Lysogenic strains of lactic Acid streptococci and lytic spectra of their temperate bacteriophages.

A total of 113 strains of mesophilic strains lactic streptococci of the species Streptococcus lactis, S. lactis subsp. diacetilactis, and S. cremoris, chosen from 291 strains that had been previously classified into six groups on the basis of their sensitivity to 132 virulent phages, were subjected to induction with mitomycin C. Among these strains, 43% produced phages capable of forming plaques of lysis on an indicator strain either spontaneously or after induction. There was a close correlation between the lytic spectra of temperate and virulent phages. Among the strains studied, 25% were shown to be indicator strains. These results emphasized the high probability of development of temperate phages in a starter culture containing mesophilic lactic streptococci and therefore their importance as a cause of accidents in cheese making.

Localization of active microorganisms in cheese by autoradiography.

By using an autoradiographic technique, one can follow, during ripening of a cheese, the distribution, size, and metabolic activity of microcolonies. Fragments of cheese were labeled with [H]leucine, fixed, and mounted in epoxy resin. After exposure and development, sections were examined by optical microscopy. In Camembert cheese, bacterial microcolonies synthesized protein rapidly during the beginning of the ripening process. At the end of the ripening process, active bacterial clusters were scarcer and of two types: (i) large microcolonies with reduced labeling, and (ii) microcolonies having the same size as those observed at the beginning of the ripening process, but with slight or no labeling.

Autoradiographic study of the localization and evolution of growth zones in bacterial colonies.

Incorporation of [3H]leucine in the bacteria of 18 to 48 h-old colonies of Pseudomonas aeruginosa, Pseudomonas putida, Bacillus thuringiensis, Staphylococcus aureus and Escherichia coli enabled the localization of bacterial multiplication sites by means of autoradiography of sagittal sections. In colonies where fast diameter expansion occurred, all the bacteria from the peripheral corona contributed to peripheral growth; in colonies where the expansion was slower, the growth rate of the bacteria in this region was heterogeneous. Besides this peripheral growth, a central region of bacterial multiplication was always found, but with variable localization and extension. In aerobic species, such as P. aeruginosa and P. putida, the central growth site was limited to the zone of oxygen penetration into the bacterial mass. However, in facultatively anaerobic species, bacterial multiplication dependend on nutrient supply. For 48 h-old colonies of S. aureus, a more complex localization of growth seemed to be affected simultaneously by nutrient penetration and accumulation of toxic substances.

[Autoradiographic study of a bacterial colony. Application to the effects of polymyxin on Pseudomonas aeruginosa colonies]. / Etude autoradiographique de la colonie bactérienne. Application aux effets de la polymyxine sur la colonie de Pseudomonas aeruginosa

The autoradiograph of a colony of Ps. a. which has been transferred, during growth, on a medium added with polymyxin and tritiate leucin makes it possible to locate an upper zone with a high metabolic activity and a basal zone with no metabolic activity. The latter, which consists of lysed cells, acts probably as a selective filter against the drug.