The cytoplasmic membrane is a primary target for the staphylocidal action of thrombin-induced platelet microbicidal protein

Antimicrobial Peptides in Infectious Diseases and Beyond-A Narrative Review

Despite recent medical research and clinical practice developments, the development of antimicrobial resistance (AMR) significantly limits therapeutics for infectious diseases. Thus, novel treatments for infectious diseases, especially in this era of increasing AMR, are urgently needed. There is ongoing research on non-classical therapies for infectious diseases utilizing alternative antimicrobial mechanisms to fight pathogens, such as bacteriophages or antimicrobial peptides (AMPs). AMPs are evolutionarily conserved molecules naturally produced by several organisms, such as plants, insects, marine organisms, and mammals, aiming to protect the host by fighting pathogenic microorganisms. There is ongoing research regarding developing AMPs for clinical use in infectious diseases. Moreover, AMPs have several other non-medical applications in the food industry, such as preservatives, animal husbandry, plant protection, and aquaculture. This review focuses on AMPs, their origins, biology, structure, mechanisms of action, non-medical applications, and clinical applications in infectious diseases.

Recent Advances in the Discovery and Function of Antimicrobial Molecules in Platelets

The conventional function described for platelets is maintaining vascular integrity. Nevertheless, increasing evidence reveals that platelets can additionally play a crucial role in responding against microorganisms. Activated platelets release molecules with antimicrobial activity. This ability was first demonstrated in rabbit serum after coagulation and later in rabbit platelets stimulated with thrombin. Currently, multiple discoveries have allowed the identification and characterization of PMPs (platelet microbicidal proteins) and opened the way to identify kinocidins and CHDPs (cationic host defense peptides) in human platelets. These molecules are endowed with microbicidal activity through different mechanisms that broaden the platelet participation in normal and pathologic conditions. Therefore, this review aims to integrate the currently described platelet molecules with antimicrobial properties by summarizing the pathways towards their identification, characterization, and functional evaluation that have promoted new avenues for studying platelets based on kinocidins and CHDPs secretion.

Human platelets and megakaryocytes express defensin alpha 1

Platelets are anucleate cells that have a role in several innate immune functions, including the secretion of proteins with antimicrobial activity. Several studies have demonstrated the ability of platelets to secrete thrombin-induced platelet microbicidal proteins and antimicrobial peptides, like hBD-1. However, the expression and secretion of defensins of the alpha family by platelets have not been fully elucidated. The aim of this study was to characterize the expression of defensin alpha 1 (DEFA1) in human platelets and megakaryocytes. Our data indicate that DEFA1 mRNA and protein are present in peripheral blood platelets and in the megakaryoblastic leukemia cell line (MEG-01). DEFA1 co-localize with α-granules of platelets and MEG-01 cells, and was also detected in cytoplasm of MEG-01 cells. The assay of our in vitro model of platelet-like particles (PLPs) revealed that MEG-01 cells could transfer DEFA1 mRNA to their differentiated PLPs. Furthermore, platelets secreted DEFA1 into the culture medium when activated with thrombin, adenosine diphosphate, and lipopolysaccharide; meanwhile, MEG-01 cells secreted DEFA1 when activated with thrombopoietin. Platelet's secreted DEFA1 can rebind to platelet's surface and have antibacterial activity against the gram-negative bacteria Escherichia coli. In summary, our data indicate that both, human platelets and megakaryocytes, can express and secrete DEFA1. These results suggest a new role of platelets and megakaryocytes in the innate immune response.

Start a fire, kill the bug: The role of platelets in inflammation and infection

Platelets are the main players in thrombosis and hemostasis; however they also play important roles during inflammation and infection. Through their surface receptors, platelets can directly interact with pathogens and immune cells. Platelets form complexes with neutrophils to modulate their capacities to produce reactive oxygen species or form neutrophil extracellular traps. Furthermore, they release microbicidal factors and cytokines that kill pathogens and influence the immune response, respectively. Platelets also maintain the vascular integrity during inflammation by a mechanism that is different from classical platelet activation. In this review we summarize the current knowledge about how platelets interact with the innate immune system during inflammation and infection and highlight recent advances in the field.

Platelets and infection

The primary function of platelets is to patrol the vasculature and seal vessel breaches to limit blood loss. However, it is becoming increasingly clear that they also contribute to pathophysiological conditions like thrombosis, atherosclerosis, stroke and infection. Severe sepsis is a devastating disease that claims hundreds of thousands of lives every year in North America and is a major burden to the public health system. Platelet surface receptors like GPIb, αIIbβ3, TLR2 and TLR4 are involved in direct platelet-bacteria interactions. Plasma proteins like fibrinogen and vWF enable indirect interactions. Furthermore, platelet granules contain a plethora of proteins that modulate the immune response as well as microbicidal agents which can directly lyse bacteria. Bacterial toxins are potent platelet activators and can cause intravascular platelet aggregation. Platelets contribute to the antibacterial response of the host involving Kupffer cells, neutrophils and the complement system. In this review we summarize the current knowledge about platelet-bacteria interactions and highlight recent advances in the field.

