Gram-positive bacteria such as Streptococcus gordonii causing life-threatening
infective endocarditis are mainly recognized by
Toll-like receptor 2 (TLR2).
Lipoteichoic acid (LTA) and
lipoproteins are representative TLR2
ligands that play important roles in
bacterial infection and in host inflammatory responses. In the present study, we generated an LTA-deficient mutant (ΔltaS) and a
lipoprotein-deficient mutant (Δlgt) and investigated the contributions of LTA and
lipoproteins to bacterial morphology and their effect on induction of proinflammatory
cytokines in THP-1 and mouse bone-marrow derived macrophages (BMDMs). Deletion of ltaS and lgt was confirmed by PCR analysis of genomic
DNA from each mutant. The mutants with absence of LTA or
lipoproteins were examined by SDS-PAGE followed by Western blotting with anti-LTA
antibodies and
silver staining, respectively. Interestingly, scanning and transmission electron microscopies showed no difference in the bacterial cell morphology or size between the wild-type and the mutants even though substantial changes in the cell size and/or morphology have been reported in other Gram-positive bacteria such as Staphylococcus aureus, Listeria monocytogenes, and Bacillus subtilis. However, S. gordonii wild-type and ΔltaS potently induced the expression of proinflammatory
cytokines including TNF-α,
IL-8, and IL-1β at the
mRNA and
protein levels, while Δlgt did not have these effects. Furthermore,
lipoproteins purified from S. gordonii also induced the expression of the aforementioned
cytokines more potently than the purified LTA. Neither LTA nor
lipoprotein induced TNF-α, KC (IL-8 counterpart in mouse), and IL-1β in TLR2-deficient BMDMs. S. gordonii Δlgt was less virulent than the wild-type or ΔltaS in a mouse intraperitoneal
infection model. Collectively, these results suggest that S. gordonii
lipoproteins, but not LTA, are mainly responsible for the
infection and inflammatory responses.