Correlation of penicillin-induced lysis of Enterococcus faecium with saturation of essential penicillin-binding proteins and release of lipoteichoic acid

Listeria monocytogenes endocarditis: case report, review of the literature, and laboratory evaluation of potential novel antibiotic synergies

Endocarditis is a rare but serious manifestation of Listeria monocytogenes (LM). However, the optimal treatment strategy for LM endocarditis has yet to be established. Current antibiotic strategies for listeriosis include penicillin G or ampicillin (AMP) monotherapy, or AMP + gentamicin combination therapy which is often favored for endocarditis. The primary objective of our investigation was to assess the utility of AMP + ceftriaxone (CRO) and AMP + daptomycin (DAP) against LM, modeling less nephrotoxic antibiotic combinations traditionally used to manage resistant enterococcal endocarditis. Here we report a case of LM endocarditis, review the world literature, and evaluate alternative treatment strategies for listeriosis utilizing in vitro and ex vivo studies. The combination of AMP + CRO and AMP + DAP were each noted to have synergistic activity against a LM endocarditis isolate. Additionally, co-incubation of the isolate with sub-lethal concentrations of antibiotics (AMP, CRO, DAP, AMP + CRO or AMP + DAP) sensitized the bacterium to whole blood killing while pretreatment with CRO and DAP (at 1/4 MIC) sensitized the bacterium to neutrophil killing. However, these effects did not reflect potentiation of antibiotic activity to human cathelicidin peptide LL-37, which is abundant in neutrophils and highly active against LM. Interestingly, AMP pretreatment of the LM endocarditis isolate resulted in increased DAP binding to the bacterium when assessed by fluorescence microscopy. These in vitro and ex vivo studies suggest further investigation of combination therapy using AMP + CRO or AMP + DAP as an alternative treatment for LM infection is warranted.

Listeria monocytogenes endocarditis: case report, review of the literature, and laboratory evaluation of potential novel antibiotic synergies

Endocarditis is a rare but serious manifestation of Listeria monocytogenes (LM). However, the optimal treatment strategy for LM endocarditis has yet to be established. Current antibiotic strategies for listeriosis include penicillin G or ampicillin (AMP) monotherapy, or AMP + gentamicin combination therapy which is often favored for endocarditis. The primary objective of our investigation was to assess the utility of AMP + ceftriaxone (CRO) and AMP + daptomycin (DAP) against LM, modeling less nephrotoxic antibiotic combinations traditionally used to manage resistant enterococcal endocarditis. Here we report a case of LM endocarditis, review the world literature, and evaluate alternative treatment strategies for listeriosis utilizing in vitro and ex vivo studies. The combination of AMP + CRO and AMP + DAP were each noted to have synergistic activity against a LM endocarditis isolate. Additionally, co-incubation of the isolate with sub-lethal concentrations of antibiotics (AMP, CRO, DAP, AMP + CRO or AMP + DAP) sensitized the bacterium to whole blood killing while pretreatment with CRO and DAP (at 1/4 MIC) sensitized the bacterium to neutrophil killing. However, these effects did not reflect potentiation of antibiotic activity to human cathelicidin peptide LL-37, which is abundant in neutrophils and highly active against LM. Interestingly, AMP pretreatment of the LM endocarditis isolate resulted in increased DAP binding to the bacterium when assessed by fluorescence microscopy. These in vitro and ex vivo studies suggest further investigation of combination therapy using AMP + CRO or AMP + DAP as an alternative treatment for LM infection is warranted.

Nafcillin enhances innate immune-mediated killing of methicillin-resistant Staphylococcus aureus

Based on in vitro synergy studies, the addition of nafcillin to daptomycin was used to treat refractory methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. Daptomycin is a de facto cationic antimicrobial peptide in vivo, with antistaphylococcal mechanisms reminiscent of innate host defense peptides (HDPs). In this study, the effects of nafcillin on HDP activity against MRSA were examined in vitro and in vivo. Exposures to β-lactam antimicrobials in general, and nafcillin in particular, significantly increased killing of S. aureus by selected HDPs from keratinocytes, neutrophils, and platelets. This finding correlated with enhanced killing of MRSA by whole blood, neutrophils, and keratinocytes after growth in nafcillin. Finally, nafcillin pretreatment ex vivo reduced MRSA virulence in a murine subcutaneous infection model. Despite the lack of direct activity against MRSA, these studies show potent, consistent, and generalized nafcillin-mediated "sensitization" to increased killing of MRSA by various components of the innate host response. The use of nafcillin as adjunctive therapy in MRSA bacteremia merits further study and should be considered in cases refractory to standard therapy.

