Observation of encapsulated lactic acid bacteria using confocal scanning laser microscopy

The confocal scanning laser microscope allows direct observation of milk and cultured milks in their natural state. The microscope was used to observe the capsules of lactic acid bacteria growing in milk. Capsule production was confirmed by microscopic observation of cells suspended in latex beads. Some strains of Streptococcus thermophilus were surrounded by a capsule 4 to 5 microns in diameter, and others by capsules 2 microns in diameter. Lactobacillus delbrueckii ssp. bulgaricus strains showed capsule sizes from 1.5 to 3 micron in diameter. Of four strains of lactococci tested, three were unencapsulated, and one had capsule sizes of 1.5 and 2 microns around some cells. Encapsulated strains produced less acid in milk than did unencapsulated strains. Growth in Elliker's broth produced smaller capsules than did growth in milk. Capsules acted as a barrier to acid diffusing from the cell.