In this study of 42 women with acute uncomplicated cystitis due to E. coli
, we assessed the relative importance of fecal abundance versus the intrinsic virulence potential in determining which of the host's intestinal E. coli
clones would cause the infection. We also undertook a novel assessment of the relationship between the clonal complexity of the fecal E. coli
population and virulence-associated characteristics of the constituent strains. For each host, we studied the urine strain and 30 colonies from a pretherapy fecal sample, thereby detecting minority E. coli
fecal populations (19
This study is, to our knowledge, the first to assess a clone's quantitative fecal prevalence, or the pauciclonal versus multiclonal nature of the source fecal sample, in relation to the phylogenetic group, virulence factor profile, and UTI status, and to compare urine and fecal clones from the same hosts according to so broad an array of molecular traits by using sophisticated statistical methods such as multilevel modeling, correspondence analysis, and PCoA. Therefore, in addition to validating our previous findings (12
) in a larger population and with greater methodological rigor, the present work also includes several novel features.
The study design allowed each woman to serve as her own control, thereby avoiding possible between-population differences that may confound comparisons of UTI isolates with fecal isolates from healthy hosts. This is an extremely powerful approach for removing the effects of known or unrecognized potential confounders, such as behavioral, environmental, physiological, or genetic differences between infected and uninfected hosts.
Our findings support three main conclusions. First, the urine clones, though almost always detectable in the host's fecal flora at the time of presentation, represented a highly biased subset of that flora, one enriched for traditionally recognized virulence traits and phylogenetic group B2, a result that supports the special-pathogenicity hypothesis. Second, however, the urine clone also tended to be the host's most abundant, or dominant, fecal clone, a result that supports the prevalence hypothesis. Finally, virulence traits and group B2 status were closely associated with fecal abundance, dominance, and pauciclonality. This suggests that prevalence and special pathogenicity are not alternative, mutually exclusive explanations for the occurrence of UTI. Instead, they may both be operational, perhaps contributing jointly to UTI pathogenesis, with virulence factors and other group B2-associated characteristics possibly promoting intestinal dominance (10
) and thereby increasing the probability that subsequent steps in pathogenesis will occur.
In this regard, within the total clonal population, fecal abundance (and its close correlate, fecal dominance) was significantly associated with the virulence score (and its close correlate, group B2 status), and each of these domains individually was significantly associated with urine clone status. However, these domains were not precisely overlapping, since some independent predictive power for either domain was evident in the multivariable modeling. This suggests that virulence and abundance, although usually aligned, may sometimes substitute for one another in promoting UTI when either occurs alone. However, among the urine clones overall, dominance within the fecal reservoir did not appreciably reduce the requirement for virulence traits and group B2 status, evidence suggesting that the UTI-promoting effect of fecal abundance is generally insufficient of itself, in intact hosts, to allow low-virulence strains to cause UTI.
The observed association of fecal pauciclonality with group B2 status and virulence is novel and of potential biological and pathogenic significance. Why women with UTI who are colonized with many different E. coli clones tend to have primarily low-virulence, non-B2 clones, whereas those colonized with fewer E. coli clones tend to have high-virulence clones primarily from group B2, is unclear and warrants further study. It may be that certain women are particularly receptive to intestinal colonization with virulent clones, which then outcompete other clones and establish a low-diversity B2-dominated flora, whereas other women can support intestinal colonization with whatever clones they encounter, which probably are mostly low-virulence, non-B2 clones. Alternatively, this phenomenon could be a function of the strains themselves, with B2 strains tending to displace other phylogenetic groups through enhanced niche fitness or inhibition of other clones. Longitudinal studies could elucidate whether intestinal clonal complexity is a relatively stable trait specific to particular hosts or varies over time for a given host; whether expansion of minority B2 clones is typically accompanied by a reduction in overall intestinal clonal complexity; and whether antecedent intestinal clonal diversity predicts UTI risk.
Additional strengths of this study include the comparatively large number of subjects, the systematic approach to recruitment, and the clinical homogeneity of the study population. Limitations include the cross-sectional design, the use of multiple comparisons (which, however, was addressed by using multivariable analysis, correspondence analysis, and PCoA to supplement standard univariate analyses), and the absence of vaginal and periurethral sampling.
In summary, our findings suggest that the relationship between the structure of the fecal E. coli population and UTI pathogenesis is complex. Fecal pauciclonality, clonal dominance or relative abundance, molecularly inferred virulence, and group B2 status are closely intertwined. Phylogenetic group B2 status and/or virulence factors may promote fecal abundance and pauciclonality, thereby contributing to upstream steps in UTI pathogenesis. This relationship, if confirmed, suggests a possible reconciliation of the prevalence and special-pathogenicity hypotheses. Furthermore, pauciclonality of the intestinal E. coli population may represent a previously unrecognized correlate of pathogenicity deserving of further investigation.