KEY MESSAGES

Nafcillin has been used as adjunctive therapy to clear persistent MRSA bacteremia. Nafcillin enhances killing of MRSA by a cadre of innate host defense peptides. Nafcillin increases binding of human cathelicidin LL-37 to the MRSA membrane. Nafcillin enhances killing of MRSA by neutrophils. Nafcillin reduces virulence of MRSA in a murine subcutaneous infection model.

Ampicillin enhances daptomycin- and cationic host defense peptide-mediated killing of ampicillin- and vancomycin-resistant Enterococcus faecium

We studied an ampicillin- and vancomycin-resistant Enterococcus faecium (VRE) isolate from a patient with endocarditis and bacteremia refractory to treatment with daptomycin (6 mg/kg of body weight) plus linezolid. Blood cultures cleared within 24 h of changing therapy to daptomycin (12 mg/kg) plus ampicillin. We examined the effects of ampicillin on daptomycin-induced growth inhibition and killing, surface charge, and susceptibility to several prototypical host defense cationic antimicrobial peptides. MICs and time-kill curves with daptomycin were assessed in the presence and absence of ampicillin. The impact of ampicillin on surface charge was assessed by flow cytometry and a poly-l-lysine binding assay. The effects of ampicillin preexposures upon VRE killing by five distinct cationic peptides of different structure, charge, origin, and mechanism of action were analyzed using the epidermal cathelicidin LL-37, thrombin-induced platelet microbicidal proteins (tPMPs), and a synthetic congener modeled after tPMP microbicidal domains (RP-1), human neutrophil peptide-1 (hNP-1), and polymyxin B (bacteria derived). Fluoroscein-Bodipy-labeled daptomycin was used to evaluate daptomycin binding to VRE membranes in the presence or absence of ampicillin. In media containing ampicillin (25 to 100 mg/liter), daptomycin MICs decreased from 1.0 to 0.38 mg/liter. Based on time-kill analysis and an in vitro pharmacodynamic model, ampicillin enhanced daptomycin activity against the study VRE from a bacteriostatic to a bactericidal profile. VRE grown in ampicillin (25 to 150 mg/liter) demonstrated an incremental reduction in its relative net positive surface charge. When grown in the presence (versus absence) of ampicillin (25 and 100 mg/liter), the VRE strain (i) was more susceptible to killing by LL-37, tPMPs, hNP-1, and RP-1 but not to polymyxin B and (ii) exhibited greater binding to Bodipy-labeled daptomycin. We conclude that ampicillin induces reductions in net positive bacterial surface charge of VRE, correlating with enhanced bactericidal effects of cationic calcium-daptomycin and a diverse range of other cationic peptides in vitro. While the mechanism(s) of such β-lactam-mediated shifts in surface charge remains to be defined, these finding suggest a potential for β-lactam-mediated enhancement of activity of both daptomycin and innate host defense peptides against antibiotic-resistant bacteria.

Use of antistaphylococcal beta-lactams to increase daptomycin activity in eradicating persistent bacteremia due to methicillin-resistant Staphylococcus aureus: role of enhanced daptomycin binding

We used daptomycin plus antistaphylococcal β-lactams (ASBL) to clear refractory MRSA bacteremia. In vitro studies showed enhanced daptomycin bactericidal activity, increased membrane daptomycin binding, and decrease in positive surface charge induced by ASBLs against daptomycin nonsusceptible MRSA. Addition of ASBLs to daptomycin may be of benefit in refractory MRSA bacteremia. (Although the official designation is "daptomycin nonsusceptiblity," we will use the term "daptomycin-resistance" in this paper for facility of presentation.).

