Fine Read­ing: When Mi­cro­bial Con­ver­sa­tions Get Phys­i­cal

by Elio

In a re­cent Opin­ion piece in Trends in Mi­cro­bi­ol­ogy, Gemma Reguera in­vites us to think out­side the box. The box in ques­tion har­bors the no­tion that com­mu­ni­ca­tion be­tween mi­crobes is chem­i­cal, and only chem­i­cal. Here, the au­thor cuts through the con­fines of this per­spec­tive to point out that mi­cro­bial com­mu­ni­ca­tion also takes place via phys­i­cal in­ter­ac­tions.

By phys­i­cal, Reguera means sound waves, elec­tro­mag­netic ra­di­a­tion (mainly light), and elec­tric cur­rents. The lat­ter, thanks to re­cent work on the func­tion mi­cro­bial nanowires, is the more fa­mil­iar one. (In­ci­den­tally, this paper's brief overview of the nanowire busi­ness is as clear as any I've seen.) Nanowires form grids be­tween cells, clearly con­sti­tut­ing an ef­fec­tive mech­a­nism of com­mu­ni­ca­tion. But fur­ther, Reguera pro­poses that many other mi­crobes may com­mu­ni­cate elec­tri­cally, all be­ing po­lar­iz­able and many be­ing known to re­spond to elec­tri­cal re­ori­en­ta­tion (i.e., gal­van­o­taxis).

In­ter­con­ver­sion of phys­i­cal sig­nals in hu­man com­mu­nication net­works (left) and mi­cro­bial analogs (right). In all pan­els, the sender is on the left, the re­ceiver on the right and in­for­ma­tion car­ri­ers in the mid­dle. In a man­ner anal­o­gous to the in­ter­con­ver­sion of sounds and elec­tric sig­nals in fixed tele­phone lines, mi­cro­bial cells might be po­lar­ized by in­com­ing sound waves of the cor­rect fre­quency to con­trol the flow of elec­trons, and elec­tric cur­rents gen­er­ated or re­ceived by the cell and vice versa (top panel). The in­ter­con­ver­sion of elec­tro­mag­netic ra­di­a­tion (ER) and sounds dur­ing ra­dio broad­cast­ing could also have a mi­cro­bial ana­log (bot­tom panel). The in­tra­cel­lu­lar move­ment of charged par­ti­cles that is in­duced by sound waves could cre­ate an elec­tro­mag­netic field and serve as a source of ER. The re­verse would also be pos­si­ble: ER might in­duce me­chan­i­cal vi­bra­tion and po­lar­iza­tion of the cell, thereby en­abling the gen­er­a­tion of sounds and elec­tric cur­rents, re­spec­tively. Source

Phys­i­cal means of com­mu­ni­ca­tion are not lim­ited to the elec­tri­cal. Sounds waves count, as does light. For one ex­am­ple, sound waves are re­ported to stim­u­late the growth of a Bacil­lus species (B. car­boniphilus) un­der stress con­di­tions. For other ev­i­dence for this sur­pris­ing propo­si­tion, I re­fer you to the pa­per. Per­haps more set­tled is the no­tion of light play­ing a role in mi­cro­bial phys­i­ol­ogy. Be­sides bi­o­lu­mi­nes­cence, some light is emit­ted by cells as bio­pho­tons, weak light emis­sions in the vis­i­ble and near in­frared, the re­sult of ex­er­gonic chem­i­cal re­ac­tions. The au­thor says: The con­tin­u­ous mo­tion of charged par­ti­cles in­side the cell gen­er­ates an elec­tro­mag­netic field, and there­fore, a po­ten­tial source of elec­tro­mag­netic ra­di­a­tion. In­ter­est­ingly, the wave­lengths of the elec­tro­mag­netic spec­trum that ap­pear to af­fect mi­crobes over­lap those of the sizes of mi­cro­bial cells (~0.1 to 100 μm).

When does phys­i­cal com­mu­ni­ca­tion trump chem­i­cal? One easy an­swer is when the sig­nal is faint and re­quires a small en­ergy in­vest­ment. Thus, phys­i­cal sig­nals work best when the cells "whis­per" to one an­other. Such sig­nals also prop­a­gate faster, not be­ing lim­ited by dif­fu­sion of mol­e­cules, and nei­ther do they re­quire mem­brane re­cep­tors. But we have just scratched the sur­face and much re­mains to be learned about these rel­a­tively un­fa­mil­iar phe­nom­ena. Reguera ends her cap­ti­vat­ing ar­ti­cle by stat­ing: Phys­i­cal sig­nal­ing could be an an­ces­tral lan­guage of all liv­ing forms and, per­haps, a key code to de­ci­pher if we want to un­der­stand the mi­cro­bial con­ver­sa­tions that have for so long re­mained in­audi­ble.