The novel antimicrobial peptide PXL150 in the local treatment of skin and soft tissue infections

Dramatic increase in bacterial resistance towards conventional antibiotics emphasises the importance to identify novel, more potent antimicrobial therapies. Antimicrobial peptides (AMPs) have emerged as a promising new group to be evaluated in therapeutic intervention of infectious diseases. Here we describe a novel AMP, PXL150, which demonstrates in vitro a broad spectrum microbicidal action against both Gram-positive and Gram-negative bacteria, including resistant strains. The potent microbicidal activity and broad antibacterial spectrum of PXL150 were not associated with any hemolytic activity. Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) failed to develop resistance towards PXL150 during continued selection pressure. PXL150 caused a rapid depolarisation of cytoplasmic membrane of S. aureus, and dissipating membrane potential is likely one mechanism for PXL150 to kill its target bacteria. Studies in human cell lines indicated that PXL150 has anti-inflammatory properties, which might be of additional benefit. PXL150 demonstrated pronounced anti-infectious effect in an in vivo model of full thickness wounds infected with MRSA in rats and in an ex vivo model of pig skin infected with S. aureus. Subcutaneous or topical application of the peptide in rats did not lead to any adverse reactions. In conclusion, PXL150 may constitute a new therapeutic alternative for local treatment of infections, and further studies are warranted to evaluate the applicability of this AMP in clinical settings.     

A novel membrane-disruptive antimicrobial peptide from frog skin secretion against cystic fibrosis isolates and evaluation of anti-MRSA effect using Galleria mellonella model

Antimicrobial peptides from amphibian skin secretion are a promising source for the development of alternative antibiotics against the urgent antibiotic resistance. Methicillin-resistant S. aureus (MRSA) has been found to persist in both early and late disease course of cystic fibrosis (CF). Japonicin-2LF was isolated from the skin secretion of Fujian Large-headed Frog (Limnonectes fujianensis) via the combination of cDNA cloning and MS/MS sequencing. The antimicrobial and anti-biofilm activities of Japonicin-2LF were evaluated using both reference and clinic isolated strains. The permeability of the cell membrane treated by the peptide was revealed by fluorescent staining. The cytotoxicity was examined by haemolysis, MTT and LDH assays. Wax moth larvae (Galleria mellonella) infection model was applied to assess the efficacy of Japonicin-2LF against the reference and clinic MRSA isolates in vivo. Japonicin-2LF exhibited potent antimicrobial activity, particularly against Gram-positive bacteria Staphylococcus aureus and MRSA, killing the bacteria via membrane permeabilisation. Additionally, Japonicin-2LF demonstrated the inhibition and eradication of biofilms, particularly against the biofilm of MRSA by eradicating the biofilm matrix as well as killing all the sessile bacteria. In the in vivo assay, Japonicin-2LF significantly decreased the mortality of MRSA acute infected larvae. In conclusion, it is a novel antimicrobial peptide discovered from the skin secretion of Limnonectes fujianensis, and particularly effective against both planktonic and sessile MRSA. The further in vivo study suggests that Japonicin-2LF could be a potential drug candidate to control the MRSA infection in cystic fibrosis patients.     

Hp1404, a New Antimicrobial Peptide from the Scorpion Heterometrus petersii

Antimicrobial peptides have attracted much interest as a novel class of antibiotics against a variety of microbes including antibiotics resistant strains. In this study, a new cationic antimicrobial peptide Hp1404 was identified from the scorpion Heterometrus petersii, which is an amphipathic α-helical peptide and has a specific inhibitory activity against gram-positive bacteria including methicillin-resistant Staphylococcus aureus. Hp1404 can penetrate the membrane of S. aureus at low concentration, and disrupts the cellular membrane directly at super high concentration. S. aureus does not develop drug resistance after multiple treatments with Hp1404 at sub MIC concentration, which is possibly associated with the antibacterial mechanism of the peptide. In addition, Hp1404 has low toxicity to both mammalian cells (HC₅₀  =  226.6 µg/mL and CC₅₀ > 100 µg/mL) and balb-c mice (Non-toxicity at 80 mg/Kg by intraperitoneal injection and LD₅₀  =  89.8 mg/Kg by intravenous injection). Interestingly, Hp1404 can improve the survival rate of the MRSA infected balb-c mice in the peritonitis model. Taken together, Hp1404 may have potential applications as an antibacterial agent.     

