Spirillum in microbiology refers to a bacterium with a cell body that twists like a spiral. It is the third distinct bacterial cell shape type besides coccus and bacillus cells. Spirillum is a genus of gram-negative bacteria comprising elongated forms having tufts of flagella at both poles and usually living in stagnant water rich in organic matter. They are spirally twisted, and aerobic, certain species are pathogenic for humans. It is the type genus for the family Spirilliaceae in some bacterial classifications. The word spirilla comes form the Greek word spirochaete which means "spiral-sheet".