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Résistance à la bacitracine chez Bacillus subtilis

Abstract : Various natural antibiotic target the bacterial cell envelope. Some of them drastically alter the cytoplasmic membrane and others affect specifically the peptidoglycan (major cell wall component) biosynthesis pathway. To overcome this problem, bacteria possess several antibiotic resistance systems that confer them a crucial advantage in their ecologic niche. One of the major mechanisms found in bacteria is the active antibiotic efflux which often involves ABC-type transporters (for ATP Binding Cassette) that extrude antibiotics using ATP as a substrate. Bacteria also possess several systems, first, to detect environmental signals (antibiotic or cell wall stress), and second, to allow the signal transduction resulting in an adaptative response. Two kinds of detection systems are involved in cell wall stress response: phosphorelay systems and extracytoplasmic function sigma factors.

Bacillus subtilis is the Gram positive model bacterium of the Firmicute group that contains several pathogen or opportunistic organisms. Some Firmicutes are also ubiquitous and many produce antibiotics. The in silico and in vivo experiments performed in the laboratory on B. subtilis and others Firmicutes have pointed out the occurrence of systems that genetically and functionally couple a phosphorelay with an ABC-type transporter. In B. subtilis, three such systems, named BceRSAB (formerly YtsABCD), YvcPQRS and YxdJKLM, present high similarity. Our goal was to test if these systems could confer resistance to cell wall active antibiotics.

Bacitracin is a non ribosomal peptide antibiotic that forms a complex with the undecaprenyl pyrophosphate (UPP) and blocks the last step of the peptidoglycan biosynthesis: regeneration of undecaprenyl phosphate (UP). Our results clearly establish that the BceRSAB system is the major component of bacitracin resistance in B. subtilis. The bceAB operon expression is activated by the phosphorelay BceRS in the presence of bacitracin. We succeed also in identifying the protein, named BcrC, as a different component of the bacitracin resistance in B. subtilis. We demonstrate that BcrC is a UPP phosphatase involved in the regeneration of UP, in opposition with bacitracin action. bcrC gene expression is not dependent on the phosphorelay BceRS, however it is controlled by three different extracytoplasmic function sigma factors. As a conclusion, B. subtilis possesses at least two independent but additive bacitracin resistance systems.
We then have turned to the regulation mechanism involved in the BceRSAB bacitracin response. Surprisingly, our results show that the BceAB ABC transporter is not dispensable for the expression of its own structural genes in response to bacitracin. In other words, both the BceRS phosphorelay and the BceAB ABC transporter are necessary to trigger the BceRSAB induction in the presence of bacitracin. Furthermore, when the UPP cellular pool decreases (overproducing the BcrC UPP phosphatase), the BceRSAB bacitracin response is also diminished. We concluded that the UPP molecule participates in the BceRSAB stimulus. Our favoured hypothesis for future work is that the BceAB ABC transporter exports the UPP/bacitracin complex to create a membrane dissymetry sensed by the BceS sensor. Nevertheless, an alternative possibility could be that the BceRSAB regulation mechanism involves a direct interaction between the sensor and the transporter in the presence of bacitracin.

Like B. subtilis, several bacteria belonging to the Firmicute group possess both BcrC-like proteins and Bce-like systems. All BcrC-like proteins contain the characteristic motif of the PAP2 phosphatase protein family and we have postulated that they all are UPP phosphatases involved in a key step of the peptidoglycan biosynthesis pathway.
The other two Bce-like systems of B. subtilis, YvcPQRS and YxdJKLM, are also induced by different cell wall active antibiotics and we know that the YvcRS transporter is also involved in the regulation mechanism. Accordingly, we propose that all Bce-like systems of Firmicutes are detoxifying units directed against cell wall active antibiotics. These systems could constitute a new family of resistance systems where ABC transporters are necessary for resistance as well as for regulation.
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Contributor : Rémi Bernard <>
Submitted on : Tuesday, January 6, 2009 - 2:50:56 PM
Last modification on : Thursday, January 18, 2018 - 1:45:48 AM
Long-term archiving on: : Tuesday, June 8, 2010 - 6:43:03 PM

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Rémi Bernard. Résistance à la bacitracine chez Bacillus subtilis. Biochimie [q-bio.BM]. Université de la Méditerranée - Aix-Marseille II, 2007. Français. ⟨tel-00350345⟩

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