Borrelia bacteria, coloured scanning electron micrograph (SEM). These bacteria (Borrelia sp.) are pathogenic, causing diseases in humans and in animals. They are transmitted by arthropods such as ticks. Lyme disease and relapsing fever are two diseases that are caused by Borrelia bacteria. The bacteria are long, coiling rope-like structures. Magnification: x3500 when printed 10cm high.