Use of Novobiocin Disk for Presumptive Identification of Staphylococcus saprophyticus
Use of Novobiocin Disk
Novobiocin ( antibiotic) differentiation disk is useful for presumptively distinguishing Staphylococcus saprophyticus from other coagulase-negative staphylococci in clinical specimens as shown above picture.
Principle of Novobiocin Susceptibility Test
S. saprophyticus is a uropathogenic bacterium that causes acute uncomplicated urinary tract infections, particularly in young, middle-aged female patients and it is intrinsic resistant to the novobiocin. Novobiocin is an antibiotic that interferes with DNA during DNA replication and the bacterial cell cycle. It binds to DNA gyrase and blocks adenosine triphosphatase (ATPase) activity. Screening of organism on the basis of the following properties-Non hemolytic on blood agar, coagulase-negative, Gram reaction having Gram-positive cocci in single, pairs and clusters from urine cultures for novobiocin resistance is reliable presumptive identification of S. saprophyticus. Novobiocin disk is allowed to equilibrate to room temperature. A suspension equivalent to a McFarland 0.5 opacity standard is prepared either in Tryptic Soy Broth or Sterile Water or Brain Heart Infusion (BHI) Broth using a pure 18-24 hour culture. Organisms are inoculated on Mueller Hinton agar plate with a sterile swab to obtain confluent growth. A disk is applied onto the inoculated agar surface and lightly press down to ensure full contact with the medium. Incubate aerobically for 18-24 hours at 35-37ºC and finally measure the diameter of the zone of inhibition around the disk, and record as susceptible or resistant. A zone of inhibition greater than 16 mm is sensitive whereas a zone of inhibition less than or equal to 16 mm is resistant.
Interpretation of Results
Sensitive – A zone of inhibition greater than 16 mm
Resistant – A zone of inhibition less than or equal to 16 mm
Limitation of Tests
- The novobiocin disk is not helpful and can give misleading results if it is performed on isolates other than those from urinary samples.
- Coagulase-negative staphylococci (CoNS) isolated from humans that are not Staphylococcus saprophyticus such as Staphylococcus cohnii, Staphylococcus xylosus or Staphylococcus pulvereri may also be resistant to novobiocin.
- Only isolated colonies of aerobic, catalase-positive, coagulase-negative, Gram-positive cocci in clusters are to be tested.
- Biochemical, immunological tests are further recommended for the conformation of organisms.
References
- https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/NovobiocinDiffDisks.htm
- https://academic.oup.com/labmed/article-abstract/16/7/422/2640286?redirectedFrom=PDF
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC272557/
- http://europepmc.org/article/PMC/2167987