10.3 Urinary Pathogens, Colony Counts, and Urinalysis Correlation

The M(ASCP) urine domain (II.I, guideline revised 2025-09-25) tests specimen sources, identification of major urinary pathogens, correlation of colony counts with clinical significance, and correlation of culture with urinalysis. This is a numbers-and-judgment section: the same 10,000 CFU/mL of Escherichia coli can be contamination, asymptomatic bacteriuria, or a real UTI depending on how the specimen was collected, who the patient is, and whether the dipstick and microscopy agree.

Specimen sources

Midstream clean-catch (MSCC) is the usual outpatient specimen. Instruct the patient to clean the periurethral area, void, then catch the middle of the stream. Even with good technique, periurethral and vaginal flora contaminate many specimens. Interpret mixed diphtheroids, lactobacilli, and coagulase-negative staphylococci from an MSCC as contamination, not as a three-pathogen infection.

Catheterized urine: an in-and-out (straight-catheter) specimen is cleaner than an indwelling Foley. If an indwelling catheter is in place, aspirate from the sampling port after disinfection—never culture the bag, and never culture a Foley catheter tip. Tips grow biofilm and do not represent bladder bacteriuria.

Suprapubic aspiration passes a needle through the abdominal wall into a full bladder. It is the gold-standard collection for infants and selected adults. Because the tract is sterile, any growth can be significant.

Nephrostomy urine comes from a percutaneous tube in the renal pelvis. Treat it as an upper-tract specimen: lower colony counts can still represent true colonization or infection of the kidney, and polymicrobial growth is more plausible than in an MSCC.

Transport within 2 hours at room temperature. If delay is unavoidable, refrigerate or use a boric-acid preservative (gray-top urine culture tube) so colony counts remain stable. Unpreserved urine at room temperature overgrows contaminants and inflates CFU/mL.

Major urinary pathogens: colony morphology and identification

Enterobacterales dominate community UTI. E. coli is a lactose-positive, typically indole-positive Gram-negative rod and causes the large majority of uncomplicated cystitis. Klebsiella spp. are mucoid lactose fermenters. Proteus spp. swarm on blood agar, are urease-positive, alkalinize urine, and are linked to struvite stones. Enterobacter and Citrobacter appear in complicated and healthcare-associated UTI. These organisms are nitrite-positive on dipstick when bladder dwell time is adequate, because they reduce nitrate.

Enterococcus spp. are Gram-positive cocci in pairs and short chains, catalase-negative, PYR-positive, and bile-esculin-positive. They grow in 6.5% NaCl. They do not reduce nitrate, so a nitrite-negative UTI is expected. They are important in catheter-associated and complicated UTI.

Streptococcus agalactiae in urine is both a uropathogen and a GBS-colonization marker in pregnancy. Beta-hemolytic catalase-negative cocci from a pregnant patient's urine should be identified. Laboratories commonly report GBS at ≥10^4 CFU/mL, and GBS bacteriuria during the current pregnancy is an indication for intrapartum prophylaxis. High-count bacteriuria in pregnancy is also treated as a UTI, not only as a colonization flag.

Candida spp. (C. albicans, C. glabrata, others) cause candiduria in catheterized, diabetic, and antibiotic-treated patients. Yeast on the plate should be correlated with yeast on urinalysis. Heavy mixed bacterial contamination plus scant yeast is not automatically a yeast UTI.

Staphylococcus saprophyticus is a coagulase-negative staphylococcus that causes community cystitis in young, sexually active women—a classic exam organism. Colonies are white to slightly yellow and usually nonhemolytic; catalase-positive, coagulase-negative. Novobiocin resistance (5-µg disk) separates it from S. epidermidis, which is novobiocin-susceptible. It is nitrite-negative. Do not discard coagulase-negative staphylococci in urine without a novobiocin result in this demographic.

Colony counts and clinical significance

Kass's classic threshold of ≥10^5 CFU/mL predicted true bacteriuria in asymptomatic women and remains the usual cutoff for asymptomatic bacteriuria (ASB). It is not a universal infection-versus-no-infection line.

