An inverse relationship between fitness and secretion efficiency in a gram-positive bacterium.

Lactiplantibacillus plantarum and other lactic acid bacteria are emerging as promising candidates for mucosal delivery of surface-displayed antigens. However, producing secreted heterologous proteins and anchoring these using LPxTG anchors can significantly reduce bacterial fitness. To understand the underlying mechanisms and limiting factors, we analyzed 11 recombinant L. plantarum strains expressing the HaloTag reporter protein with the same LPxTG anchor but varying signal peptides. By labeling the reporter protein with fluorescent ligands, this approach allowed simultaneous detection of correctly folded intracellular and extracellular protein, revealing signal peptide-dependent variation in the relative amounts of intra- and extracellularly folded HaloTag. Furthermore, this analysis uncovered an unexpected correlation between secretion efficiency and bacterial fitness. Strains with better growth showed more premature intracellular folding and reduced protein translocation and surface display. Conversely, strains with a higher fraction of surface-displayed protein, i.e. strains with greater secretion efficiency, exhibited impaired growth, likely due to increased interactions between the signal peptide and the secretion machinery, leading to secretion overload. Correlation analyses and confirmation of observed correlations by mutational studies of the signal peptides showed that signal peptide hydrophobicity is positively correlated with higher secretion efficiency but is accompanied by a trade-off in fitness. These findings underscore the critical role of signal peptides in balancing protein secretion and bacterial viability, offering valuable insights for optimizing protein secretion and anchoring in gram-positive bacteria.