Urinary Tract Infection (UTI): Causes, Urine Culture, and Treatment

A young woman's urine culture grows three different organisms at 10⁴ CFU/mL each. The intern is ready to start antibiotics. The bench technologist reads the same plate and reports it as a contaminated specimen, no significant growth, please recollect. Both are looking at the same plate. The difference between them is not the organism list, it is knowing what a urine culture can and cannot tell you. That judgment is what this article is about.

Bacteria can invade and cause UTI via three major routes: ascending, hematogenous, and lymphatic. The ascending route is the most common in females. Hematogenous spread accounts for fewer than 5% of UTIs and rarely occurs with gram-negative bacilli.

Polymicrobial UTI in the absence of anatomic abnormalities, foreign bodies, or trauma is rare. Three or more organisms in large quantities usually indicates a contaminated specimen, a point that becomes central when you interpret the culture later in this article.

Etiology

*Escherichia coli*

Escherichia coli is the most common cause of community-acquired symptomatic UTIs, especially cystitis. Uropathogenic E. coli (UPEC) has remained the predominant uropathogen (80%) isolated in acute community-acquired uncomplicated UTIs. Escherichia coli can adhere to the urethral and bladder mucosa via pili.

*Staphylococcus aureus*

Staphylococcus aureus can cause pyelonephritis (infection of the renal parenchyma). In the case of bacteremic patients, S. aureus reaches the kidney via hematogenous spread or the descending route.

Because S. aureus typically reaches the kidney hematogenously, S. aureus in urine should prompt a question about bloodstream infection rather than a simple ascending UTI. This is why any count of S. aureus from a suprapubic or catheter specimen can be significant (see interpretation below).

*Staphylococcus saprophyticus*

It is common in young women. Approximately 10% to 15% of cases of UTIs in women of reproductive age groups are caused by S. saprophyticus. S. saprophyticus is the one coagulase-negative staphylococcus you do not dismiss as skin contaminant in a symptomatic young woman.

Mycobacteria

Mycobacteria can cause UTI in HIV-positive patients.

*Pseudomonas aeruginosa*

Pseudomonas aeruginosa can cause urinary tract infections, most common in patients staying in healthcare settings for longer. Patients with anatomic or neurologic abnormalities affecting their urinary tract or heavily antibiotic-experienced patients are also predisposed to UTIs from Pseudomonas aeruginosa.

Enterococci

Infrequently cause uncomplicated cystitis and pyelonephritis.

Other bacteria

Other bacteria commonly isolated from patients with UTIs are Klebsiella spp., Proteus spp. Enterobacter spp. Acinetobacter, Citrobacter, beta-hemolytic streptococci etc.

*Candida*

Candida species can cause UTI in patients with extensive prior antibiotic use and indwelling Foley catheters. Other high-risk patients are diabetic, immunocompromised, and immunosuppressive therapy patients.

Viruses

Viruses rarely cause UTIs. Adenovirus, BK virus, and cytomegalovirus can cause hemorrhagic cystitis. These viruses almost exclusively cause cystitis in immunocompromised hosts such as those who have undergone stem cell transplants.

Factors affecting prevalence of etiological agents

The etiology of UTI is also affected by underlying host factors that complicate UTI, such as

1. Age: The most common organisms isolated in children with uncomplicated UTI are Enterobacteriaceae. Gram-positive organisms are common, and polymicrobial infections account for up to 1 in 3 infections in the elderly.
2. Gender: Women are affected more often than men (about 40 to 50 times) because of the shorter female urethra (4 cm) compared with the male urethra (20 cm) i.e., infectious agents reach the bladder more easily in females.
3. Diabetes: Etiologic pathogens associated with UTI among patients with diabetes include Klebsiella spp., Group B streptococci, and Enterococcus spp., as well as E. coli.
4. Spinal cord injury or urinary catheterization: Patients with spinal cord injuries commonly have E. coli infections.

**Complicated vs. Uncomplicated UTI**

Complicated UTI has a more diverse etiology than uncomplicated UTI, and organisms that rarely cause disease in healthy patients can cause significant disease in hosts with anatomic, metabolic, or immunologic underlying diseases.

When to send urine for culture

Send urine for culture only when there is strong clinical suspicion of UTI, to avoid detecting asymptomatic bacteriuria. Culture is specifically indicated in suspected pyelonephritis (loin pain and fever), suspected UTI in men, recurrent UTI, pregnancy, and failed empiric treatment or persistent symptoms.

Common symptoms are frequency, urgency, dysuria, suprapubic pain, cloudy or bloody or strong-smelling urine, and flank pain.

Urine Sample Collection

Both noninvasive and invasive methods for collecting urine samples are available. Samples obtained from invasive methods are reliable as they are less likely to be contaminated and easy to interpret.

