668
U N I T 7
Kidney and Urinary Tract Function
of the urinary tract, to evade the destructive effects of
the host’s immune system, and to develop resistance to
antimicrobial agents. Not all bacteria are capable of
adhering to and infecting the urinary tract. Of the many
strains of
E
.
coli
, only those with increased ability to
adhere to the epithelial cells of the urinary tract are able
to produce UTIs. These bacteria have fine protein fila-
ments, called
pili
or
fimbriae
, that help them adhere to
receptors on the lining of urinary tract structures.
Obstruction and Reflux
Obstruction and reflux are other factors that increase
the risk for UTIs. Any microorganisms that enter the
bladder normally are washed out during voiding. When
outflow is obstructed, urine remains in the bladder and
acts as a medium for microbial growth; the microorgan-
isms in the contaminated urine can then ascend along
the ureters to infect the kidneys. The presence of resid-
ual urine correlates closely with bacteriuria and with its
recurrence after treatment. Another aspect of bladder
outflow obstruction and bladder distention is increased
intravesical pressure, which compresses blood vessels in
the bladder wall, leading to a decrease in the mucosal
defenses of the bladder.
In UTIs associated with stasis of urine flow, the
obstruction may be anatomic or functional. Anatomic
obstructions include urinary tract stones, prostatic
hyperplasia, pregnancy, and malformations of the ure-
terovesical junction. Functional obstructions include
neurogenic bladder, infrequent voiding, detrusor (blad-
der) muscle instability, and constipation.
A phenomenon called
urethrovesical reflux
occurs
when urine from the urethra moves into the bladder. In
women, urethrovesical reflux can occur during activities
such as coughing or squatting, in which an increase in
intra-abdominal pressure causes the urine to be squeezed
into the urethra and then to flow back into the bladder
as the pressure decreases.
40
This also can happen when
voiding is abruptly interrupted. Because the urethral ori-
fice frequently is contaminated with bacteria, the reflux
mechanism may cause bacteria to be drawn back into
the bladder. The
vesicoureteral reflux
, which occurs at
the level of the bladder and ureter, allows urine and
bacteria to ascend from the bladder to the kidney and
is associated with pyelonephritis and infections of the
upper urinary tract. (see Chapter 25, Fig. 25-11).
Catheter-Induced Infection
Urinary catheters are a source of urethral irritation
and provide a means for entry of microorganisms
into the urinary tract. Catheter-associated bacteriuria
remains the most frequent cause of gram-negative sep-
ticemia in hospitalized patients. Studies have shown
that bacteria adhere to the surface of the catheter and
initiate the growth of a biofilm that then covers the
surface of the catheter (see Chapter 14, Fig. 14-6).
41
The biofilm tends to protect the bacteria from the
action of antibiotics and makes treatment difficult. A
closed drainage system (i.e., closed to air and other
sources of contamination) and careful attention to
perineal hygiene (i.e., cleaning the area around the
urethral meatus) help to prevent infections in persons
who require an indwelling catheter. Careful hand-
washing and early detection and treatment of UTIs
also are essential.
Clinical Features
The manifestations of UTI depend on whether the infec-
tion involves the lower (bladder) or upper (kidney) uri-
nary tract and whether the infection is acute or chronic.
An acute episode of cystitis (bladder infection) is char-
acterized by frequency of urination, lower abdominal
or back discomfort, and burning and pain on urination
(i.e., dysuria).
34,38
Occasionally, the urine is cloudy and
foul smelling. In adults, fever and other signs of infec-
tion usually are absent. If there are no complications,
the symptoms disappear within 48 hours of treatment.
The symptoms of cystitis also may represent urethritis
caused by
Chlamydia trachomatis
,
Neisseria gonor-
rhoeae
, or herpes simplex virus, or vaginitis attribut-
able to
Trichomonas vaginalis
or
Candida
species (see
Chapter 41).
Diagnosis andTreatment
The diagnosis of UTI usually is based on symptoms and
on examination of the urine for the presence of microor-
ganisms. When necessary, x-ray films, ultrasonography,
and CT and renal scans are used to identify contributing
factors, such as obstruction.
Urine tests are used to establish the presence of bac-
teria in the urine and a diagnosis of UTI. A commonly
accepted criterion for diagnosis of a UTI is the presence
of 100,000 colony-forming units (CFU) or more bac-
teria per milliliter (mL) of urine.
35
Colonization usu-
ally is defined as the multiplication of microorganisms
in or on a host without apparent evidence of invasive-
ness or tissue injury. Pyuria (the presence of less than
five to eight leukocytes per high-power field) indicates
a host response to infection rather than asymptomatic
bacterial colonization. A Gram stain may be done to
determine the type (gram positive or gram negative) of
organism that is present.
Chemical screening (urine dipstick) for markers of
infection may provide useful information but is less sen-
sitive than microscopic analysis.
35,36
These tests are rela-
tively inexpensive, are easy to perform, and can be done
in the clinic setting or even in the home. A urine culture
may be done to confirm the presence of pathogenic bacte-
ria in urine specimens, allow for their identification, and
permit the determination of their sensitivity to specific
antibiotics.
The treatment of UTI is based on the pathogen causing
the infection and the presence of contributing host–
agent factors. Other considerations include whether
the infection is acute, recurrent, or chronic. Most acute
lower UTIs, which occur mainly in women and are gen-
erally caused by
E
.
coli
, are treated successfully with
a short course of antimicrobial therapy. Forcing fluids
