MRSA new treatments on the horizon: current status

A Computational and Modeling Study of the Reaction Mechanism of Staphylococcus aureus Monoglycosyltransferase Reveals New Insights on the GT51 Family of Enzymes

Bacterial glycosyltransferases of the GT51 family are key enzymes in bacterial cell wall synthesis. Inhibiting cell wall synthesis is a very effective approach for development of antibiotics, as this can lead to either bacteriostatic or bactericidal effects. Even though the existence of this family has been known for over 50 years, only one potent inhibitor exists, which is an analog of the lipid IV product and derived from a natural product. Drug development focused on bacterial transglycosylase has been hampered due to little being know about its structure and reaction mechanism. In this study, Staphylococcus aureus monoglycosyltransferase was investigated at an atomistic level using computational methods. Classical molecular dynamics simulations were used to reveal information about the large-scale dynamics of the enzyme-substrate complex and the importance of magnesium in structure and function of the protein, while mixed mode quantum mechanics/molecular mechanics calculations unveiled a novel hypothesis for the reaction mechanism. From these results, we present a new model for the binding mode of lipid II and the reaction mechanism of the GT51 glycosyltransferases. A metal-bound hydroxide catalyzed reaction mechanism yields an estimated free energy barrier of 16.1 ± 1.0 kcal/mol, which is in line with experimental values. The importance of divalent cations is also further discussed. These findings could significantly aid targeted drug design, particularly the efficient development of transition state analogues as potential inhibitors for the GT51 glycosyltransferases.

Complete genome sequence analysis of a novel Staphylococcus phage StAP1 and proposal of a new species in the genus Silviavirus

Bacteriophage StAP1 was isolated from a soil sample infecting Staphylococcus aureus and S. xylosus. Its genome was found to be 135,502 base pairs (bp) long with 30.00 mol% G+C content and 192 open reading frames. While no tRNA encoding genes were identified, 7 mobile elements were found to interrupt five StAP1 open reading frames. Comparative genomic and proteomic analysis consistently supports the establishment of a new species in the genus Silviavirus.

Discovery of Potent Pantothenamide Inhibitors of Staphylococcus aureus Pantothenate Kinase through a Minimal SAR Study: Inhibition Is Due to Trapping of the Product

The potent antistaphylococcal activity of N-substituted pantothenamides (PanAms) has been shown to at least partially be due to the inhibition of Staphylococcus aureus's atypical type II pantothenate kinase (SaPanK_II), the first enzyme of coenzyme A biosynthesis. This mechanism of action follows from SaPanK_II having a binding mode for PanAms that is distinct from those of other PanKs. To dissect the molecular interactions responsible for PanAm inhibitory activity, we conducted a mini SAR study in tandem with the cocrystallization of SaPanK_II with two classic PanAms (N5-Pan and N7-Pan), culminating in the synthesis and characterization of two new PanAms, N-Pip-PanAm and MeO-N5-PanAm. The cocrystal structures showed that all of the PanAms are phosphorylated by SaPanK_II but remain bound at the active site; this occurs primarily through interactions with Tyr240' and Thr172'. Kinetic analysis showed a strong correlation between k_cat (slow PanAm turnover) and IC₅₀ (inhibition of pantothenate phosphorylation) values, suggesting that SaPanK_II inhibition occurs via a delay in product release. In-depth analysis of the PanAm-bound structures showed that the capacity for accepting a hydrogen bond from the amide of Thr172' was a stronger determinant for PanAm potency than the capacity to π-stack with Tyr240'. The two new PanAms, N-Pip-PanAm and MeO-N5-PanAm, effectively combine both hydrogen bonding and hydrophobic interactions, resulting in the most potent SaPanK_II inhibition described to date. Taken together, our results are consistent with an inhibition mechanism wherein PanAms act as SaPanK_II substrates that remain bound upon phosphorylation. The phospho-PanAm-SaPanK_II interactions described herein may help future antistaphylococcal drug development.

