These results indicate that beta-defensins and LL37 have versatile antibacterial activity against oral bacteria.

Lipoteichoic acid (LTA) is one of two anionic polymers on the surface of the gram-positive bacterium Staphylococcus aureus. LTA is critical for the bacterium-host cell interaction and has recently been shown to be required for cell growth and division. To determine additional biological roles of LTA, we found it necessary to identify permissive conditions for the growth of an LTA-deficient mutant. We found that an LTA-deficient S. aureus ⌬ltaS mutant could grow at 30°C but not at 37°C. Even at the permissive temperature, ⌬ltaS mutant cells had aberrant cell division and separation, decreased autolysis, and reduced levels of peptidoglycan hydrolases. Upshift of ⌬ltaS mutant cells to a nonpermissive temperature caused an inability to exclude Sytox green dye. A high-osmolarity growth medium remarkably rescued the colony-forming ability of the ⌬ltaS mutant at 37°C, indicating that LTA synthesis is required for growth under low-osmolarity conditions. In addition, the ⌬ltaS mutation was found to be synthetically lethal with the ⌬tagO mutation, which disrupts the synthesis of the other anionic polymer, wall teichoic acid (WTA), at 30°C, suggesting that LTA and WTA compensate for one another in an essential function.

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The antimicrobial peptides human ␤-defensin-1 (hBD1), hBD2, hBD3, and CAP18 expressed by keratinocytes have been implicated in mediation of the innate defense against bacterial infection. To gain insight into Staphylococcus aureus infection, the susceptibility of S. aureus, including methicillin-resistant S. aureus (MRSA), to these antimicrobial peptides was examined. Based on quantitative PCR, expression of hBD2 mRNA by human keratinocytes was significantly induced by contact with S. aureus, and expression of hBD3 and CAP18 mRNA was slightly induced, while hBD1 mRNA was constitutively expressed irrespective of the presence of S. aureus. Ten clinical S. aureus isolates, including five MRSA isolates, induced various levels of expression of hBD2, hBD3, and CAP18 mRNA by human kertinocytes. The activities of hBD3 and CAP18 against S. aureus were found to be greater than those of hBD1 and hBD2. A total of 44 S. aureus clinical isolates, including 22 MRSA strains, were tested for susceptibility to hBD3 and CAP18. Twelve (55%) and 13 (59%) of the MRSA strains exhibited more than 20% survival in the presence of hBD3 (1 g/ml) and CAP18 (0.5 g/ml), respectively. However, only three (13%) and two (9%) of the methicillin-sensitive S. aureus isolates exhibited more than 20% survival with hBD3 and CAP18, respectively, suggesting that MRSA is more resistant to these peptides. A synergistic antimicrobial effect between suboptimal doses of methicillin and either hBD3 or CAP18 was observed with 10 MRSA strains. Furthermore, of several genes associated with methicillin resistance, inactivation of the fmtC gene in MRSA strain COL increased susceptibility to the antimicrobial effect mediated by hBD3 or CAP18.