Detection of salmonellas in confectionery products by conductance.

A modified lysine decarboxylase broth has been developed which could be used with a Bactometer M123 to differentiate salmonellas from other bacteria by the characteristics of the conductance detection curve. The medium was used in combination with a selenite cystine trimethylamine oxide dulcitol medium to screen 50 strains of salmonellas and 42 strains of other organisms to establish detection curve magnitude and rate values which could be used to identify curves specific to salmonellas. The combination of media detected all salmonellas tested except Salmonella pullorum. The two media were used to screen 100 inoculated product samples with the Bactometer instrument, in parallel with traditional plating procedures, and using various combinations of pre-enrichment and selective enrichment incubation periods. After 24 h pre-enrichment, the Bactometer system detected more positive samples than the conventional plating procedures after pre-enrichment and selective enrichment. It is considered that these media used in parallel in the Bactometer after conventional pre-enrichment could provide a 48 h screening procedure for salmonellas with a sensitivity comparable to present plating procedures.

Reproducibility of pyrolysis mass spectrometry: effect of growth medium and instrument stability on the differentiation of selected Bacillus species.

The discrimination of a set of 53 strains, taken from four closely related Bacillus species (Bacillus subtilis, B. pumilus, B. licheniformis and B. amyloliquefaciens), was examined using pyrolysis mass spectrometry. Strains were grown on six different media to examine the effect of media variation, especially batch-to-batch variation of a single medium, on the pyrolysis mass spectra and strain discrimination achieved. Long-term reproducibility over a period of 14 months was also examined. It was shown that batch-to-batch media variation is insufficient to affect spectra and strain discrimination significantly, but different media types do affect this. It was shown that species groups could still be recovered from the data, however, with an appropriate data-handling system. It was not possible to directly compare spectra produced 14 months apart, but the strain and species discrimination achieved using each data-set were highly comparable.

Curie-point pyrolysis mass spectrometry applied to characterization and identification of selected Bacillus species.

The use of pyrolysis mass spectrometry in the characterization and identification of Bacillus species was studied. Fifty-three strains of four closely related groups, Bacillus subtilis, B. pumilus, B. licheniformis and 'B. amyloliquefaciens', were used in a study of both sporulated and nonsporulated cultures. Pyrolysis was carried out using a Pyromass 8-80, a novel pyrolysis mass spectrometer specifically designed for fingerprinting complex samples. The pyrolysis data obtained were analysed using multivariate statistical techniques. All four groups could be differentiated using data from non-sporulated cultures but the data from sporulated cultures did not separate B. subtilis from 'B. amyloliquefaciens' or B. pumilus. In contrast, B. licheniformis was more clearly differentiated from the other three species using these data. Culture maturity affected the mass spectra obtained from non-sporulated cultures.

Discrimination of some Gram-negative bacteria by direct probe mass spectrometry.

The technique of direct probe mass spectrometry is described together with its application to the analysis of 50 strains of Gram-negative bacteria representing seven genera. Thirty-six of these strains were analysed in duplicate, and the 72 spectra held in a computer library. The intensities of 63 ions from each of the spectra were analysed by discriminant analysis techniques and all seven groups could be distinguished by as few as six ions. A set of 29 strains, 15 of which already in the data base, were used as unknowns to challenge the library on two separate occasions. The success rate of these challenges was 97% and 90% using the full spectra, but only 72% and 62% using the selection of six ions. Possible explanations for this are discussed as well as the scope and limitations of the method as a means of characterizing micro-organisms.

Effect of different growth conditions on the discrimination of three bacteria by pyrolysis gas-liquid chromatography.

High-resolution pyrolysis gas-liquid chromatography was applied to three bacteria (Escherichia coli NCTC 9001, Pseudomonas putida (NCIB 9494, and Staphylococcus aureus NCTC 8532) grown under a variety of conditions. Changing the culture medium drastically altered the quantitative aspects of the pyrograms of all three organisms, but the effects of culture time and incubation temperature were less severe. Mathematical analysis of the relative peak heights showed that four peaks could be used to discriminate the three bacteria however they were cultured.

Discrimination of micro-organisms using direct probe mass spectrometry.

Direct probe mass spectrometry of single colonies of different bacteria produced spectra which could be distinguished by calculations based on the intensities of 25 selected ions. Direct probe mass spectrometry has enormous potential as a rapid technique for characterizing micro-organisms.

Use of canonical variates analysis in differentiation of bacteria by pyrolysis gas-liquid chromatography.

Low-resolution pyrolysis gas-liquid chromatography (p.g.l.c.) can differentiate genera of aerobic food spoilage bacteria. Multivariate statistical techniques were applied but neither principal components nor furthest neighbour cluster analysis produced a consistent differentiation although both confirmed the reproducibility of p.g.l.c. When the distance between genera was redefined in terms of Mahalanobis D2- a generalized concept taking into account scatter around the mean-good differentiation was observed and could be displayed graphically by plotting the genus group means relative to the first two canonical variate axes. The coefficients of the canonical variates provide a strategy for discriminating between the genus groups. Some practical problems in the identification of unknowns using this technique are discussed.