Fine Read­ing: No Growth!

by Roberto

One of my most mem­o­rable early lessons in mi­cro­bial phys­i­ol­ogy (and one that I still love to chal­lenge gen­eral au­di­ences with ) was be­ing asked how long would it take E. coli  cells to reach the mass of the Earth (un­der con­di­tions of un­re­stricted growth). It was and re­mains a won­der­ful ex­er­cise that al­lows the learner to be­gin to dis­cover the re­mark­able dy­nam­ics of ex­po­nen­tial growth. For STC read­ers who have not yet per­formed this sim­ple ex­er­cise, I im­plore to do so. It's fun! (Oth­ers should feel free to skip ahead to the next para­graph.) To start, make these three sim­ple as­sump­tions: (1) one E. coli  cell has a mass of 10^–13 grams, (2) Earth has a mass of 10²⁷ grams, and (3) the dou­bling time of E. coli  is 20 min­utes. How long, then, will it take for the prog­eny from a sin­gle E. coli  cell to reach the mass of Earth? Once you have the an­swer, keep read­ing.

How long un­til E. coli  reaches the mass of Earth? By R. Kolter

I have to ad­mit that when I first was con­fronted with this ques­tion I had a feel­ing that it would be a rel­a­tively short time. Rel­a­tive, that is, to the ge­o­log­i­cal age of Earth. But… less than two days? Wow! As the say­ing goes, I was floor­ed! Know­ing full well that only a tiny frac­tion of Earth has the nu­tri­ents to sus­tain cel­lu­lar growth, it was ev­i­dent that a freely grow­ing E. coli  would fin­ish those in a mat­ter of a few hours. Clearly, for the ex­tant pop­u­la­tions of Bac­te­ria and Ar­chaea on Earth, that "un­seen ma­jor­ity" thought to num­ber some 10³⁰ cells, the pe­ri­ods of un­re­stricted growth must be few and far be­tween. The rest of their ex­is­tence is, by ne­ces­sity, spent in pe­ri­ods be­ing en­ergy de­prived and not di­vid­ing, yet ready to start di­vid­ing if and when con­di­tions be­come pro­pi­tious for growth. In gen­eral, our un­der­stan­ding of cells in such states is very poor. For one, mi­cro­bi­ol­o­gists in­ter­ested in de­scrib­ing mol­e­c­u­lar mech­a­nisms of reg­u­la­tion at play in mi­crobes have largely avoided study­ing non-grow­ing cells. And many of those in­ter­ested in en­ergy-lim­ited states have not fo­cused their at­ten­tion on such things as reg­u­la­tion of gene ex­pres­sion.

It is in the hope of bridg­ing this gap, as a way to stim­u­late (in their words) "di­alec­tic be­tween en­vi­ronmental and mol­e­c­u­lar mi­cro­bi­ol­ogy per­spec­tives," that Megan Bergkessel, David Basta and Di­anne New­man from Cal­tech of­fer us a won­der­ful re­view en­ti­tled "The phys­i­ol­ogy of growth ar­rest: unit­ing mol­e­c­u­lar and en­vi­ron­men­tal mi­cro­bi­ol­ogy." In my opin­ion, this should be re­quired read­ing for all mi­cro­bi­ol­o­gists. This pa­per is way more than a sim­ple re­view of the lit­er­a­ture. In each of the three main ar­eas of growth ar­rest phys­i­ol­ogy that the au­thors ex­plore, en­ergy me­ta­bo­lism, reg­u­la­tion of gene ex­pres­sion, and DNA main­te­nance, they not only crit­i­cally re­view pub­lished re­sults, they of­fer their views on fu­ture di­rec­tions that they feel the work should take. The­re­in, mi­cro­bi­ol­o­gists every­where can find some key di­rec­tions to in­spire their fu­ture work. In ad­dition, three very nicely "boxes" cover the ba­sics of non-grow­ing states, pop­u­la­tion het­ero­gene­ity and new tools for study­ing non-grow­ing states. Very very fine read­ing in­deed!

Bergkessel M, Basta DW, New­man DK. The phys­i­ol­ogy of growth ar­rest: unit­ing mol­e­c­u­lar and environ­mental mi­cro­bi­ol­ogy. Nat Rev Mi­cro­biol. 2016 Aug 11;14(9):549–62. doi: 10.1038/nrmicro.2016.107. PMID: 27510862; PMCID: PMC10069271 (Open Ac­cess)