Which antimicrobial is most appropriate for mastitis based on pathogen — Staph aureus vs Strep vs gram-negative?

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For gram-positive mastitis, antimicrobial choice hinges on whether the pathogen is a staphylococcus or streptococcus; gram-negative mastitis often resolves without antimicrobials. --- Staphylococcus aureus is best treated with penicillin-based or cephalosporin-based intramammary therapy when susceptibility is confirmed. In North America, resistance among S. aureus isolates remains very low across all commonly used intramammary drugs: penicillin-novobiocin (0% resistance), ceftiofur (0.1%), oxacillin (0.2%), erythromycin (0.7%), cefoperazone (1.2%), and pirlimycin (2.8%).13 In southeastern Australia, antimicrobial resistance among Staph. aureus is similarly uncommon despite decades of use.1 The exception is large Chinese dairy herds, where Staph. aureus resistance to penicillin reaches 66% and to tetracycline 59%, making empirical penicillin unreliable in that context.2 Subclinical Staph. aureus isolates carry a higher probability of penicillin resistance than clinical isolates — 5.16 times higher for penicillin and 4.70 times higher for amoxicillin — so culture and susceptibility testing is especially important before treating subclinical cases.1 Methicillin-resistant Staph. aureus (MRSA) is rare in North American dairy cattle (0.2% of isolates with oxacillin MIC ≥4 µg/mL) and has not increased substantially between 2002 and 2022.13 When MRSA is suspected, oxacillin or cefoxitin disk diffusion is the appropriate phenotypic confirmatory test.6 First-generation cephalosporins or amoxicillin-clavulanate are preferred over third- and fourth-generation cephalosporins for staphylococcal mastitis; use of third-generation (cefoperazone, ceftiofur) and fourth-generation (cefquinome) cephalosporins should be reserved for gram-negative infections resistant to non-critically important antimicrobials.6 Streptococcal mastitis is reliably treated with beta-lactams in North America and Australia. Streptococcus dysgalactiae resistance to penicillin-novobiocin is 0%, to ceftiofur 0.1