Correspondence: David M. Donovan, ABBL, BARC, Agricultural Research Service, US Department of Agriculture, 10300 Baltimore Avenue, Building 230, BARC-EAST, Beltsville, MD 20705, USA. Tel.: 301 504 9150; fax: 301 504 8571;
Peptidoglycan hydrolases are an effective new source of antimicrobials. A chimeric fusion protein of the Ply187 endopeptidase domain and LysK SH3b cell wall–binding domain is a potent agent against Staphylococcus aureus in four functional assays.
Staphylococcus aureus is a pathogen that causes a broad spectrum of human and animal diseases and has adapted to antibiotic selective pressures resulting in a high prevalence of multidrug resistant strains (de Lencastre et al., 2007). The spread of these antibiotic-resistant strains is a threat to public health and a critical concern to healthcare providers worldwide. Thus, the search for novel antimicrobials against pathogens that are refractory to resistance development is essential.
Phage endolysins are cell wall hydrolases that are produced near the end of the phage lytic cycle to help the nascent phage escape the infected host. Endolysins are ideally suited as antimicrobials for several reasons as described previously (Loessner, 2005; Donovan et al., 2009). It has been postulated that phage endolysins have co-evolved with their host such that they target cell wall bonds that are believed difficult for the host cell to alter, and thus, bacterial resistance to phage endolysins is unlikely (Fischetti, 2005). Due to the absence of an outer membrane as in Gram-negative bacteria, endolysins are able to access the Gram-positive peptidoglycan ‘from without’ and lyse these bacteria when exposed externally.
The S. aureus bacteriophage 187 endolysin (Ply187) gene was initially reported by Loessner et al. (1999). Ply187 consists of 628 amino acids with a calculated molecular mass of 71.6 kDa. Typically, endolysins from a Gram-positive background have a modular structure with an N-terminal catalytic domain for peptidoglycan hydrolysis and a C-terminal cell wall–binding domain (CBD; Loessner, 2005). However, the Pfam domain database indicates that the amino terminus of Ply187 harbors a cysteine, histidine-dependent amidohydrolases/peptidases (CHAP) domain (Bateman & Rawlings, 2003; Rigden et al., 2003), and the C-terminus contains a glucosaminidase domain with no known CBD (Loessner et al., 1999; Fig. 1a). CBDs are essential for some endolysins' lytic activity and often determine specificity (Baba & Schneewind, 1996; Loessner et al., 2002; Grundling & Schneewind, 2006; Lu et al., 2006; Sass & Bierbaum, 2007). Ply187 is, thus, somewhat unique among lysins of Gram-positive phages in that it lacks a CBD. The Ply187 CHAP domain shows weak homology (40% identity) with the CHAP domain of other well-characterized lysins, such as LysK, an endolysin from phage K (O'Flaherty et al., 2005). Previous experimental data indicate that cloned full-length Ply187 is nearly inactive, while C-terminal truncated Ply187 (1-157aa; Ply187AN) is much more active than the full-length protein, suggesting an inhibitory domain at the C-terminus (Loessner et al., 1999). In an effort to improve the Ply187 lytic activity, the Ply187AN sequences were fused to the LysK SH3b CBD (KSH3b) to generate a chimeric Ply187AN-KSH3b fusion protein (Fig. 1a), similar to work reported from this laboratory with the streptococcal LambdaSa2 (LSA2) endolysin N-terminal lytic domain (Becker et al., 2009a). In a series of functional assays, we have demonstrated that this fusion protein is much more active than the parental Ply187AN truncated enzyme.
Materials and methods
All Gram-positive strains used to determine lysin target range are described in Table 1. All plasmid constructs were created and cryopreserved in Escherichia coli DH5α (Invitrogen, Carlsbad, CA) and expressed from freshly transformed E. coli BL21 (DE3; Invitrogen) as described below.
Table 1. Susceptibility of multiple bacterial strains to lysis by the parental enzyme LysK and the fusion protein Ply187AN-KSH3b
Smallest amount of protein in a volume of 10 μL causing a lysis zone after overnight incubation: ++++, 0.1 pmol; +++, 1 pmol; ++, 10 pmol; +, 100 pmol. ‘(+)’represents a faint lysis zone; ‘−’, no lysis zone at the highest amount tested (100 pmol). Scores represent averaged results from two separate experiments.
Jean C. Lee, Channing Laboratory, Brigham and Women's Hospital, Boston, MA.
Yasunori Tanji, Tokyo Institute of Technology, Yokohama, Japan; bovine mastitis isolates.
