Staphylococcus saprophyticus: Properties, Pathogenesis, and Lab Diagnosis

A young woman, recently married, develops burning on urination and a constant urge to go, five days after becoming sexually active. Her urine grows not the E. coli you would expect, but a Staphylococcus. This is Staphylococcus saprophyticus, and the pattern is so characteristic it earned the name honeymoon cystitis.

The interesting question is not just how the lab identifies it, but why this one organism, out of dozens of harmless staphylococci living on our skin, is the one that reliably infects the bladder of young women.

The answer is written into the organism itself: a set of tools that let it grip the bladder wall, survive in urine, and resist being flushed out. This article works through what those tools are and how they turn a skin resident into a urinary pathogen.

Staphylococcus saprophyticus is one of the pathogenic species of staphylococci, the other two are S. aureus and S. epidermidis.  Staphylococcus epidermidis and S. saprophyticus are often referred to as coagulase-negative staphylococci (CONS).

Figure: Flow chart for the identification of Staphylococcus saprophyticus

S. saprophyticus causes urinary tract infections (UTI), especially cystitis in sexually active young women, second only to E. coli as a cause of community-acquired UTI in this group. The infection often follows sexual intercourse closely enough that it has a clinical nickname worth knowing: honeymoon cystitis.

General Features

1. In Gram staining, Staphylococcus saprophyticus appears as Gram-positive cocci in clusters.
2. Catalase test: Positive
3. Coagulase production: No
4. Typical hemolysis: None
5. Resistant to novobiocin in contrast to S. epidermidis which is sensitive.
6. Causes community-acquired UTI in young, sexually active women

Transmission

Staphylococcus saprophyticus is a normal flora of the perineum, rectum, urethra, cervix, and gastrointestinal tract. It is found primarily on the mucosa of the genital tract in young women and from that site can ascend into the urinary bladder to cause urinary tract infections. Most women with this infection have had sexual intercourse within the previous 24 hours. Patients usually present with dysuria, pyuria, and hematuria.

Why This Organism Causes Disease Where It Does

Almost everything about S. saprophyticus as a pathogen follows one logic: it is built to colonize and hold onto the urinary tract, a niche most staphylococci cannot occupy.

Three tools explain why this particular coagulase-negative Staphylococcus, out of dozens living harmlessly on skin, is the one that consistently causes UTIs. Read them as a sequence: stick, survive, persist.

• Adhesins (including a surface protein called Aas) let it stick tightly to uroepithelial cells lining the bladder, resisting the mechanical washout that urination would otherwise provide.
• Urease, present in essentially all strains, hydrolyzes urea into ammonia, which alkalinizes the urine. This is the same mechanism behind Proteus-associated struvite stones. S. saprophyticus is a less common but real cause of infection-associated struvite (magnesium ammonium phosphate) stones, worth remembering as an exception to "Proteus is always the urease answer."
• Biofilm formation lets it persist on the bladder lining and on indwelling catheters, contributing to why recurrence is common.

Although S. saprophyticus usually causes lower urinary tract infection (cystitis), the same adhesins and urease that let it colonize the bladder can, less commonly, allow ascending infection toward the kidney (pyelonephritis), particularly when infection is untreated or recurrent. Its urease also makes it an uncommon but real cause of infection-associated struvite stones, the same stone chemistry usually attributed to Proteus.

Sexual intercourse mechanically displaces the organism from its normal home, the perineum, rectum, and periurethral area, into the short female urethra, which is the direct explanation for honeymoon cystitis and why symptoms typically appear within 24 hours of intercourse.

Laboratory Diagnosis

Sample: Clean-catch mid-stream urine sample.

Find detailed information about the urine sample collection procedure here.

Colony Morphology in 5% sheep blood agar

S. saprophyticus usually gives white colonies in blood agar, but colonies can be yellow to orange. Colonies are large; entire, very glossy, smooth, opaque, butyrous, and convex.

