Ag43 Promotes Persistence of Uropathogenic Escherichia coli Isolates in the Urinary Tract

Uropathogenic Escherichia coli (UPEC) harbors an arsenal of virulence factors which enable it to infect the host. Adhesion factors of E. coli, facilitating the initial colonization of epithelia and eventually leading to invasion of the host cell, have been described extensively elsewhere (2, 3). More recently, factors supporting E. coli persistence have also attracted increasing interest (6, 7, 11).

Variants of the autotransporter protein Ag43 are expressed by pathogenic as well as commensal E. coli strains (10). By using a mouse infection model, differential functions during pathogenesis were revealed for the two Ag43 proteins in UPEC strain CFT073, encoded by the CFT073 fluA gene (fluA_CFT073) and fluB_CFT073 (9). Whereas Ag43a displayed higher-level autoaggregative properties and promoted persistence in the urinary bladder, Ag43b impaired initial colonization but did not affect later stages of infection. We here investigated clinical UPEC isolates from sporadic versus recurrent urinary tract infections (UTI) to demonstrate the relevance of this observation for human tissue.

A total of 118 E. coli isolates were collected from 47 children with sporadic and recurrent UTI (5). Biochemical differentiation and macrorestriction analyses identified 85 different E. coli strains; 12 strains were repeatedly recovered from the same patients at visits at least 1 month apart (recurrent strains), whereas the remaining strains were isolated only once (sporadic strains). All isolates were tested for the presence of flu (9) and the two CFT073-specific gene variants fluA_CFT073 and fluB_CFT073 (8).

The flu gene was detected in 55 of 85 isolates (65%), and we observed a significant association with isolates from recurrent infections (11 [92%] of 12 such isolates had the gene, versus 44 [60%] of 73 isolates from sporadic infections [P < 0.05]). The overall prevalence for fluA_CFT073 and fluB_CFT073 was 27% each (each gene was present in 23 of 85 isolates). Although neither gene was linked to recurrent strains, the common presence of both genes in one strain was more often found in recurrent than in sporadic strains of the tested collection (5 of 12 recurrent strains versus 10 of 73 sporadic strains [P < 0.05]). Thus, our data support the previously reported linkage between Ag43 proteins and intracellular persistence (1). We further sought for characteristics which could possibly explain the differential roles of fluA_CFT073 and fluB_CFT073 observed during mouse UTI (9).

The neutrophil-attractant chemokine interleukin-8 (IL-8) is assigned a crucial role in bacterial clearance from the urinary tract (4). The IL-8/creatinine ratios in urine samples from children infected with fluA_CFT073-positive strains were significantly lower than those in samples from children infected with strains lacking this gene (P < 0.05) (Fig. 1), and isolates carrying only fluA_CFT073 were associated with lower IL-8 levels than isolates expressing only fluB_CFT073 (P < 0.05) (Fig. 2). These findings suggest that fluB_CFT073 mediates a more immunogenic bacterial phenotype, which could explain the previously reported interference of Ag43b with early stages of infection (9). In contrast, a higher degree of bacterial autoaggregation in strains expressing fluA_CFT073 (9) might reduce the exposure of immunogenic surface structures, thereby allowing evasion of immune recognition and longer bacterial persistence in the host. Taken together, clinical data from the UPEC collection investigated here support the suggested roles of Ag43 variants in type strain CFT073 and underline the impact of Ag43 proteins on bacterial persistence in the urinary tract. They further indicate that both fluA_CFT073 and fluB_CFT073 might be of advantage for UPEC.

FIG. 1.

IL-8 levels in urine from children with UTI caused by E. coli strains, in relation to the presence (n = 23) or absence (n = 57) of fluA_CFT073. Individual levels and medians are presented. The difference is significant (by the Mann-Whitney U test).

FIG. 2.

IL-8 levels in urine from children with UTI caused by E. coli strains carrying only fluA_CFT073 (n = 8) or only fluB_CFT073 (n = 8). Individual levels and medians are presented. The difference is significant (by the Mann-Whitney U test).