3-Phenyl substituted 6,7-dimethoxyisoquinoline derivatives as FtsZ-targeting antibacterial agents

The emergence of multidrug-resistant bacteria has created an urgent need for antibiotics with a novel mechanism of action. The bacterial cell division protein FtsZ is an attractive target for the development of novel antibiotics. The benzo[c]phenanthridinium sanguinarine and the dibenzo[a,g]quinolizin-7-ium berberine are two structurally similar plant alkaloids that alter FtsZ function. The presence of a hydrophobic functionality at either the 1-position of 5-methylbenzo[c]phenanthridinium derivatives or the 2-position of dibenzo[a,g]quinolizin-7-ium derivatives is associated with significantly enhanced antibacterial activity. 3-Phenylisoquinoline represents a subunit within the ring-systems of both of these alkaloids. Several 3-phenylisoquinolines and 3-phenylisoquinolinium derivatives have been synthesized and evaluated for antibacterial activity against Staphylococcus aureus and Enterococcus faecalis, including multidrug-resistant strains of methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faecalis (VRE). A number of derivatives were found to have activity against both MRSA and VRE. The binding of select compounds to S. aureus FtsZ (SaFtsZ) was demonstrated and characterized using fluorescence spectroscopy. In addition, the compounds were shown to act as stabilizers of SaFtsZ polymers and concomitant inhibitors of SaFtsZ GTPase activity. Toxicological assessment of select compounds revealed minimal cross-reaction mammalian β-tubulin as well as little or no human cytotoxicity.     

Antibiotic and synergistic effect of Leu-Lys rich peptide against antibiotic resistant microorganisms isolated from patients with cholelithiasis

Pseudomonas aeruginosa has eventually developed resistance against flomoxef sodium, isepamicin and cefpiramide. Therefore, in this study, the antibacterial activity and synergistic effects of the amphipathic-derived P5-18mer antimicrobial peptide were tested against pathogens associated with cholelithiasis that have developed resistance against commonly used antibiotics. The results were then compared with the activities of the amphipathic-derived peptide, P5-18mer, melittin and common antibiotics. Growth inhibition of planktonic bacteria was tested using the National Committee for Clinical Laboratory Standards (NCCLS). The bactericidal activity of the antimicrobial peptides was measured using time-kill curves. Synergistic effects were evaluated by testing the effects of P5-18mer alone and in combination with flomoxef sodium, isepamicin or cefpiramide at 0.5 × MIC. P5-18mer peptide displayed strong activity against pathogens and flomoxef sodium, isepamicin and cefpiramide-resistant bacteria cell lines obtained from a patient with gallstones; however, it did not exert cytotoxicity against the human keratinocyte HaCat cell line. In addition, the results of time-kill curves indicated that P5-18mer peptide exerted bactericidal activity against four strains of P. aeruginosa. Finally, the use of P5-18mer and antibiotics exerted synergistic effects against cell lines that were resistant to commonly used antibiotics. These results indicate that this class of peptides has a rapid microbicidal effect on flomoxef sodium, isepamicin and cefpiramide-resistant strains of P. aeruginosa. Therefore, these peptides may be used as a lead drug for the treatment of acquired pathogens from patients with cholelithiasis who are affected with antibiotic-resistant bacteria.     

