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Antimicrobial Agents and Chemotherapy, January 2009, p. 298-302, Vol. 53, No. 1
0066-4804/09/$08.00+0     doi:10.1128/AAC.00657-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Klebsiella pneumoniae OmpA Confers Resistance to Antimicrobial Peptides{triangledown}

Enrique Llobet,1,2,{dagger} Catalina March,1,2,{dagger} Paloma Giménez,2 and José A. Bengoechea1,2,3,4*

Program Infection and Immunity, Fundación Caubet-CIMERA Illes Balears,1 Centro Investigación Biomédica en Red de Enfermedades Respiratorias (CibeRes), Bunyola,2 Área de Microbiología, Facultad Biología, Universitat Illes Balears, Palma Mallorca,3 Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain4

Received 19 May 2008/ Returned for modification 25 July 2008/ Accepted 5 November 2008


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ABSTRACT
 
A Klebsiella pneumoniae ompA mutant was more susceptible to antimicrobial peptides (APs) than the wild type. Susceptibility did not result from surface changes other than the absence of OmpA. Our data suggest that OmpA is implicated in the activation of yet-unknown systems dedicated to ameliorating AP cytotoxicity.


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INTRODUCTION
 
Outer membrane proteins (OMPs) are important for membrane integrity, transport of molecules, and pathogenesis (22). OmpA is a well-characterized OMP (39), and it has been shown to mediate adhesion to and/or invasion of eukaryotic cells (32, 36) and serum resistance (31, 40) and to protect bacteria against lung collectins (31, 40). OmpA is also targeted by the innate immune system. Neutrophil elastase degrades OmpA, resulting in cell death (3), whereas binding of serum amyloid protein A to OmpA increases bacterial phagocytosis (18, 35).

Klebsiella pneumoniae infections are particularly devastating among immunocompromised patients (34). The increased prevalence of multidrug-resistant strains complicates the treatment of these infections (37). Studies indicate that recognition of K. pneumoniae OmpA by the innate immune system helps to clear Klebsiella infections (21, 25).

Antimicrobial peptides (APs) belong to the array of oxygen-independent mechanisms for killing pathogens (14, 16). Nearly all APs are amphipathic and cationic (5, 17, 27). When APs are used in concentrations close to the MIC, bacteria do not develop resistance easily (15). Since proteins from the innate immune system target OmpA, here we explore whether K. pneumoniae OmpA contributes to susceptibility to APs.

An ompA mutant was constructed by insertion mutagenesis. A 378-bp ompA fragment was PCR amplified (using genomic DNA from wild-type K. pneumoniae 52145 [26], Taq polymerase [Promega], and the primers IntOmpAKpnF and IntOmpAKpnR) (Table 1), cloned into pGEM-T Easy (Promega), verified by DNA sequencing, and then subcloned as an EcoRI fragment into pKNOCK-Cm (2) to obtain pKNOCKIntKpnOmpA, which was inserted into 52145 by homologous recombination to obtain strain 52OmpA2. Correct insertion was confirmed by PCR and Southern blotting. OMPs were extracted from bacteria grown in Luria-Bertani broth and were analyzed as described previously (11). 52OmpA2 did not express OmpA (Fig. 1A). The growth rates of 52145 and 52OmpA2 in Luria-Bertani, determined by measuring the optical density at 540 nm at different time points, were not significantly different (data not shown).


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  		TABLE 1. Primers


Figure 1
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  		FIG. 1. SDS-polyacrylamide gel electrophoresis (the acrylamide concentration was 4% in the stacking gel and 12% in the separation one) followed by Coomasie brilliant blue staining of OMPs from 52145, 52OmpA2, and 52OmpA2Com (A) or 52145-{Delta}wcaK2, 52145-{Delta}wcaK2ompA, and 52145-{Delta}wcaK2ompACom (B). MW, molecular size marker. A white arrow marks the OmpA protein, and a black arrow marks OmpK36 (13, 19).

Susceptibilities to APs were tested by a survival assay (6, 7). 52OmpA2 was more susceptible to protamine (Fig. 2A), human neutrophil {alpha}-defensin 1 (HNP-1) (Fig. 2B), and polymyxin B (Fig. 2C) than 52145. The structures of the APs used are different, indicating that the susceptibility of 52OmpA2 was not dependent on the AP tested. To complement 52OmpA2, we used the mini-Tn7 delivery vector pUC18R6KT-mini-Tn7T, whose insertion occurs at a single Tn7 attachment site downstream of glmS (8). For selection of the transposition event, a kanamycin cassette was PCR amplified from the plasmid pKD4 (10) using primers cassette-F1 and cassette-R1 and cloned into EcoRV-digested pUC18R6KT-mini-Tn7T (8) to give pUC18R6KT-mini-Tn7TKm. A 1.5-kb fragment encompassing ompA and its promoter was PCR amplified (using the primers KpnOmpAcomF and KpnOmpAcomR and Vent polymerase [New England Biolabs]) and cloned into SmaI-digested pUC18R6KT-mini-Tn7TKm to give pUC18R6KT-mini-Tn7TKmKpnOmpA. Tn7 delivery was performed as described previously (8), and insertion was verified by colony PCR with the primer pairs KpnglmSup/Ptn7L and KpnglmSdown/Ptn7R (8). 52OmpA2Com, the strain obtained, expressed amounts of OmpA similar to those of 52145 (Fig. 1A). 52OmpA2Com susceptibility to APs was similar to that of 52145 (Fig. 2A to C).