Antimicrobial and anti-biofilm effect of Bac8c on major bacteria associated with dental caries and Streptococcus mutans biofilms

Dental caries is a common oral bacterial infectious disease. Its prevention and treatment requires control of the causative pathogens within dental plaque, especially Streptococcus mutans (S. mutans). Antimicrobial peptides (AMPs), one of the promising substitutes for conventional antibiotics, have been widely tested and used for controlling bacterial infections. The present study focuses on evaluating the potential of the novel AMPs cyclic bactenecin and its derivatives against bacteria associated with dental caries. The results indicate that Bac8c displayed highest activity against the bacteria tested, whereas both cyclic and linear bactenecin had weak antimicrobial activity. The cytotoxicity assay showed that Bac8c did not cause detectable toxicity at concentrations of 32-128μg/ml for 5min or 32-64μg/ml for 60min. S. mutans and Lactobacillus fermenti treated with Bac8c showed variable effects on bacterial structure via scanning electron microscopy and transmission electron microscopy. There appeared to be a large amount of extracellular debris and obvious holes on the cell surface, as well as loss of cell wall and nucleoid condensation. The BioFlux system was employed to generate S. mutans biofilms under a controlled flow, which more closely resemble the formation process of natural biofilms. Bac8c remarkably reduced the viability of cells in biofilms formed in the BioFlux system. This phenomenon was further analyzed and verified by real-time PCR results of a significant suppression of the genes involved in S. mutans biofilm formation. Taken together, this study suggests that Bac8c has a potential clinical application in preventing and treating dental caries.

Sensitization of gram-negative bacteria by targeting the membrane potential

Toward generating new tools for fighting multidrug-resistant (MDR) bacteria, we assessed the ability of a membrane-active peptide to sensitize gram-negative bacteria to various antibiotics. The mechanism for affecting inner and/or outer membrane functions was assessed by complementary biophysical methods (SPR, DSC, ITC). The implication of efflux pumps was examined using Acr-AB mutants, as tested with representative antibiotics, host defense peptides, and synthetic mimics. The ability to affect disease course systemically was compared for a single therapy and combination therapy, using the mouse thigh-infection model. The data show that potent antibiotic action can be provoked in vitro and in vivo, by a treatment combining two antibacterial compounds whose individual inefficiency against gram-negative bacteria stems from their efflux. Thus, at subminimal inhibitory concentrations, the lipopeptide-like sequence, N(α)(ω7)dodecenoyl-lysyl-[lysyl-aminododecanoyl-lysyl]-amide (designated C12(ω7)K-β12), has, nonetheless, rapidly achieved a transient membrane depolarization, which deprived bacteria of the proton-motive force required for active efflux. Consequently, bacteria became significantly sensitive to intracellular targeting antibiotics. Collectively, these findings suggest a potentially useful approach for expanding the antibiotics sensitivity spectrum of MDR gram-negative bacteria to include efflux substrates.

Role of the LytSR two-component regulatory system in adaptation to cationic antimicrobial peptides in Staphylococcus aureus

Many host defense cationic antimicrobial peptides (HDPs) perturb the staphylococcal cell membrane (CM) and alter transmembrane potential (ΔΨ) as key parts of their lethal mechanism. Thus, a sense-response system for detecting and mediating adaptive responses to such stresses could impact organism survival; the Staphylococcus aureus LytSR two-component regulatory system (TCRS) may serve as such a ΔΨ sensor. One well-known target of this system is the lrgAB operon, which, along with the related cidABC operon, has been shown to be a regulator in the control of programmed cell death and lysis. We used an isogenic set of S. aureus strains: (i) UAMS-1, (ii) its isogenic ΔlytS and ΔlrgAB mutants, and (iii) plasmid-complemented ΔlytSR and ΔlrgAB mutants. The ΔlytS strain displayed significantly increased in vitro susceptibilities to all HDPs tested (neutrophil-derived human neutrophil peptide 1 [hNP-1], platelet-derived thrombin-induced platelet microbicidal proteins [tPMPs], and the tPMP-mimetic peptide RP-1), as well as to calcium-daptomycin (DAP), a cationic antimicrobial peptide (CAP). In contrast, the ΔlrgAB strain exhibited no significant changes in susceptibilities to these cationic peptides, indicating that although lytSR positively regulates transcription of lrgAB, increased HDP/CAP susceptibilities in the ΔlytS mutant were lrgAB independent. Further, parental UAMS-1 (but not the ΔlytS mutant) became more resistant to hNP-1 and DAP following pretreatment with carbonyl cyanide m-chlorophenylhydrazone (CCCP) (a CM-depolarizing agent). Of note, lytSR-dependent survival against CAP/HDP killing was not associated with changes in either surface positive charge, expression of mprF and dlt, or CM fluidity. The ΔlytS strain (but not the ΔlrgAB mutant) displayed a significant reduction in target tissue survival in an endocarditis model during DAP treatment. Collectively, these results suggest that the lytSR TCRS plays an important role in adaptive responses of S. aureus to CM-perturbing HDPs/CAPs, likely by functioning as a sense-response system for detecting subtle changes in ΔΨ.

In vitro cross-resistance to daptomycin and host defense cationic antimicrobial peptides in clinical methicillin-resistant Staphylococcus aureus isolates

We investigated the hypothesis that methicillin-resistant Staphylococcus aureus (MRSA) isolates developing reduced susceptibilities to daptomycin (DAP; a calcium-dependent molecule acting as a cationic antimicrobial peptide [CAP]) may also coevolve reduced in vitro susceptibilities to host defense cationic antimicrobial peptides (HDPs). Ten isogenic pairs of clinical MRSA DAP-susceptible/DAP-resistant (DAP(s)/DAP(r)) strains were tested against two distinct HDPs differing in structure, mechanism of action, and origin (thrombin-induced platelet microbicidal proteins [tPMPs] and human neutrophil peptide-1 [hNP-1]) and one bacterium-derived CAP, polymyxin B (PMB). Seven of 10 DAP(r) strains had point mutations in the mprF locus (with or without yyc operon mutations), while three DAP(r) strains had neither mutation. Several phenotypic parameters previously associated with DAP(r) were also examined: cell membrane order (fluidity), surface charge, and cell wall thickness profiles. Compared to the 10 DAP(s) parental strains, their respective DAP(r) strains exhibited (i) significantly reduced susceptibility to killing by all three peptides (P < 0.05), (ii) increased cell membrane fluidity, and (iii) significantly thicker cell walls (P < 0.0001). There was no consistent pattern of surface charge profiles distinguishing DAP(s) and DAP(r) strain pairs. Reduced in vitro susceptibility to two HDPs and one bacterium-derived CAP tracked closely with DAP(r) in these 10 recent MRSA clinical isolates. These results suggest that adaptive mechanisms involved in the evolution of DAP(r) also provide MRSA with enhanced survivability against HDPs. Such adaptations appear to correlate with MRSA variations in cell membrane order and cell wall structure. DAP(r) strains with or without mutations in the mprF locus demonstrated significant cross-resistance profiles to these unrelated CAPs.