Design,synthesis and biological evaluation of spiropyrimidinetriones oxazolidinone derivatives as antibacterial agents

Gram-positive bacteria are among the most common human pathogens associated with clinical infections which range from mild skin infections to sepsis. Resistance towards existing class of drugs by Gram-positive bacteria including methicillin resistant Staphylococcus aureus (MRSA), Staphylococcus epidermidis (MRSE) and vancomycin resistant enterococci (VRE) is a growing concern. There is an urgent need to discover new antibiotics which are active against resistant strains of Gram positive bacteria. We report herein a novel class of spiropyrimidinetrione oxazolidinone derivatives as novel antibacterial agents. Key step towards the synthesis of title compounds involved the use of tert-amino reaction with [1,5]-hydride shift leading to the new CC bond formation. Compound 30n has demonstrated potent antibacterial activity against a panel of Gram-positive microbial strains including MRSA, MRSE, and LNZ and vancomycin resistant strains of E. faecalis. Further, molecular docking studies suggest that 30n has binding mode similar to that of LNZ in 50S RNA ribosome.     

Evaluation of antimicrobial activity of bacterial symbionts isolated from wild field cockroach Blattella vaga from Saudi Arabia

Drug-resistant pathogens form the main threat to global health during the current century. Annually, a lot of patients die in hospitals due to infection with one or more drug-resistant bacteria especially Staphylococcus aureus (MRSA). In the absence of new effective antimicrobial drugs, the number of deaths said to be increased. Searching for new antibiotics in our backyard form a part of scientist strategies to solve such serious health problem. Insects consider one of such interesting sources of the new era of antimicrobial drugs. Cockroaches as an example can live and adapt in a polluted area for a long time, so through this work field cockroach, Blattella vaga was collected from two semi-wild areas around Riyadh, Saudi Arabia for isolation of gut bacteria searching for new antimicrobial agents. Three species of bacteria were identified from field cockroach gut: Bacillus licheniformis, Bacillus subtilis, and Kocuria rosea. The three species were isolated, purified, and tested for their antimicrobial activity against four drug-resistant pathogens (three bacteria: Salmonella enterica (ATCC25566), Staphylococcus aureus (MRSA) (Clinical strain), and Streptococcus mutans (RCMB 017(1) ATCC ® 25175™) and one fungus: Candida albicans (RCMB005003(1) ATCC® 10231™)). The results show no antimicrobial activity of Bacillus subtilis and very good activity Bacillus licheniformis and Kocuria rosea. Bacillus licheniformis gives very effective activity against Candida albicans while Kocuria rosea is effective against MRSA and Streptococcus mutans. None of the gut isolated bacteria show any activity against Salmonella enterica. Such results revealed that the metabolites of these bacteria could be used as substitutes to the already used antibiotics to overcome the problem of multidrug-resistant human pathogens.     

Antimicrobial peptides from C-terminal amphipathic region of E. coli FtsA

Antimicrobial peptides constitute an indispensable component of innate immune system in organisms ranging from bacteria to man. Despite this, peptides lag far behind the conventional antibiotics in treating infections. The menace of multidrug-resistant bacteria, however, has revived the antimicrobial peptide research. We reasoned that the membrane-binding regions of bacterial proteins could be purposed to combat them. Here, we identify potent antimicrobial peptides from the C-terminal amphipathic helix of E. coli FtsA protein. The 11 and 13-residue peptides exhibited activity against E. coli, gentamicin-resistant MRSA, and C. albicans. The activity is little affected by the presence of salt and divalent cations. The peptides preferentially bind to the negatively-charged membranes as indicated by tryptophan fluorescence studies. The peptides permeabilize the E. coli outer and inner membranes at very promising concentrations suggesting membrane-disruption as one of the mechanisms of killing.     

