Tn 7860 , a novel chromosomal pseudo-compound transposon carrying the OptrA gene in Enterococcus faecalis

Linezolid is a last-resort treatment for infections caused by multidrug-resistant Gram-positive bacteria, particularly in severe cases involving VRE. However, resistance to linezolid has been gradually increasing, primarily due to indirect co-selection imposed by the use of florfenicol in intensive livestock farming.1,2 Currently, eight different acquired resistance genes have been identified, including cfr, its cfr-like variants, optrA, poxtA and poxtA2.1,2 In particular, the optrA gene codes for an ABC-F protein, leading to a decreased susceptibility to phenicols and oxazolidinones (including tedizolid) by a ribosomal protection mechanism.1,2 Several studies indicate that the optrA gene is the most widespread determinant responsible for linezolid resistance in enterococci of human and animal origin.1,2 It is well-known that in most bacterial genera, the optrA genetic contexts are located on conjugative plasmids or, less frequently, on chromosomal transposons or pseudo-compound transposons, such as Tn6674, Tn6823, Tn6261, Tn7363 and Tn7695.2–8 This study was carried out in a slaughterhouse located in Central Italy, with a capacity of 2500 pigs per day, sampling caecal contents of slaughtered pigs. Caecal content samples were inoculated into buffered peptone water, homogenized and incubated at 37°C for 24 h. Subsequently, 10 µL of the cultures was plated into the Slanetz–Bartley agar supplemented with florfenicol (10 mg/L) and incubated at 37°C for 48 h. Selected colonies were identified using MALDI-TOF mass spectrometry (MALDI Biotyper, Bruker Daltonics, Bremen, Germany). The enterococcal isolates were tested by PCR for the presence of cfr, optrA and poxtA genes.9 Interestingly, one Enterococcus faecalis isolate (named V2548) was optrA positive. Susceptibility testing, carried out by the broth microdilution method and interpreted according to the CLSI clinical breakpoints (https://em100.edaptivedocs.net/GetDoc.aspx?doc=CLSI%20M100%20ED35:2025&scope=ser), revealed that E. faecalis V2548 was nonsusceptible to tedizolid (MIC 1 mg/L), resistant to florfenicol and chloramphenicol (MICs both 64 mg/L), intermediate to linezolid (MIC 4 mg/L) and erythromycin (MIC 1 mg/L) and susceptible to vancomycin and tetracycline (MICs both 1 mg/L).