A continuum of anionic charge: structures and functions of D-alanyl-teichoic acids in gram-positive bacteria

Teichoic acids (TAs) are major wall and membrane components of most gram-positive bacteria. With few exceptions, they are polymers of glycerol-phosphate or ribitol-phosphate to which are attached glycosyl and D-alanyl ester residues. Wall TA is attached to peptidoglycan via a linkage unit, whereas lipoteichoic acid is attached to glycolipid intercalated in the membrane. Together with peptidoglycan, these polymers make up a polyanionic matrix that functions in (i) cation homeostasis; (ii) trafficking of ions, nutrients, proteins, and antibiotics; (iii) regulation of autolysins; and (iv) presentation of envelope proteins. The esterification of TAs with D-alanyl esters provides a means of modulating the net anionic charge, determining the cationic binding capacity, and displaying cations in the wall. This review addresses the structures and functions of D-alanyl-TAs, the D-alanylation system encoded by the dlt operon, and the roles of TAs in cell growth. The importance of dlt in the physiology of many organisms is illustrated by the variety of mutant phenotypes. In addition, advances in our understanding of D-alanyl ester function in virulence and host-mediated responses have been made possible through targeted mutagenesis of dlt. Studies of the mechanism of D-alanylation have identified two potential targets of antibacterial action and provided possible screening reactions for designing novel agents targeted to D-alanyl-TA synthesis.

Antibiotic induced endotoxin release and clinical sepsis: a review

Sepsis and peritonitis have not lost much of their danger for patients. The mortality rate in peritonitis has only marginally decreased during the last 30 years despite aggressive surgical and sophisticated intensive care treatment. In intra-abdominal infection and peritonitis source control remains the mainstay of treatment, although general principles and denominators of successful source control need to be established. Endotoxin has been recognized as a major player in the pathogenesis of sepsis and its significance in clinical disease has been investigated in clinical studies for more than 20 years. Since the Sixties there is a growing interest in the effect of antibiotics and other compounds on the release of endotoxin. The effect of antibiotics on the release of endotoxin and inflammatory parameters, e.g., cytokines, remains to be clarified despite a growing body of in-vitro studies, animal studies and a few clinical studies. The purpose of this review is to evaluate the evidence of endotoxin release in clinical studies and the effect that antibiotic treatment may have in-vitro, in-vivo and in clinical studies on endotoxin and cytokine release. In-vitro antibiotic-induced endotoxin release may depend on antibiotic class, presence of serum, type of organism, site of antibiotic action and Gram-stain. Endotoxin release may be different in late or early lysis, proportional to the number of killed pathogens. Morphology of bacteria may have an impact on endotoxin release and phagocytosis. Antibiotic-treated animals may show higher endotoxin levels with a higher survival rate than untreated animals. Plasma endotoxin may increase despite decreasing bacteremia. There may be a similar killing rate by different antibiotics but a difference in endotoxin release. Intestinal endotoxin does not necessarily correlate to the level of gram-negative bacteria. However, the alteration of the gut content by pretreatment may be associated with reduced endotoxemia and increased survival. Antibiotic-induced endotoxin release may be different depending on the type of infection, the location of infection, the virulence of strains, Gram-stain, mode of application and dosage of antibiotic. Different antibiotics may induce the release of different forms of endotoxin which may be lethal for sensitized animals. The combination of antibiotics with inhibitors of endotoxin or the pro-inflammatory response may be responsible for increased survival by decrease of endotoxin release. The clinical significance of antibiotic-induced endotoxin release is documented only in a few clinical disorders, e.g., meningitis, urosepsis. The difference in endotoxin release by PBP 2-specific antibiotics, e.g., imipenem, and PBP 3-specific antibiotics, e.g., ceftazidime, may not be visible in each study. Patients with increased multi-organ failure (MOF) scores may profit from treatment with antibiotics known to decrease endotoxin. In conclusion, the clinical significance of antibiotic-induced endotoxin release remains to be clarified. Type of pathogen and its virulence may be more important than recently suggested. gram-positive pathogens were just recently recognized as an important factor for the development of the host response. In case of fever of unknown origin in intensive care patients either failure of treatment, e.g., failure of source control in intra-abdominal infection, or a side effect of antibiotic treatment, e.g., endotoxin release, should be considered as a cause of the fever.

Penicillin tolerance and modification of lipoteichoic acid associated with expression of vancomycin resistance in VanB-type Enterococcus faecium D366

Induction of vancomycin resistance in Enterococcus faecium D366, which exhibits a VanB-type resistance, as well as its constitutive expression in MT9, a derivative of D366, was associated with penicillin tolerance as shown by decreased lysis and killing of the cells. This phenomenon was linked neither to decreased expression of the different autolysins nor to their decreased lytic activity on the different cell walls. The only change observed was that almost twice the normal amount of D-alanine was attached to the lipoteichoic acid.

