question about bacterial colonies. what do the + and minus signs mean?

[mrh125]

like if there's a colony with A+/A- and A+/A+ or His-/His-?

Does the - mean it's obligate for A or histidine or whatever? and what does the + mean?

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[jetsfan1234]

like if there's a colony with A+/A- and A+/A+ or His-/His-?

Does the - mean it's obligate? and what does the + mean?

Depends in context but usually (+) means working gene is present while(-) lacks functioning gene. E.g., his- bacteria is unable to synthesize histidine, so the medium must contain histidine for colonies to grow.