Prediction of penicillin resistance in Staphylococcus aureus isolates from dairy cows with mastitis, based on prior test results

Abstract

AIM: To gauge how well prior laboratory test results predict in vitro penicillin resistance of Staphylococcus aureus isolates from dairy cows with mastitis.

METHODS: Population-based data on the farm of origin (n=79), genotype based on pulsed-field gel electrophoresis (PFGE) results, and the penicillin-resistance status of Staph. aureus isolates (n=115) from milk samples collected from dairy cows with mastitis submitted to two diagnostic laboratories over a 6-month period were used. Data were mined stochastically using the all-possible-pairs method, binomial modelling and bootstrap simulation, to test whether prior test results enhance the accuracy of prediction of penicillin resistance on farms.

RESULTS: Of all Staph. aureus isolates tested, 38% were penicillin resistant. A significant aggregation of penicillin-resistance status was evident within farms. The probability of random pairs of isolates from the same farm having the same penicillin-resistance status was 76%, compared with 53% for random pairings of samples across all farms. Thus, the resistance status of randomly selected isolates was 1.43 times more likely to correctly predict the status of other isolates from the same farm than the random population pairwise concordance probability (p=0.011). This effect was likely due to the clonal relationship of isolates within farms, as the predictive fraction attributable to prior test results was close to nil when the effect of within-farm clonal infections was withdrawn from the model.

CONCLUSIONS: Knowledge of the penicillin-resistance status of a prior Staph. aureus isolate significantly enhanced the predictive capability of other isolates from the same farm. In the time and space frame of this study, clinicians using previous information from a farm would have more accurately predicted the penicillin-resistance status of an isolate than they would by chance alone on farms infected with clonal Staph. aureus isolates, but not on farms infected with highly genetically heterogeneous bacterial strains.

KEY WORDS:

• Penicillin
• resistance
• Staphylococcus aureus
• bovine
• mastitis

Acknowledgements

We thank Graham Young and Liz Burrows from Gribbles Diagnostic (New Zealand) Ltd for the Staph. aureus isolates, Megan Leyland for the PFGE, and Massey University for funding this work. MIC results are part of Mr Martin Situmbeko's Masters Degree project at Massey University, under the supervision of A Grinberg. This paper was significantly enhanced by the contributions of the reviewers and editors.

Notes

¹ H Heffernan, Institute of Environmental Science and Research Limited, Porirua, New Zealand