Ag+ (silver cation) has antimicrobial properties and is used in a broad range of applications (urinary catheter, burn wound cream, etc) as a topical agent. Resistance can develop in some Gram-negative bacteria.

Susceptible bacteria showed a silver nitrate MIC <= 8 mg/L.

High level resistance is indicated by a silver nitrate MIC > 128 mg/L


There may be 2 forms of resistance:

(1) overt

(2) cryptic (inducible), seen in Enterobacter and Klebsiella species


The cryptic resistance is associated with the sil operon.


The cryptic form can be identified by plating a saturated culture of a Gram-negative isolate onto agar containing supra-inhibitory concentrations of silver ions (128 mg silver nitrate per L) with overnight incubation.

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