Témoin S, Roche S M, Grépinet O, Fardini Y, Velge P
INRA, UR1282, Infectiologie Animale et Santé Publique, Centre de Recherche de Tours, Nouzilly 37380, France.
Microbiology (Reading). 2008 Mar;154(Pt 3):939-948. doi: 10.1099/mic.0.2007/011106-0.
In order to understand the causes of the low virulence of Listeria monocytogenes field strains, five low-virulence strains were analysed. These five strains showed changes in relation to invasion, phosphatidyl-inositol phospholipase C (PI-PLC) activity, plaque formation and in vivo virulence. Molecular analyses revealed the same mutations in the plcA, inlA and inlB genes in all five strains. The Thr262Ala substitution in the PI-PLC protein was responsible for the absence of PI-PLC activity. This residue, conserved in certain L. monocytogenes species, is located at the outer rim of the active site pocket and could impair the cleavage activity of the enzyme. The low invasion rate of these strains was due to a nonsense codon leading to a lack of InlA protein synthesis, and to an Ala117Thr substitution in the leucine-rich repeat of InlB, which altered the interaction with the Met receptor. Single trans complementation with the inlA(EGDe), inlB(EGDe) or plcA(EGDe) genes restored the capacity of low-virulence strains either to enter epithelial and fibroblastic cells or to express PI-PLC activity. Complementation by allelic exchange of the plcA(EGDe) gene on the chromosome and trans complementation with either the inlA(EGDe) or the inlB(EGDe) gene restored the ability to form plaques, but only partly restored the in vivo virulence, suggesting that there were other gene mutation(s) with consequences that could mainly be observed in vivo. These results indicate that the low virulence of L. monocytogenes strains can be explained by point mutations in a number of virulence genes; these could therefore be important for detecting low-virulence strains. Moreover, the fact that all the strains had the same substitutions suggests that they have a common evolutionary pathway.
为了解单核细胞增生李斯特菌田间菌株毒力低的原因，对五株低毒力菌株进行了分析。这五株菌株在侵袭、磷脂酰肌醇磷脂酶C（PI-PLC）活性、噬斑形成和体内毒力方面均表现出变化。分子分析显示，所有五株菌株的plcA、inlA和inlB基因存在相同的突变。PI-PLC蛋白中的苏氨酸262被丙氨酸取代导致PI-PLC活性缺失。该残基在某些单核细胞增生李斯特菌物种中保守，位于活性位点口袋的外缘，可能会损害该酶的切割活性。这些菌株的低侵袭率是由于一个无义密码子导致InlA蛋白合成缺失，以及InlB富含亮氨酸重复序列中的丙氨酸117被苏氨酸取代，这改变了与Met受体的相互作用。用inlA(EGDe)、inlB(EGDe)或plcA(EGDe)基因进行单转互补恢复了低毒力菌株进入上皮细胞和成纤维细胞或表达PI-PLC活性的能力。通过染色体上plcA(EGDe)基因的等位基因交换互补以及用inlA(EGDe)或inlB(EGDe)基因进行转互补恢复了形成噬斑的能力，但仅部分恢复了体内毒力，这表明存在其他基因突变，其后果主要在体内观察到。这些结果表明，单核细胞增生李斯特菌菌株的低毒力可以通过多个毒力基因中的点突变来解释；因此，这些对于检测低毒力菌株可能很重要。此外，所有菌株都有相同的取代这一事实表明它们有共同的进化途径。
