Protein Spectra and Antigen Detection

Think about what each method actually measures. MALDI-TOF measures the masses of ribosomal proteins. The laser and matrix turn those proteins into ions, and the flight time through the tube sorts them by mass, producing a spectrum of peaks. That spectrum is compared with a library, and the closest match names the organism. Because it reads proteins rather than growth, it is fast, but it needs a pure colony and a library that contains the organism. Antigen tests work differently: an antibody binds one specific surface molecule, and binding produces a visible clump or color. That makes them fast and usable directly on body fluid, but each test only recognizes the organism it was designed for, so it is a targeted question, not a general identification.

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MALDI-TOF: identifying by protein mass

MALDI-TOF mass spectrometry (matrix-assisted laser desorption/ionization time-of-flight) identifies an isolate from its ribosomal protein content. A small amount of a pure colony is mixed with a matrix compound, dried on a target plate, and struck by a laser pulse. The matrix absorbs the energy and vaporizes the sample, ionizing the proteins. The ions then travel through a flight tube, and the time they take to reach the detector depends on their mass-to-charge ratio: lighter ions arrive first. The instrument records a spectrum of peaks, each corresponding to a ribosomal protein of a particular mass. That spectrum is the fingerprint. The software compares it against a reference library of spectra from known organisms and reports the closest match with a score. Because the method measures proteins rather than metabolic activity, it does not require the organism to grow during the test, and a result can be available within minutes of obtaining a colony. The requirements are strict, though: the colony must be pure, because a mixed sample produces a composite spectrum that matches nothing cleanly, and the organism must be present in the library. An organism absent from the database cannot be named, only reported as no reliable match.

Antigen-based identification

Antigen-based methods use antibodies that bind a specific molecule on the organism's surface, such as a capsular polysaccharide or a cell wall antigen. The antibody is labeled, often with a colored particle or a fluorescent tag, so binding produces a visible signal. A latex agglutination test, for example, mixes antibody-coated particles with a suspension of the organism; if the antigen is present, the particles cross-link and clump visibly. The strength of this approach is speed and directness. Because it detects a preformed surface molecule rather than a growth product, it can identify an organism from a colony within minutes, and some antigen tests work directly on body fluid such as cerebrospinal fluid or urine, before any culture is available. The limitation is scope: each test recognizes only the antigen it was designed for, so antigen testing is used when a specific organism is already suspected, not as a general identification method.

MALDI-TOF: identifying by protein mass

Antigen-based identification

Two routes to a name

MALDI-TOF

Antigen detection