When Antimicrobial Susceptibility Results Change: A Practical Look at Caprine Mastitis

Mastitis is an important disease in dairy goats and can lead to reduced milk yield, poor milk quality, early removal of animals, and increased treatment costs¹. Subclinical mastitis is particularly common in goats. Although milk may appear normal and there may be no obvious mammary abnormalities, subclinical infection can progress to clinical disease or act as a reservoir of pathogens for healthy animals². 

Staphylococci are frequently associated with caprine mastitis, with Staphylococcus aureus linked to clinical disease and coagulase-negative Staphylococci (CNS) commonly associated with subclinical mastitis. Streptococcus spp. are also important causes of clinical and subclinical mastitis in goats^3,4,5. 

For veterinarians managing these infections, antimicrobial susceptibility results can provide useful information. However, the conditions used to determine susceptibility can influence the observed antimicrobial activity. 

Why the Testing Environment Matters⁵ 

Minimum inhibitory concentration (MIC) testing identifies the lowest antimicrobial concentration that completely inhibits bacterial growth. The work reported in this paper compared standard Clinical and Laboratory Standards Institute (CLSI) conditions with a modified approach in which the culture medium was supplemented with 25% goat serum. 

A total of 107 mastitis isolates were evaluated: 39 CNS, 37 S. aureus, 11 Streptococcus spp., 10 Escherichia coli, and 10 Mycoplasma agalactiae. The antimicrobials evaluated were tildipirosin, gamithromycin, oxytetracycline, and danofloxacin. 

The results showed appreciable changes in MIC values after goat serum was added, particularly for tildipirosin, gamithromycin, and oxytetracycline against several pathogens. Gamithromycin and tildipirosin showed reduced MIC50 values against CNS and S. aureus with serum supplementation. 

For M. agalactiae, serum supplementation produced a marked reduction in the MIC90 of gamithromycin and tildipirosin. The MIC90 decreased by approximately five log2 dilutions, from greater than 16 µg/ml under standard conditions to 0.5 µg/ml after serum supplementation. 

What the Pathogen-Specific Findings Showed⁵ 

Danofloxacin demonstrated the lowest MIC50 and MIC90 values against E. coli under both standard conditions and after goat serum supplementation. Serum addition also reduced the MIC90 of tildipirosin and gamithromycin against E. coli. 

For M. agalactiae, danofloxacin and oxytetracycline showed the lowest MIC90 values under standard conditions, both at 0.5 µg/ml. After adding 25% goat serum, the MIC90 values became more similar across the four antimicrobials, with values of 0.5 µg/ml for tildipirosin and gamithromycin and 0.25 µg/ml for danofloxacin and oxytetracycline. 

The findings for S. aureus were different. The tested strains showed high MIC90 values for tildipirosin, gamithromycin, and oxytetracycline, while danofloxacin produced a MIC90 of 2 µg/ml. 

Practical Clinical Insights 

For veterinarians interpreting antimicrobial susceptibility results in caprine mastitis, the testing environment deserves attention. The findings indicate that artificial culture media may produce different MIC values from media containing biological components such as goat serum. The results were particularly notable for the macrolides, whose MICs were markedly lower against several pathogens when serum was present. 

This is relevant when considering how closely an in-vitro susceptibility result reflects conditions within an infected mammary gland. The findings also reinforce that in-vitro sensitivity does not always correspond directly with treatment effectiveness under field conditions. 

The work concluded that MICs for antibiotics used against mastitis should be determined under conditions that more closely resemble intramammary infections to obtain representative susceptibility patterns. Under standard incubation conditions, caprine mastitis pathogens were broadly susceptible to danofloxacin, while the potency of macrolides against susceptible pathogens increased when serum was present in the culture medium. 

References 

1. Gelasakis AI, Mavrogianni VS, Petridis IG, Vasileiou NG, Fthenakis GC. Mastitis in sheep–The last 10 years and the future of research. Veterinary microbiology. 2015 Dec 14;181(1-2):136-46. https://www.researchgate.net/profile/Athanasios-Gelasakis/publication/280164574 

2. Persson Y, Olofsson I. Direct and indirect measurement of somatic cell count as indicator of intramammary infection in dairy goats. Acta Veterinaria Scandinavica. 2011 Mar 4;53(1):15. https://link.springer.com/content/pdf/10.1186/1751-0147-53-15.pdf 

3. Virdis S, Scarano C, Cossu F, Spanu V, Spanu C, De Santis EP. Antibiotic resistance in Staphylococcus aureus and coagulase negative staphylococci isolated from goats with subclinical mastitis. Veterinary medicine international. 2010;2010(1):517060. https://onlinelibrary.wiley.com/doi/pdf/10.4061/2010/517060 

4. Mishra AK, Sharma N, Kumar A, Kumar N, Bayyappa MG, Kumar S. Isolation, characterization and therapeutic potential assessment of bacteriophages virulent to Staphylococcus aureus associated with goat mastitis. Iranian journal of veterinary research. 2014;15(4):320. https://pmc.ncbi.nlm.nih.gov/articles/PMC4789206/pdf/ijvr-15-320.pdf 

5. Galecio JS, Escudero E, Corrales JC, García-Romero E, De la Fe C, Hernandis V, Marin P. Susceptibility of caprine mastitis pathogens to tildipirosin, gamithromycin, oxytetracycline, and danofloxacin: effect of serum on the in vitro potency of current macrolides. World Journal of Microbiology and Biotechnology. 2022 Dec;38(12):221. https://link.springer.com/content/pdf/10.1007/s11274-022-03407-0.pdf