Novobiocin Susceptibility Test: Principle, Procedure, and Results
The Novobiocin Susceptibility Test is a widely utilized and essential diagnostic procedure in the clinical microbiology laboratory. It is categorized as a differential biochemical test, primarily employed for the presumptive identification and clear differentiation of various species within the genus *Staphylococcus*. Specifically, this test is critical for distinguishing the clinically significant uropathogen *Staphylococcus saprophyticus* from other, often commensal or contaminating, coagulase-negative staphylococci (CONS), most notably *Staphylococcus epidermidis*. The rapid and accurate result provided by this test is a cornerstone for guiding the early management and treatment of urinary tract infections.
Principle of Novobiocin Action and the Disk Diffusion Method
Novobiocin, an antibiotic belonging to the aminocoumarin class, is a natural product derived from the bacterium *Streptomyces niveus*. Its mechanism of action is based on the inhibition of bacterial DNA synthesis. Novobiocin functions as a potent inhibitor of the bacterial enzyme DNA gyrase, which is a type II topoisomerase. DNA gyrase is indispensable for essential cellular processes, including DNA replication, transcription, and repair, as it introduces negative supercoils into the DNA helix, relieving torsional stress. Novobiocin targets the GyrB subunit of this enzyme, competitively binding to the ATP-binding site and thereby inhibiting the ATPase activity that powers the enzyme’s function. Without a functional DNA gyrase, the bacterial cell cannot replicate its DNA or complete cell division, leading to the inhibition of growth.
The test itself is a standard disk diffusion assay, often following the Kirby-Bauer method. A filter paper disk impregnated with a standardized concentration (typically 5 µg) of the novobiocin antibiotic is placed on the surface of an agar plate that has been heavily seeded with the test organism. As the plate is incubated, the antibiotic diffuses radially outward from the disk into the agar medium, creating a concentration gradient. If the test organism is susceptible to the antibiotic, the novobiocin concentration prevents bacterial growth, resulting in a clear ring around the disk known as the “zone of inhibition.” Conversely, if the organism is resistant, it continues to grow right up to the margin of the disk because the novobiocin fails to inhibit its DNA gyrase and subsequent cell replication.
Test Objective and Specific Clinical Utility
The main purpose of the Novobiocin Susceptibility Test is rooted in the epidemiology of urinary tract infections (UTIs). *Staphylococcus saprophyticus* is a well-established cause of acute uncomplicated UTIs, ranking as the second most frequent bacterial cause after *Escherichia coli*, especially in young, sexually active female outpatients. When a coagulase-negative staphylococcus is isolated from a urine specimen, the laboratory must determine if it is the true pathogen, *S. saprophyticus*, or a common skin commensal contaminant, such as *S. epidermidis*. The Novobiocin Susceptibility Test provides a simple and rapid presumptive answer to this question.
The distinction is based on a fixed, intrinsic metabolic pattern: *S. saprophyticus* is innately and uniformly resistant to novobiocin, while *S. epidermidis* and the majority of other CONS species, including *S. lugdunensis* and *S. warneri*, are susceptible (sensitive). Therefore, the test allows for the essential grouping of CONS into novobiocin-resistant organisms (presumptively *S. saprophyticus*) and novobiocin-susceptible organisms (presumptively *S. epidermidis* or other CONS). This rapid differentiation is a critical step in providing timely clinical reporting, particularly because the positive predictive accuracy for identifying *S. saprophyticus* from urine isolates of young women based on novobiocin resistance is exceptionally high.
Detailed Procedure of the Novobiocin Susceptibility Test
The procedure must be executed meticulously to ensure accurate results. A pure culture of the test organism, ideally grown for 18–24 hours, is required. First, a bacterial suspension must be prepared to a standardized density, typically a 0.5 McFarland opacity standard, using a sterile diluent such as tryptic soy broth, saline, or sterile distilled water. This standardization ensures a uniform and consistent inoculum size.
Next, a suitable agar medium, most commonly Mueller-Hinton Agar (MHA) or 5% Sheep Blood Agar (BAP), is used. A sterile swab is dipped into the standardized suspension, and the excess fluid is removed. The entire surface of the agar plate is then inoculated by streaking the swab over the plate in three different directions to create a confluent lawn of growth. The plate surface must be allowed to dry for 5 to 15 minutes before the next step.
Using sterile forceps, one 5 µg novobiocin disk is aseptically placed onto the center of the inoculated agar surface. The disk is gently pressed down to ensure complete contact and adherence with the medium. The inoculated plate is then inverted and incubated aerobically at a temperature of 35°C to 37°C for an optimal period, typically 18 hours (for MHA) or 24 hours (for BAP).
Reading and Interpretation of Test Results
After the incubation period, the plate is examined for growth. The results are determined by measuring the diameter of the zone of inhibition surrounding the novobiocin disk, using a ruler or sliding calipers calibrated in millimeters. The clear zone is the area where the antibiotic has prevented visible bacterial growth.
The interpretation relies on established zone diameter cutoffs, which may vary slightly based on the medium used and the standard followed (e.g., Clinical and Laboratory Standards Institute – CLSI). When using Mueller-Hinton Agar, the following criteria are commonly applied:
Sensitive (Susceptible) Result: A zone of inhibition diameter greater than 16 mm. This result indicates that the bacterial DNA gyrase is successfully inhibited by the novobiocin. This is the expected result for *Staphylococcus epidermidis* and the majority of non-pathogenic coagulase-negative staphylococci.
Resistant Result: A zone of inhibition diameter less than or equal to 16 mm. This result indicates that the organism is resistant to the antibiotic’s action. This is the expected and definitive result for *Staphylococcus saprophyticus*.
A simple visual guide is often used: if there is an obvious, large clear zone around the disk, the organism is sensitive. If the confluent lawn of growth extends right up to the edge of the disk, or only a very small, non-significant zone is visible, the organism is resistant. For Blood Agar, a common susceptible cutoff is a zone size of 12 mm or greater, with resistance being less than 12 mm.
Mechanistic Basis for Intrinsic Resistance
The resistance of *S. saprophyticus* is not acquired through mobile genetic elements but is an intrinsic, chromosomal property. This innate resistance is genetically encoded by a novobiocin-resistant form of the DNA gyrase B subunit (GyrB). Comparative genetic studies have demonstrated that two specific amino acid residues, Glycine-85 and Lysine-140, in the *S. saprophyticus* GyrB protein are directly responsible for conferring a high minimum inhibitory concentration (MIC) to novobiocin. These residues effectively alter the binding pocket of the enzyme, reducing the antibiotic’s affinity for the target and allowing the bacteria to maintain DNA gyrase function even in the presence of novobiocin. This molecular difference is the consistent biological basis that makes the Novobiocin Susceptibility Test a reliable diagnostic tool for this species.
Summary and Quality Control
In summary, the Novobiocin Susceptibility Test is a rapid, cost-effective, and highly reliable test for the presumptive identification of *S. saprophyticus* when isolated from a suspected UTI. It provides a quick metabolic signature—resistance to 5 µg of novobiocin—that distinguishes it from other common CONS. However, it must be noted that the test should be used in conjunction with other traditional identification tests (such as catalase and coagulase results) for definitive species identification. To ensure the reliability of the test results, quality control (QC) is performed with known strains. The sensitive control, *Staphylococcus aureus* ATCC 25923, and the resistant control, *Staphylococcus saprophyticus* ATCC 15305, are routinely tested to confirm that the media and the antibiotic disks are functioning with the expected zone diameters.