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Electron micrograph of GAS strain A-20 entry into cultured A-549 cells. Monolayers were infected with 5 × 107 CFU for 120 min before they were washed and exposed to penicillin. A bacterium (B) is enclosed in a vacuole (V). The nucleus (N) and microvilli (MV) are indicated. Magnification, ×10,000.
