A disk (disc) in microbiology is a small circular piece of filter paper or polymer impregnated with a defined amount of an antimicrobial or biochemical agent, used in diffusion based assays to assess microbial susceptibility or enzyme activity.
Background and Method
Disk diffusion tests are widely used in clinical microbiology to determine the susceptibility of bacteria to antibiotics. The method traces back to the Kirby–Bauer technique, which standardized the diameter of the paper disks, the concentration of antimicrobial, and the composition of the agar medium. In this assay, a bacterial lawn is inoculated on the surface of Mueller–Hinton agar, and antibiotic disks are placed on the agar. During incubation the antimicrobial diffuses outward, creating a concentration gradient. Susceptible bacteria are inhibited, forming a clear zone around the disk known as the zone of inhibition. The diameter of this zone is measured and compared with standardized tables to interpret the isolate as susceptible, intermediate or resistant. Disks are manufactured with precise antibiotic content to ensure reproducibility. The term “disc” is used interchangeably with “disk,” reflecting British and American spelling preferences. In addition to antibiotic susceptibility testing, disks impregnated with chemicals such as bacitracin, optochin or novobiocin are used in diagnostic schemes to differentiate closely related species. Filter paper disks can also be saturated with enzymes or substrates to detect enzyme activity, such as disks impregnated with oxidase reagent for the oxidase test. The reliability of disk based assays depends on proper storage of disks, maintaining dryness and correct temperature to prevent loss of potency.
Diagnostic Uses and Variants
The optochin disc test distinguishes Streptococcus pneumoniae from other alpha hemolytic streptococci, as S. pneumoniae is inhibited by optochin. Bacitracin discs are used to identify Streptococcus pyogenes. Novobiocin discs help differentiate Staphylococcus saprophyticus (resistant) from S. epidermidis (sensitive). Impregnated disks delivering growth factors such as X factor (hemin) and V factor (nicotinamide adenine dinucleotide) are used to differentiate Haemophilus influenzae from other Haemophilus species. In mycology, discs containing nystatin or amphotericin B assess fungal susceptibility. Specialized synergy tests use multiple disks spaced near each other to detect extended‑spectrum beta‑lactamase production or inducible clindamycin resistance. These examples illustrate the versatility of disk based assays in microbiology.
Disk based methods provide a simple and standardized way to evaluate antimicrobial susceptibility and identify organisms. Proper preparation, storage and interpretation of disk tests are essential for accurate laboratory results.
Related Terms: Kirby–Bauer test, zone of inhibition, antibiotic susceptibility testing, antimicrobial susceptibility testing, filter paper disk