Topic overview

Start with the big picture

Bacteria occur in several basic shapes, including cocci, rods, coccobacilli, curved rods, spirilla, spirochetes, and filamentous forms. Their arrangements can offer additional clues, such as cocci in chains, clusters, pairs, or tetrads, and rods in palisades. Morphology also includes differences in size and cell envelopes: Gram-positive and Gram-negative bacteria stain differently because their wall structures differ, while acid-fast and cell wall–deficient bacteria have distinctive features. Specialized structures add further context, including endospores, capsules, slime layers, flagella, pili, and inclusion bodies. Recognizing these patterns helps connect microscopic appearance with bacterial classification and relevant biological functions.