A Bioengineered Nisin Derivative to Control Biofilms of Staphylococcus pseudintermedius

Antibiotic resistance and the shortage of novel antimicrobials are among the biggest challenges facing society. One of the major factors contributing to resistance is the use of frontline clinical antibiotics in veterinary practice. In order to properly manage dwindling antibiotic resources, we must identify antimicrobials that are specifically targeted to veterinary applications. Nisin is a member of the lantibiotic family of antimicrobial peptides that exhibit potent antibacterial activity against many gram-positive bacteria, including human and animal pathogens such as Staphylococcus, Bacillus, Listeria, and Clostridium. Although not currently used in human medicine, nisin is already employed commercially as an anti-mastitis product in the veterinary field. Recently we have used bioengineering strategies to enhance the activity of nisin against several high profile targets, including multi-drug resistant clinical pathogens such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) and also against staphylococci and streptococci associated with bovine mastitis. However, newly emerging pathogens such as methicillin resistant Staphylococcus pseudintermedius (MRSP) pose a significant threat in terms of veterinary health and as a reservoir for antibiotic resistance determinants. In this study we created a nisin derivative with enhanced antimicrobial activity against S. pseudintermedius. In addition, the novel nisin derivative exhibits an enhanced ability to impair biofilm formation and to reduce the density of established biofilms. The activities of this peptide represent a significant improvement over that of the wild-type nisin peptide and merit further investigation with a view to their use to treat S. pseudintermedius infections.     

A Rapid and Quantitative Flow Cytometry Method for the Analysis of Membrane Disruptive Antimicrobial Activity

We describe a microbial flow cytometry method that quantifies within 3 hours antimicrobial peptide (AMP) activity, termed Minimum Membrane Disruptive Concentration (MDC). Increasing peptide concentration positively correlates with the extent of bacterial membrane disruption and the calculated MDC is equivalent to its MBC. The activity of AMPs representing three different membranolytic modes of action could be determined for a range of Gram positive and negative bacteria, including the ESKAPE pathogens, E. coli and MRSA. By using the MDC50 concentration of the parent AMP, the method provides high-throughput, quantitative screening of AMP analogues. A unique feature of the MDC assay is that it directly measures peptide/bacteria interactions and lysed cell numbers rather than bacteria survival as with MIC and MBC assays. With the threat of multi-drug resistant bacteria, this high-throughput MDC assay has the potential to aid in the development of novel antimicrobials that target bacteria with improved efficacy.     

Antibacterial Activity of THAM Trisphenylguanide against Methicillin-Resistant Staphylococcus aureus

This study investigated the potential antibacterial activity of three series of compounds synthesized from 12 linear and branched polyamines with 2–8 amino groups, which were substituted to produce the corresponding guanides, biguanides, or phenylguanides, against Acinetobacter baumannii, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. Antibacterial activity was measured for each compound by determining the minimum inhibitory concentration against the bacteria, and the toxicity towards mammalian cells was determined. The most effective compound, THAM trisphenylguanide, was studied in time-to-kill and cytoplasmic leakage assays against methicillin-resistant Staphylococcus aureus (MRSA, USA300) in comparison to chlorhexidine. Preliminary toxicity and MRSA challenge studies in mice were also conducted on this compound. THAM trisphenylguanide showed significant antibacterial activity (MIC ∼1 mg/L) and selectivity against MRSA relative to all the other bacteria examined. In time-to-kill assays it showed increased antimicrobial activity against MRSA versus chlorhexidine. It induced leakage of cytoplasmic content at concentrations that did not reduce cell viability, suggesting the mechanism of action may involve membrane disruption. Using an intraperitoneal mouse model of invasive MRSA disease, THAM trisphenylguanide reduced bacterial burden locally and in deeper tissues. This study has identified a novel guanide compound with selective microbicidal activity against Staphylococcus aureus, including a methicillin-resistant (MRSA) strain.     

