Using viruses to detect bacteria!

The FDA just approved a new rapid detection test for MRSA and MSSA from blood cultures that are positive for Gram positive cocci (GPC). The most widely used tests for rapid MRSA or Staphylococcus aureus detection from blood cultures are real-time PCR tests by BD and Cepheid. These tests are limited by both cost and by some performance issues, one of which I referred to here.

The interesting thing about the new Microphage test is that it utilizes bacteriophages specific for S. aureus. The advantage of the test is that it is a simple immunoassay, requiring no special equipment or platform—the immunoassay detects phage antigens, which accumulate only if their target (S. aureus) is present. The MRSA/MSSA distinction is made in a similar manner, but using cefoxitin to inhibit the MSSA that the phages would otherwise feast upon. From the data in the package insert and in this ICAAC abstract from last year, the main limitation is a sensitivity of 91.8% for detection of S. aureus. If the result is positive for S. aureus, the assay is very accurate for distinguishing MRSA from MSSA (accurate enough to meet FDA standards for any commercial susceptibility test). So positive results should be useful, while negative results may not be (negative predictive value will vary based upon the proportion of all blood cultures positive for GPC that are S. aureus).

I could find no peer-reviewed literature on this test (point me to it if you can find anything), so we’ll have to see if real-life performance matches the trial data presented in the package insert. Cost will also be a big issue, and my understanding is that they haven’t settled on a price at this point.

Regardless of how it pans out, you have to love the concept of using bacteriophages for diagnostic purposes…compared with our nucleic acid detection tests, proteomics, etc., it just seems so…..old school!