Part 2: What happens to the sample you submit to the NDSU-VDL for culture?

The NDSU-VDL bacteriology section uses matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry to assist in identifying bacterial and fungal colonies isolated from cultures. Identification on the MALDI takes only a few minutes whereas traditional methods may take up to several days.

A colony is taken with a toothpick, spread onto a MALDI plate and allowed to air-dry. Then, a “matrix” is added to the colony to extract proteins from the bacteria. The plate is loaded on the MALDI, which pulses the matrix-bacteria mixture with a UV laser. The laser causes the positively charged ion to release from the plate (desorption). The ion flies through a tube to a detector inside the MALDI. Ions of different masses and charges will travel faster/slower depending on the size of the molecule; the time it takes to reach the detector is the “time of flight” (TOF of MALDI-TOF). These different ions will create a molecular “fingerprint” that the instrument uses to provide an identification for the microbiologist to interpret.

Whether the organism is considered a pathogen varies depending on the species, the sample source, and quantity isolated. For example, some bacteria are always considered pathogens (anthrax) while others are only pathogens if they out compete the good bacteria normally present at a certain body site. Microbiologists are experts in deciding what is normal and what should be considered a pathogen, but they can also consult our pathologists or the submitting veterinarian to aid in deciding which bacteria to workup.

If an organism is determined to be the likely pathogen, the microbiologist may setup antibiotic sensitivity testing to determine which antimicrobials will be best for the patient. Some bacteria have a predictable pattern of “sensitivity” to an antibiotic and do not require sensitivity testing while others vary and should be tested to make sure the antibiotic prescribed will work against that pathogen.

Kelli Maddock, MLS (ASCP)CM

A microbiologist spreads a bacterial colony on a MALDI-TOF plate.
Lynn Schaan, MT (diagnostic microbiologist) loads a plate on the MALDI.