The Ply187 protein sequence is available (Y07740) through GenBank. The truncated Ply187 N-terminal domain (Ply187AN; 1–157 aa) was commercially synthesized based on published sequence (Loessner et al., 1999). To enhance the heterologous expression of Ply187 endolysin in E. coli, the sequences encoding the Ply187AN were converted to an E. coli codon bias, commercially synthesized, and subcloned into pUC57 with engineered 5′ NdeI (CATATG; ATG = start of translation) and 3′ XhoI (CTCGAG; codes for aa's LE) restriction enzyme sites (Genscript, Piscataway, NJ). These sites did not introduce translational stop codons and were used to subclone the protein-coding sequences into pET21a for expression and purification in E. coli B21 (DE3; EMD Biosciences, San Diego, CA). Subcloning of the Ply187AN construct into the pET21a expression vector was via conventional means, effectively adding six C-terminal His codons to the protein-coding sequences, using a strategy similar to previous constructs from the Donovan lab ( Becker et al., 2009a,b). Similarly, the chimeric Ply187AN (Ply187AN-KSH3b) was fused to the LysK SH3b by subcloning the Ply187AN NdeI-XhoI DNA fragment harboring all coding sequences into a similarly digested preconstructed pET21a-KSH3b vector described previously (Becker et al., 2009b). Recombinant LysK was used in this work as a positive control (Becker et al., 2009a, b).
Recombinant protein expression and purification
For protein expression, E. coli B21 (DE3; EMD Biosciences) expression cultures were grown at 37 °C in Luria–Bertani (LB) broth containing ampicillin (100 μg mL−1) to an OD600 nm of 0.4–0.6, chilled on ice for 30 min, induced by addition of isopropyl-β-D-thiogalactopyranoside (IPTG) to a final concentration of 1 mM, and grown at 10 °C for 20 h. Escherichia coli harvested from 500 mL cultures were lysed via sonication and His-tagged proteins isolated using nickel chromatography Ni-NTA (Qiagen, Valencia, CA) as described previously (Becker et al., 2009a, b). Briefly, wash and elution profiles were empirically determined to be 10 mL of 10 mM imidazole, 20 mL of 20 mM imidazole and elution with 0.5 mL of 250 mM imidazole in the same phosphate-buffered saline (PBS; 50 mM NaH2PO3, 300 mM NaCl, pH 8.0) with 30% glycerol to prevent precipitation of the purified protein. All samples were then desalted with Zeba desalting column (Pierce, Rockford, IL) equilibrated in 2× PBS buffer and filter sterilized. Sterilized protein preparation was stored at 4 °C in 2× PBS buffer until the time of assay.
SDS-PAGE and zymogram analysis of Ply187 constructs
Four microgram per lane of each construct and Kaleidoscope protein standards (Bio-Rad, Hercules, CA) were analyzed using 15% SDS-PAGE. Zymograms were prepared identically as the SDS-PAGE gels, but with 300 mL culture equivalents of mid-log-phase S. aureus cells (OD600 nm of 0.4–0.6). The SDS-PAGE and zymogram were running simultaneously. SDS gels were Coomassie-stained and zymograms were washed in excess water for 1 h and incubated at room temperature in PBS, resulting in areas of clearing in the turbid gel where a lytic protein had localized.
Plate lysis assay
Plate lysis assays are described previously (Becker et al., 2009a, b). In brief, purified proteins for each construct were sterilized and diluted in sterile PBS buffer with 30% glycerol. Ten microlitre containing 10, 1, or 0.1 μg of test protein were spotted onto a freshly spread lawn of mid-log phase (OD600c. 0.4–0.6) of either S. aureus Newman or S. aureus 305 cells that had air-dried for 10 min on tryptic soy agar plates. The spotted plates were air-dried for 10 min in a laminar flow hood and incubated overnight at 37 °C. Scoring of the cleared spots occurred within 20 h of plating the cells.
Turbidity reduction assay
Turbidity reduction assays are described previously (Becker et al., 2009a, b). In brief, assays with staphylococcal strain S. aureus Newman was grown to logarithmic phase (OD600 nm = 0.4–0.6) at 37 °C in brain–heart infusion broth (DIFCO, Franklin Lakes, NJ). Assay was performed in a 96-well dish and analyzed in a plate reader. Rates of lysis that occurred with 0.2 mL of S. aureus cells at an OD600 nm = 1.0 were determined every 20 s for 5 min. Peaks in ∆OD were determined on a sliding scale and reported results are within the linear range of the assay. Results are presented as specific activities (ΔOD600 nm μmol−1 min−1).
