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Gram stains were performed on strains of Actinomyces bovis, Actinomyces viscosus, Arthrobacter globiformis, Bacillus brevis, Butyrivibrio fibrisolvens, Clostridium tetani, Clostridium thermosaccharolyticum, Corynebacterium parvum, Mycobacterium phlei, and Propionibacterium acnes, using a modified Gram regimen that allowed the staining process to be observed by electron microscopy (J. A. Davies, G. K. Anderson, T. J. Beveridge, and H. C. Clark, J. Bacteriol. 156:837-845, 1983). Furthermore, since a platinum salt replaced the iodine mordant of the Gram stain, energy-dispersive X-ray spectroscopy could evaluate the stain intensity and location by monitoring the platinum signal. These gram-variable bacteria could be split into two groups on the basis of their staining responses. In the Actinomyces-Arthrobacter-Corynebacterium-Mycobacterium-Propionibacterium group, few cells became gram negative until the exponential growth phase; by mid-exponential phase, 10 to 30% of the cells were gram negative. The cells that became gram negative were a select population of the culture, had initiated septum formation, and were more fragile to the stress of the Gram stain at the division site. As cultures aged to stationary phase, there was a relatively slight increase toward gram negativity (now 15 to 40%) due to the increased lysis of nondividing cells by means of lesions in the side walls; these cells maintained their rod shape but stained gram negative. Those in the Bacillus-Butyrivibrio-Clostridium group also became gram negative as cultures aged but by a separate set of events. These bacteria possessed more complex walls, since they were covered by an S layer. They stained gram positive during lag and the initial exponential growth phases, but as doubling times increased, the wall fabric underlying the S layer became noticeably thinner and diffuse, and the cells became more fragile to the Gram stain. By stationary phase, these cultures were virtually gram negative.