Co-location of the erm(T) gene and blaROB-1 gene on a small plasmid in Haemophilus parasuis of pig origin.

Sir, Haemophilus parasuis is the causative agent of Glässer’s disease, characterized by fibrinous polyserositis, polyarthritis and meningitis in pigs, which causes considerable economic losses in the swine industry. Macrolide antibiotics inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit. Although macrolides are widely used in the treatment of Gram-positive infections, they are also of clinical relevance in the treatment of infections caused by Gram-negative bacteria such as H. parasuis. The major mechanism of resistance to macrolides is the methylation of the adenine at position A2058 in domain V of the 23S rRNA and the methylases encoded by erm genes generally confer cross-resistance to macrolide, lincosamide and streptogramin B (MLSB) antibiotics (http://faculty.washington.edu/ marilynr/). In the present study, we report that the erm(T) gene is present in a blaROB-1-positive H. parasuis isolated in China. So far, the erm(T) gene has been described only in lactobacilli, streptococci, enterococci and staphylococci, – 6 all of which are Gram-positive bacteria. A total of 145 H. parasuis isolates were isolated from pigs suffering from polyserositis, pneumonia or meningitis in China during August 2010 to July 2011. Isolates were confirmed by biochemical tests and 16S diagnostic PCR. Due to the unavailability of a CLSI-approved method for H. parasuis, susceptibility tests were conducted using the broth microdilution method according to the CLSI recommendations for Actinobacillus pleuropneumoniae. The reference strain A. pleuropneumoniae ATCC 27090 served as a quality control strain. Among the 145 H. parasuis isolates, only strain FS39 exhibited high MICs of erythromycin (64 mg/L), lincomycin (64 mg/L), penicillin (32 mg/L), amoxicillin (256 mg/L) and cefaclor (32 mg/L), but showed low MIC values of ceftiofur (,0.064 mg/L) and cefotaxime (,0.064 mg/L). Subsequently, PCR screening for the genes erm(A), erm(B), erm(C), erm(F) and mef(A), which have been reported to occur in Research letters