Staphylococcus saprophyticus is a bacterium commonly encountered in clinical settings, particularly as a cause of infection. Correctly classifying this microorganism is a foundational step in microbiology, providing initial clues that guide laboratory identification and inform medical treatment decisions. Determining the Gram stain status is an initial, rapid process that helps narrow down identification possibilities. This article addresses the specific Gram stain classification of S. saprophyticus and explains the underlying biological reasons for this classification.
Staphylococcus saprophyticus and Gram Staining
Staphylococcus saprophyticus is definitively classified as a Gram-positive bacterium, appearing as purple or blue cocci (spherical cells) often arranged in clusters resembling bunches of grapes. The Gram stain is a differential staining technique used to separate bacteria into two major groups based on the physical properties of their cell walls. The process involves applying crystal violet dye, fixing it with iodine, and then washing it with alcohol or acetone. Gram-positive bacteria retain the purple color, while Gram-negative bacteria lose the purple stain and pick up a pink counterstain, like safranin. This initial Gram-positive classification immediately distinguishes the organism from Gram-negative pathogens and directs the selection of subsequent, more specific biochemical tests needed for a final diagnosis.
Understanding the Bacterial Cell Wall Structure
The reason S. saprophyticus stains purple lies in the structure of its cell wall. Gram-positive bacteria possess a thick, multi-layered cell wall composed primarily of peptidoglycan. This extensive mesh-like peptidoglycan layer effectively traps the crystal violet-iodine complex, preventing its wash-out by the alcohol decolorizer. Unlike Gram-negative organisms, Gram-positive bacteria lack an outer lipopolysaccharide (LPS) membrane. This structural arrangement ensures the organism retains the deep purple color despite the decolorization step.
Clinical Significance and Management
The Gram-positive classification of S. saprophyticus has significant implications for its clinical management, especially because it is a frequent cause of urinary tract infections (UTIs). It is the second most common cause of community-acquired UTIs, especially in young, sexually active women. Knowing the organism is Gram-positive immediately informs the choice of initial, or empiric, antibiotic treatment. Antibiotics used to treat Gram-positive infections often target the thick peptidoglycan layer, a structure absent in Gram-negative bacteria.
Laboratory Differentiation
Beyond the Gram stain, S. saprophyticus is further identified in the lab as a coagulase-negative staphylococcus (CoNS) that is uniquely resistant to the antibiotic novobiocin. This novobiocin resistance is a defining feature that differentiates it from other common, but usually non-pathogenic, CoNS like Staphylococcus epidermidis. This specific resistance is due to intrinsic structural changes in the drug target protein, DNA gyrase B subunit (GyrB).
Treatment
For uncomplicated UTIs caused by S. saprophyticus, the preferred oral treatments are typically nitrofurantoin or trimethoprim-sulfamethoxazole (TMP-SMX). This targeted approach, guided by the initial Gram stain and subsequent novobiocin differentiation, ensures that an effective antibiotic is chosen promptly, contrasting with the different drug classes often used to treat UTIs caused by the most common Gram-negative culprit, Escherichia coli.