Haemophilus ducreyi is resistant to human antimicrobial peptides

Abstract

We examined the susceptibility of Haemophilus ducreyi to antimicrobial peptides likely to be encountered in vivo during human infection. H. ducreyi was significantly more resistant than Escherichia coli to the bactericidal effects of all peptides tested. Class I and II H. ducreyi strains exhibited similar levels of resistance to antimicrobial peptides.

FIG. 1.

H. ducreyi is significantly more resistant than E. coli to the bactericidal effects of α-defensins. Percent survival of bacteria exposed to α-defensins HNP-1 (A), HNP-2 (B), HNP-3 (C), and HD-5 (D). H. ducreyi 35000HP, black bars; E. coli ML35, gray bars. Data represent the means ± standard errors of the results of three independent assays. Asterisks represent statistically significant differences between strains at the indicated concentration of AP, with P < 0.0001 (HNP-1 and HD-5), P = 0.0017 (HNP-2), and P = 0.033 (HNP-3).

FIG. 2.

H. ducreyi is significantly more resistant than E. coli to the bactericidal effects of β-defensins. Percent survival of bacteria exposed to β-defensins HBD-2 (A), HBD-3 (B), and HBD-4 (C). H. ducreyi 35000HP, black bars; E. coli ML35, gray bars. Data represent the means ± standard errors of the results of three independent assays. Asterisks represent statistically significant differences between strains at the indicated concentration of AP, with P < 0.0001 (HBD-2, HBD-3, and HBD-4 at 20 μg/ml) and P = 0.0089 (HBD-4 at 2 μg/ml).

FIG. 3.

H. ducreyi is significantly more resistant than E. coli to the bactericidal effects of the human cathelicidin LL-37. Percent survival of bacteria exposed to 10-fold (A) and 2-fold (B) serial dilutions of LL-37. H. ducreyi 35000HP, black bars; E. coli ML35, gray bars. Data represent the means ± standard errors of the results of three independent assays. Asterisks represent statistically significant differences between strains at the indicated concentration of AP, with P < 0.0001 (A) and P = 0.0012 (B).

FIG. 4.

Representative class I and class II H. ducreyi strains are both resistant to APs. Susceptibility of class I 35000HP and class II CIP542 ATCC to α-defensin HNP-1 (A), β-defensin HBD-3 (B), and cathelicidin LL-37 (C). Class I results are shown in black, and class II results are shown in gray. Data represent the means ± standard errors of the results of three independent assays. Asterisks represent statistically significant differences between strains at the indicated concentration of AP, with P < 0.02 for HBD-3 and P = 0.0036 for LL-37.