Minimal inhibitory concentration assay
A classical microdilution broth method for determination of the minimal inhibitory concentration (MIC) was used (Jones et al., 1985) with modifications as described previously (Becker et al., 2009b). All MIC values represent four assays with three replicates in each assay.
Fresh milk samples obtained from the USDA BARC, Beltsville, MD, dairy herd were pasteurized for 30 min at 63 °C. The milk was brought to 37 °C and was spiked with exponentially growing (OD600 nm = 0.4–0.6) cells of the mastitis causing S. aureus strain Newbould 305 at a concentration of 1 × 103 CFU mL−1. Immediately after inoculation, purified enzyme was added at a concentration of 8 μM, and the milk was incubated at 37 °C without shaking for 3 h. Samples were taken immediately before and immediately after addition of enzyme, as well as after 30, 60, 120, and 180 min, and the CFUs determined by direct plating on duplicate TSA plates. A sample spiked with lysin buffer alone without enzyme served as a control. The absence of CFUs in pasteurized milk was verified by direct plating on TSA plates.
SDS-PAGE and zymogram analysis of Ply187-derived lysins
Nickel chromatography purified proteins were analyzed using 15% SDS-PAGE and Kaleidoscope protein standards (Bio-Rad; Fig. 1b), with or without 300 mL culture equivalents of mid-log-phase S. aureus cells (OD600 nm = 0.4–0.6) embedded in the gel as described previously (Becker et al., 2009b), to verify the absence of co-purifying lytic contaminants. Coomassie-stained SDS-PAGE of each purified protein C-His-Ply187AN and C-His-Ply187AN-KSH3b indicated that the two constructs were able to be expressed in E. coli and purified at > 95% purity (Becker et al., 2009a, b). Zones of lysis on the zymogram gel run in parallel with the SDS-PAGE indicate that the predicted protein in each preparation is the only protein with staphylolytic activity (Fig. 1b).
Chimeric Ply187AN-KSH3b lysin is active in killing live S. aureus in plate lysis, turbidity reduction, and MIC assays
To evaluate the fusion construct's potential as an antimicrobial agent, we verified and quantitated the lytic activity of Ply187AN-KSH3b, the parental truncation (Ply187AN), and a strong antimicrobial endolysin, LysK (O'Flaherty et al., 2005; Becker et al., 2009b) in three different antimicrobial assays. The plate lysis assay results in Fig. 2a demonstrate that both 10 and 1 μg LysK [179 and 17.9 pmol, respectively; molecular weight (MW): 55.8 kD] produced a zone of clearing, indicating that 1 μg LysK in 10 μL of buffer is effective at eliminating the S. aureus lawn, consistent with previous reports (O'Flaherty et al., 2005; Becker et al., 2009b). In contrast, only 10 μg of Ply187AN (529 pmol; MW: 18.9 kD) produced a zone of clearing in the plate lysis assay, indicating that Ply187AN is less effective than LysK in this assay. Surprisingly, Ply187AN-KSH3b produced a zone of clearing at 10, 1, and 0.1 μg (327, 32.7, and 3.27 pmol, respectively; MW: 30.6 kD), indicating that Ply187AN-SH3b is more active than Ply187AN and LysK. All bacterial strain susceptibility tests are described in Table 1. The staphylococcal strains included bovine mastitis isolates, MRSA strains, and coagulase-negative staphylococci. Both the fusion protein and LysK were able to lyse all staphylococcal strains tested, with Ply187AN-KSH3b exhibiting higher activity than LysK against many strains when compared on a molar basis. In contrast, both enzymes were inactive against nonstaphylococcal strains with the exception of Ply187AN-KSH3b showing weak activity against Streptococcus dysgalactiae.
