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

HyperosmolarityLow pHVery dilute urine fails to grow most

bacteriaMen have prostatic fluid that is

inhibitoryFlow has a washing effect

Risk Factors:Age 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

Adults to 65Low incidence unless genital-

urinary abnormalities

Risk Factors:Age

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 concentrationLowered pHDecreased 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

Nonspecific symptoms: failure to thrive, vomiting, lethargy, fever

Children > 2 years 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 epidermidisPredominant

StreptococciAlphaNonhemolytic

Lactobillus species Diphtheroids Yeast

Microbial Agents of UTIs

Specimen Collection 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

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

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: MacConkey1 Nonselective agar: BloodOR Bi-Plate

Urine Culture Procedure 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?

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 pyuriaPresence or absence of

symptoms

References 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&prodoid=J116

Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.