OmpA of uropathogenic Escherichia coli promotes postinvasion pathogenesis of cystitis

Abstract

Type 1 pilus directs bladder epithelial binding and invasion by uropathogenic Escherichia coli (UPEC) in the initial stage of cystitis, but the bacterial determinants of postinvasion events in the pathogenesis of cystitis are largely undetermined. We show here that the UPEC outer membrane protein A (OmpA), a monomeric, major, integral protein component of the bacterial outer membrane, functions as a critical determinant of intracellular virulence for UPEC, promoting persistent infection within bladder epithelium. Using a murine urinary tract infection (UTI) model, we demonstrate that whereas deletion of the UPEC ompA gene did not disrupt initial epithelial binding and invasion by UPEC, it did preclude completion of the intracellular bacterial community (IBC) pathway, accompanied by diminishing bacterial loads in the bladder. This defect in epithelial persistence of the ompA mutant was enhanced in competitive infections with wild-type UPEC. Microscopic examinations revealed that the ompA mutant formed significantly fewer IBCs, and those that were initiated were unable to progress past the early stages of maturation. These defects could be corrected by complementation of ompA. In addition, expression of ompA during wild-type UTI was sharply increased at time points correlated with IBC development and the arrival of host immune effector cells. Our findings establish OmpA as a key UPEC virulence factor that functions after epithelial invasion to facilitate IBC maturation and chronic bacterial persistence.

FIG. 1.

Type 1 pilus expression and function in the UPEC ompA mutant. Immunogold electron microscopy demonstrates equivalent pilus expression in wild-type UTI89 (A) and the ompA mutant (B). (C) The hemagglutination of guinea pig erythrocytes by wild-type (WT) and ompA mutant UTI89 is equivalent; the UTI89 fimH mutant is included as a negative control. In ex vivo gentamicin protection assays, CFU recovered 1 h after infection with either wild type or ompA mutant UTI89 are equivalent in the luminal and intracellular compartments (D). Scale bar for panels A and B, 200 nm.

FIG. 2.

Bacterial loads in the bladders of C3H/HeN mice at the indicated time points after infection with either wild-type UTI89 (▪), the ompA mutant (○), or the complemented ΔompA/pKW5 strain (░⃞), and the dashed line indicates the limit of detection (10 CFU/bladder). Bacterial loads of the ompA mutant begin to fall at 48 h and are significantly lower than the wild type at 1 week and 2 weeks postinfection (*, P < 0.001), and this difference is complemented when ompA is provided in trans.

FIG. 3.

CFU recovered at the indicated time points after competitive coinfection of C3H/HeN mice with both wild-type and ompA mutant UTI89. The competitive fitness of the ompA mutant is sharply attenuated as early as 24 h (*, P = 0.002 to 0.015).

FIG. 4.

Early IBC formation in C3H/HeN mice. (A) The numbers of IBCs detected at 6 h by LacZ staining are significantly lower after infection with the ompA mutant, compared to wild-type UTI89 (P = 0.007) or the complemented ΔompA/pKW5 strain (P = 0.04). By light microscopy, wild-type early IBCs are readily detected in hematoxylin-and-eosin-stained bladder sections (B); modest intracellular replication was evident in occasional ompA mutant IBCs (C).

FIG. 5.

Confocal microscopic examination of IBC maturation and filamentation. Consistent with previous results, mature IBCs were readily identified 16 h after wild-type UPEC infection (A), whereas a rare, stunted intracellular collection was found at this time point after infection with the ompA mutant (C). Similarly, robust filamentous bacterial populations were evident 16 h after wild-type infection (B), in contrast to the sparse filamentous forms seen after infection with the ompA mutant (D). In mice infected with the complemented ΔompA/pKW5 strain, the formation of mature IBCs (E) and filaments (F) was restored. Scale bars, 10 μm.

FIG. 6.

RT-PCR analysis of ompA expression in bladder RNA extracts. (A) RT-PCR performed on whole-bladder extracts demonstrates upregulation of ompA expression at 16 and 24 h, which subsides to still-elevated levels at 48 h to 2 weeks after infection with UTI89. (B) RNA isolated separately from the luminal and gentamicin-protected (intracellular) fractions reveals upregulation of ompA in both compartments at 16 and 48 h. The dashed line in each graph depicts the baseline (inoculum) level of ompA expression, used for comparison.