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Fourier Transform Infrared Spectroscopy Detection of Lipid Ion‐Pairing in the Staphylococcus aureus Plasma Membrane
Author(s) -
Duran Andrea T,
Marbach Helene,
Rasul Bahar,
Andrew Chan K. L.,
Harvey Richard D
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.877.1
Subject(s) - fourier transform infrared spectroscopy , chemistry , membrane , staphylococcus aureus , chromatography , lipid bilayer , analytical chemistry (journal) , bacteria , biochemistry , chemical engineering , biology , engineering , genetics
Ion‐pairing between lipids in the Staphylococcus aureus plasma membrane has been hypothesized to aid survival of this bacterium at low pH and potentially facilitate antimicrobial drug resistance through increased lipid ordering. Fourier transform infrared spectroscopy (FTIR) is a useful and rapid technique that can potentially characterize the physical properties of phospholipid membranes. The purpose of this study was to determine if FTIR can detect ion‐pairing in natural lipids extracted from bacteria grown under different conditions and also in representative synthetic models. S. aureus reference strain 476 and methicillin‐resistant S. aureus (MRSA) strain A7 were cultured in Brain Heart Infusion broth at pH 7.4 and pH 5.5. The phospholipids were isolated from each of the samples using an organic solvent extraction process. Synthetic lipid models composed of binary mixtures of dipalmitoyl‐3‐aza‐dehydroxy‐lysyl‐phosphatidylglycerol (DP3adLPG) and dipalmitoylphosphatidylglycerol (DPPG) were also created, to mimic the compositions of the S. aureus plasma membrane at pH 7.4 (DP3adLPG/DPPG [30:70]) and pH 5.5 (DP3adLPG/DPPG [60:40]). Humidity chambers containing saturated NaCl solutions were created from T25 flasks, onto which two CaF 2 FTIR windows were fixed, onto the inner face of one, the dried lipid sample films were deposited from solution in chloroform. Ion‐pairing was detected by FTIR analysis in both the lipid extracts and synthetic models, by analyzing the ν (PO 4 ) − symmetric and ν (PO 4 ) − asymmetric stretch regions of each FTIR spectrum. The DP3adLPG/DPPG [30:70] model had a ν (PO 4 ) − symmetric (1056.41 cm −1 ) in between that of the DPPG (1052.93 cm −1 ) and DP3adLPG (1063.84 cm −1 ) standards, with a peak closer to that of DPPG. The ν (PO 4 ) − symmetric stretch of the DP3ad‐LPG/DPPG [30:70] model suggests that both ion‐pairing between the two lipids took place, as well as self‐pairing within the headgroup of the DP3adLPG. These results are similar to the ν (PO 4 ) − symmetric stretch observed in Strain A7 at pH 7.4 (1055.34 cm −1 ), demonstrating that, in terms of the lipid interactions, the synthetic model accurately represents the S. aureus membrane at pH 7.4. The results of the current study established that FTIR can be used to detect ion‐pairing in S. aureus lipid extracts. Furthermore, the similar ν (PO 4 ) − symmetric in the DP3adLPG/DPPG models relevant to the lipid extracts at pH 7.4 and pH 5.5 provides physicochemical evidence that the synthetic DP3adLPG/DPPG models can accurately represent ion‐pairing within the S. aureus membrane. Support or Funding Information Supported in part by grant MHIRT 2T37MD001368 from the National Institute on Minority Health and Health Disparities, National Institute of Health