A simple and efficient centrifugation filtration method for bacterial concentration and isolation prior to testing liquid specimens with laser-induced breakdown spectroscopy

Abstract

An inexpensive filtration device was designed and constructed to rapidly concentrate bacteria in a liquid suspension on the surface of a disposable filter medium while at the same time separating the bacterial cells from larger contaminants in the suspension on the basis of their size. The device consists of a two-stage insert that is held rigidly in a standard tube during bacterial suspension centrifugation. The filters can be easily removed from the insert for subsequent testing with laser-induced breakdown spectroscopy in a process that takes only 3 min. Filter media of 0.45 μm pore size was found to capture approximately 90–95% of the cells in suspension. A limit of detection of 90,000 cells per laser shot was calculated by constructing a calibration curve from multiple suspensions of varying concentration. Deposition of the bacterial lawn across the surface of the 9.5 mm diameter filters was found to be uniform to within ±20% of the mean as determined by the total measured optical breakdown emission. Use of 5 μm pore size filter media in the first stage of the insert was found to remove close to 100% of a 12 μm grain size tungsten contamination from the suspension while removing 10% of the bacterial cells. This mounting protocol provides a very convenient method for sample preparation that makes use of common techniques, apparatus, and procedures that would be familiar to clinicians or microbiological pathology laboratory personnel.