A 32-year-old mother on postpartum day 10 presents with a painful, red, swollen area in the upper outer quadrant of her left breast. She has fever (38.8°C), chills, and malaise. She is exclusively breastfeeding. What is the most common causative organism of her mastitis?

Explanation

## Most Common Causative Organism of Postpartum Mastitis ### Epidemiology of Lactational Mastitis **Key Point:** Staphylococcus aureus is the most common causative organism of postpartum (lactational) mastitis, accounting for 40–50% of all cases. It is a normal skin commensal that enters the breast through cracked nipples or via the infant's oral flora. ### Pathogenesis of S. aureus Mastitis 1. **Portal of entry:** Damaged nipple epithelium (fissures, abrasions) or ascending infection from infant's mouth 2. **Virulence factors:** Produces alpha-toxin, protein A, and other exotoxins that cause tissue inflammation and abscess formation 3. **Milk stasis:** Incomplete milk removal creates an ideal environment for bacterial proliferation 4. **Biofilm formation:** S. aureus can form biofilms on breast tissue, making antibiotic penetration difficult ### Comparison of Common Causative Organisms | Organism | Frequency | Clinical Features | Antibiotic Resistance | Notes | |----------|-----------|-------------------|----------------------|-------| | **S. aureus** | 40–50% | Acute onset, high fever, localized abscess | MRSA emerging | Most common; can cause abscess | | **Streptococcus agalactiae (GBS)** | 10–15% | Subacute presentation, less severe | Penicillin-sensitive | Associated with neonatal infection | | **E. coli** | 5–10% | Often with systemic toxicity | Variable | Associated with fecal contamination | | **Candida albicans** | 5–10% | Chronic, burning pain, white patches | Antifungal-resistant strains rare | Rare in acute mastitis; more common in thrush | ### Clinical Presentation of S. aureus Mastitis **High-Yield:** Classic presentation includes: - **Unilateral** localized erythema and induration (often upper outer quadrant) - **Acute onset** (typically day 5–28 postpartum) - **High fever** (≥38.5°C) with chills and malaise - **Wedge-shaped** area of involvement - **Possible abscess** formation with fluctuance (in 5–11% of cases) ### Management Approach **Mnemonic: BREAST** — **B**reast culture before antibiotics, **R**est and support, **E**mpty breast frequently, **A**ntibiotics (anti-staphylococcal), **S**upport and NSAIDs, **T**herapy continuation for 10–14 days. 1. **Culture:** Obtain breast milk culture (gold standard) before starting antibiotics 2. **First-line antibiotics:** Dicloxacillin or cephalexin (covers S. aureus) 3. **Continue breastfeeding:** Safe for infant; helps drain infected milk 4. **Supportive care:** Warm compresses, frequent emptying, NSAIDs 5. **Monitor for abscess:** If no improvement in 48–72 hours despite antibiotics, ultrasound is indicated **Clinical Pearl:** Breastfeeding should continue during mastitis treatment. The infant is not at risk from the infection, and continued milk removal is essential for resolution. Antibiotics are safe during breastfeeding. ### Why S. aureus Is Most Common S. aureus is ubiquitous on skin and in the infant's oral flora. It readily colonizes damaged nipple epithelium and produces potent toxins that cause acute inflammation. Its prevalence has increased with the emergence of community-acquired MRSA (CA-MRSA), making empiric coverage important in high-prevalence regions. [cite:Williams Obstetrics 26e Ch 37; Harrison 21e Ch 129]