Symptomatic women with cystitis can have true infection at 10^2–10^4 CFU/mL of a single uropathogen. Many laboratories still use 10^4 as a default workup threshold for MSCC, but the M exam expects you to lower the threshold when symptoms, pyuria, and a single uropathogen line up. Catheter-associated infection guidelines treat ≥10^3 CFU/mL of a species plus symptoms as significant. Suprapubic and other surgically obtained urine: any growth may be significant. IDSA ASB definitions use ≥10^5 CFU/mL on voided specimens (two consecutive specimens with the same species in women; one in men) and generally recommend not treating ASB except in pregnancy and before selected urologic procedures.

GBS in pregnancy: report at ≥10^4 CFU/mL per CDC laboratory guidance. Clinically, GBS bacteriuria this pregnancy triggers IAP, and high-count bacteriuria is treated as a UTI as well.

Calibrated loop math

Quantitative urine culture uses a calibrated loop, not a streak-for-isolation from an unmeasured drop.

• A 0.001 mL (1 µL) loop: each colony = 1,000 CFU/mL. Ten colonies = 10^4 CFU/mL. One hundred colonies = 10^5 CFU/mL. Thirty-five colonies of a single morphology = 3.5 × 10^4 CFU/mL.
• A 0.01 mL (10 µL) loop: each colony = 100 CFU/mL. This loop detects the 10^3 range needed for catheters, children, and symptomatic cystitis.

Worked example: a 0.001 mL loop yields 80 colonies of E. coli = 80,000 CFU/mL (8 × 10^4). If the patient is a symptomatic woman with a positive leukocyte esterase and 20 WBC/hpf, this count is significant even though it is just under 10^5. If the same count arrives with three additional skin-flora morphologies and a negative microscopic exam, think contamination.

Second worked example: a 0.01 mL loop yields 25 colonies of S. saprophyticus = 2,500 CFU/mL. In a young woman with dysuria and pyuria, that count can be a real UTI; the same count of mixed diphtheroids from an MSCC is not.

Confluent growth is reported as greater than 10^5 CFU/mL (or the laboratory's equivalent). Do not try to count a lawn and then multiply incorrectly.

Urinalysis correlation

Leukocyte esterase (LE) detects neutrophil esterase and is a proxy for pyuria. Microscopic WBC counts of about ≥5–10/hpf support inflammation. Nitrite detects bacterial reduction of urinary nitrate; it is useful when positive, mainly for Enterobacterales with adequate bladder time. Nitrite is often negative in infection due to Enterococcus, S. saprophyticus, Streptococcus, Acinetobacter, yeast, or a short time since the last void. A negative nitrite never excludes UTI.

Correlate, do not rubber-stamp:

• Single uropathogen at a significant count plus LE/WBC: typical UTI.
• ≥10^5 CFU/mL with no pyuria in an asymptomatic nonpregnant adult: ASB or contamination; do not automatically force susceptibilities into a treatment cascade.
• Pyuria with negative culture: prior antibiotics, urethritis (C. trachomatis, N. gonorrhoeae, M. genitalium), tuberculosis, fastidious organisms, or noninfectious inflammation.
• Bacteria on an uncentrifuged Gram stain of urine roughly correspond to ≥10^5 CFU/mL.

Mixed flora versus polymicrobial infection

MSCC with three or more morphologies, especially lactobacilli, diphtheroids, and mixed coagulase-negative staphylococci, is contamination—particularly if LE is negative. Recollect rather than identify everything. True polymicrobial UTI occurs with long-term catheters, stones, obstruction, fistula, or ileal conduit: two uropathogens (for example E. coli and Klebsiella pneumoniae) at significant counts with pyuria deserve workup. The exam distinction is mixed skin or vaginal flora from a dirty catch versus a limited number of recognized uropathogens from a catheter or stone patient.

In practice, read the source, do the loop math, look at the UA, then decide whether the plate is a UTI, ASB, contamination, or a true polymicrobial infection. That sequence is the II.I skill the exam is testing.