Synergism between Medihoney and Rifampicin against Methicillin-Resistant Staphylococcus aureus (MRSA)

Skin and chronic wound infections caused by highly antibiotic resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) are an increasing and urgent health problem worldwide, particularly with sharp increases in obesity and diabetes. New Zealand manuka honey has potent broad-spectrum antimicrobial activity, has been shown to inhibit the growth of MRSA strains, and bacteria resistant to this honey have not been obtainable in the laboratory. Combinational treatment of chronic wounds with manuka honey and common antibiotics may offer a wide range of advantages including synergistic enhancement of the antibacterial activity, reduction of the effective dose of the antibiotic, and reduction of the risk of antibiotic resistance. The aim of this study was to investigate the effect of Medihoney in combination with the widely used antibiotic rifampicin on S. aureus. Using checkerboard microdilution assays, time-kill curve experiments and agar diffusion assays, we show a synergism between Medihoney and rifampicin against MRSA and clinical isolates of S. aureus. Furthermore, the Medihoney/rifampicin combination stopped the appearance of rifampicin-resistant S. aureus in vitro. Methylglyoxal (MGO), believed to be the major antibacterial compound in manuka honey, did not act synergistically with rifampicin and is therefore not the sole factor responsible for the synergistic effect of manuka honey with rifampicin. Our findings support the idea that a combination of honey and antibiotics may be an effective new antimicrobial therapy for chronic wound infections.     

Antimicrobial activity of amphiphilic neamine derivatives: Understanding the mechanism of action on Gram-positive bacteria

Amphiphilic aminoglycoside derivatives are potential new antimicrobial agents mostly developed to fight resistant bacteria. The mechanism of action of the 3′,6-dinonyl neamine, one of the most promising derivative, has been investigated on Gram-negative bacteria, including P. aeruginosa. In this study, we have assessed its mechanism of action against Gram-positive bacteria, S. aureus and B. subtilis. By conducting time killing experiments, we assessed the bactericidal effect induced by 3′,6-dinonyl neamine on S. aureus MSSA and MRSA. By measuring the displacement of BODIPY?-TR cadaverine bound to lipoteichoic acids (LTA), we showed that 3′,6-dinonyl neamine interacts with these bacterial surface components. We also highlighted the ability of 3′,6-dinonyl neamine to enhance membrane depolarization and induce membrane permeability, by using fluorescent probes, DiSC₃C(5) and propidium iodide, respectively. These effects are observed for both MSSA and MRSA S. aureus as well as for B. subtilis. By electronic microscopy, we imaged the disruption of membrane integrity of the bacterial cell wall and by fluorescence microscopy, we demonstrated changes in the localization of lipids from the enriched-septum region and the impairment of the formation of septum. At a glance, we demonstrated that 3′,6-dinonyl neamine interferes with multiple targets suggesting a low ability of bacteria to acquire resistance to this agent. In turn, the amphiphilic neamine derivatives are promising candidates for development as novel multitarget therapeutic antibiotics.     

Design and synthesis of cell selective α/β-diastereomeric peptidomimetic with potent in vivo antibacterial activity against methicillin resistant S. Aureus

Design of therapeutically viable antimicrobial peptides with cell selectivity against microorganisms is an important step towards the development of new antimicrobial agents. Here, we report four de novo designed, short amphipathic sequences based on a α-helical template comprising of Lys, Trp and Leu or their corresponding D-and/or β-amino acids. Sequence A-12 was protease susceptible whereas its α/β-diastereomeric analogue UNA-12 was resistant to trypsin and proteinase K up to 24 h. A-12 and UNA-12 exhibited broad-spectrum antibacterial activity (MIC: 2–32 µg/mL) against pathogens including methicillin resistant S. aureus (MRSA) and methicillin-resistant S. epidermidis (MRSE). Interestingly, A-12 was found to be most toxic (>50% haemolytic at 250 µg/mL) whereas UNA-12 was found to be non cytotoxic among the all analogues against hRBCs and human keratinocytes. Interaction studies with artificial membranes by tryptophan fluorescence and acrylamide quenching assay demonstrated A-12 interacted equally in bacterial as well as mammalian mimic membrane whereas UNA-12 was found to be more selective towards bacterial mimic membrane. Further microscopic tool has revealed membrane damaging ability of A-12 and UNA-12 with bactericidal mode of action against MRSA. Encouragingly, peptidomimetics analogue UNA-12 showed remarkable safety and efficacy against MRSA in in-vivo neutropenic mice thigh infection model. In summary, simultaneous replacement of the natural amino acids with D-/β-congeners is a promising strategy for designing of potent, cell selective and protease stable peptide based antibiotics.     

