Urinary Tract
Infections
MLAB 2434 –Microbiology
Keri Brophy-Martinez
Definitions



UTI = Urinary Tract Infection
 Spectrum of diseases caused by microbial invasion
of the genitourinary tract
Upper UT includes renal parenchyma (pyelonephritis)
and ureters (ureteritis)
 Symptoms include: fever, flank pain & tenderness
Lower UT includes bladder (cystitis), urethra
(urethritis), and, in males, the prostrate (prostatitis)


Symptoms include: pain on urination, increased
frequency, urgency, suprapubic tenderness
Bacteriuria = presence of bacteria in urine; may be
symptomatic or asymptomatic
Anatomy of the Urinary
Tract
Urinary System


Resistant to colonization and infection
Characteristics of urine
Hyperosmolarity
 Low pH
 Very dilute urine fails to grow most
bacteria
 Men have prostatic fluid that is
inhibitory
 Flow has a washing effect

Risk Factors:
Age


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Infants
 Boys have higher incidence rates due to
uncircumcision
Pre-school age
 Girls infected more than boys
 Most renal damage due to UTI at this age
School-age children
 Girls more prone to develop UTI upon
sexual activity
Risk Factors:
Age

Adults to 65

Low incidence unless genital-urinary
abnormalities
Risk Factors:
Age

Over age 65
 UTIs increase dramatically in both genders
 Atypical presentation
• Fever, delirium, failure to thrive


Males
• Prostate changes & increased catherization
• Neuromuscular changes
Females
• Fecal soiling & increased catherization
• Neuromuscular changes
• Bladder prolapse
Risk Factors:
Other

Institutionalized care

Increase in UTIs
• Instrumentation/catherization
• Genital-urinary tract abnormalities
Pregnancy
 Renal transplant

Risk Factors:
Other

Urinary conditions
High ammonia concentration
 Lowered pH
 Decreased blood flow in renal
medulla


Results in:
• Reduced chemotaxis of WBCs
• Reduced bactericidal activity of WBCs
Clinical Signs and
Symptoms

Infants and children < 2 years age

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Children > 2 years

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Nonspecific symptoms: failure to thrive,
vomiting, lethargy, fever
Likely to have localized symptoms:
Dysuria, frequency, abdominal or flank
pain
Adults with lower UT infections

Dysuria, frequency, urgency, and
sometimes suprapubic tenderness
Clinical Signs and
Symptoms (cont’d)

Adults with Upper UTIs


Especially those acute pyelonephritis,
include LUTI symptoms along with flank
pain and tenderness and fever
AGN (Acute Glomerulonephritis)

Results from immune response to S.
pyogenes (Group A) infections, either
respiratory or pyodermal
• Edema around eyes
• Hematuria
• RBC and WBC casts
Pathogenesis of UTIs

Three access routes
• Ascending (most significant)
• Usually seen in females since ureter is shorter
• Descending
• Also referred to as Hematogenous/Blood-borne
• Occurs as a result of bacteremia
• Less than 5% of UTI’s
• Lymphatic
• Increased pressure on bladder causes a redirect of
lymph fluid to kidney
• Infection dependent on size of the bacteria, strength
of the bacteria present, and how strong the body's
defense mechanisms are at the time.
• Very rare
Flora of Normal Voided Urine

Staphylococcus epidermidis


Predominant
Streptococci
Alpha
 Nonhemolytic

Lactobillus species
 Diphtheroids
 Yeast

Microbial Agents of UTIs
Specimen Collection
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Need to collect specimen to prevent
normal vaginal, perianal, and urethral
flora
Mid-stream clean catch – if self
collected, patient needs GOOD
instructions
Catheterized- sample must come from
port NOT bag
Suprapubic aspiration ( only for anaerobic
culture)
Specimen Collection (cont’d)


Additives – even with additive, time from
collection to processing should not exceed 24
hours
 Grey top culture tubes( sodium borate)
keep sample integrity for up to 48 hours
Transport
 If not processed or preserved, urine
should be cultured within 2 hours
 If refrigerated, urine can be held for 24
hours
Preculture Screening

Manual screening: Routine Urinalysis

Chemical screening
• Leukocyte Esterase and Nitrate on urine
dipstick

Urine microscopic
• 5 to 10 WBC/hpf is upper limit of normal
• Presence of bacteria

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Automated methods – expensive, except
in large volume labs
Gram stains generally not performed on
urines
Causes for Rejection


Inadequate method of collection or transport
Labeling incomplete

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name, source, acc # etc.
Insufficient volume
Fecal contamination
24 hour urines, pooled urines, and Foley
catheter tips must be rejected for culture
Setup of Urine Culture

Setup
1 Selective agar: MacConkey
 1 Nonselective agar: Blood
 OR Bi-Plate

Urine Culture Procedure
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Inoculation using either a 0.001ml(x1000) OR a
0.01 ml (x100) loop onto selective/nonselective
media, such as BAP and MAC
Dip calibrated loop into well-mixed urine. Quickly
make a single streak down the middle of the BAP
with the loop containing urine
Streak back and forth across the plate
perpendicular to the original inoculum, this creates
a “lawn”
With the same calibrated loop, do the same with
the MAC plate
Incubate at 35oC for 24-48 hours
Urine Streaking Technique
Interpretation of Urine
Cultures

Is there growth?

If no growth:
• At 24 hours:
• Preliminary report: no growth at 24 hours
• Reincubate plates
• At 48 hours:
• Final report: no growth at 48 hours
• Discard plates
Interpretation of Urine
Cultures

If there is growth, what media has
it grown on?
BAP only: rules out the enteric
GNR’s, colonies may be GPC, GPR,
GNDC
 BAP and MAC: most likely an enteric
GNR or Pseudomonas. If multiple
colony types, a gram stain must be
done.

Interpretation of Urine
Cultures

How many colony types are growing?


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Specimen with ≥ three organisms is
probably contamination and should not be
identified unless specifically requested by
physician
One or two pathogens ≥ 100,000 CFU/ml
should be identified and sensitivities done
One or two pathogens ≥ 100 CFU/ml should
be identified only if clinical situation
warrants or specimen is catheterized or
suprapubic aspiration
Determining the CFU
Count the numbers of colonies of
the plate
 Multiply that number by the dilution
factor of the loop

Test YOUR Understanding
A clean catch urine is collected
from a pregnant patient with
symptoms of urinary tract infection.
The urine is inoculated onto blood
and MacConkey agar with a 0.001
loop. After 24 hour incubation, 72
colonies grew on the blood plate.
 What is the colony count?

Interpretation of Urine
Cultures

Things to consider in UTI’s
Colony count of pure or predominant
organism
 Measurement of pyuria
 Presence or absence of symptoms

References
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Engelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory Diagnosis of
Infectious Diseases: Essentials of Diagnostic Microbiology . Baltimore, MD:
Lippincott Williams and Wilkins.
https://catalog.hardydiagnostics.com/cp_prod/CatNav.aspx?oid=7405&prodo
id=J116
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of
Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.