Bacterial membrane activity of α-peptide/β-peptoid chimeras: influence of amino acid composition and chain length on the activity against different bacterial strains

BACKGROUND

Characterization and use of antimicrobial peptides (AMPs) requires that their mode of action is determined. The interaction of membrane-active peptides with their target is often established using model membranes, however, the actual permeabilization of live bacterial cells and subsequent killing is usually not tested. In this report, six α-peptide/β-peptoid chimeras were examined for the effect of amino acid/peptoid substitutions and chain length on the membrane perturbation and subsequent killing of food-borne and clinical bacterial isolates.

RESULTS

All six AMP analogues inhibited growth of twelve food-borne and clinical bacterial strains including Extended Spectrum Beta-Lactamase-producing Escherichia coli. In general, the Minimum Inhibitory Concentrations (MIC) against Gram-positive and -negative bacteria were similar, ranging from 1 to 5 μM. The type of cationic amino acid only had a minor effect on MIC values, whereas chain length had a profound influence on activity. All chimeras were less active against Serratia marcescens (MICs above 46 μM). The chimeras were bactericidal and induced leakage of ATP from Staphylococcus aureus and S. marcescens with similar time of onset and reduction in the number of viable cells. EDTA pre-treatment of S. marcescens and E. coli followed by treatment with chimeras resulted in pronounced killing indicating that disintegration of the Gram-negative outer membrane eliminated innate differences in susceptibility. Chimera chain length did not influence the degree of ATP leakage, but the amount of intracellular ATP remaining in the cell after treatment was influenced by chimera length with the longest analogue causing complete depletion of intracellular ATP. Hence some chimeras caused a complete disruption of the membrane, and this was parallel by the largest reduction in number of viable bacteria.

CONCLUSION

We found that chain length but not type of cationic amino acid influenced the antibacterial activity of a series of synthetic α-peptide/β-peptoid chimeras. The synthetic chimeras exert their killing effect by permeabilization of the bacterial cell envelope, and the outer membrane may act as a barrier in Gram-negative bacteria. The tolerance of S. marcescens to chimeras may be due to differences in the composition of the lipopolysaccharide layer also responsible for its resistance to polymyxin B.

Carotenoid-related alteration of cell membrane fluidity impacts Staphylococcus aureus susceptibility to host defense peptides

Carotenoid pigments of Staphylococcus aureus provide integrity to its cell membrane (CM) and limit oxidative host defense mechanisms. However, the role of carotenoids in staphylococcal resistance to nonoxidative host defenses has not been characterized. The current study examined the relationship among CM carotenoid content, membrane order, and in vitro susceptibility to daptomycin or to prototypic neutrophil-derived, platelet-derived, or bacterium-derived cationic antimicrobial peptides (human neutrophil defensin-1 [hNP-1], platelet microbicidal proteins [PMPs], or polymyxin B, respectively). A previously characterized methicillin-susceptible Staphylococcus aureus (MSSA) isogenic clinical strain set was used, including a parental isolate with an intact carotenoid biosynthetic operon (crtOPQMN) containing the crtM gene encoding early steps in staphyloxanthin biosynthesis, a crtM deletion mutant, and a crtMN multicopy plasmid-complemented variant. Compared to the parental and crtM knockout strains, the crtMN-complemented strain exhibited (i) increased carotenoid production, (ii) increased CM rigidity (P < 0.001), and (iii) uniformly reduced susceptibility to killing by the above-mentioned range of cationic peptides (statistically significant for hNP-1 [20 μg/ml]; P = 0.0037). There were no significant differences in phospholipid composition and asymmetry, fatty acid profiles, surface charge, or cell wall thickness among the strain set. Collectively, these data support the concept that carotenoid biosynthesis can contribute to the ability of S. aureus to subvert nonoxidative host defenses mediated by cationic peptides, potentially by increasing target membrane rigidity.

Analysis of cell membrane characteristics of in vitro-selected daptomycin-resistant strains of methicillin-resistant Staphylococcus aureus

Our previous studies of clinical daptomycin-resistant (Dap(r)) Staphylococcus aureus strains suggested that resistance is linked to the perturbations of several key cell membrane (CM) characteristics, including the CM order (fluidity), phospholipid content and asymmetry, and relative surface charge. In the present study, we examined the CM profiles of a well-known methicillin-resistant Staphylococcus aureus (MRSA) strain (MW2) after in vitro selection for DAP resistance by a 20-day serial passage in sublethal concentrations of DAP. Compared to levels for the parental strain, Dap(r) strains exhibited (i) decreased CM fluidity, (ii) the increased synthesis of total lysyl-phosphatidylglycerol (LPG), (iii) the increased flipping of LPG to the CM outer bilayer, and (iv) the increased expression of mprF, the gene responsible for the latter two phenotypes. In addition, we found that the expression of the dlt operon, which also increases positive surface charge, was enhanced in the Dap(r) mutants. These phenotypic and genotypic changes correlated with reduced DAP surface binding, mirroring observations made in clinical Dap(r) isolates. In this strain, serial exposure to DAP induced an increase in vancomycin MICs into the vancomycin-intermediate S. aureus (VISA) range (4 microg/ml) in parallel with increasing DAP MICs. Also, this Dap(r) strain exhibited significantly thicker cell walls than the parental strain, potentially correlating with the coevolution of the VISA phenotype and implicating cell wall structure and/or function in the Dap(r) phenotype. Importantly, despite the overexpression of mprF and dlt, the relative net positive surface charge was decreased in the Dap(r) mutants, suggesting that other factors contribute to the surface charge alterations and that a simple charge repulsion mechanism could not entirely explain the Dap(r) phenotype in these strains.

