Infectious multiple drug resistance / S. Falkow.

  • Falkow, Stanley.
Date:
1975
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Infectious multiple drug resistance / S. Falkow. Source: Wellcome Collection.

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    20 Chapter 2 shorter and plumper and has become 'bald' via a chromosomal mutation in one of the Type 1 pili genes. The experiments of Brinton and his associates, and numerous subsequent studies by others, have clearly established that F-pili are essential for specific pair formation. For example, if F-pili are removed from donor cells by blending, the bacteria can no longer form specific pairs with recipients until the pili are resynthesized. In terms of specific pair formation, donor pili have been likened to grappling hooks. It is implicit, of course, that the tip of the donor pilus and the recipient cell surface must undergo some interaction. As yet, it has not been possible to describe this interaction in any definite morphological or biochemical terms. One can only state that upon mixing donor and recipient cells, pairs form which are bound together over a distance by F-püi, and that these pairs are stable enough to resist gentle pipetting and dñution. 2.5.2 Effective pair formation Just because two cells form a specific pair does not necessarily mean that a successful conjugal event will follow. Indeed, specific pairs can form even between cells that are dead. A step must follow specific pair formation that initiates a series of biochemical sequences which ensure that genetic material can pass from the donor to the recipient cell. This second step, effective pair formation, is the least-well-understood stage of bacterial conjugation because it is difficult to study experimentally. The central question concerning effective pair formation revolves around the nature of the actual bridge between cells used to conduct the DNA from the donor to the recipient cell. Since the discovery of donor pili, Brinton and others have asserted that the F-pili, with their central axial hole, serve as both the organelle for specific pair formation and as a presynthesized conduit by which the DNA subsequently enters the recipient. This attractive hypothesis has not been confirmed, because no investigator has yet been able to find DNA present within the pilus channel during conjugation. This is by no means a telling argument against the model, since it seems likely that DNA within a donor pilus might easily be lost in isolation of the pili. Roy Curtiss III, in an excellent review (1969) of bacterial conjugation, has some substantial doubts about the pilus conduction model. He notes, for example, that a feature of the pilus model is that genetic transfer can occur even though mating pairs are separated by several microns from each other. Curtiss cites several lines of evidence to suggest that actual wall-to- wall contact is required for efficient genetic transfer to occur. He proposes therefore that the pilus conduction model must at least be modified to bring about wall-to-wall contact, either by insertion of the pilus into the female or withdrawal of the pilus into the donor. Curtiss' own model does not involve transfer of DNA through the pilus. He visualizes that the formation of effective pairs involves the two-step establishment of a conjugal bridge through which genetic material can be transferred. In the