Bacteria are grouped in a number of different ways. Most bacteria are of one of three typical shapes—rod-shaped (bacillus), round (coccus, e.g., streptococcus), and spiral (spirillum). An additional group, vibrios, appear as incomplete spirals. The number and shape of the mitochondria, as well as the numbers of cristae they have, can differ widely from cell type to cell type. Even within one type of tissue, the shape of the mitochondria can vary depending on their functional status. Mitochondria are mobile, plastic organelles.