Some remarks about laser-induced mass spectrometry of bacteria

In order to investigate the availability of laser-induced mass spectrometry for the determination of bacteria, ten strains of the genera Escherichia and Streptococcus were chosen. Gram-negative as well as Gram-positive lysogenic and non-lysogenic, pathogenic and non-pathogenic strains were examined. Giant pulses of a Nd: YAG laser (frequency quadrupled, wave-length 265 nm) were used to vaporize and partly ionize the bacterial material. The time-of-flight mass spectra obtained could be discriminated by using the stepwise discriminant analysis provided by BMDP software package. Two different methods relating centrifugation and washing procedures were used. With the first method a correct classification between 50.0 and 90.9 per cent (S. sanguis) by means of mass spectra of positive ions and a correct classification between 27.1 and 80.4 per cent (S. lactis-25) by means of mass spectra of negative ions was observed. Using the second method a correct classification between 40.7 and 93.3 per cent (S. lactis-25) by means of mass spectra of positive ions and a correct classification between 56.7 and 95.0 per cent by means of mass spectra of negative ions was obtained. Here S. sanguis with 93.3 and S. lactis-25 with 81.7 per cent were significantly differentiated. In this context it must be emphasized that a correct classification of 27 per cent must be considered statistically significant, if 10 different strains are used.