Novel bacterial metabolite merochlorin A demonstrates in vitro activity against multi-drug resistant methicillin-resistant Staphylococcus aureus

Background

We evaluated the in vitro activity of a merochlorin A, a novel compound with a unique carbon skeleton, against a spectrum of clinically relevant bacterial pathogens and against previously characterized clinical and laboratory Staphylococcus aureus isolates with resistance to numerous antibiotics.

Methods

Merochlorin A was isolated and purified from a marine-derived actinomycete strain CNH189. Susceptibility testing for merochlorin A was performed against previously characterized human pathogens using broth microdilution and agar dilution methods. Cytotoxicity was assayed in tissue culture assays at 24 and 72 hours against human HeLa and mouse sarcoma L929 cell lines.

Results

The structure of as new antibiotic, merochlorin A, was assigned by comprehensive spectroscopic analysis. Merochlorin A demonstrated in vitro activity against Gram-positive bacteria, including Clostridium dificile, but not against Gram negative bacteria. In S. aureus, susceptibility was not affected by ribosomal mutations conferring linezolid resistance, mutations in dlt or mprF conferring resistance to daptomycin, accessory gene regulator knockout mutations, or the development of the vancomycin-intermediate resistant phenotype. Merochlorin A demonstrated rapid bactericidal activity against MRSA. Activity was lost in the presence of 20% serum.

Conclusions

The unique meroterpenoid, merochlorin A demonstrated excellent in vitro activity against S. aureus and C. dificile and did not show cross-resistance to contemporary antibiotics against Gram positive organisms. The activity was, however, markedly reduced in 20% human serum. Future directions for this compound may include evaluation for topical use, coating biomedical devices, or the pursuit of chemically modified derivatives of this compound that retain activity in the presence of serum.     

Combined Use of Bacteriophage K and a Novel Bacteriophage To Reduce Staphylococcus aureus Biofilm Formation

Biofilms are major causes of impairment of wound healing and patient morbidity. One of the most common and aggressive wound pathogens is Staphylococcus aureus, displaying a large repertoire of virulence factors and commonly reduced susceptibility to antibiotics, such as the spread of methicillin-resistant S. aureus (MRSA). Bacteriophages are obligate parasites of bacteria. They multiply intracellularly and lyse their bacterial host, releasing their progeny. We isolated a novel phage, DRA88, which has a broad host range among S. aureus bacteria. Morphologically, the phage belongs to the Myoviridae family and comprises a large double-stranded DNA (dsDNA) genome of 141,907 bp. DRA88 was mixed with phage K to produce a high-titer mixture that showed strong lytic activity against a wide range of S. aureus isolates, including representatives of the major international MRSA clones and coagulase-negative Staphylococcus. Its efficacy was assessed both in planktonic cultures and when treating established biofilms produced by three different biofilm-producing S. aureus isolates. A significant reduction of biofilm biomass over 48 h of treatment was recorded in all cases. The phage mixture may form the basis of an effective treatment for infections caused by S. aureus biofilms.     

Novel inhibitors of Staphylococcus aureus RnpA that synergize with mupirocin

We recently discovered RnpA as a promising new drug discovery target for methicillin-resistant S. aureus (MRSA). RnpA is an essential protein that is thought to perform two required cellular processes. As part of the RNA degrasome Rnpa mediates RNA degradation. In combination with rnpB it forms RNase P haloenzymes which are required for tRNA maturation. A high throughput screen identified RNPA2000 as an inhibitor of both RnpA-associated activities that displayed antibacterial activity against clinically relevant strains of S. aureus, including MRSA. Structure-activity studies aimed at improving potency and replacing the potentially metabotoxic furan moiety led to the identification of a number of more potent analogs. Many of these new analogs possessed overt cellular toxicity that precluded their use as antibiotics but two derivatives, including compound 5o, displayed an impressive synergy with mupirocin, an antibiotic used for decolonizing MSRA whose effectiveness has recently been jeopardized by bacterial resistance. Based on our results, compounds like 5o may ultimately find use in resensitizing mupirocin-resistant bacteria to mupirocin.     