Salicylanilide carbamates: Promising antibacterial agents with high in vitro activity against methicillin-resistant Staphylococcus aureus (MRSA)

A series of twenty-one salicylanilide N-alkylcarbamates was assessed for novel antibacterial characteristics against three clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and S. aureus ATCC 29213 as the reference and quality control strain. The minimum inhibitory concentration was determined by the broth dilution micro-method with subsequent subcultivation of aliquots to assess minimum bactericidal concentration. The bactericidal kinetics was established by time-kill assay. Ampicillin, ciprofloxacin and vancomycin were used as reference antibacterial drugs. All the tested compounds exhibited highly potent anti-MRSA activity (⩽ 0.008-4 μg/mL) comparable or up to 250× higher than that of vancomycin, the standard in the treatment of serious MRSA infections. 4-Chloro-2-(3,4-dichlorophenylcarbamoyl)phenyl butylcarbamate and 4-chloro-2-(3,4-dichlorophenylcarbamoyl)phenyl ethylcarbamate were the most active compounds. In most cases, compounds provided reliable bacteriostatic activity, except for 4-chloro-2-(4-chlorophenylcarbamoyl)phenyl decylcarbamate exhibiting bactericidal effect at 8h (for clinical isolate of MRSA 63718) and at 24h (for clinical isolates of MRSA SA 630 and MRSA SA 3202) at 4× MIC. Structure-activity relationships are discussed.

Synthesis and Biological Evaluation of N-Alkoxyphenyl-3-hydroxynaphthalene-2-carboxanilides

A series of fifteen new N-alkoxyphenylanilides of 3-hydroxynaphthalene-2-carboxylic acid was prepared and characterized. Primary in vitro screening of the synthesized compounds was performed against Staphylococcus aureus, three methicillin-resistant S. aureus strains, Mycobacterium tuberculosis H37Ra and M. avium subsp. paratuberculosis. Some of the tested compounds showed antibacterial and antimycobacterial activity against the tested strains comparable with or higher than that of the standards ampicillin or rifampicin. 3-Hydroxy-N-(2-propoxyphenyl)naphthalene-2-carboxamide and N-[2-(but-2-yloxy)-phenyl]-3-hydroxynaphthalene-2-carboxamide had MIC = 12 µM against all methicillin-resistant S. aureus strains; thus their activity is 4-fold higher than that of ampicillin. The second mentioned compound as well as 3-hydroxy-N-[3-(prop-2-yloxy)phenyl]-naphthalene-2-carboxamide had MICs = 23 µM and 24 µM against M. tuberculosis respectively. N-[2-(But-2-yloxy)phenyl]-3-hydroxynaphthalene-2-carboxamide demonstrated higher activity against M. avium subsp. paratuberculosis than rifampicin. Screening of the cytotoxicity of the most effective antimycobacterial compounds was performed using THP-1 cells, and no significant lethal effect was observed for the most potent compounds. The compounds were additionally tested for their activity related to inhibition of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. N-(3-Ethoxyphenyl)-3-hydroxynaphthalene-2-carboxamide (IC₅₀ = 4.5 µM) was the most active PET inhibitor. The structure-activity relationships are discussed.

In vitro bactericidal activity of 4- and 5-chloro-2-hydroxy-N-[1-oxo-1-(phenylamino)alkan-2-yl]benzamides against MRSA

A series of nine substituted 2-hydroxy-N-[1-oxo-1-(phenylamino)alkan-2-yl]benzamides was assessed as prospective bactericidal agents against three clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and S. aureus ATCC 29213 as the reference and quality control strain. The minimum bactericidal concentration was determined by subculturing aliquots from MIC determination onto substance-free agar plates. The bactericidal kinetics of compounds 5-chloro-2-hydroxy-N-[(2S)-3-methyl-1-oxo-1-{[4-(trifluoromethyl)phenyl]amino}butan-2-yl]benzamide (1f), N-{(2S)-1-[(4-bromophenyl)amino]-3-methyl-1-oxobutan-2-yl}-4-chloro-2-hydroxybenzamide (1g), and 4-chloro-N-{(2S)-1-[(3,4-dichlorophenyl)amino]-3-methyl-1-oxobutan-2-yl}-2-hydroxybenzamide (1h) was established by time-kill assay with a final concentration of the compound equal to 1x, 2x, and 4x MIC; aliquots were removed at 0, 4, 6, 8, and 24 h time points. The most potent bactericidal agent was compound 1f exhibiting remarkable rapid concentration-dependent bactericidal effect even at 2x MIC at 4, 6, and 8 h (with a reduction in bacterial count ranging from 3.08 to 3.75 log₁₀ CFU/mL) and at 4x MIC at 4, 6, 8, and 24 h (5.30 log₁₀ CFU/mL reduction in bacterial count) after incubation against MRSA 63718. Reliable bactericidal effect against other strains was maintained at 4x MIC at 24 h.

Complete genome of Staphylococcus aureus phage SA11

A novel lytic bacteriophage, SA11, infecting Staphylococcus aureus was isolated, and the whole genome was sequenced. It belongs to the siphoviridae based on electron microscopic observation. It has a linear double-stranded DNA genome of 136,326 bp. Genomic analysis showed that it is distantly related to Staphylococcus phages A5W, K, ISP, Sb-1, and G1.