The molecular mechanisms of resistance to fluoroquinolones, tetracyclines, an aminocyclitol, macrolides, a lincosamide, a phenicol, and pleuromutilins were investigated in Mycoplasma bovis . For the identification of mutations responsible for the high MICs of certain antibiotics, whole-genome sequencing of 35 M. bovis field isolates and 36 laboratory-derived antibiotic-resistant mutants was performed. In vitro resistant mutants were selected by serial passages of M. bovis in broth medium containing subinhibitory concentrations of the antibiotics. Mutations associated with high fluoroquinolones MICs were found at positions 244 to 260 and at positions 232 to 250 (according to Escherichia coli numbering) of the quinolone resistance-determining regions of the gyrA and parC genes, respectively. Alterations related to elevated tetracycline MICs were described at positions 962 to 967, 1058, 1195, 1196, and 1199 of genes encoding the 16S rRNA and forming the primary tetracycline binding site. Single transversion at position 1192 of the rrs1 gene resulted in a spectinomycin MIC of 256 μg/ml. Mutations responsible for high macrolide, lincomycin, florfenicol, and pleuromutilin antibiotic MICs were identified in genes encoding 23S rRNA. Understanding antibiotic resistance mechanisms is an important tool for future developments of genetic-based diagnostic assays for the rapid detection of resistant M. bovis strains.

The origins and relationships of Newcastle disease virus (NDV) vaccine strains Hertfordshire OI) andMukteswar, and the virulent Herts'33 were studied using partial sequence analysis of the fusion protein gene.The mesogenic strain H was obtained by egg passages of a field virus isolated in England in 1933 (later knownas Herts'33). Different lines of the strain Herts'33, however, divided into two distinct groups: genotype IV, and ahitherto undescribed lineage, which comprised the Weybridge line (Herts'33/56). Vaccine strain H and the twoclusters comprising viruses designated Herts'33 displayed 6.5 to 6.8% and 15.6 to 16.3% mutational distances,respectively, which precluded parent-offspring relationships with either of them. In contrast, the different linesof the vaccine strain Mukteswar, which was reportedly derived from an Indian field isolate in the mid-1940s,showed 98.9 to 100% sequence similarity to strain H. It is therefore probable that the two vaccines were derivedfrom the same virus stock.