Roles of Serine Accumulation and Catabolism in the Colonization of the Murine Urinary Tract by Escherichia coli CFT073

Abstract

In this study we show that deletion of the genes encoding L-serine deaminases SdaA and SdaB resulted in a mutant that accumulates higher intracellular levels of L-serine than CFT073. CFT073 sdaA sdaB has a mild competitive colonization defect whereas a CFT073 dsdA sdaA sdaB triple mutant shows a greater loss in competitive colonization ability. Thus, the inability to generate serine-specific catabolic products does not result in hypercolonization and the ability to catabolize serine represents a positive physiological trait during murine UTI. CFT073 dsdC and CFT073 dsdC dsdA mutants continue to outcompete the wild type in the UTI model. These results confirm that loss of DsdA activity results in the hypercolonization phenotype and that DsdC does not play a direct role in the elevated-colonization phenotype. Interestingly, a CFT073 dsdA mutant with deletions of D-serine transporter genes dsdX and cycA shows wild-type colonization levels of the bladder but is attenuated for kidney colonization. Thus, D-serine acts as a signal for hypercolonization and virulence gene expression by CFT073 dsdA, whereas overall catabolism of serine represents a positive Escherichia coli fitness trait during UTI.Urinary tract infections (UTIs) in adult women impose an estimated cost of $2.4 billion per year in the United States (12). Most women will experience at least one UTI in their lifetime, resulting in an estimated 6.8 million physician visits, 1.2 million emergency room visits, and nearly a quarter million hospitalizations each year. Escherichia coli remains by far the primary causative agent of community-acquired UTIs (11).The urinary tract is a normally sterile environment, and it poses daunting challenges to colonization by E. coli and other microorganisms. In addition to the cleansing flow of urine, numerous innate and acquired immune factors challenge the growth of uropathogenic Escherichia coli (UPEC) in the urinary tract. The host defense involves phagocytic attack, antimicrobial peptides, complement lytic and opsonizing factors, and reactive oxygen and nitrogen species. In addition, the urinary tract offers high-salt and high-osmolarity conditions while limiting E. coli nutrients common to the intestinal tract, especially neutral sugars and iron (4). Thus, we hypothesize that the ability of UPEC to import and metabolize the available carbon and nitrogen sources present in the urinary tract plays a special role in its ability to colonize and cause disease at that site.From a bacterial nutritional standpoint, urine is a dilute mixture of amino acids and small peptides, quite similar to tryptone broth, with the notable exception of the abundance of urea in urine (4). The growth of E. coli in tryptone broth is well characterized, where growing cells preferentially and sequentially utilize serine and then aspartate while secreting acetate. Once these amino acids are depleted, cells then import and use tryptophan and acetate, followed by alanine, glutamine, and threonine (31). This order of nutrient preference holds true for ...

Uropathogenic Escherichia coli CFT073 Is Adapted to Acetatogenic Growth but Does Not Require Acetate during Murine Urinary Tract Infection

“…We have previously shown that D-serine is able to act as an intracellular signal, enhancing urinary tract colonization, but it also can serve as a key nutrient in the absence of L-serine catabolic potential (4). As shown in this work and in that of other laboratories, the catabolism of serine leads to the generation of acetyl phosphate (40,53), among other compounds, which aids signal transduction via several two-component regulatory systems (25,46,51,53).…”

“…The dashed line indicates a competitive index of 1.0. (4). When these observations are paired with our previous observations of the importance of the periplasmic stress response in colonization of the mouse model (42,43), the alterations in colonization associated with the dissimilatory acetate metabolism mutants seem likely due to an inability to accumulate acetyl phosphate, an inappropriate acetyl-CoA level, or an altered stress response.…”

“…Further studies demonstrate that multiple CFT073 mutants able to accumulate D-serine intracellularly are at a competitive advantage in the urinary tract (4). A CFT073 dsdA sdaA sdaB strain unable to utilize either enantiomer of serine is at a competitive disadvantage in the murine model (4). This suggests there is a role for serine as a key nutrient in vivo.…”

“…CFT073 with a dsdA mutation is at a competitive advantage relative to wild-type CFT073 in the murine model of UTI (44). Further studies demonstrate that multiple CFT073 mutants able to accumulate D-serine intracellularly are at a competitive advantage in the urinary tract (4). A CFT073 dsdA sdaA sdaB strain unable to utilize either enantiomer of serine is at a competitive disadvantage in the murine model (4).…”

High Metabolic Potential May Contribute to the Success of ST131 Uropathogenic Escherichia coli

“…It has recently been suggested that bacterial metabolic capability enhances fitness and contributes to pathogenesis, which could provide an alternative explanation for the success of prevalent UPEC clones (16), and previous reports have shown that certain metabolic enzymes may also enhance virulence (22). For example, the ability of UPEC to catabolize the amino acid D-serine during UTI supports bacterial growth and acts as a signaling mechanism to trigger virulence gene expression (2,24).…”