Flow cytometry has become a valuable tool in food microbiology. By analysing large numbers of cells individually using light-scattering and fluorescence measurements, this technique reveals both cellular characteristics and the levels of cellular components. Flow cytometry has been developed to rapidly enumerate microorganisms; to distinguish between viable, metabolically active and dead cells, which is of great importance in food development and food spoilage; and to detect specific pathogenic microorganisms by conjugating antibodies with fluorochromes, which is of great use in the food industry. In addition, high-speed multiparametric data acquisition, analysis and cell sorting, which allow other characteristics of individual cells to be studied, have increased the interest of food microbiologists in this technique. This mini-review gives an overview of the principles of flow cytometry and examples of the application of this technique in the food industry.

Genetic diversity and genetic relationships among 42 Pseudomonas stutzeri strains belonging to several genomovars and isolated from different sources were investigated in an examination of 20 metabolic enzymes by multilocus enzyme electrophoresis analysis. Forty-two distinct allele profiles were identified, indicating that all multilocus genotypes were represented by a single strain. All 20 loci were exceptionally polymorphic, with an average of 15.9 alleles per locus. To the best of our knowledge, this P. stutzeri sample exhibited the highest mean genetic diversity (H ‫؍‬ 0.876) found to date in all bacterial species studied by multilocus enzyme electrophoresis. A high frequency of occurrence of null alleles was identified. The index of association (I A ) for the P. stutzeri strains analyzed was 1.10. The I A values were always significantly different from zero for all subgroups studied, including clinical and environmental isolates and strains classified as genomovar 1. These results suggest that the population structure of P. stutzeri is strongly clonal, indicating that there is no significant level of assortative recombination that might destroy linkage disequilibrium.Pseudomonas stutzeri was first isolated by Burri and Stutzer (6) as Bacillus denitrificans II and named P. stutzeri by Van Niel and Allen (47). It has an unusual colony shape and consistency when directly isolated, being described as wrinkly, dry, and tenaciously coherent. P. stutzeri, a gram-negative rod-shaped bacterium that is mobile by means of a single polar flagellum, is a nonpigmented denitrifier that liberates nitrogen gas from nitrate, is amylase positive and gelatinase negative, and is able to grow on maltose and starch (4, 43). P. stutzeri has a wide environmental distribution but is found mainly in soil and water. Many strains have been isolated from clinical specimens (20). The members of the species share physiological characteristics that make P. stutzeri of special interest in ecological studies. This species shows high metabolic versatility (35) including the degradation of environmental pollutants (1, 37) and high-molecular-weight polyethylene glycols (30). P. stutzeri serves as a model for the study of the biochemistry and genetics of denitrification and natural transformation processes.Pseudomonas species are grouped on the basis of rRNA-DNA hybridization studies (31). P. stutzeri is a nonfluorescent denitrifying species of the genus Pseudomonas included in the rRNA group I. P. stutzeri forms a homogeneous group within the genus Pseudomonas, with phenotypic traits that permit description to the species level. However, P. stutzeri is a heterogeneous species with respect to many phenotypic characteristics and DNA composition. Several studies have demonstrated that P. stutzeri consists of a complex collection of strains that might be distributed in more than one species (2, 25, 31, 35). DNA-DNA hybridization studies (35,43) have shown the existence of at least eight genomic groups, called genomovars.Confirmation of this syste...

The synthesis of prodigiosin by non-proliferating cells of Serratia marcescens was examined at various pH values between 5.5 and 9.5. During incubation in unbuffered medium, pH changed and prodigiosin production was similar regardless of the initial pH. Variations in pigment production were noted when buffers were employed in cultures of non-proliferating cells. The optimum pH for prodigiosin production was 8.0-8.5. Proline oxidase was also measured. The results suggest that the effect of pH may be related to the amount of proline which can be incorporated into prodigiosin.

The production of prodigiosin by non‐proliferating cells of Serratia marcescens is inhibited by addition of glucose or different carbon sources to the induction medium. The induction in acidic external pH, mimicking the effects produced by the carbon sources, reduced prodigiosin synthesis, and the prodigiosin production seems to be related to the length of the low pH period. Buffering at pH 7·5 increased pigment production in media with repressing carbon sources. This study reveals that the inhibitory effect of carbon sources on prodigiosin production may be due to a lowering of the pH of the medium.

Buffering capacity and membrane conductance to H+ were measured in Enterococcus faecalis and Lactobacillus acidophilus by a pulse technique. The magnitude of these parameters varied between one species and another. Over the pH range studied, from pH 3.72 to 7.74, the acidophile Lactobacillus acidophilus showed higher values of buffering capacity and membrane H+ conductance than the neutrophile Enterococcus faecalis. These results support the idea that acidophiles have high cytoplasmic buffering capacity, which may allow them to resist changes in cytoplasmic pH.