Background: Bacteria play an important role in the formation of brown pigment stones through adherence and biofilm formation. Scanning electron microscopy of cross sections of these stones reveals a laminated appearance and various bacteria in the different layers. Our postulation was that different bacteria might be involved at different stages of stone formation. Methods: By using standard bacteriologic cultures, the composition, morphology, and antibiotic sensitivity patterns of bacteria isolated from paired stone were compared with bile samples from 70 patients with acute cholangitis. A further comparison was made between bacteria isolated from the periphery and center of 3 randomly selected brown pigment stones. Results: Ninety-one percent of bile and 99% of stone samples yielded positive cultures, with a total of 151 and 149 bacteria isolated from bile and stones, respectively. In 22 patients (33%), the bacteria isolated from the paired bile and stone samples were totally different. The mean percentage similarity of bacteria isolated from bile and stones was 39% (range 0%-100%). Of the 59 pairs of similar bacteria isolated, the antibiotic sensitivity patterns were different in 24 (41%) cases. Of the 3 brown stones studied, either different bacterial species or the same bacteria but different strains with different antibiotic sensitivities were isolated from the center and periphery of the stones. Conclusions: Bacteria present in the different layers of brown pigment stones may represent the bacterial flora in bile at different times. Simple bile culture may not identify bacteria trapped inside the stone.
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