In Jain Dharma, mushrooms are avoided not because of whether the visible “plant” is harmed, but because of the level of life and mode of existence involved.

Key Jain points:

• Mushrooms are considered non-vegetable, earthy, and impurity-associated foods in traditional Jain dietary discipline.
• They are often regarded as samurchima jiva-related or as foods that arise in damp, decaying, low-vitality environments, which makes them unsuitable for a Jain diet.
• Jain ahimsa is not only about avoiding obvious injury to a large organism. It also aims to minimize harm to subtle life forms and avoid foods linked with higher violence or impurity.
• The fact that you can disperse spores does not remove the issue, because Jain restraint is not based only on reproduction. It is based on non-violence, purity, and renunciation of foods considered inappropriate in Jain conduct.

About your bacteria point:

• Yes, bacteria exist in many foods Jains eat. But Jainism does not claim that one can eliminate all microscopic life from every action.
• The principle is to avoid unnecessary harm and avoid foods that are especially associated with violence, decay, or unsuitable conditions.
• So the presence of tiny life in permitted foods does not make all foods equally acceptable. Jain dietary discipline uses a graded ethical approach, not a claim of total life-free consumption.

In short: Mushrooms are avoided in Jainism because they are not treated as ordinary plant food, and because their nature, origin, and association with low forms of life make them unsuitable for ahimsa-based living.