Comparison of virulence in community-associated methicillin-resistant Staphylococcus aureus pulsotypes USA300 and USA400 in a rat model of pneumonia

BACKGROUND

The predominant genetic background of community-associated methicillin-resistant Staphylococcus aureus has transitioned from USA400 to USA300 in most US communities. The explanation for this shift is unclear. We hypothesized that USA300 must be more pathogenic--specifically, that USA300 would have increased virulence when compared with USA400 in an animal model.

METHODS

Rats were inoculated intratracheally with 1 of 6 S. aureus isolates from the USA300 and USA400 backgrounds. We assessed mortality, in vivo bacterial growth, and histopathology. We assessed the in vitro expression of capsule and of selected genes believed to be important in virulence in S. aureus, including agr, saeRS, sarA, alpha-toxin (hla), and Panton-Valentine leukocidin (pvl).

RESULTS

USA300 isolates were more lethal, produced more severe pneumonia, and had higher in vivo bacterial density in the lung than did USA400 isolates. In vitro expression of agr, saeRS, sarA, hla, and pvl were greater in USA300 isolates. USA300 isolates were unencapsulated, whereas 2 of 3 USA400 isolates produced capsule.

CONCLUSIONS

USA300 isolates were more virulent than USA400 isolates in a model of necrotizing pneumonia. The explanation for this is unclear, but it likely results from increased expression of S. aureus regulatory systems (e.g., agr, saeRS, and sarA) and the resultant upregulation of key virulence factors including alpha-toxin and PVL.

Human alpha-defensins inhibit BK virus infection by aggregating virions and blocking binding to host cells

BK virus (BKV) is a polyomavirus that establishes a lifelong persistence in most humans and is a major impediment to success of kidney grafts. The function of the innate immune system in BKV infection and pathology has not been investigated. Here we examine the role of antimicrobial defensins in BKV infection of Vero cells. Our data show that alpha-defensin human neutrophil protein 1 (HNP1) and human alpha-defensin 5 (HD5) inhibit BKV infection by targeting an early event in the viral lifecycle. HD5 treatment of BKV reduced viral attachment to cells, whereas cellular treatment with HD5 did not. Colocalization studies indicated that HD5 interacts directly with BKV. Ultrastructural analysis revealed HD5-induced aggregation of virions. HD5 also inhibited infection of cells by other related polyomaviruses. This is the first study to demonstrate polyomavirus sensitivity to defensins. We also show a novel mechanism whereby HD5 binds to BKV leading to aggregation of virion particles preventing normal virus binding to the cell surface and uptake into cells.

In vitro susceptibility of Staphylococcus aureus to thrombin-induced platelet microbicidal protein-1 (tPMP-1) is influenced by cell membrane phospholipid composition and asymmetry

Thrombin-induced platelet microbicidal proteins (e.g. tPMP-1) are small cationic peptides released from mammalian platelets. As the cytoplasmic membrane (CM) is a primary target of tPMPs, distinct CM characteristics are likely to affect the cells' susceptibility profiles. In Staphylococcus aureus, CM surface charge and hydrophobicity are principally determined by the content and distribution of its three major phospholipid (PL) constituents: negatively charged phosphatidylglycerol (PG) and cardiolipin (CL) and positively charged lysyl-PG (LPG). PL composition profiles, and inner vs outer CM leaflet PL distributions, were compared in an isogenic tPMP-susceptible (tPMP(S)) and -resistant (tPMP(R)) S. aureus strain pair (ISP479C vs ISP479R respectively). All PLs were asymmetrically distributed between the outer and inner CM leaflets in both strains. However, in ISP479R, the outer CM leaflet content of LPG was significantly increased vs ISP479C (27.3+/-11.0 % vs 18.6+/-7.0 % respectively; P=0.05). This observation correlated with reduced binding of the cationic proteins cytochrome c, poly-L-lysine, tPMP-1 and the tPMP-1-mimetic peptide, RP1, to tPMP-1(R) whole cells and to model liposomal CMs with LPG content and distribution similar to that of tPMP-1(R) strains. Collectively, selected CM parameters correlated with reduced staphylocidal capacities of tPMP-1 against certain S. aureus strains, including relative increases in outer CM leaflet positive charge and reduced surface binding of cationic molecules. These findings offer new insights into mechanisms of antimicrobial peptide susceptibility and resistance in S. aureus.

A synthetic congener modeled on a microbicidal domain of thrombin- induced platelet microbicidal protein 1 recapitulates staphylocidal mechanisms of the native molecule

Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a staphylocidal peptide released by activated platelets. This peptide initiates its microbicidal activity by membrane permeabilization, with ensuing inhibition of intracellular macromolecular synthesis. RP-1 is a synthetic congener modeled on the C-terminal microbicidal alpha-helix of tPMP-1. This study compared the staphylocidal mechanisms of RP-1 with those of tPMP-1, focusing on isogenic tPMP-1-susceptible (ISP479C) and -resistant (ISP479R) Staphylococcus aureus strains for the following quantitative evaluations: staphylocidal efficacy; comparative MIC; membrane permeabilization (MP) and depolarization; and DNA, RNA, and protein synthesis. Although the proteins had similar MICs, RP-1 caused significant killing of ISP479C (<50% survival), correlating with extensive MP (>95%) and inhibition of DNA and RNA synthesis (>90%), versus substantially reduced killing of ISP479R (>80% survival), with less MP (55%) and less inhibition of DNA or RNA synthesis (70 to 80%). Interestingly, RP-1-induced protein synthesis inhibition was equivalent in both strains. RP-1 did not depolarize the cell membrane and caused a relatively short postexposure growth inhibition. These data closely parallel those previously reported for tPMP-1 against this strain set and exemplify how synthetic molecules can be engineered to reflect structure-activity relationships of functional domains in native host defense effector molecules.

