The Gram stain allows for recognition of the shape and pattern or arrangement of bacteria. The shapes are cocci (Sing: coccus; round), bacilli (sing: bacillus; rod-shaped), and spirilla (sing: spirillum; spiral).
Based on how the bacteria divides during replication, different patterns or arrangements maybe produced. The cellular morphology, or shapes, of the individual cells and their arrangement in pairs, chains, or clusters are useful in the identification of the bacteria .
Bacilli vary in size from very long oblong to short rods. They may even have squared off ends or one end larger than the other, forming a club. The patterns or arrangement they form are:
A. diplobacilli – pairs
B. streptobacilli - chains