Synergistic effect of amoxicillin and cefotaxime against Enterococcus faecalis

The antibacterial efficacy of the combination of amoxicillin and cefotaxime was assessed against 50 clinical strains of Enterococcus faecalis. For 48 of 50 strains, the MIC of amoxicillin that inhibited 50% of isolates tested decreased from 0.5 microgram/ml (range, 0.25 to 1 microgram/ml) to 0.06 microgram/ml (range, 0.01 to 0.25 microgram/ml) in the presence of only 4 micrograms of cefotaxime per ml. Alternatively, the MIC of cefotaxime that inhibited 50% of isolates tested decreased from 256 micrograms/ml (range, 8 to 512 micrograms/ml) to 1 micrograms/ml (range, 0.5 to 16 micrograms/ml) in the presence of only 0.06 microgram of amoxicillin per ml. For JH2-2, a reference strain of E. faecalis, the MICs of amoxicillin, cefotaxime, and amoxicillin in the presence of cefotaxime (4 micrograms/ml) were 0.5, 512, and 0.06 microgram/ml, respectively. By using a penicillin-binding protein (PBP) competition assay, it was shown that with cefotaxime, 50% saturation of PBPs 2 and 3 was obtained at very low concentrations (< 1 microgram/ml), while 50% saturation of PBPs 1, 4, and 5 was obtained with > or = 128 micrograms/ml. With amoxicillin, 50% saturation of PBPs 4 and 5 was obtained at 0.12 and 0.5 microgram/ml, respectively. Therefore, the partial saturation of PBPs 4 and 5 by amoxicillin combined with the total saturation of PBPs 2 and 3 by cefotaxime could be responsible for the observed synergy between these two compounds.

Overproduction of a low-affinity penicillin-binding protein and high-level ampicillin resistance in Enterococcus faecium

Five ampicillin-resistant clinical isolates of Enterococcus faecium were analyzed for a correlation between overproduction of the low-affinity penicillin-binding protein (PBP 5) and the level of ampicillin resistance. Comparison was made with one susceptible clinical isolate and its ampicillin-resistant derivative obtained in the laboratory by selection with increasing concentrations of penicillin. Overproduction of the low-affinity PBP relative to the susceptible isolate was noted in moderately resistant strains (MIC, 32 micrograms/ml) but not in highly resistant strains (MIC, 128 micrograms/ml). Polyclonal antibodies specifically reacting with the low-affinity PBP of Enterococcus hirae, Enterococcus faecalis, and Enterococcus faecium (M. Ligozzi, M. Aldegheri, S. C. Predari, and R. Fontana, FEMS Microbiol. Lett. 83:335-340, 1991) were used to determine the amount of this PBP in the E. faecium isolates. In all strains, the antibody preparation reacted with a membrane protein of the same molecular mass as PBP 5. The amount of this protein was very small in the susceptible strain but large in all of the resistant strains. These results suggest that the highly resistant strains also overproduced the low-affinity PBP, which, compared with PBP 5 of moderately resistant strains, appeared to be modified in its penicillin-binding capability.

Mechanism of action of BAY v 3522, a new cephalosporin with unusually good activity against enterococci

The in vitro activity of BAY v 3522, a new cephalosporin with unusually good activity against enterococci, was tested on 100 clinical isolates of Enterococcus faecalis. The MIC for 86.3% of the strains was 4 micrograms/ml, whereas the MIC for 13.7% ranged from 8 to 16 micrograms/ml. No differences were found between MICs determined with low- or high-density inocula. The bactericidal activity of BAY v 3522 was tested on eight clinical strains; most strains showed a ca. 3-log decrease of the original inoculum at two to eight times the MIC. The interaction of BAY v 3522 and of other beta-lactams with penicillin-binding proteins (PBPs) was studied with a laboratory strain, E. hirae ATCC 9790, producing a discernible amount of PBP 5, a protein belonging to the family of low-affinity PBPs, responsible for the low susceptibility of enterococci to beta-lactams. PBPs 3 and 5 of ATCC 9790 showed the highest affinity for the new cephalosporin. Bay V 3522 at the MIC (8 micrograms/ml) saturated these two PBPs without any significant binding to the other PBPs. This result may explain the good antienterococcal activity of BAY v 3522.