Urine culture and the colony-count exception: the standard cutoff for significant bacteriuria is 100,000 (10⁵) CFU/mL, a threshold derived largely from E. coli infections. S. saprophyticus is a recognized exception. It frequently causes genuine, symptomatic infection at counts below 10⁵ CFU/mL, so applying the usual cutoff will cause real infections to be dismissed as insignificant.

In a young, sexually active woman with typical cystitis symptoms, a lower count of a catalase-positive, coagulase-negative, novobiocin-resistant Staphylococcus should be taken seriously, not discarded.

When gram-positive cocci in clusters are recovered from urine, identification follows a short sequence: catalase positive confirms Staphylococcus, coagulase negative places it among the coagulase-negative staphylococci, and the novobiocin test then separates the two that matter here. A novobiocin-resistant isolate is presumptively S. saprophyticus (the uropathogen); a novobiocin-sensitive isolate is usually S. epidermidis (a skin contaminant).

For the breakpoint, procedure, and why this test is reliable only on urinary isolates, see the novobiocin test article. Because S. saprophyticus is a true pathogen in this population, an isolate should have antimicrobial susceptibility testing performed and the antibiogram reported to the physician.

Figure: Staphylococcus saprophyticus growing on Mannitol Salt Agar (Image source: Gary E. Kaiser)

Where students actually get confused

• A colony count below the usual 100,000 CFU/mL threshold doesn't rule this organism out. S. saprophyticus UTIs are a recognized exception to the standard significant-bacteriuria cutoff, lower counts can still be clinically significant, especially in sequential specimens. Treating this organism with the same threshold logic as E. coli will cause you to dismiss real infections.
• Catalase-positive, coagulase-negative is not the end of the workup. Both S. epidermidis and S. saprophyticus fit that description. Novobiocin is the test that actually separates them, not catalase or coagulase.
• "CoNS in urine" used to mean contaminant by default. Historically true for most CoNS, but S. saprophyticus specifically is a real uropathogen in young women, not a skin contaminant, context (young, sexually active, symptomatic) matters as much as the organism ID.
• Proteus is not the only urease-positive stone former. S. saprophyticus is urease-positive and is an uncommon but real cause of infection-associated struvite stones. On an exam, "urease-positive uropathogen" is not automatically Proteus, the young woman with a Staphylococcus UTI can be the answer.

Key exam facts

Treatment

Staphylococcus saprophyticus urinary tract infection is treated as uncomplicated cystitis. First-line options are nitrofurantoin or trimethoprim-sulfamethoxazole, with the choice guided by local resistance patterns.

Fluoroquinolones such as ciprofloxacin are effective but are generally reserved rather than used first, given fluoroquinolone toxicity concerns and the need to preserve them. Note that S. saprophyticus is often reported as resistant to fosfomycin, unlike E. coli, which is a practical point when a Staphylococcus rather than a coliform is the cause.

References

1. Kuroda M, Yamashita A, Hirakawa H, et al. Whole genome sequence of Staphylococcus saprophyticus reveals the pathogenesis of uncomplicated urinary tract infection. Proc Natl Acad Sci U S A. 2005;102(37):13272-13277. https://doi.org/10.1073/pnas.0502950102
2. Gatermann S, John J, Marre R. Staphylococcus saprophyticus urease: characterization and contribution to uropathogenicity in unobstructed urinary tract infection of rats. Infect Immun. 1989;57(1):110-116. https://doi.org/10.1128/IAI.57.1.110-116.1989
3. Raz R, Colodner R, Kunin CM. Who are you, Staphylococcus saprophyticus? Clin Infect Dis. 2005;40(6):896-898. https://doi.org/10.1086/428353
4. Gupta K, Hooton TM, Naber KG, et al. International clinical practice guidelines for the treatment of acute uncomplicated cystitis and pyelonephritis in women: a 2010 update by the IDSA and ESCMID. Clin Infect Dis. 2011;52(5):e103-e120. https://doi.org/10.1093/cid/ciq257
5. Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.