Figure 2
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  		FIG. 2. A K. pneumoniae OmpA mutant is more susceptible to APs than the wild-type strain. Bacteria were exposed to protamine (A), HNP-1 (fixed concentration of 20 µg/ml) (B), or polymyxin B (C and D). Bacteria were exposed to polymyxin B and protamine during 1 h and to HNP-1 during 3 h. Each point in panels A, C, and D represents the mean and standard deviation for six samples from three independently grown batches of bacteria and significant survival differences (P < 0.05; two-tailed t test) between 52145 and 52145-{Delta}wcaK2, and the respective ompA mutants are indicated by asterisks. Error bars in panel B display standard deviations from the means of three experiments, each one run in duplicate, and an asterisk indicates a significant difference (P < 0.05; two-tailed t test) between 52145 and 52OmpA2. Symbols: {circ}, 52145; •, 52OmpA2; {blacktriangleup}, 52OmpA2Com; {square}, 52145-{Delta}wcaK2; {blacksquare}, 52145-{Delta}wcaK2OmpA; {blacktriangledown}, 52145-{Delta}wcaK2OmpACom.

Since K. pneumoniae capsule polysaccharide (CPS) mediates resistance to APs (7), we examined whether the absence of OmpA decreases the expression of CPS, thereby increasing the susceptibility to APs. However, the two strains expressed similar amounts of cell-bound CPS (7, 33) (339 ± 10 µg per 106 cells and 345 ± 15 µg per 106 cells, respectively; P > 0.05 [two-tailed t test]). We investigated whether the absence of OmpA increases the susceptibility to APs of a CPS mutant. pKNOCKIntKpnOmpA was introduced into 52145-{Delta}wcaK2, a 52145-derived CPS mutant (construction will be described elsewhere). 52145-{Delta}wcaK2ompA did not express OmpA (Fig. 1B), and its expression was restored by integration of pUC18R6KT-mini-Tn7TKmKpnOmpA (Fig. 1B). 52145-{Delta}wcaK2ompA was more susceptible to polymyxin B than 52145-{Delta}wcaK2 (Fig. 2D), and both strains were more susceptible to polymyxin B than the corresponding CPS-expressing strains (Fig. 2A and D). Our data indicate that changes in CPS expression do not account for the increased susceptibility of 52OmpA2 to APs.

We explored whether 52OmpA2 possesses a fragile outer membrane (OM), which could increase the susceptibility to APs (28, 38). However, the MICs of ciprofloxacin, imipenem, cefotaxime, amoxicillin, and gentamicin for 52145 and 52OmpA2 were similar (Table 2). The MIC of polymyxin B for 52OmpA2 was higher than that for 52145 (Table 2). We tested the susceptibilities to sodium dodecyl sulfate (SDS) and novobiocin by the disk diffusion method (4). Both strains were resistant to SDS (100 µg per disk), and 52OmpA2 was as susceptible as 52145 to novobiocin (80 µg per disk; diffusion halo, 0.85 ± 0.1 cm and 0.9 ± 0.1 cm respectively; P > 0.05). Both strains did not uptake propidium iodide, which penetrates only bacteria with damaged OMs (fluorescence spectroscopy protocol, Live/Dead BAcLight bacterial viability kit; Molecular Probes).


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  		TABLE 2. Susceptibilities of K. pneumoniae strains to antibiotics

We investigated whether the absence of OmpA increases the binding of polymyxin B to whole bacteria. Bacteria (108) were incubated with polymyxin B (16 µg/ml; 30 min), and the amount of AP bound was determined (7). 52145 and 52OmpA2 adsorbed similar amounts of the peptide (7.4 ± 0.1 and 8.1 ± 0.1 µg/ml, respectively; P > 0.05 [two-tailed t test]). We also determined the polymyxin B bound by 52145-{Delta}wcaK2 and 52145-{Delta}wcaK2ompA lipopolysaccharides (LPSs), obtained as described previously (7, 41). LPSs were purified from CPS mutants to avoid copurification of CPS (12). LPSs (50 µg/ml) were incubated with polymyxin B (16 µg/ml; 30 min), and the peptide bound was measured (7). 52145-{Delta}wcaK2 and 52145-{Delta}wcaK2ompA LPSs adsorbed similar amounts of polymyxin B (4.8 ± 0.1 and 4.4 ± 0.3 µg/ml, respectively; P > 0.05 [two-tailed t test]). These observations suggest that susceptibility of 52OmpA2 to APs is not caused by changes in the OM other than the absence of OmpA.