Functional interrelationships between cell membrane and cell wall in antimicrobial peptide-mediated killing of Staphylococcus aureus

Perturbation of the Staphylococcus aureus cytoplasmic membrane (CM) is felt to play a key role in the microbicidal mechanism of many antimicrobial peptides (APs). However, it is not established whether membrane permeabilization (MP) alone is sufficient to kill susceptible staphylococci or if the cell wall (CW) and/or intracellular targets contribute to AP-induced lethality. We hypothesized that the relationships between MP and killing may differ for distinct APs. In this study, we investigated the association between AP-induced MP and lethality in S. aureus whole cells versus CW-free protoplasts, and in comparison to the MP of liposomes modeled after whole CMs in terms of phospholipid composition, fluidity and charge. Four APs with different structure-activity relationships were examined: thrombin-induced platelet microbicidal protein 1 (tPMP-1), human neutrophil protein 1 (hNP-1), gramicidin D, and polymyxin B. MP was quantified fluorometrically by calcein release. All APs tested, except polymyxin B, caused concentration-dependent MP and killing of whole cells, but not of protoplasts. The reduced AP susceptibility of protoplasts was associated with increased cardiolipin and lysyl-phosphatidylglycerol content and reduced fluidity of their CMs. However, liposomal MP induced by tPMP-1, hNP-1, and gramicidin D paralleled that of whole cells. Collectively, these results indicate that (i) structurally distinct APs likely exert their staphylocidal effects by differing mechanisms, (ii) MP is not the sole event leading to AP-induced staphylocidal activity, (iii) a complex interrelationship exists between the CM and CW in AP-induced killing, and (iv) liposomes modeled upon whole cell or protoplast CMs can recapitulate the respective susceptibilities to killing by distinct APs.

Penicillin-induced killing and postantibiotic effect in oral streptococci are enhanced by platelet microbicidal proteins

Thrombin-induced platelet microbicidal proteins (tPMP) are alpha-granule-derived cationic antimicrobial proteins released from platelets by stimulation with thrombin. tPMP has potent microbicidal activities against a broad spectrum of common microbial pathogens in infective endocarditis. We studied in vitro interactions of tPMP with penicillin against oral streptococci, Streptococcus rattus BHT (a tPMP-susceptible organism) and Streptococcus gordonii DL1 (a tPMP-insusceptible organism). tPMP was prepared by stimulating rabbit platelets with thrombin. tPMP plus penicillin showed a synergistic bactericidal effect on both S. rattus BHT and S. gordonii DL1, in contrast to either agent alone. Sequential exposure of both S. rattus BHT and S. gordonii DL1 to tPMP followed by exposure to penicillin at 10x the MICs resulted in a significant extension of the postantibiotic-effect duration compared with antibiotic exposure alone. The combined data indicate that tPMP exerts cooperative bactericidal and growth-inhibiting effect in concert with penicillin.

Role of blood platelets in infection and inflammation

Blood platelets are here presented as active players in antimicrobial host defense and the induction of inflammation and tissue repair in addition to their participation in hemostasis. Megakaryopoiesis is inhibited after acute infection with viruses or bacteria. In contrast, chronic inflammation is often associated with reactive thrombocytosis. Platelets can bind and internalize pathogens and release microbicidal proteins that kill certain bacteria and fungi. By making cell-cell contacts with leukocytes and endothelial cells, platelets assist white blood cells in rolling, arrest and transmigration. On stimulation by bacteria or thrombin, platelets release the content of their alpha-granules, which include an arsenal of bioactive peptides, such as CC-chemokines and CXC-chemokines and growth factors for endothelial cells, smooth muscle cells and fibroblasts. Thus, integral to innate immunity, the tiny little platelets may become bombshells when irritated by pathogens.

Diversity in antistaphylococcal mechanisms among membrane-targeting antimicrobial peptides

Many antimicrobial peptides permeabilize the bacterial cytoplasmic membrane. However, it is unclear how membrane permeabilization and antimicrobial activity are related for distinct peptides. This study investigated the relationship between Staphylococcus aureus membrane permeabilization and cell death due to the following antistaphylococcal peptides: thrombin-induced platelet microbicidal protein 1 (tPMP-1), gramicidin D, and protamine. Isogenic S. aureus strains ISP479C and ISP479R (tPMP-1 susceptible and resistant, respectively), were loaded with the fluorochrome calcein and exposed to a range of concentrations of each peptide. Flow cytometry was then used to monitor membrane permeabilization by quantifying the release of preloaded calcein. Killing was determined by quantitative culture at time points simultaneous to measurement of membrane permeabilization. Membrane permeabilization and killing caused by tPMP-1 occurred in a time- and concentration-dependent manner, reflecting the intrinsic tPMP-1 susceptibilities of ISP479C and ISP479R. In comparison, gramicidin D killed both S. aureus strains to equivalent extents in a concentration-dependent manner between 0.5 to 50 microg/ml, but cell permeabilization only occurred at the higher peptide concentrations (25 and 50 microg/ml). Protamine permeabilized, but did not kill, either strain at concentrations up to 10 mg/ml. Regression analyses revealed different relationships between membrane permeabilization and staphylocidal activity for the distinct antimicrobial peptides. Taken together, these findings demonstrate that permeabilization, per se, does not invariably result in staphylococcal death due to distinct antimicrobial peptides. Thus, although each of these peptides interacts with the S. aureus cytoplasmic membrane, diversity exists in their mechanisms of action with respect to the relationship between membrane permeabilization and staphylocidal activity.

