Traditional bacterial testing methods rely either on culturing the microbes or on the polymerase chain reaction, better known as PCR, to study their DNA; the former takes days, while the latter cannot generally distinguish live from dead cells. An alternative approach called phage amplification detection lets bacterial viruses, or phages, do all the work. The method works because phages are picky about the bacterial strains they invade. Once inside their preferred hosts, the phages put the cells to work making new viruses. So if scientists inoculate a sample with a specific phage strain and then observe an explosion of viruses, they know that the phages' bacteria of choice are present.
Researchers usually detect the new viruses by mass spectrometry. But all generations of phages look the same in these tests. To establish that detected viruses are the result of invading a bacterial host, researchers must inoculate at concentrations below the detection limit of the spectrometer and wait for the viruses to multiply.
To speed up the method, John Barr and Jon Rees of the CDC, with Facundo Fernández and graduate student Carrie Pierce of Georgia Tech, infected cultures of a lab strain of Staphylococcus aureus with a virus called bacteriophage 53 that was labeled with a heavy isotope of nitrogen, 15N. Within infected cells, the labeled virus directed the production of unlabeled 14N progeny. Thanks to their different isotopes, the parent and progeny phages became easy to distinguish using a technique called matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, or MALDI TOF MS.
