Analysis of Lactobacillus sakei mutants selected after adaptation to the gastrointestinal tracts of axenic mice

Abstract

We recently showed that Lactobacillus sakei, a natural meat-borne lactic acid bacterium, can colonize the gastrointestinal tracts (GIT) of axenic mice but that this colonization in the intestinal environment selects L. sakei mutants showing modified colony morphology (small and rough) and cell shape, most probably resulting from the accumulation of various mutations that confer a selective advantage for persistence in the GIT. In the present study, we analyzed such clones, issued from three different L. sakei strains, in order to determine which functions were modified in the mutants. In the elongated filamentous cells of the rough clones, transmission electron microscopy (TEM) analysis showed a septation defect and dotted and slanted black bands, suggesting the presence of a helical structure around the cells. Comparison of the cytoplasmic and cell wall/membrane proteomes of the meat isolate L. sakei 23K and of one of its rough derivatives revealed a modified expression for 38 spots. The expression of six oxidoreductases, several stress proteins, and four ABC transporters was strongly reduced in the GIT-adapted strain, while the actin-like MreB protein responsible for cell shaping was upregulated. In addition, the expression of several enzymes involved in carbohydrate metabolism was modified, which may correlate with the observation of modified growth of mutants on various carbon sources. These results suggest that the modifications leading to a better adaptation to the GIT are pleiotropic and are characterized in a rough mutant by a different stress status, a cell wall modification, and modified use of energy sources, leading to an improved fitness for the colonization of the GIT.

FIG. 1.

Morphology of cells issued from S and R clones of the L. sakei 23K, FLEC01, or LTH5590 strain observed by transmission electron microscopy. L. sakei FLEC01 wild type (A) and its rough clone derivative FLEC01-J15RN2 (B and C); L. sakei 23K wild type (D) and its rough clone derivative L. sakei 23K-J30RO2 (E); L. sakei LTH5590 wild type (F) and it small clone derivative L. sakei LTH5590-J21SO2 (G) grown in MRS liquid medium.