Once the stain is developed, three observations are read together: the Gram reaction, the cell shape, and how the cells are grouped. Shape is described as coccus (round) or bacillus (rod), and arrangement reflects how the cells divide and stay attached. Cocci in irregular clusters point toward staphylococci, cocci in chains toward streptococci, and cocci in pairs toward organisms such as Neisseria. Rods may appear as single cells, in chains, or in branching filaments. These patterns are not species-specific, but they sharply narrow the field and tell the laboratory which culture conditions are worth setting up.
The second half of the reading is the host side of the smear. Neutrophils (polymorphonuclear leukocytes) are the inflammatory cells recruited to a site of bacterial infection, so a field crowded with neutrophils and organisms is strong evidence of true infection rather than surface colonization. Squamous epithelial cells, by contrast, are shed from skin and mucous membranes, and their presence signals that the specimen was contaminated during collection. The quantity of organisms matters as well: seeing many organisms in a normally sterile fluid such as cerebrospinal fluid carries far more weight than seeing a few organisms in sputum, where a resident population is expected. Reading the smear therefore means combining the organism's appearance with the evidence that the host is actually responding to it.