Experimentally, the spirochete can penetrate human fibroblasts and live intracellularly, even when the extracellular medium contains ceftriaxone at concentrations well above bacteriocidal levels. Although intracellular organisms have never been demonstrated in vivo, this may be one mechanism by which the organism eludes host defense mechanisms. Clinically, B burgdorferi has been cultured from skin lesions of patients with acrodermatitis chronica atrophicans 10 years after initial infection.