Urine collected by noninvasive methods passes through the contaminated milieu, so semi-quantitative culture is used to diagnose urinary tract infections (UTI) and to discriminate between contamination, colonization (asymptomatic bacteriuria), and clinical infection.

Suprapubic aspiration

Suprapubic aspiration is the best method to avoid contamination of specimens with bacteria in the distal urethra. Urine is withdrawn directly into a syringe through a percutaneously inserted needle.  This type of collection technique may be indicated in pediatric practice.

Use: Used infrequently because it is not indicated clinically (except in rare circumstances). It is invasive and uncomfortable and requires too much time and resources to be practical.

Straight catheter technique

Collection of urine by use of a single catheter (straight catheter technique) is the next-best technique for obtaining urine specimens with minimal contamination. It is an invasive technique with added disadvantages because inserting a catheter through the urethra can introduce bacteria into the bladder (and thereby cause UTI), and rare complications have been reported.

Use: It is not indicated clinically for most patients because it is too labor-intensive and costly for routine use.

Clean Catch Midstream Urine

It is the least invasive technique and is used widely. It has an obvious disadvantage compared to the techniques mentioned above. The chances of contamination from normal vaginal, perineal, and anterior urethral flora are high. The urine sample passes through the distal urethra and can become contaminated with commensal bacteria.

Commensal flora (resident flora) found in the urine samples are

1. Anaerobic cocci
2. Anaerobic gram-negative bacilli
3. Coagulase-negative staphylococci (excluding S. saprophyticus)
4. Commensal Mycobacterium spp.
5. Commensal Mycoplasma spp.
6. Diphtheroids (Corynebacterium spp.)
7. Lactobacilli
8. Nonpathogenic Neisseria spp.
9. Propionibacterium spp.
10. Viridans and non-hemolytic streptococci

Use: Most urine specimens are obtained from adult patients via the clean-catch midstream technique

**How to collect Midstream Specimen of Urine (MSU)?**

Instructions for male patients

1. Remove undergarments.
2. Wash hands.
3. Retract the foreskin completely.
4. Wipe the head of the penis in a single motion with the first towelette. Repeat with a second towelette. If not circumcised, hold foreskin back before cleansing.
5. Void 20 to 25 ml into the toilet and catch a portion of the remaining urine in the cup without stopping the stream. Do not touch the cup with the penis.
6. Place the lid on the cup securely
7. Immediately transfer to the microbiology laboratory or follow the procedure as indicated by the Hospital personnel.

Indwelling Catheter

Specimen collection from patients with indwelling catheters requires a scrupulous aseptic technique. The catheter tubing should be clamped off above the port to allow the collection of freshly voided urine. The catheter port or wall of the tubing should then be cleaned vigorously with 70% ethanol, and urine aspirated via a needle or syringe; the integrity of the closed drainage system must be maintained to prevent the introduction of organisms into the bladder.

Processing of Urine Sample

Calibrated Loop: Semi-Quantitative Urine Culture

The calibrated loop is the most clinically significant type for diagnostic microbiology. Unlike the standard 4 mm loop that picks up an indeterminate volume of specimen, calibrated loops deliver a precise, reproducible volume, enabling bacterial counting without full serial dilution.

Standard volumes and their applications:

How the calibrated loop urine culture works:

1. Mix the urine specimen thoroughly (invert tube gently 5–10 times).
2. Hold the calibrated loop vertically: the loop must be perpendicular to the surface of the urine to pick up a consistent volume by surface tension. Tilting the loop reduces the volume picked up.
3. Inoculate CLED agar (or blood agar + MacConkey) using a continuous back-and-forth streak across the full plate diameter, then streaking perpendicular lines across the primary streak.
4. Incubate at 37°C for 18–24 hours.
5. Count colonies on the primary streak area for the semi-quantitative result.

The key technique point: Vertical loop angle is everything. A loop held at 45° will underdeliver volume. A loop dipped more than 2–3 mm into the urine will overdeliver. Practice with water and a calibrated volume until the technique is consistent; a reproducible result depends entirely on reproducible loop angle and immersion depth.

Interpretation:

NOTE:

In certain conditions, even growth between 10³–10⁵ CFU/mL is considered significant. Those conditions are:

• Patient is on diuretics
• Patients being on antimicrobial

In certain conditions, even any count is considered significant. Those conditions are

• Specimen obtained from catheter tubing
• Suprapubic aspirate
• Suspected hematogenously acquired infection (e.g., S. aureus)

Culture

The standard protocol plates 1 µL of well-mixed urine onto 5% sheep blood agar and MacConkey agar (or CLED as a single-plate alternative), incubated aerobically at 35–37°C for 18–24 hours. Blood agar supports most uropathogens, MacConkey differentiates lactose fermenters and suppresses swarming Proteus, and CLED prevents Proteus swarming while allowing colony counting on one plate.