In Vitro Activity of Sodium New Houttuyfonate Alone and in Combination with Oxacillin or Netilmicin against Methicillin-Resistant Staphylococcus aureus

Background

Staphylococcus aureus can cause severe infections, including bacteremia and sepsis. The spread of methicillin-resistant Staphylococcus aureus (MRSA) highlights the need for novel treatment options. Sodium new houttuyfonate (SNH) is an analogue of houttuynin, the main antibacterial ingredient of Houttuynia cordata Thunb. The aim of this study was to evaluate in vitro activity of SNH and its potential for synergy with antibiotics against hospital-associated MRSA.

Methodology

A total of 103 MRSA clinical isolates recovered in two hospitals in Beijing were evaluated for susceptibility to SNH, oxacillin, cephalothin, meropenem, vancomycin, levofloxacin, minocycline, netilmicin, and trimethoprim/sulfamethoxazole by broth microdilution. Ten isolates were evaluated for potential for synergy between SNH and the antibiotics above by checkerboard assay. Time-kill analysis was performed in three isolates to characterize the kill kinetics of SNH alone and in combination with the antibiotics that engendered synergy in checkerboard assays. Besides, two reference strains were included in all assays.

Principal Findings

SNH inhibited all test strains with minimum inhibitory concentrations (MICs) ranging from 16 to 64 µg/mL in susceptibility tests, and displayed inhibition to bacterial growth in concentration-dependent manner in time-kill analysis. In synergy studies, the combinations of SNH-oxacillin, SNH-cephalothin, SNH-meropenem and SNH-netilmicin showed synergistic effects against 12 MRSA strains with median fractional inhibitory concentration (FIC) indices of 0.38, 0.38, 0.25 and 0.38 in checkerboard assays. In time-kill analysis, SNH at 1/2 MIC in combination with oxacillin at 1/128 to 1/64 MIC or netilmicin at 1/8 to 1/2 MIC decreased the viable colonies by ≥2log₁₀ CFU/mL.

Conclusions/Significance

SNH demonstrated in vitro antibacterial activity against 103 hospital-associated MRSA isolates. Combinations of sub-MIC levels of SNH and oxacillin or netilmicin significantly improved the in vitro antibacterial activity against MRSA compared with either drug alone. The SNH-based combinations showed promise in combating MRSA.     

A computational model of antibiotic-resistance mechanisms in methicillin-resistant Staphylococcus aureus (MRSA)

An agent-based model of bacteria-antibiotic interactions has been developed that incorporates the antibiotic-resistance mechanisms of Methicillin-Resistant Staphylococcus aureus (MRSA). The model, called the Micro-Gen Bacterial Simulator, uses information about the cell biology of bacteria to produce global information about population growth in different environmental conditions. It facilitates a detailed systems-level investigation of the dynamics involved in bacteria-antibiotic interactions and a means to relate this information to traditional high-level properties such as the Minimum Inhibitory Concentration (MIC) of an antibiotic. The two main resistance strategies against β-lactam antibiotics employed by MRSA were incorporated into the model: β-lactamase enzymes, which hydrolytically cleave antibiotic molecules, and penicillin-binding proteins (PBP2a) with reduced binding affinities for antibiotics. Initial tests with three common antibiotics (penicillin, ampicillin and cephalothin) indicate that the model can be used to generate quantitatively accurate predictions of MICs for antibiotics against different strains of MRSA from basic cellular and biochemical information. Furthermore, by varying key parameters in the model, the relative impact of different kinetic parameters associated with the two resistance mechanisms to β-lactam antibiotics on cell survival in the presence of antibiotics was investigated.