In bacteriology, the region where the prokaryotic chromosome is concentrated (without a surrounding membrane) is called the:

Introduction / Context:
Unlike eukaryotes, bacteria do not possess a membrane-bound nucleus. Instead, their chromosome resides in a distinct but non-membrane-bound region. Recognizing the correct term for this region is fundamental vocabulary in microbiology.

Given Data / Assumptions:

• Prokaryotes lack a nuclear envelope.
• The bacterial genome is typically a single, circular DNA molecule associated with nucleoid-associated proteins.
• We must choose the standard term used in textbooks and research.

Concept / Approach:
The correct term is nucleoid. It describes the DNA-dense region in the cytoplasm where the chromosome is found. It is functionally analogous to a nucleus in concentrating genetic material, but it is not enclosed by a membrane and has a different organization.

Step-by-Step Solution:

Verification / Alternative check:
Electron micrographs and fluorescence imaging show a DNA-dense area without a surrounding membrane; molecular studies refer to it as the nucleoid.

Why Other Options Are Wrong:
Nucleus: Requires a nuclear membrane; absent in bacteria. Nuclein / nucleose: Historical or chemical terms, not the structural region in modern usage.

Common Pitfalls:
Equating DNA presence with a nucleus; in prokaryotes, genomic DNA is organized differently.

Final Answer:
nucleoid