Potential role of phase variation of type 1 pili in urinary tract infection. and bacterial prostatitis

Uropathogenic Escherichia coli triggers oxygen-dependent apoptosis in human neutrophils through the cooperative effect of type 1 fimbriae and lipopolysaccharide

Type 1 fimbriae are the most commonly expressed virulence factor on uropathogenic Escherichia coli. In addition to promoting avid bacterial adherence to the uroepithelium and enabling colonization, type 1 fimbriae recruit neutrophils to the urinary tract as an early inflammatory response. Using clinical isolates of type 1 fimbriated E. coli and an isogenic type 1 fimbria-negative mutant (CN1016) lacking the FimH adhesin, we investigated if these strains could modulate apoptosis in human neutrophils. We found that E. coli expressing type 1 fimbriae interacted with neutrophils in a mannose- and lipopolysaccharide (LPS)-dependent manner, leading to apoptosis which was triggered by the intracellular generation of reactive oxygen species. This induced neutrophil apoptosis was abolished by blocking FimH-mediated attachment, by inhibiting NADPH oxidase activation, or by neutralizing LPS. In contrast, CN1016, which did not adhere to or activate the respiratory burst of neutrophils, delayed the spontaneous apoptosis in an LPS-dependent manner. This delayed apoptosis could be mimicked by adding purified LPS and was also observed by using fimbriated bacteria in the presence of d-mannose. These results suggest that LPS is required for E. coli to exert both pro- and antiapoptotic effects on neutrophils and that the difference in LPS presentation (i.e., with or without fimbriae) determines the outcome. The present study showed that there is a fine-tuned balance between type 1 fimbria-induced and LPS-mediated delay of apoptosis in human neutrophils, in which altered fimbrial expression on uropathogenic E. coli determines the neutrophil survival and the subsequent inflammation during urinary tract infections.

Assessment of virulence of uropathogenic Escherichia coli type 1 fimbrial mutants in which the invertible element is phase-locked on or off

Type 1 fimbria is a proven virulence factor of uropathogenic Escherichia coli (UPEC), causing urinary tract infections. Expression of the fimbria is regulated at the transcriptional level by a promoter situated on an invertible element, which can exist in one of two different orientations. The orientation of the invertible element that allows the expression of type 1 fimbriae is defined as "on," and the opposite orientation, in which no transcription occurs, is defined as "off." During the course of a urinary tract infection, we have observed that the infecting E. coli population alternates between fimbriated and nonfimbriated states, with the fimbriated on orientation peaking at 24 h. We propose that the ability of the invertible element to switch orientations during infection is itself a virulence trait. To test this hypothesis, nucleotide sequence changes were introduced in the left inverted repeat of the invertible element of UPEC pyelonephritis strain CFT073 that locked the invertible elements permanently in either the on or the off orientation. The virulence of these mutants was assessed in the CBA mouse model of ascending urinary tract infection at 4, 24, 48, and 72 h postinoculation (hpi). We conducted independent challenges, in which bladders of mice were inoculated with either a single mutant or the wild type, and cochallenges, in which a mutant and the wild type were inoculated together to allow direct competition in the urinary tract. In both sets of experimental infections, the locked-off mutant was recovered from the urine, bladder, and kidneys in significantly lower numbers than the wild type at 24 hpi (P < or = 0.05), demonstrating its attenuation. Conversely, the locked-on mutant was recovered in higher numbers than the wild type at 24 hpi (P < or = 0.05), showing enhanced virulence of this mutant. No significant differences were seen between the mutants and wild type in the urine or the bladder at 48 or 72 hpi. However, the wild type outcompeted the locked-off mutant in the kidneys during the cochallenge experiment at 72 hpi (P = 0.009). Overall, these data suggest that the ability of the invertible element controlling type 1 fimbria expression to phase vary contributes significantly to virulence early (24 hpi) in the course of a urinary tract infection by UPEC and most profoundly influences colonization of the bladder.

