Molecular epidemiology of Pasteurella multocida in dairy and beef calves.

The molecular epidemiology of Pasteurella multocida has rarely been studied at the farm level in cattle. The aim of this study was to determine whether single or multiple strains of P. multocida tend to exist within farms. Molecular characterisation was carried out on isolates obtained from nasal swabs from 105 calves from 32 randomly selected beef and dairy farms located throughout Scotland, and from 131 calves from 20 farms in the Mayenne region of France, where sampling occurred in response to respiratory disease outbreaks. P. multocida isolates were characterised by random-amplified polymorphic DNA (RAPD) typing and pulsed-field gel electrophoresis (PFGE) using restriction enzyme ApaI. In addition, isolates representative of each farm/RAPD profile combination were typed by multilocus sequence typing (MLST). Among 105 Scottish isolates, 15 RAPD profiles were distinguished. The majority of farms (27/32) had indistinguishable profiles in all positive animals. Five farms had two profiles. Among 140 French isolates, 23 RAPD profiles were distinguished. More within-farm heterogeneity was observed although 10/20 farms had just one profile (E4) in sampled calves. Profile E4 accounted for 60% (84/140) of French isolates. PFGE was more discriminatory than RAPD but confirmed results with respect to within farm homogeneity or heterogeneity of strains, whereas MLST was not discriminatory enough for farm level epidemiology. As in other host species, either several strains or one dominant strain of P. multocida may exist within farms, with evidence for a role of management factors such as movements onto the farm in the number of strains detected.

Genotypic and phenotypic detection of macrolide and lincosamide resistance in Streptococcus uberis.

Streptococcus uberis isolates (n = 55) were obtained from milk samples of cases of mild clinical mastitis in 55 dairy cows from 35 herds serviced by one veterinary practice in Mayenne, France. Isolates were tested for macrolide and lincosamide resistance by using phenotypic and genotypic methods. Erythromycin resistance was detected in 12 of the 55 (22%) isolates based on agar disc diffusion testing and MIC measurements, and was encoded by ermB. This gene also conferred phenotypic resistance to pirlimycin based on MIC measurements, but the D-test was needed for detection of the resistance phenotype in the agar disc diffusion test. Isolates with ermB were also highly resistant to the macrolide antibiotic spiramycin. Seventeen of the 55 isolates (31%) were classified as resistant to spiramcyin only and as having intermediate susceptibility to spiramycin based on agar disc diffusion testing and MIC measurements, respectively. The genetic mechanism behind this phenotype and its clinical relevance are unknown. The efflux pump gene mefA was not detected in any of the 55 isolates in this study. Pirlimycin resistance without macrolide resistance was encoded by the lincosamide resistance gene linB in 4 isolates. Based on current guidelines, some linB-positive isolates would be classified as susceptible by using phenotypic tests, and alternative values for the interpretation of the agar disc diffusion test are suggested. We conclude that the agar disc diffusion test is a useful indicator for macrolide and lincosamide resistance of Strep. uberis in veterinary practice, provided that the D-test is used for detection of pirlimycin resistance.