A new assay detects living Pseudomonas syringae through the proteins it injects into plants

An ELISA finds its target with antibodies raised in animals against that target ^[7]. ELCA finds it with a protein the bacterium already makes ^[11]. When Pseudomonas syringae, a bacterium of many crops, vegetables and woody ornamentals ^[2], attacks a plant, it injects effector proteins into host cells to get past its defenses ^[9]. Those effectors are fragile, so the bacterium carries chaperone proteins that bind each one tightly, like a lock and key, and shield it from degradation ^[10]. Rachel Herschlag, the lead author, built the test around that binding step ^[5][11].

Carolee Bull, a co-author and professor of bacterial systematics and plant pathology at Penn State, said many diagnostics use antibodies produced in animals to fight an infection ^[7]. "She went on to develop this technique by using the same mechanism that many pathogens use to invade their hosts to allow us to recognize them," Bull said ^[8].

The readout of ELCA ^[3] is a color change. "Unlike other diagnostics, this new method uses a bacterial chaperone to recognize and bind to its matching effector protein," Herschlag said ^[11]. "This then triggers a colorless solution to turn yellow, providing an easy visual signal that the target is present" ^[12].

What a seed tester wants to know is whether the bacteria are alive ^[18]. DNA-based tests pick up fragments of the cell that persist after the bacteria die, so they can report a positive when nothing is alive ^[13]. Effectors, the authors say, are made only by living cells ^[14]. "Because effectors are thought to degrade quickly, effectors from dead bacteria likely do not survive very long," Herschlag said ^[15]. "Indeed, we showed that living bacteria had higher levels of detection than dead bacteria" ^[16].

That is a difference in signal, not a clean cutoff ^[16]. The viability logic rests on an assumption the authors state as belief, that effectors degrade quickly once a cell dies ^[15]. The published account does not put a number on the detection limit or on how much higher the live signal ran than the dead. It is a proof of concept on one organism ^[5], and the team lists reducing background signal among its next steps ^[17].

The reach, if the assay holds, is wide. Any bacterium that uses a comparable chaperone-effector system could in principle get the same treatment ^[17]. "This could include some animal and human pathogens in addition to those affecting plants, expanding its potential utility in clinical, agricultural, environmental and biotechnology applications," Herschlag said ^[6].