Plasmid-encoded tet B tetracycline resistance in Haemophilus parasuis

The complete sequence of two plasmids, pHS-Tet (5.1 kb) and pHS-Rec (9.5 kb), isolated from Haemophilus parasuis strain HS1543 has been obtained. Plasmid pHS-Tet contains four open reading frames including a tet(B) tetracycline resistance gene which unusually did not have an associated tetR repressor gene. From a total of 45 H. parasuis isolates surveyed (15 international reference strains, 15 field isolates selected for their genetic diversity, and 15 recent Australian field isolates), 2 tetracycline-resistant field isolates (HS226 and HS1859) were identified. Analysis of three additional isolates from the same disease outbreak as strain HS1859 revealed a further tetracycline-resistant H. parasuis strain (HS1857, serovar 8) and a tetracycline-resistant Actinobacillus pleuropneumoniae strain (HS1861). An approximately 10.6-kb plasmid was identified in field isolate HS226 and outbreak strains HS1857, HS1859, and HS1861. Southern hybridization analysis of these plasmids showed that the Tet B determinant was present, and restriction digest comparisons suggest that these plasmids are related. This is believed to be the first report of native H. parasuis plasmids and Tet B-mediated tetracycline resistance in this microorganism.