Bacterial arrangements: the cocci that characteristically form chains are called:

Introduction / Context:
Microscopic arrangement is a quick clue to bacterial identity. Cocci can arrange as pairs, chains, tetrads, or clusters depending on division planes and post-division adhesion.

Given Data / Assumptions:

• Pairs of cocci are termed diplococci.
• Chains of cocci are termed streptococci.
• Grape-like clusters are termed staphylococci; groups of four are tetracocci (tetrads).

Concept / Approach:
Strepto- derives from Greek for twisted chain and refers to the chain-like arrangement formed after repeated division in a single plane with continued adherence of daughter cells.

Step-by-Step Solution:

Verification / Alternative check:
Classic examples include Streptococcus pyogenes in chain formations on Gram stain.

Why Other Options Are Wrong:
Diplococci: Pairs, not chains. Staphylococci: Clusters, not chains. Tetracocci: Packets of four.

Common Pitfalls:
Confusing streptococci (chains) with staphylococci (clusters) due to similar-sounding names.

Final Answer:
Streptococci