An innovative method of shining a laser on a minute drop of blood, mucus, or wastewater may enable the light reflecting back to positively identify the bacteria in the sample. The study, published Wednesday in the journal Nano Letters, details an approach that could lead to inexpensive, accurate, and almost immediate microbial assays of virtually any fluid a researcher might want to test for microbes.
Culturing methods in use today can take hours or even days to complete. A tuberculosis culture takes 40 days. The new test enables researchers to find out which specific bacteria -- such as E. coli, Staphylococcus, Streptococcus, Salmonella, or anthrax -- are present in a sample. It may facilitate better and faster infection diagnoses, improved use of antibiotics, safer foods, enhanced environmental monitoring, and faster drug development.
Each type of bacterium demonstrates a unique pattern of light. However, virtually every other molecule or cell in a given sample does as well. Red blood cells, white blood cells, and other sample components all send back their own signals, making it difficult to distinguish their microbial patterns from the signals of other cells. The team's breakthrough was in distinguishing those optical fingerprints from the broad array of light reflecting from each sample.
A milliliter of blood the size of a raindrop can contain billions of cells, only a few of which might be microbes. The team sought to separate and amplify the light reflecting from the bacteria alone. To do that, they combined a four-decade-old technology -- the inkjet printer -- with two cutting-edge technologies: nanoparticles and artificial intelligence (AI).
The researchers found that the key to separating bacterial spectra from other signals was to portion the cells into extremely small samples. They utilized modified inkjet printing principles to print out thousands of tiny dots of blood instead of investigating a single large sample. Each printed blood dot was just two trillionths of a liter in volume -- a billion times smaller than a raindrop, droplets so small they may hold just a few dozen cells.
The researchers then infused the samples with gold nanorods that attach themselves to bacteria. Somewhat like antennas, they draw the laser light toward the bacteria and amplify the signal about 1,500 times. Appropriately isolated and amplified, the bacterial spectra stand out. Machine learning then compares the few spectra reflecting from each printed fluid dot to telltale bacteria signatures.
While this technique was used with blood samples, the researchers say it can also be applied to other fluids containing target cells, with applications including testing drinking water or spotting viruses more quickly, accurately, and inexpensively than present methods.
“It’s an innovative solution with the potential for life-saving impact. We are now excited for commercialization opportunities that can help redefine the standard of bacterial detection and single-cell characterization,” said Cairo University professor and senior co-author Amr A.E. Saleh, PhD, in a statement.