Basic microbiology — Agrobacterium tumefaciens, the agent of crown gall disease and a key tool in plant genetic engineering, is classified as what type of microorganism?

Introduction / Context:
Agrobacterium tumefaciens transfers T-DNA to plant cells, causing crown gall tumors and enabling engineering of transgenic plants. Understanding its microbial classification underpins choices of antibiotics, culture conditions, and vector design in the lab.

Given Data / Assumptions:

• Agrobacterium belongs to Alphaproteobacteria.
• Gram reaction influences outer membrane characteristics and sensitivity to certain drugs.
• It is a bacterium, not a eukaryote or virus.

Concept / Approach:
Agrobacterium tumefaciens (now also referred to within the Rhizobium/Agrobacterium complex) is Gram-negative, possessing an outer membrane and thin peptidoglycan layer. This aligns with its staining properties and genetic toolkits used for binary vector systems.

Step-by-Step Solution:

Verification / Alternative check:
Standard microbiology references and lab protocols list Agrobacterium as Gram-negative rods growing on specific media like LB or YEB.

Why Other Options Are Wrong:

• Gram-positive: would show thick peptidoglycan and lack an outer membrane, which is incorrect here.
• Fungi/yeast/virus: wrong domains and biology.

Common Pitfalls:
Confusing plant pathogens broadly with fungi; Agrobacterium is bacterial.

Final Answer:
gram (-) bacteria