We wondered whether the absence of other OMPs would increase the susceptibility to APs. We constructed a mutant lacking OmpK36, another K. pneumoniae OMP (1), and tested the susceptibility to APs. A 524-bp PCR amplicon (using primers KnpOmpK36F and KpnOmpK36R) from ompK36 was cloned into pGEM-T Easy, verified by DNA sequencing, and subcloned into EcoRI-digested pKNOCK-Cm to obtain pKNOCKIntKpnOmpK36, which was inserted into 52145 by homologous recombination to obtain 52OmpK36. Correct insertion was verified by PCR and Southern blotting. 52OmpK36 did not express OmpK36, whereas the expression of other OMPs was not affected (Fig. 3A). A lack of OmpK36 did not increase the susceptibility to polymyxin B (Fig. 3B) or HNP-1 (Fig. 3C).


Figure 3
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  		FIG. 3. (A) SDS-PAGE (the acrylamide concentration was 4% in the stacking gel and 12% in the separation one) followed by Coomasie brilliant blue staining of OMPs from 52145 and 52OmpK36. MW, molecular size marker. The black arrow marks OmpK36 (13, 19). (B) The K. pneumoniae ompK36 mutant (•) is as resistant as the wild type ({circ}) to polymyxin B. Bacteria were exposed to different concentrations of polymyxin B during 1 h. Each point represents the mean and standard deviation for six samples from three independently grown batches of bacteria. (C) The K. pneumoniae ompK36 mutant is as resistant as the wild type to HNP-1 (3-h incubation with a fixed concentration of 20 µg/ml). Error bars display standard deviations from the means of three experiments, each one run in duplicate.

We speculated that the absence of OmpA may interfere with the expression of systems dedicated to ameliorating AP cytotoxicity. If this is so, a brief exposure of K. pneumoniae to polymyxin B should increase its resistance to APs in an OmpA-dependent manner. In contrast to results for 52OmpA2, a 1-h treatment of 52145 with polymyxin B (1 µg/ml) during growth increased its survival with the peptide (Fig. 4). These K. pneumoniae OmpA-requiring systems are presently unknown. Bacterial countermeasures to resist APs include alteration of surface charge, active efflux, and proteolytic degradation (29, 30). Hence, an absence of OmpA may affect the activation/function of these systems. Considering the connection between OMPs and activation of the {sigma}E regulon (24) and the connection between the latter and susceptibility to APs (9, 20, 23), we hypothesize that a lack of activation of the {sigma}E regulon might underlie the susceptibility of 52OmpA2 to APs.


Figure 4
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  		FIG. 4. Exposure of K. pneumoniae 52145 but not the ompA mutant to polymyxin B (1 h during growth; final concentration, 1 µg/ml) increases the resistance to polymyxin B. After polymyxin B treatment, bacteria were washed and the susceptibility to polymyxin B tested by the same survival assay as for Fig. 2 and 3. Each point represents the mean and standard deviation for six samples from three independently grown batches of bacteria, and significant survival differences (P < 0.05; two-tailed t test) between bacteria pretreated with polymyxin B (black symbols) and nontreated bacteria (white symbols) are indicated by asterisks. Symbols: {circ} or •, 52145; {triangleup} or {blacktriangleup}, 52OmpA2Com; {square} or {blacksquare}, 52OmpA2.

Our findings may help in developing new therapeutic strategies. Compounds directed to block OmpA could render bacteria susceptible to APs, and identification of the systems activated in an OmpA-dependent fashion may lead to new targets for design of antimicrobials.


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ACKNOWLEDGMENTS
 
We are grateful to members of the Bengoechea lab for helpful discussions and to Christian Frank for critically reading the manuscript. We are indebted to Herbert Schweizer for sending us mini-Tn7 constructs.

This work was funded by grants from Fondo de Investigación Sanitaria (PI05/2311) and Govern Balear (Grant for Competitive Groups) to J.A.B.


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FOOTNOTES
 
* Corresponding author. Mailing address: Program Infection and Immunity, Fundació Caubet-CIMERA Illes Balears, Recinto Hospital Joan March, Carretera Sóller Km 12, 07110 Bunyola. Spain. Phone: 34 971 011780. Fax: 34 971 011797. E-mail: bengoechea{at}caubet-cimera.es Back

{triangledown} Published ahead of print on 17 November 2008. Back

{dagger} These authors contributed equally to this work. Back


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Antimicrobial Agents and Chemotherapy, January 2009, p. 298-302, Vol. 53, No. 1
0066-4804/09/$08.00+0     doi:10.1128/AAC.00657-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.




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