Establishment of a persistent Escherichia coli reservoir during the acute phase of a bladder infection

The vast majority of urinary tract infections are caused by strains of uropathogenic Escherichia coli that encode filamentous adhesive organelles called type 1 pili. These structures mediate both bacterial attachment to and invasion of bladder epithelial cells. However, the mechanism by which type 1 pilus-mediated bacterial invasion contributes to the pathogenesis of a urinary tract infection is unknown. Here we show that type 1-piliated uropathogens can invade the superficial epithelial cells that line the lumenal surface of the bladder and subsequently replicate, forming massive foci of intracellular E. coli termed bacterial factories. In response to infection, superficial bladder cells exfoliate and are removed with the flow of urine. To avoid clearance by exfoliation, intracellular uropathogens can reemerge and eventually establish a persistent, quiescent bacterial reservoir within the bladder mucosa that may serve as a source for recurrent acute infections. These observations suggest that urinary tract infections are more chronic and invasive than generally assumed.

Tamm-Horsfall protein binds to type 1 fimbriated Escherichia coli and prevents E. coli from binding to uroplakin Ia and Ib receptors

The adherence of uropathogenic Escherichia coli to the urothelial surface, a critical first step in the pathogenesis of urinary tract infection (UTI), is controlled by three key elements: E. coli adhesins, host receptors, and host defense mechanisms. Although much has been learned about E. coli adhesins and their urothelial receptors, little is known about the role of host defense in the adherence process. Here we show that Tamm-Horsfall protein (THP) is the principal urinary protein that binds specifically to type 1 fimbriated E. coli, the main cause of UTI. The binding was highly specific and saturable and could be inhibited by d-mannose and abolished by endoglycosidase H treatment of THP, suggesting that the binding is mediated by the high-mannose moieties of THP. It is species-conserved, occurring in both human and mouse THPs. In addition, the binding to THP was much greater with an E. coli strain bearing a phenotypic variant of the type 1 fimbrial FimH adhesin characteristic of those prevalent in UTI isolates compared with the one prevalent in isolates from the large intestine of healthy individuals. Finally, a physiological concentration of THP completely abolished the binding of type 1 fimbriated E. coli to uroplakins Ia and Ib, two putative urothelial receptors for type 1 fimbriae. These results establish, on a functional level, that THP contains conserved high-mannose moieties capable of specific interaction with type 1 fimbriae and strongly suggest that this major urinary glycoprotein is a key urinary anti-adherence factor serving to prevent type 1 fimbriated E. coli from binding to the urothelial receptors.

Phase variation of the lpf operon is a mechanism to evade cross-immunity between Salmonella serotypes

Conventional wisdom holds that phase variation is a mechanism for immune evasion. However, despite fimbrial phase variation, mice previously exposed to Salmonella typhimurium are protected against a subsequent challenge. We evaluated whether lpf phase variation instead may be a mechanism to evade cross-immunity between Salmonella serotypes. Mice were immunized orally with S. typhimurium aroA mutants either that expressed the lpf operon (phase-on variant) or in which the entire lpf operon had been removed by deletion. During a subsequent challenge with virulent Salmonella enteritidis a selection against lpf phase-on variants was observed in mice previously exposed to S. typhimurium long polar fimbriae. Vaccination with S. typhimurium did not confer protection against challenge with S. enteritidis, presumably because lpf phase-off variants were able to evade cross-immunity. We propose that lpf phase variation is a mechanism to evade cross-immunity between Salmonella serotypes, thereby allowing their coexistence in a host population.