To further quantify the degree of lytic enhancement obtained from the fusion of the Ply187 CHAP domain to the KSH3b domain, we compared the staphylolytic activities in both turbidity reduction (Fig. 2b) and MIC assays (Fig. 2c). A standardized turbidity assay modified from Donovan et al. (2006) with staphylococcal strains grown to logarithmic phase (OD600 nm = 0.4–0.6) at 37 °C in brain–heart infusion broth (DIFCO) was performed in a 96-well dish and analyzed in a plate reader as described previously (Becker et al., 2009a, b). The specific activity for LysK, Ply187AN, and Ply187AN-KSH3b was 1.2 ± 0.4, 0.08 ± 0.03, and 1.2 ± 0.2 ∆OD600 nm μmol−1 min−1, respectively. Consistent with the plate lysis assay, Ply187AN was only about 10% as active as LysK. However, the addition of the KSH3b domain to the Ply187 CHAP domain yielded a 10-fold increase in specific activity (Fig. 2b). The resulting activity of Ply187AN-KSH3b was similar to that of the recombinant LysK protein. A classical microdilution broth method for determination of the MIC was used (Jones et al., 1985) with modifications as described previously (Becker et al., 2009b) to determine the MIC for each construct. In the MIC assay, LysK inhibited growth of S. aureus Newman at concentrations of 35.6 ± 10.5 μg mL−1, corresponding to 0.6 ± 0.2 nmol mL−1, which is comparable to previous results (Becker et al., 2009b). The MIC for Ply187AN is 53.5 ± 12.0 μg mL−1 (2.8 ± 0.6 nmol mL−1). Similar to the prior two antimicrobial assays, Ply187AN-KSH3b is more active than Ply187AN, with an MIC of only 7.6 ± 2.9 μg mL−1 (0.24 ± 0.09 nmol mL−1), indicating that this construct yields a 10-fold increase in specific activity compared to Ply187AN. The fusion's MIC is fivefold lower than that of LysK's (and twofold lower when compared on a molar basis). We know that LysK is a very potent antimicrobial, showing higher activities in turbidity reduction assays than lysostaphin (Becker et al., 2009b). These data indicate that Ply187AN-KSH3b is a potent staphylolytic agent.
Chimeric Ply187AN-KSH3b lysin is active in pasteurized cow's milk
The lytic activity of Ply187-AN-KSH3b, Ply187-AN, and LysK was determined via serial diluting and plating of timed samples of milk inoculated with S. aureus, and either a lytic enzyme or enzyme buffer alone (Fig. 3). Immediately after addition of enzyme, Ply187-AN-KSH3b samples showed immediate eradication of all CFUs at time zero and remained undetectable throughout the 3-h period. Ply187-AN and LysK showed no significant difference from the buffer alone control at any time point.
Although the bacterial SH3b domain is readily identified in multiple domain databases, it is still poorly understood at the level of the site of binding. SH3b domains often determine an endolysin's specificity (Baba & Schneewind, 1996; Grundling & Schneewind, 2006; Lu et al., 2006; Sass & Bierbaum, 2007; Becker et al., 2009a, b). Low et al., (2005) have proposed a model to explain the role of the CBD in lysin lytic activity. This model suggested that the CBD folds back, binds to, and inhibits the lytic domain until it recognizes and binds to peptidoglycan when it releases the lytic domain allowing digestion of the peptidoglycan. Although believed to play a role in substrate recognition and binding specificity, its role must be empirically determined. In deletion experiments, Horgan et al. (2009) suggested that deletion of the LysK SH3b domain enhanced LysK CHAP domain enzymatic activity, while Becker et al. (2009b) showed that fusion of the LysK SH3b domain to the LysK CHAP domain was necessary for CHAP domain activity. Other laboratories have demonstrated that lysostaphin's SH3b domain binds to the S. aureus pentaglycine bridge (Baba & Schneewind, 1996; Grundling & Schneewind, 2006; Lu et al., 2006). Despite the lack of a consensus binding site, SH3b domains and other CBDs have been used for decades to develop novel antimicrobials (Diaz et al., 1990, 1991; Croux et al., 1993; Sheehan et al., 1996; Lopez et al., 1997). Recently, the Fischetti group used a non-SH3b CBD to generate a chimeric staphylolytic lysin using the Twort phage endolysin CHAP domain (Daniel et al., 2010). Our laboratory used staphylococcal SH3b domains in fusions with the streptococcal phage LambdaSA2 endolysin endopeptidase domain to shift the activity from Streptococcus specificity to an enzyme that recognized both streptococcal and staphylococcal cell walls (Becker et al., 2009b). Despite the numerous CBD fusion constructs reported, the Ply187-KSH3 fusion in this study represents the first time that the activity of a lytic domain from an endolysin that naturally lacks a CBD was enhanced by adding a known CBD.
In conclusion, we have generated a potent chimeric Ply187AN-KSH3b protein by fusing the CHAP endopeptidase domain of endolysin Ply187 from phage 187 and SH3b CBD of LysK from phage K. This chimeric Ply187AN-KSH3b is a more effective antimicrobial than the full-length Ply187, the Ply187 truncation (Ply187AN), and also outperforms the known high activity lysin, LysK, in three of four functional assays.
This work was supported in part by Viridax, Inc., Baco Raton, FL, as well as NIH grant 1RO1AI075077-01A1; NRI grant 2007-35204-18395 and US State Department funds supporting a US-Pakistani and US-Russian collaboration. All awards are to DMD. Mentioning of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture.