Urinary Tract Infection: Organisms, UA & Treatment

Creator: Progress Note
Published: 2026-08-27
Keywords: urinary tract infection, urinary tract infection, uti organisms, e coli p fimbriae, staphylococcus saprophyticus, proteus urease, leukocyte esterase nitrite, wbc casts, asymptomatic bacteriuria pregnancy, cystitis treatment, sterile pyuria

bolt The 60-second version

check_circleE. coli causes most UTIs; type 1 fimbriae bind bladder epithelium and P fimbriae bind kidney epithelium (pyelonephritis).
check_circleStaphylococcus saprophyticus is the second most common cause in young sexually active women; it is coagulase-negative and novobiocin-resistant.
check_circleProteus and Klebsiella make urease → alkaline urine and struvite stones; Enterococcus and Pseudomonas are common in catheter-associated and hospital UTIs.
check_circleUrinalysis: leukocyte esterase = pyuria; nitrite = Enterobacterales that reduce nitrate (often negative with Enterococcus and S. saprophyticus).
check_circleCystitis: dysuria, frequency, urgency, suprapubic pain, no fever. Fever, flank pain, and WBC casts indicate pyelonephritis.
check_circleTreat asymptomatic bacteriuria only in pregnancy and before urologic procedures that breach the mucosa.

target What you'll be able to do · aligned to USMLE Step 1 · Renal

MICROIdentify common urinary pathogens and their virulence factors and lab features.
RENALInterpret leukocyte esterase, nitrite, casts, and culture results.
PATHDistinguish cystitis from pyelonephritis.
PHARMChoose appropriate treatment and identify when to treat asymptomatic bacteriuria.

How UTIs are tested on Step 1

Questions name a patient group and a lab clue (nitrite, urine pH, catalase, novobiocin) and ask for the organism or the virulence factor.

Ascending infection. Urine flow, normal bladder emptying, and urothelial defenses normally clear bacteria.

Organisms and clues

The setting and lab tests identify the organism.

Organism	Setting	Lab or clue
Escherichia coli	Most UTIs in all groups	Lactose fermenter, nitrite-positive; fimbriae
Staphylococcus saprophyticus	Young sexually active women	Gram-positive, catalase-positive, coagulase-negative, novobiocin-resistant
Klebsiella pneumoniae	Hospital, diabetes	Mucoid capsule; urease
Proteus mirabilis	Catheters, stones	Urease → alkaline urine, struvite; swarming motility
Enterococcus faecalis	Catheters, instrumentation	Gram-positive cocci; nitrite-negative; intrinsically resistant to cephalosporins
Pseudomonas aeruginosa	Hospital, catheters	Oxidase-positive, non-lactose fermenter

Catheter-associated infections are often polymicrobial and resistant.

Risk factors. Vesicoureteral reflux predisposes children to recurrent pyelonephritis and renal scarring.

Reading the urinalysis

Dipstick and microscopy each add information.

Finding	Meaning
Leukocyte esterase	Neutrophils in urine (pyuria)
Nitrite	Gram-negative organisms that reduce nitrate to nitrite (needs time in the bladder)
WBC casts	Inflammation within the kidney: pyelonephritis or interstitial nephritis
Culture ≥ 10⁵ CFU/mL	Classic threshold for significant bacteriuria (lower counts can count in symptomatic patients)
Sterile pyuria	Urethritis (Chlamydia, gonorrhea), tuberculosis, interstitial nephritis, stones
Alkaline pH with infection	Urease-producing organism (Proteus, Klebsiella)

A negative nitrite does not exclude UTI; Enterococcus and S. saprophyticus usually do not produce nitrite.

Cystitis vs pyelonephritis and treatment

Systemic signs mean the kidney is involved.

Lower vs upper tract. Nitrofurantoin does not reach adequate kidney tissue levels and is not used for pyelonephritis.

Situation	Typical options
Uncomplicated cystitis	Nitrofurantoin, TMP-SMX (if local resistance is low), fosfomycin
Pregnancy	Nitrofurantoin (avoid at term), cephalexin, amoxicillin-clavulanate; avoid fluoroquinolones
Asymptomatic bacteriuria	Treat in pregnancy or before urologic procedures; otherwise do not treat

Fluoroquinolones are not first-line for uncomplicated cystitis because of adverse effects and resistance.

Worked example

Use the lab profile to name the organism.

functions Worked example — young woman with dysuria

A 20-year-old sexually active woman has dysuria and frequency. Urinalysis: leukocyte esterase positive, nitrite negative. Culture grows gram-positive cocci in clusters that are catalase-positive, coagulase-negative, and novobiocin-resistant. Organism?

Gram-positive cocci in clusters, catalase-positive = Staphylococcus.
Coagulase-negative excludes S. aureus.
Novobiocin resistance separates S. saprophyticus from S. epidermidis (sensitive).
Nitrite is negative because staphylococci do not reduce nitrate.

Answer. Staphylococcus saprophyticus, the second most common cause of cystitis in young sexually active women.

Next in the roadmap

Pyelonephritis

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Try it now

2 real questions on Urinary Tract Infection

Straight from the Progress Note question bank, each with a cited explanation. No account needed.

A 68-year-old man receiving intravenous gentamicin for a complicated urinary tract infection develops a rising serum creatinine on hospital day 6, with urinalysis showing granular casts but no cellular casts. Renal biopsy, photomicrograph shown below, from a related case shows necrosis and sloughing of proximal tubular epithelial cells into the tubular lumen with preserved glomeruli. The renal injury observed in this patient results primarily from aminoglycoside accumulation in which of the following renal structures, and by what mechanism?

Image for question 1 — Amadalvarez, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

From our NCLEX bank

A nurse is caring for a client who has an indwelling urinary catheter. Which nursing action best reduces the risk of a catheter-associated urinary tract infection?

Frequently asked questions

What is the most common cause of UTI?expand_more

Escherichia coli, using fimbriae to adhere to the urothelium.

Why can a UTI have a negative nitrite test?expand_more

Some organisms (Enterococcus, S. saprophyticus) do not reduce nitrate, and urine may not stay in the bladder long enough for conversion.

When is asymptomatic bacteriuria treated?expand_more

In pregnancy and before urologic procedures that cause mucosal bleeding. Otherwise it is not treated.

What do WBC casts indicate?expand_more

Inflammation in the kidney itself, such as pyelonephritis or interstitial nephritis, rather than the bladder.

References & further reading

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