Biosynthesis of the glycolipid anchor in lipoteichoic acid of Staphylococcus aureus RN4220: role of YpfP, the diglucosyldiacylglycerol synthase

In Staphylococcus aureus RN4220, lipoteichoic acid (LTA) is anchored in the membrane by a diglucosyldiacylglycerol moiety. The gene (ypfP) which encodes diglucosyldiacylglycerol synthase was recently cloned from Bacillus subtilis and expressed in Escherichia coli (P. Jorasch, F. P. Wolter, U. Zahringer, and E. Heinz, Mol. Microbiol. 29:419-430, 1998). To define the role of ypfP in this strain of S. aureus, a fragment of ypfP truncated from both ends was cloned into the thermosensitive replicon pVE6007 and used to inactivate ypfP. Chloramphenicol-resistant (ypfP::cat) clones did not synthesize the glycolipids monoglucosyldiacylglycerol and diglucosyldiacylglycerol. Thus, YpfP would appear to be the only diglucosyldiacylglycerol synthase in S. aureus providing glycolipid for LTA assembly. In LTA from the mutant, the glycolipid anchor is replaced by diacylglycerol. Although the doubling time of the mutant was identical to that of the wild type in Luria-Bertani (LB) medium, growth of the mutant in LB medium containing 1% glycine was not observed. This inhibition was antagonized by either L- or D-alanine. Moreover, viability of the mutant at 37 degrees C in 0.05 M phosphate (pH 7.2)-saline for 12 h was reduced to <0.1%. Addition of 0.1% D-glucose to the phosphate-saline ensured viability under these conditions. The autolysis of the ypfP::cat mutant in the presence of 0.05% Triton X-100 was 1.8-fold faster than that of the parental strain. Electron microscopy of the mutant revealed not only a small increase in cell size but also the presence of pleomorphic cells. Each of these phenotypes may be correlated with either (or both) a deficiency of free glycolipid in the membrane or the replacement of the usual glycolipid anchor of LTA with diacylglycerol.

Modification of autolysis by synthetic peptides derived from the presumptive binding domain of Staphylococcus aureus autolysin

The autolytic cell wall hydrolase of Staphylococcus aureus, Atl, contains three highly cationic repeats in the central region of the amino acid sequence, and the repeats are presumed to have the role of binding the enzyme to some components on the cell surface. To explain the possible function of the repeats, we synthesized a number of 10- to 30-mer oligopeptides based on the Atl amino acid sequence (Thr432-Lys610) containing repeat 1, and examined their effects on the autolysis of S. aureus cells. When the peptides were added to a cell suspension of S. aureus under low ionic strength conditions, five peptides, A10, A11, A14, A16 and B9, showed immediate increases in optical density (OD) of the cell suspension accompanied by decreases in viable cell counts. After the immediate increases, the ODs for A10 and A14 changed little in the first 2 hr. In contrast, the ODs for A11 and A16 decreased rapidly. When peptide A10 was added to suspensions of heat-killed whole cells, crude cell walls and a crude peptidoglycan preparation, their ODs were increased approximately 2-fold. In contrast, the OD was not increased when the peptide was added to a suspension of pure peptidoglycan from which anionic polymers had been removed. Light microscopic and transmission electron microscopic study showed that A10 and A14 inhibited autolysis and that A11 and A16 induced autolysis earlier than the control. These results suggest strongly that the peptides adsorb to and precipitate on the anionic cell surface polymers such as teichoic acid and lipoteichoic acid via ionic interaction. The effects of peptides on the autolysis may be the results of the modification of S. aureus autolysin activities. These peptides, especially the 10-mer peptide B9 (PGTKLYTVPW) that represents the C-terminal half of A10 and N-terminal half of A11, may be important segments for Atl to bind to the cell surface.

Staphylococcus aureus, Platelets, and the Heart

Infective endocarditis (IE) caused by Staphylococcus aureus is serious, burgeoning frequency, and growing increasingly resistant to antibiotics. S. aureus IE is associated with high morbidity and mortality rates in nosocomial and community-acquired settings. S. aureus is the most common, most virulent IE etiologic pathogen. S. aureus IE pathogenesis depends upon complex interaction among the pathogen, platelets, plasma proteins, and vascular endothelial cells. S. aureus coordinates the expression of key virulence factors required for the specific pathogenic phases of IE. Platelets, now appear to play an important role in antimicrobial host defense against S. aureus IE and other endovascular infections. Platelet microbicidal proteins are believed to significantly contribute to the antimicrobial properties of platelets; however, abnormal disposition of native or prosthetic cardiac valves is an important risk factor in S. aureus IE establishment and severity. Thus, the need to define the molecular mechanisms of S. aureus pathogenesis and host defense against IE is urgent. Understanding these mechanisms will yield new approaches for the prevention and treatment of such life-threatening cardiovascular infections due to S. aureus.

