Modulation of surface antigen expression by Klebsiella pneumoniae in response to growth environment.

Growth in pooled human body fluids [urine, serum and peritoneal dialysate (HPD)] modulated the expression of cell envelope antigens in virulent (serotype O1:K1) and avirulent (serotype O1:K66) Klebsiella pneumoniae strains. Marked variations in the outer membrane protein (OMP) and lipopolysaccharide (LPS) profiles were noted when broth-grown cells were compared with those of bacteria cultured in body fluids. In particular, for the O1:K1 serotype strain, growth in the latter resulted in: (a) the expression of at least five iron-regulated OMPs in the 74-87 kDa range, the pattern of which was medium dependent; (b) alterations in the migration of the LPS core polysaccharide; and (c) the reversion of isogenic O-:K+ and O-:K- mutants to the O+ phenotype after growth in fresh serum but not in heat-inactivated serum, urine or HPD. Similar results were obtained for the O1:K66 serotype, although no variation in the migration of the LPS core was noted. For both O1:K1 and O1:K66 serotypes, neither the surface exposure of O1 serotype LPS nor the production of K-antigen (capsular polysaccharide) was affected by growth in body fluids. No reversion of K- mutants to the K+ phenotype was observed. These data illustrate the phenotype flexibility of this opportunistic pathogen and emphasise the crucial role of the O- rather than the K-antigen in protecting K. pneumoniae from complement-mediated serum killing.