST23 complex.In clinical Escherichia coli isolates, overexpression of the chromosomal AmpC ␤-lactamase was first largely attributed to different mutations located in strategic regions of the promoter, conferring various levels of resistance to cephalosporins (2). Structural modifications in the vicinity of the active site of the enzyme have also been recently described for extendedspectrum AmpC (ESAC) ␤-lactamases. They are characterized by increased catalytic efficiency against oxyiminocephalosporins, including cefepime and cefpirome (1, 5, 6, 7). The prevalence of ESAC-producing E. coli clinical isolates was investigated at Nantes University Hospital during the 5-year period of 2004 to 2008. Multilocus sequence types (MLST) and phylogenetic groups were determined for all the ESAC-producing isolates. A total of 41 strains (0.16% of the E. coli clinical isolates collected during the study period) were analyzed on the basis of their ␤-lactam resistance phenotypes compatible with AmpC overproduction (MIC Ն 64 mg/liter for ceftazidime using the Vitek2 system [bioMérieux] and negative double-disk synergy test). MICs of cephalosporins were determined by the Etest method. A search for several plasmidmediated AmpC ␤-lactamases (bla CMY , bla ACC , and bla DHA ) was performed under standard PCR conditions, using specific primers (8). PCR amplifications of the chromosomal ampC promoter and gene were performed with the primers AB1/ampC2 and Int-B1/Int-HN (2, 5). The nucleotide and deduced protein sequences were analyzed to search for nucleotide or amino acid replacements leading to extension of the hydrolysis spectrum.