Hataraku Saikin

Cells are not the only things at work in your body! Deep in your gut, a war rages...between good bacteria, bad bacteria, and opportunistic pathogens—and occasionally, against outside invaders! And if bitter rivals Clostridium Perfringens, leader of the bad bacteria, and Bifidum Bacteria, leader of the good, get their way, the battle for intestinal supremacy and safety will never end!

(Source: Kodansha USA)