Bacilli are rod-shaped when viewed under a microscope.
球菌 describes round, spherical bacteria, whereas 桿菌 describes rod-shaped bacteria—the two main morphological categories of bacteria.
Standard kanji spelling for 'bacillus'.
Variant kanji form; 杆 is a simplified variant of 桿.
From 桿 (rod, shaft) + 菌 (bacterium, fungus). The kanji 桿 is used for its meaning of a rod-like shape.