MS Imaging of Antibiotics within Staph. epidermidis Bacterial Biofilms by Laser Desorption Postionization

Biofilms can cause persistent and antibiotic-resistant infections, whose treatment is of major medical concern. Microbial biofilms composed of S. epidermidis are a major source of hospital infections. To address the many questions surrounding bacterial biofilms, MS imaging is needed. MS imaging allowed correlation of chemical information with biologically relevant structures, while maintaining spatial and chemical integrity. Vacuum ultraviolet single photon ionization laser desorption postionization mass spectrometry (LDPI-MS) was used to postionize the abundant gas-phase neutral molecules ejected by laser desorption from bacterial biofilms. LDPI-MS was chemically selective for low ionization energy species, such as antibiotics. Sensitivity was also improved by increasing the neutral desorption yields via addition of a matrix, analogous to the improvement in ion yields observed in MALDI-MS. LDPI-MS with vacuum and extreme ultraviolet radiation from 8 to 24 eV generated at a synchrotron was used to postionize laser desorbed neutrals of antibiotic-treated biofilms and a modified fullerene. LDPI-MS imaging characterized the distribution of antibiotics within intact S. epidermidis bacterial biofilms and displayed heterogeneous features of its microstructure. Spatial resolution of MS imaging experiments of doped biofilms was ~30 microns. Cross sectioning was used to depth profile antibiotics within biofilm slices with depth resolution of ~30 microns. Matrix addition provided nearly a ten fold enhancement of LDPI-MS signal producing limits of detection near 20 fmol.