Research report: Antibiotic susceptibility of mastitis pathogens in Finnish dairy cows in 2022–2023: concerning changes in the penicillin susceptibility of Streptococcus uberis

In a comprehensive study published in Acta Veterinaria Scandinavica, researchers reported a concerning shift in the antibiotic susceptibility profiles of common mastitis pathogens in Finnish dairy cows. The investigation, which analyzed milk specimens collected between December 2021 and July 2023, revealed that while overall antimicrobial resistance remained relatively low, the proportion of Streptococcus uberis isolates classified as non-wild type (NWT) to penicillin had increased significantly over the past decade. This finding stands out against a backdrop of generally favorable susceptibility results for other major bacterial species, highlighting a specific area of concern for veterinary antibiotic stewardship in the Nordic region.

The study aimed to assess the current state of antibiotic resistance among the most common bacterial causes of intramammary infections (IMI) in Finland, a country where multiplex PCR assays have largely replaced traditional bacterial culture for routine diagnosis. Because these rapid PCR tests do not provide information on antibiotic susceptibility, there has been a lack of recent data on antimicrobial resistance (AMR) in Finnish mastitis pathogens. The research team addressed this gap by combining PCR screening with traditional culture and susceptibility testing, allowing for a detailed evaluation of resistance mechanisms and minimum inhibitory concentrations (MICs) for a representative sample of farms.

Sampling was conducted across 220 dairy farms, from which 502 bacterial isolates were successfully cultured and tested. The most prevalent species identified was Staphylococcus aureus, followed by non-aureus staphylococci (NAS), S. uberis, Streptococcus dysgalactiae, and Escherichia coli. The isolates were tested using the broth microdilution method, with results categorized as wild type (WT) or non-wild type (NWT) based on epidemiological cut-off values (ECOFFs) defined by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). This approach allows for the identification of acquired resistance mechanisms, although it is important to note that ECOFFs do not directly define clinical resistance or treatment failure.

For Staphylococcus aureus, the results indicated a continued decline in penicillin resistance, with only 4% of isolates producing beta-lactamase. This is a marked decrease from previous studies in Finland, which reported resistance rates of approximately 25% in 2005 and 2012. No methicillin-resistant S. aureus (MRSA) isolates were detected, and all isolates were classified as WT to cefoxitin and oxacillin. In contrast, beta-lactamase production among NAS isolates was more common, with 38% of isolates testing positive. This proportion varied significantly by species, with S. epidermidis showing the highest rate at 68%, while other species such as S. chromogenes and S. simulans showed no resistance in this specific cohort.

The most notable finding of the study was the high proportion of S. uberis isolates classified as NWT to penicillin, reaching 42% (95% CI: 31.5–52.8). This represents a substantial increase from the 4% reported in 2012, suggesting a rapid evolution of resistance in this pathogen. The researchers noted that a tentative ECOFF was used for penicillin and S. uberis, which introduces some uncertainty into the interpretation of these results. However, the trend aligns with observations from Denmark, where NWT proportions for S. uberis also rose sharply between 2016 and 2021. In comparison, Norway reported a much lower proportion of NWT S. uberis isolates in 2020–2021, highlighting significant geographical variation in resistance patterns.

For Gram-negative bacteria, including E. coli and Klebsiella species, resistance levels remained generally low. All E. coli isolates were classified as WT to fluoroquinolones, and no extended-spectrum beta-lactamase (ESBL)-producing isolates were detected. Two Klebsiella pneumoniae isolates with elevated colistin MICs were subjected to whole-genome sequencing to investigate potential resistance mechanisms. While no mobile colistin resistance (mcr) genes were found, the isolates harbored other acquired resistance genes and chromosomal point mutations, underscoring the importance of continued monitoring even for antibiotics that are banned in veterinary medicine in Finland.

The study also highlighted the limitations of relying solely on PCR for mastitis diagnosis. In 16 cases, the PathoProof™ PCR test detected the blaZ gene, which is associated with penicillin resistance in staphylococci, but the cultured isolates were negative for this gene and did not produce beta-lactamase. This discrepancy suggests that the PCR test may detect genetic material from low-abundance bacteria or remnants of previous infections, potentially leading to overestimation of resistance if used in isolation. The researchers emphasized the need for periodic culture and susceptibility testing to complement PCR results and maintain an accurate picture of AMR trends.

Despite the concerning rise in penicillin NWT S. uberis, the overall antibiotic susceptibility among mastitis pathogens in Finland remains favorable. However, the authors stressed that the clinical relevance of the elevated penicillin MICs in S. uberis remains unclear due to a lack of intramammary pharmacokinetic-pharmacodynamic data. Further investigations are needed to determine whether these MIC changes impact the efficacy of penicillin therapy in clinical settings.

The findings of this study underscore the importance of continued surveillance of antimicrobial resistance in dairy cattle. As PCR-based diagnostics become more widespread, the risk of missing emerging resistance phenotypes increases. The researchers recommended that regular susceptibility testing be integrated into national monitoring programs to guide policy decisions and treatment recommendations. By maintaining a close watch on the evolving resistance profiles of mastitis pathogens, the dairy industry can work to minimize antibiotic use and preserve the effectiveness of essential veterinary medicines.

Subject of Research: Antibiotic susceptibility of mastitis pathogens in Finnish dairy cows in 2022–2023: concerning changes in the penicillin susceptibility of Streptococcus uberis

Article Title: Antibiotic susceptibility of mastitis pathogens in Finnish dairy cows in 2022–2023: concerning changes in the penicillin susceptibility of Streptococcus uberis

Article References: Nykäsenoja, S., Grönthal, T., Rajala-Schultz, P., & Autio, T. (2026). Antibiotic susceptibility of mastitis pathogens in Finnish dairy cows in 2022–2023: concerning changes in the penicillin susceptibility of Streptococcus uberis. Acta Veterinaria Scandinavica, 68(1), Article 42. https://doi.org/10.1186/s13028-026-00875-4

Image Credits: AI Generated

DOI: 10.1186/s13028-026-00875-4

Keywords: Antibiotic, susceptibility, mastitis, pathogens, Finnish, dairy, cows, concerning, changes, penicillin, Streptococcus, uberis