The first look at the two mem­branes of Gram-neg­a­tives

Pic­tures Con­sid­ered #34

by Elio

Fig­ure 1. Three sep­a­rate struc­tures sur­round the cell: the outer mem­brane (OM), solid mem­brane ℠, and the plasma mem­brane (PM). The outer mem­brane, a unit mem­brane, ap­pears ex­tremely con­vo­luted, whereas the solid mem­brane tightly fol­lows the pe­riph­ery of the pro­to­plas­mic con­stituents. Sev­eral cir­cu­lar struc­tures are also ev­i­dent in the outer mem­brane. Mag­ni­fi­ca­tion 120,000×. Source

In a re­cent "Clas­sic Spot­light" in the Jour­nal of Bac­te­ri­o­lo­gy, the ed­i­tor in chief, Tom Sil­havy, pointed out that the first vi­sual proof that Gram-neg­a­tive bac­te­ria have two mem­branes came from work by Bladen and Mer­gen­ha­gen. In their 1964 pa­per, they clearly showed in thin sec­tion of the Gram-neg­a­tive anaer­obe Veilonella three en­velope lay­ers, the outer mem­brane, the pep­ti­do­gly­can cell wall, and the cy­to­plas­mic mem­brane. They went on to show that lysozyme de­stroys the mid­dle layer, and, by se­lec­tive lipid ex­trac­tion, that lipopolysac­cha­ride re­sides in the outer mem­brane, a term they coined. Within a few years, tech­niques for the sep­a­ra­tion of the in­ner and outer mem­brane by den­sity cen­trifu­ga­tion be­came avai­lable, mak­ing it pos­si­ble to an­a­lyze these frac­tions bio­che­mically. These con­firmed the broad con­clu­sions based on mor­pho­log­i­cal stud­ies in the orig­i­nal pa­per.

 

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