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 Welch bacillus, leader of the bad bacteria, and Bifidobacterium, leader of the good, get their way, the battle for intestinal supremacy and safety will never end!
(Source: Kodansha USA)
Note: Chapter count includes 2 extra chapters.