A 32-year-old multiparous woman on postpartum day 8 develops unilateral breast pain, erythema, and a palpable tender lump. She has fever (38.8°C) and feels systemically unwell. She is exclusively breastfeeding. What is the most common causative organism of her condition?

Explanation

## Microbiology of Postpartum Mastitis **Key Point:** Staphylococcus aureus is the most common causative organism of infectious mastitis in lactating women, accounting for 40–50% of cases. ### Pathogenesis of Mastitis 1. **Source of infection:** Skin flora of mother or baby's oral flora 2. **Entry route:** Cracked nipples, poor latch, incomplete breast emptying 3. **Pathophysiology:** Milk stasis → bacterial proliferation → inflammation and abscess formation ### Causative Organisms in Postpartum Mastitis | Organism | Frequency | Source | Clinical Notes | |----------|-----------|--------|----------------| | **Staphylococcus aureus** | 40–50% | Skin flora | Most common; can cause abscess; may be MRSA | | **Streptococcus agalactiae (GBS)** | 10–15% | Maternal GIT/vaginal flora | Associated with maternal GBS colonization | | **Escherichia coli** | 5–10% | Maternal GIT; fecal contamination | Less common; often in immunocompromised | | **Candida albicans** | Rare (<5%) | Baby's oral thrush or maternal skin | Causes painful, burning sensation; white patches | **Mnemonic:** **SAC** — **S**taphylococcus aureus (most common), **A**lternatives (GBS, E. coli), **C**andida (rare, fungal) **High-Yield:** Staphylococcus aureus is the causative organism in the majority of cases of infectious mastitis. It is a gram-positive coccus that colonizes the skin and can enter via cracked nipples or poor latch. MRSA is an emerging concern in some regions. **Clinical Pearl:** If mastitis does not respond to standard antibiotics (dicloxacillin or cephalexin) within 48–72 hours, consider MRSA or obtain culture. Empiric coverage for MRSA (clindamycin or trimethoprim-sulfamethoxazole) may be warranted in high-risk populations.