Staphylococcus saprophyticus is a type of bacteria commonly found in the environment and on the bodies of humans and animals. This organism is generally considered part of the normal microbial community living on the skin and mucosal surfaces without causing harm. However, it is recognized as an organism capable of causing specific infections when it travels to other parts of the body.
Characteristics and Natural Habitat
Staphylococcus saprophyticus is classified as a Gram-positive coccus, which means the individual bacterial cells are spherical and retain a violet stain during a standard laboratory test. These cells typically group together in clusters that resemble bunches of grapes when viewed under a microscope. It is further categorized as a coagulase-negative staphylococcus (CNS) because it does not produce the enzyme coagulase, which is a key distinction from more widely known organisms like Staphylococcus aureus.
A defining feature of this species is its natural resistance to the antibiotic Novobiocin, a property highly unusual among other coagulase-negative staphylococci. This intrinsic resistance is a permanent trait of the species and is an important tool for laboratory identification.
The natural reservoirs of S. saprophyticus include the skin, the perineum, and the gastrointestinal and genitourinary tracts of humans. It is also frequently found in the gut flora of domestic animals, such as pigs and cows, suggesting a potential route of transmission to humans through consumption of contaminated meat products. In humans, colonization of the rectum and the area surrounding the urethra provides a source for potential infection if the bacteria are displaced.
Role as a Urinary Pathogen
The most significant role of Staphylococcus saprophyticus in human health is its propensity to cause acute, uncomplicated urinary tract infections (UTIs). The organism is a common cause of these infections, and is second only to Escherichia coli as a cause of community-acquired UTIs in young women. The infection rate is particularly high in sexually active females between the ages of 16 and 25, where S. saprophyticus can account for 10% to 20% of all cases.
Infection often begins when the bacteria, which may be colonizing the perineum or genital tract, are mechanically transferred into the urethra. From there, the bacteria ascend into the bladder, a process that is facilitated by bacterial surface proteins called adhesins. These adhesins allow the organism to tightly bind to the cells lining the urinary tract, helping it to colonize the tissue and resist being flushed out by urination.
The resulting infection is characterized by symptoms of cystitis, which is inflammation of the bladder. Patients commonly experience dysuria, which is painful or burning urination, along with increased urinary frequency and an urgent need to urinate. Lower abdominal discomfort or suprapubic pain is also frequently reported, reflecting the irritation of the bladder wall. Infections caused by this organism can also display a seasonal pattern, with a higher incidence observed during the late summer and early fall months.
Identification and Treatment
Identifying an infection caused by S. saprophyticus begins with a urine sample collected via the clean-catch mid-stream method. A urinalysis can reveal the presence of white blood cells and sometimes red blood cells, indicating an inflammatory response in the urinary tract. Definitive diagnosis requires a urine culture, where the bacteria are grown in a laboratory setting to confirm their identity.
A key difference in diagnosis is that a lower number of colony-forming units (CFUs) in the culture may be considered significant compared to the threshold used for other uropathogens. Once a staphylococcus is isolated and confirmed to be coagulase-negative, the defining Novobiocin resistance test is performed. The ability of the bacteria to grow in the presence of the antibiotic confirms the isolate as S. saprophyticus, distinguishing it from other non-pathogenic staphylococci.
Treatment for uncomplicated UTIs caused by this organism relies on oral antibiotics that concentrate well in the urinary tract. Common first-line options include nitrofurantoin, prescribed for a short course, or trimethoprim-sulfamethoxazole (TMP-SMX). Fluoroquinolone antibiotics are reserved to reduce the risk of resistance development. Susceptibility testing is performed on the isolated bacteria to ensure the chosen antibiotic is effective.