Gram Staining and Infection Investigations Quiz

See Bacterial Shapes. Option D is correct. Bacteria are described by their shape and their arrangement. A coccus (plural cocci) is round, a bacillus (plural bacilli) is a rod, and a spirillum is spiral. Cocci can sit singly, in pairs (diplococci), in chains or in clusters. Streptococci are Gram-positive cocci that usually assemble linearly to form chains, so on a Gram stain you see purple spheres lined up like beads on a string. Staphylococci, in contrast, form irregular bunches like grapes. Arrangement is useful because several species look similar on a Gram stain, so it narrows the possibilities: purple cocci in chains point to Streptococcus, whereas purple cocci in clusters point to Staphylococcus. The pairs of cocci seen with Streptococcus pneumoniae are the commonest exception to learn, since this organism usually appears as diplococci. A memory aid in the SimpleMed article is that streps are strips (chains), and staphylococci look like a bunch of grapes.

• Streptococci: Gram-positive cocci that usually form chains.
• Staphylococci: Gram-positive cocci that form irregular clusters like grapes.
• Shapes: cocci are round, bacilli are rods and spirilla are spiral.