Antibacterial action of structurally diverse cationic peptides on gram-positive bacteria

Antimicrobial cationic peptides are ubiquitous in nature and are thought to be a component of the first line of defense against infectious agents. It is widely believed that the killing mechanism of these peptides on bacteria involves an interaction with the cytoplasmic membrane. Cationic peptides from different structural classes were used in experiments with Staphylococcus aureus and other medically important gram-positive bacteria to gain insight into the mechanism of action. The membrane potential-sensitive fluorophore dipropylthiacarbocyanine was used to assess the interactions of selected antimicrobial peptides with the cytoplasmic membrane of S. aureus. Study of the kinetics of killing and membrane depolarization showed that, at early time points, membrane depolarization was incomplete, even when 90% or more of the bacteria had been killed. CP26, a 26-amino-acid alpha-helical peptide with a high MIC against S. aureus, still had the ability to permeabilize the membrane. Cytoplasmic-membrane permeabilization was a widespread ability and an action that may be necessary for reaching an intracellular target but in itself did not appear to be the killing mechanism. Transmission electron microscopy of S. aureus and Staphylococcus epidermidis treated with CP29 (a 26-amino-acid alpha-helical peptide), CP11CN (a 13-amino-acid, proline- and tryptophan-rich peptide), and Bac2A-NH(2) (a linearized version of the 12-amino-acid loop peptide bactenecin) showed variability in effects on bacterial structure. Mesosome-like structures were seen to develop in S. aureus, whereas cell wall effects and mesosomes were seen with S. epidermidis. Nuclear condensation and abherrent septation were occasionally seen in S. epidermidis. Our experiments indicated that these peptides vary in their mechanisms of action and that the mechanism of action likely does not solely involve cytoplasmic-membrane permeabilization.

In vitro resistance of Staphylococcus aureus to thrombin-induced platelet microbicidal protein is associated with alterations in cytoplasmic membrane fluidity

Platelet microbicidal proteins (PMPs) are small, cationic peptides which possess potent microbicidal activities against common bloodstream pathogens, such as Staphylococcus aureus. We previously showed that S. aureus strains exhibiting resistance to thrombin-induced PMP (tPMP-1) in vitro have an enhanced capacity to cause human and experimental endocarditis (T. Wu, M. R. Yeaman, and A. S. Bayer, Antimicrob. Agents Chemother. 38:729-732, 1994; A. S. Bayer et al., Antimicrob. Agents Chemother. 42:3169-3172, 1998; V. K. Dhawan et al., Infect. Immun. 65:3293-3299, 1997). However, the mechanisms mediating tPMP-1 resistance in S. aureus are not fully delineated. The S. aureus cell membrane appears to be a principal target for the action of tPMP-1. To gain insight into the basis of tPMP-1 resistance, we compared several parameters of membrane structure and function in three tPMP-1-resistant (tPMP-1(r)) strains and their genetically related, tPMP-1-susceptible (tPMP-1(s)) counterpart strains. The tPMP-1(r) strains were derived by three distinct methods: transposon mutagenesis, serial passage in the presence of tPMP-1 in vitro, or carriage of a naturally occurring multiresistance plasmid (pSK1). All tPMP-1(r) strains were found to possess elevated levels of longer-chain, unsaturated membrane lipids, in comparison to their tPMP-1(s) counterparts. This was reflected in corresponding differences in cell membrane fluidity in the strain pairs, with tPMP-1(r) strains exhibiting significantly higher degrees of fluidity as assessed by fluorescence polarization. These data provide further support for the concept that specific alterations in the cytoplasmic membrane of S. aureus strains are associated with tPMP-1 resistance in vitro.

Membrane permeabilization by thrombin-induced platelet microbicidal protein 1 is modulated by transmembrane voltage polarity and magnitude

Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a small, cationic peptide generated from rabbit platelets when they are exposed to thrombin in vitro. It has potent microbicidal activity against a broad spectrum of bacterial and fungal pathogens, including Staphylococcus aureus. Previous in vitro studies involving whole staphylococcal cells and planar lipid bilayers (as artificial bacterial membrane models) suggested that membrane permeabilization by tPMP-1 is voltage dependent (S.-P. Koo, M. R. Yeaman, and A. S. Bayer, Infect. Immun. 64:3758-3764, 1996; M. R. Yeaman, A. S. Bayer, S. P. Koo, W. Foss, and P. M. Sullam, J. Clin. Investig. 101:178-187, 1998). Thus, the aims of the present study were to specifically characterize the electrophysiological events associated with membrane permeabilization by tPMP-1 by using artificial planar lipid bilayer membranes. We assessed the influence of transmembrane voltage polarity and magnitude on the initiation and modulation of tPMP-1 membrane permeabilization at various concentrations of tPMP-1 (range, 1 to 100 ng/ml) added to the cis side of the membranes. The incidence of membrane permeabilization induced by tPMP-1 at all of the concentrations tested was more frequent at -90 mV than at +90 mV. It is noteworthy that membrane permeabilization due to 1-ng/ml tPMP-1 was successfully initiated at -90 mV but not at +90 mV. Further, the mean onset times of induction of tPMP-1 activity were comparable under the various conditions. Modulation of ongoing membrane permeabilization was dependent on voltage and tPMP-1 concentration. Membrane permeabilization at a low tPMP-1 concentration (1 ng/ml) was directly correlated with trans-negative voltages, while a higher tPMP-1 concentration (100 ng/ml) induced conductance which was more dependent on trans-positive voltages. Collectively, these data indicate that the mechanism of tPMP-1 microbicidal activity at the bacterial cytoplasmic membrane may involve distinct induction and propagation stages of membrane permeabilization which, in turn, are modulated by transmembrane potential, as well as peptide concentration.

In vitro antibacterial activities of platelet microbicidal protein and neutrophil defensin against Staphylococcus aureus are influenced by antibiotics differing in mechanism of action