The next day, read the plates for colony count (against the interpretation table above), colony morphology, and any sign of contamination. Only specimens with significant growth are processed further for identification and susceptibility testing.

Treatment of Urinary Tract Infections

Choice of the antibiotics for the treatment of any infections depends on patients’ specific factors (age, underlying diseases/abnormalities) local resistance patterns of the etiological agents, and cost & availability of the drugs).

There are several national and international guidelines that help to choose an empiric regimen for UTIs. You can refer to 2010 IDSA/ESCMID guidelines (they preferred nitrofurantoin, trimethoprim-sulfamethoxazole, fosfomycin, and pivmecillinam for uncomplicated cystitis in women) or local guidelines of your country/hospital regarding the choice of antibiotics for the treatment of UTI.

Drugs used for the empiric treatment of uncomplicated UTIs are:

1. Ciprofloxacin: Avoid fluoroquinolones for uncomplicated UTIs when alternative antibiotics are possible. Fluoroquinolones should not be used if the local prevalence of resistance of the uropathogen exceeds 10%.
2. Nitrofurantoin: In patients with reduced renal function, nitrofurantoin should only be used if local resistance data suggests a high resistance to alternative agents. Due to lower drug levels in the renal parenchyma, nitrofurantoin and fosfomycin should be avoided in cases of suspected pyelonephritis.
3. Trimethoprim-sulfamethoxazole (co-trimoxazole): should not be used if the local prevalence of resistance of uropathogen (E.coli) exceeds 20%.

How to Remember

• The three C's of a positive urine culture — Contamination, Colonization, Clinical infection. A positive culture never means "UTI" by itself. It means one of three things: organisms from outside the bladder (contamination), organisms living in the bladder without disease (colonization, i.e., asymptomatic bacteriuria), or organisms causing disease (clinical infection). The count narrows it; the patient's symptoms decide it. The word "clinical" in the third C is the whole point. Symptoms are what separate it from colonization.
• "10⁵ or it didn't happen." For a clean-catch midstream specimen, ≥10⁵ CFU/mL is the significance threshold. Every exception loosens it, and they loosen it for a reason: the specimen is cleaner (suprapubic aspirate, catheter tube → any count) or the patient is primed so organisms are washed out (diuretics, prior antimicrobials → 10³–10⁵ counts), or the route bypasses the urethra entirely (hematogenous S. aureus → any count).

Key exam facts in one table

Where Students Get Confused

• A positive culture means the patient has a UTI. No. It means one of three things: contamination, colonization (asymptomatic bacteriuria), or clinical infection. The count and the symptoms decide which.
• Three organisms must be a bad polymicrobial infection. Usually the opposite: three organisms in a clean-catch specimen almost always signals contamination, not severe infection.
• Any coagulase-negative staph in urine is skin contaminant. S. saprophyticus is the exception: significant in a symptomatic young woman.
• The 10⁵ threshold is absolute. It's the default for clean-catch. It loosens (to any count, or to 10³) for cleaner specimens (suprapubic, catheter) and primed patients (diuretics, prior antibiotics).
• Loop angle is a detail. It's the whole result. A loop held at 45° or dipped too deep delivers the wrong volume and invalidates the count.
• Nitrofurantoin treats everything. It reaches poor levels in the renal parenchyma, making it the wrong choice for pyelonephritis.

References

1. Minardi, D., d'Anzeo, G., Cantoro, D., Conti, A., & Muzzonigro, G. (2011). Urinary tract infections in women: etiology and treatment options. International Journal of General Medicine, 4, 333–343. https://doi.org/10.2147/IJGM.S11767
2. Chu, C. M., & Lowder, J. L. (2018). Diagnosis and treatment of urinary tract infections across age groups. American Journal of Obstetrics and Gynecology, 219(1), 40–51. https://doi.org/10.1016/j.ajog.2017.12.231
3. Kaur, R., & Kaur, R. (2021). Symptoms, risk factors, diagnosis and treatment of urinary tract infections. Postgraduate Medical Journal, 97(1154), 803–812. https://doi.org/10.1136/postgradmedj-2020-139090
4. Llor, C., Moragas, A., Aguilar-Sánchez, M., García-Sangenís, A., Monfà, R., & Morros, R. (2023). Best methods for urine sample collection for diagnostic accuracy in women with urinary tract infection symptoms: a systematic review. Family Practice, 40(1), 176–182. https://doi.org/10.1093/fampra/cmac058
5. Karacan, C., Erkek, N., Senel, S., Akin Gunduz, S., Catli, G., & Tavil, B. (2010). Evaluation of urine collection methods for the diagnosis of urinary tract infection in children. Medical Principles and Practice, 19(3), 188–191. https://doi.org/10.1159/000273068
6. Sinawe, H., & Casadesus, D. Urine Culture. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. https://www.ncbi.nlm.nih.gov/books/NBK557569/