The cumulative effect of disinfection, storage, histological fixation and demineralization on number and staining ability of gram-positive bacteria.

It has previously been shown experimentally using S. faecalis that the relative number of blue-staining Gram-positive bacteria was reduced by demineralizing agents as well as by disinfection (Wijnbergen & van Mullem 1987, van Mullem & Wijnbergen 1989). In this study the cumulative effect of disinfection, experimental period, fixation and demineralization was investigated. The use of additives to the fixative and the demineralizing agents to limit the loss of blue-staining bacteria was also studied. The numbers of bacteria were determined using a haemocytometer, and the percentage of blue-staining organisms were ascertained from smears. When compared with the start of the experiment, the sequence of disinfection with chlorhexidine, storage in PBS, fixation with neutral formaldehyde and demineralization with formic acid or EDTA appeared to reduce the relative numbers of Gram-positive staining bacteria. For storage periods of 0 and 4 days the reduction factors were 100 and 600, respectively, using formic acid. These factors were 50 and 95, respectively, using EDTA. Addition of cetyl pyridinium chloride to the fixative and the demineralizing agents, and addition of neutral formaldehyde to the demineralizing agents lowered these reduction factors to 80 and 200, respectively, for formic acid, and 40 and 85, respectively, for EDTA. If the results are extrapolated to animal experiments where disinfection, experimental period, fixation and demineralization form part of the experimental framework, even the lowest reduction in the number of blue-staining bacteria could lead to false interpretation of tissue sections.