Differences in outer membrane characteristics between gallstone-associated bacteria and normal bacterial flora

Abstract

Previous studies with scanning electron microscopy (SEM) have suggested that pigment gallstones contain bacteria. We set out to culture these bacteria and to study their membrane characteristics. We studied gallstones from 54 patients (36 men, 18 women; mean age 55·4 years) admitted consecutively to two hospitals for cholecystectomy. SEM detected bacteria in all of 14 brown pigment stones, 2 of 14 black pigment stones, and in the pigmented centres of 9 of 19 mixed cholesterol stones; no bacteria were detected in 14 pure cholesterol stones or within the cholesterol portions of mixed stones. We were able to culture bacteria from all gallstones with bacteria seen on SEM and for which sufficient material was available (n=16). 20 bacterial species were recovered from these stones. Gallstones containing bacteria were associated with clinical sepsis and cholangitis. All bacteria obtained from gallstones agglutinated human O P1 erythrocytes, which reflects the presence of P1-specific fimbriae. 5 strains were positive for Forssman-antigen-specific fimbriae. None showed evidence of mannose-specific fimbriae. All of the organisms bound anti-Gal, a ubiquitous naturally occurring IgG specific for α-galactosyl residues. The presence of P1 fimbriae and α-galactosyi residues and the absence of mannose-specific fimbriae distinguish these organisms from gut flora. We postulate that possession of these unusual properties may enhance the ability of bacteria to colonise the biliary tree and initiate pigment gallstone formation.