Thrombin-induced platelet microbicidal protein-1 (tPMP-1) and human neutrophil defensin-1 (HNP-1) are small, cationic antimicrobial peptides. These peptides exert potent in vitro microbicidal activity against a broad spectrum of human pathogens, including Staphylococcus aureus. Evidence suggests that tPMP-1 and HNP-1 target and disrupt the bacterial membrane. However, it is not yet clear whether membrane disruption itself is sufficient to kill the bacterium or whether subsequent, presumably intracellular, events are also involved in killing. We investigated the staphylocidal activities of tPMP-1 and HNP-1 in the presence or absence of pretreatment with antibiotics that differ in their mechanisms of action. The staphylocidal effects of tPMP-1 and HNP-1 on control cells (no antibiotic pretreatment) were rapid and concentration dependent. Pretreatment of S. aureus with either penicillin or vancomycin (bacterial cell wall synthesis inhibitors) significantly enhanced the anti-S. aureus effects of tPMP-1 compared with the effects against the respective control cells over the entire tPMP-1 concentration range tested (P < 0.05). Similarly, S. aureus cells pretreated with these antibiotics were more susceptible to HNP-1 than control cells, although the difference in the effects against cells that received penicillin pretreatment did not reach statistical significance (P < 0.05 for cells that received vancomycin pretreatment versus effects against control cells). Studies with isogenic pairs of strains with normal or deficient autolytic enzyme activities demonstrated that enhancement of S. aureus killing by cationic peptides and cell wall-active agents could not be ascribed to a predominant role of autolytic enzyme activation. Pretreatment of S. aureus cells with tetracycline, a 30S ribosomal subunit inhibitor, significantly decreased the staphylocidal effect of tPMP-1 over a wide peptide concentration range (0.16 to 1.25 microgram/ml) (P < 0.05). Furthermore, pretreatment with novobiocin (an inhibitor of bacterial DNA gyrase subunit B) and with azithromycin, quinupristin, or dalfopristin (50S ribosomal subunit protein synthesis inhibitors) essentially blocked the S. aureus killing resulting from exposure to tPMP-1 or HNP-1 at most concentrations compared with the effects against the respective control cells (P < 0.05 for a tPMP-1 concentration range of 0.31 to 1.25 microgram/ml and for an HNP-1 concentration range of 6.25 to 50 microgram/ml). These findings suggest that tPMP-1 and HNP-1 exert anti-S. aureus activities through mechanisms involving both the cell membrane and intracellular targets.

Antimicrobial peptides from platelets

The fact that platelets play a key role in host defense against infection has been demonstrated by the following observations(1): (a) platelets rapidly respond to sites of endovascular trauma and chemotactic stimuli associated with microbial colonization, and they are the earliest and predominant cells at sites of microbial colonization of vascular endothelium; (b) platelets have surface receptors and cytoplasmic granules comparable in structure and function to those of neutrophils, monocytes, or macrophages; (c) platelets adhere directly to, and may internalize, microbial pathogens, thereby enhancing their clearance from the bloodstream and limiting their potential for hematogenous dissemination; (d) bacterial, fungal, and protozoal pathogens are damaged or killed by activated platelets in vitro; (e) platelets are capable of initiating or amplifying complement fixation in the presence of microorganisms; (f) platelets generate oxygen metabolites which likely contribute to their antimicrobial activity; (g) platelets and leukocytes interact synergistically to exert enhanced antimicrobial functions in vitro; (h) thrombocytopenia increases susceptibility to and severity of certain infections. Importantly, rabbit and human platelets are now known to contain and release microbicidal proteins (termed platelet microbicidal proteins [PMPs] or thrombin-induced PMPs [tPMPs]) when stimulated with microorganisms or platelet agonists associated with infection in vitro. It is hypothesized that these microbicidal peptides accumulate locally at sites of endovascular damage or infection. Recent investigations have confirmed that tPMP-susceptible pathogens are less capable of proliferation or hematogenous dissemination in vivo as compared with their isogenic counterpart strains that are resistant to PMPs. Collectively, the above observations strongly suggest that platelets play key and multi-faceted roles in antimicrobial host defense which appear to be significantly mediated by PMPs and tPMPs. Copyright 1999 Harcourt Publishers Ltd.

Platelet microbicidal proteins and neutrophil defensin disrupt the Staphylococcus aureus cytoplasmic membrane by distinct mechanisms of action

Platelet microbicidal proteins (PMPs) are hypothesized to exert microbicidal effects via cytoplasmic membrane disruption. Transmission electron microscopy demonstrated a temporal association between PMP exposure, damage of the Staphylococcus aureus cytoplasmic membrane ultrastructure, and subsequent cell death. To investigate the mechanisms of action of PMPs leading to membrane damage, we used flow cytometry to compare the effects of two distinct PMPs (thrombin-induced PMP-1 [tPMP-1] or PMP-2) with human neutrophil defensin-1 (hNP-1) on transmembrane potential (Deltapsi), membrane permeabilization, and killing of S. aureus. Related strains 6850 (Deltapsi -150 mV) and JB-1 (Deltapsi -100 mV; a respiration-deficient menadione auxotroph of 6850) were used to assess the influence of Deltapsi on peptide microbicidal effects. Propidium iodide (PI) uptake was used to detect membrane permeabilization, retention of 3,3'-dipentyloxacarbocyanine (DiOC5) was used to monitor membrane depolarization (Deltapsi), and quantitative culture or acridine orange accumulation was used to measure viability. PMP-2 rapidly depolarized and permeabilized strain 6850, with the extent of permeabilization inversely related to pH. tPMP-1 failed to depolarize strain 6850, but did permeabilize this strain in a manner directly related to pH. Depolarization, permeabilization, and killing of strain JB-1 due to PMPs were significantly less than in strain 6850. Growth in menadione reconstituted Deltapsi of JB-1 to a level equivalent to 6850, and was associated with greater depolarization due to PMP-2, but not tPMP-1. Reconstitution of Deltapsi also enhanced permeabilization and killing of JB-1 due to tPMP-1 or PMP-2. Both PMP-2 and tPMP-1 caused significant reductions in viability of strain 6850. In contrast to tPMP-1 or PMP-2, defensin hNP-1 depolarized, permeabilized, and killed both strains 6850 and JB-1 equally, and in a manner directly related to pH. Collectively, these data indicate that membrane dysfunction and cell death due to tPMP-1, PMP-2, or hNP-1 likely involve different mechanisms. These findings may also reveal new insights into the microbicidal activities versus mammalian cell toxicities of antimicrobial peptides.