A Mas­ter­piece of Bac­te­r­ial Struc­ture

by Elio

Terry Bev­eridge of Guelph Uni­ver­sity died re­cently. We re­gret the loss of this out­stand­ing struc­tural microbiolo­gist who made stel­lar con­tri­bu­tions to var­ied top­ics such as the struc­ture of the S‑layer, the in­ter­ac­tion of micro­bial sur­faces with met­als and an­tibi­otics, bac­te­r­ial cell di­vi­sion, Gram stain and its vari­abil­ity, and many more. He was a very fine per­son, be­fit­ting his early train­ing with Bob Mur­ray, a great friend of this blog. I am prompted to honor Terry's mem­ory by re­count­ing an im­por­tant piece of work he pub­lished in 1991. It still has much cur­rency.

Thin-sec­tion com­pos­ite show­ing cell spacer plugs (A) and an end plug (B). P, Plug; S, sheath; W, cell wall; AM, amor­phous ma­trix. Bars = 100 nm. Source

Terry and his col­leagues were study­ing an ar­chaeon, Methanospir­il­lum hun­gatei. (The species name hon­ors an­other one of my he­roes, Bob Hun­gate, whom I will speak about at an­other time.) M. hun­gatei is sheathed and forms chains some 9–12 cells long. What makes them un­usual is that they have pro­teina­ceous plugs at the ends of each cell. Fur­ther­more, the plugs at the ends of each fil­a­ment stain Gram-pos­i­tive, the rest of the fil­a­ment Gram-neg­a­tive. So, is M. hun­gatei Gram-pos­i­tive or Gram-neg­a­tive? The an­swer is both, as was pointed out re­cently in a com­ment to this blog by SMC.

El­e­gantly, Beveridge's lab stud­ied this phe­nom­e­non by re­plac­ing Gram's io­dine with an elec­tron-dense plat­inum com­pound and then view­ing the cells us­ing high res­o­lu­tion elec­tron mi­croscopy. They found that the plugs, which could be iso­lated from the sheaths for study, con­sist of three lay­ers with highly-or­dered mol­e­c­u­lar ar­rays. All three lay­ers con­tain holes, and one of them (the "ho­ley layer") has sig­nif­i­cantly larger holes. They pro­posed that these holes, which make the plugs 3–6 times more porous than the sheath, al­low pas­sage of com­pounds of higher mol­e­c­u­lar weight – thus the se­lec­tive Gram-pos­i­tive stain­ing of the fil­a­ment ends. Once through the ter­mi­nal plug, such com­pounds can dif­fuse along the fil­a­ment axis to the rest of the cells, pass­ing through the in­te­rior plugs along the way. This arrange­ment may re­flect the need for cau­tion on the part of the cell re­gard­ing what it lets in. By lim­it­ing the en­try for large and pos­si­bly toxic com­pounds to the ends of fil­a­ments, the in­te­rior cells may have a bet­ter chance to ei­ther es­cape or counter their tox­i­c­ity.

This pa­per is full of lovely high res­o­lu­tion mi­cro­graphs and com­put­er­ized re­con­struc­tions which show­case the nat­ural sym­me­try and pat­tern of these un­usual struc­tures. And we were so naïve as to sug­gest (in our Tal­mu­dic Ques­tion # 21) that Ar­chaea tend to be less struc­turally so­phis­ti­cated!

Note: See more elec­tron mi­cro­graphs of Terry Beveridge's lab in Pic­tures Con­sid­ered #45 : A Gau­ze Ban­dage.