- Volume 145, Issue 12, 1999

Three strains of Corynebacterium producing various amounts of PS2 S-layer protein were studied. For all strains, more PS2 was produced if the bacteria were grown in minimal medium supplemented with lactate than if they were grown in minimal medium supplemented with glucose. The consumption of substrate and PS2 production was studied in cultures with mixed carbon sources. It was found that the inhibitory effect of glucose consumption was stronger than the stimulatory effect of lactate in one strain, but not in the other two strains. The regulation of gene expression involved in S-layer formation may involve metabolic pathways, which probably differ between strains. S-layer organization was also studied by freeze-fracture electron microscopy. It was found that low levels of PS2 production correlated with the partial covering of the cell surface by a crystalline array. Finally, it was found that PS2 production was mainly regulated by changes in gene expression and that secretion was probably not a limiting step in PS2 accumulation.

The cell walls of Gram-positive bacteria consist primarily of a macromolecular matrix comprising similar amounts of peptidoglycan and covalently attached anionic polymers. Under most growth conditions the anionic polymers of Bacillus subtilis are principally teichoic acids; in strain 168 these include a polyglycerol teichoic acid and a glucose/galactosamine-containing teichoic acid. However, when cultures are subjected to phosphate stress the bacterium induces a complex series of responses, one of which is the replacement of at least part of the wall teichoic acid with teichuronic acid, a non-phosphate- containing anionic polymer. In this paper the construction of a transcriptional reporter strain that facilitates the monitoring of the promoter region upstream of the tua operon involved in teichuronic acid synthesis and its controlled expression are reported. The expression of the tua operon was monitored in both phosphate-starved, non-growing batch cultures and phosphate-limited continuous cultures. We show that the transcription of the operon correlates well with the anionic polymer composition of the cell walls.

The plant-pathogenic bacterium Erwinia carotovora subsp. carotovora causes plant disease mainly through a number of extracellular plant-cell-wall-degrading enzymes. In this study, the ability of an rpoS mutant of the Er. carotovora subsp. carotovora strain SCC3193 to infect plants and withstand environmental stress was characterized. This mutant was found to be sensitive to osmotic and oxidative stresses in vitro and to be deficient in glycogen accumulation. The production of extracellular enzymes in vitro was similar in the mutant and in the wild-type strains. However, the rpoS mutant caused more severe symptoms than the wild-type strain on tobacco plants and also produced more extracellular enzymes in planta, but did not grow to higher cell density in planta compared to the wild- type strain. When tested on plants with reduced catalase activities, which show higher levels of reactive oxygen species, the rpoS mutant was found to cause lower symptom levels and to have impaired growth. In addition, the mutant was unable to compete with the wild- type strain in planta and in vitro. These results suggest that a functional rpoS gene is needed mainly for survival in a competitive environment and during stress conditions, and not for effective infection of plants.