In vivo phase variation of Escherichia coli type 1 fimbrial genes in women with urinary tract infection

Type 1 fimbriae, expressed by most Escherichia coli strains, are thought to attach to human uroepithelium as an initial step in the pathogenesis of urinary tract infections (UTI). Numerous reports using both in vitro and murine models support this role for type 1 fimbriae in colonization. Unfortunately, only a limited number of studies have directly examined the expression of fimbriae in vivo. To determine whether type 1 fimbrial genes are transcribed during an acute UTI, we employed a modification of an established method. The orientation (ON or OFF) of the invertible promoter element, which drives transcription of type 1 fimbrial genes, was determined by PCR amplification using primers that flank the invertible element, followed by SnaBI digestion. The orientation of the type 1 fimbrial switch was determined under three experimental conditions. First, E. coli strains from different clinical sources (acute pyelonephritis patients, cystitis patients, and fecal controls) were tested under different in vitro culture conditions (agar versus broth; aerated versus static). The genes in the more-virulent strains (those causing acute pyelonephritis) demonstrated a resistance, in aerated broth, to switching from OFF to ON, while those in fecal strains readily switched from OFF to ON. Second, bladder and kidney tissue from CBA mice transurethrally inoculated with E. coli CFT073 (an established murine model of ascending UTI) was assayed. The switches directly amplified from infected bladder and kidney tissues were estimated to be 33 and 39% ON, respectively, by using a standard curve. Finally, bacteria present in urine samples collected from women with cystitis were tested for type 1 fimbria switch orientation. For all 11 cases, an average of only 4% of the switches in the bacteria in the urine were ON. In 7 of the 11 cases, we found that all of the visible type 1 fimbrial switches were in the OFF position (upper limit of detection of assay, 98% OFF). Strains recovered from these urine samples, however, were shown after culture in vitro to be capable of switching the fimbrial gene to the ON position and expressing mannose-sensitive hemagglutinin. The results from experimental infections and cases of cystitis in women suggest that type 1 fimbrial genes are transcribed both in the bladder and in the kidney. However, those bacteria found in the urine and not attached to the uroepithelium are not transcriptionally active for type 1 fimbrial genes.

In vitro binding of type 1-fimbriated Escherichia coli to uroplakins Ia and Ib: relation to urinary tract infections

Urinary tract infections, caused mainly by Escherichia coli, are among the most common infectious diseases. Most isolates of the uropathogenic E.coli can express type 1 and P fimbriae containing adhesins that recognize cell receptors. While P fimbriae recognize kidney glycolipid receptors and are involved in peyelonephritis, the urothelial for type 1 fimbriae were not identified. We show that type 1-fimbriated E. coli recognize uroplakins Ia and Ib, two major glycoproteins of urothelial apical plaques. Anchorage of E. coli to urothelial surface via type 1 fimbriae-uroplakin I interactions may play a role in its bladder colonization and eventual ascent through the ureters, against urine flow, to invade the kidneys.

Prostatic fluid inflammation in prostatitis

We studied expressed prostatic secretions from 106 patients with prostatitis to determine the longitudinal course of prostatic fluid inflammation. Prostatic fluid specimens were collected from 14 patients with acute bacterial, 13 with chronic bacterial and 79 with abacterial prostatitis. White blood cells per high power microscopic field of the expressed prostatic secretion were counted under a cover slip. Inflammation in the expressed prostatic secretion was considered to be consistent with prostatitis if there were 10 or more white blood cells per high power field. The 14 patients with acute bacterial prostatitis had a mean of 10 visits with a mean followup of 59 months. Inflammation resolved within 1 month in 9 patients with acute bacterial prostatitis but it recurred in 5 other patients in association with urinary tract infection. The 13 patients with chronic bacterial prostatitis had a mean of 10 visits with a mean followup of 58 months. Episodic inflammation in the expressed prostatic secretion associated with urinary tract infection was seen in all patients during followup. The 79 patients with abacterial prostatitis had a mean of 7 visits with a mean followup of 40 months. Resolution of inflammation in the expressed prostatic secretion occurred in 9 patients (11%). Inflammation in the expressed prostatic secretion at followup was seen in 70 patients (89%), and 27 of the 79 patients (34%) had 10 or more white blood cells per high power field of expressed prostatic secretion in all subsequent specimens. In cases of abacterial prostatitis, neither the initial expressed prostatic secretion white blood count nor the presence of symptoms reliably predicted subsequent inflammation. The data suggest that prostatic inflammation resolves in most patients with acute bacterial prostatitis and is episodic in patients with chronic bacterial or abacterial prostatitis.