Objectives

The aim of this study was to determine the mechanisms and molecular characterisation of one strain (HPS412) of Haemophilus parasuis, which exhibited high MICs of antimicrobial susceptibility.

Methods

A total of 113 H. parasuis strains isolated from pigs suffering from polyserositis, pneumonia or meningitis in China and screened them for antimicrobial susceptibility. Susceptibility testing of the minimal inhibitory concentrations (MIC) was determined in fastidious medium consisting of tryptone soya broth (TSB) containing 5% bovine serum and 10 μg/mL NAD in 96-well microtiter plates. The genomic DNA was completely sequenced by combining PacBio RS II and Illumina HiSeq 4000 platforms. Gene prediction was performed using Glimmer v.3.02 with Hidden Markov models.

Results

One strain (HPS412) exhibited high MICs of sulfamethoxazole (256 μg/mL), tetracycline (128 μg/mL), streptomycin (128 μg/mL), gentamicin (128 μg/mL), amoxicillin (128 μg/mL), chloramphenicol (64 μg/mL), penicillin (64 μg/mL) and cefaclor (64 μg/mL). Sequence analysis showed that numerous drug-resistance genes including tet(B), bla_ROB-1, sul2, catIII, aph(3″)-Ib, aph(6)-Id and aph(3′)-Ia were present in a chromosomal gene island as adjacent duplicate copies and the rep-orf3-bla_ROB-1 structure most likely had a direct plasmid origin. The tet(B) and bla_ROB-1 were flanked on one or both by ISApl1 elements.

Conclusions

The acquisition of bla_ROB-1 and the other antibiotic resistance genes was related to the presence of ISApl1. ISApl1 plays important roles in the horizontal transmission of antibiotic resistance genes.