Enumeration of subgingival species on primary isolation plates using colony lifts

Abstract

This study evaluated the feasibility of using a colony lift method and DNA probes to enumerate bacterial species cultured on primary isolation plates. Fourteen digoxigenin-labeled whole chromosomal DNA probes representing 12 subgingival species were validated by hybridization with colony lifts prepared from 249 reference strains of 51 species grown on Trypticase soy agar plates supplemented with 5% sheep blood. Colonies of reference strains were lifted onto Nytran filters from plates and treated to lyse cells, remove cellular proteins, denature and fix microbial DNA to the filters. Positive reactions were detected with an anti-digoxigenin antibody conjugated to alkaline phosphatase and revealed by bromo-chloro-indolyl phosphate and nitroblue tetrazolium. Cross-reactions were not observed for 13/14 probes, but 2 strains of Streptococcus mitis reacted with the probe to Streptococcus sanguis II. Subgingival plaque samples were taken by means of a sterile curette from mesiobuccal surfaces of teeth present in each of 26 subjects with differing periodontal disease states. Samples were dispersed, diluted, plated and incubated anaerobically for 7 d at 35°C. Colonies were lifted as described above. Filters were cut into sections and hybridized with the 14 digoxigenin-labeled DNA probes. The probes were used to enumerate the test species and the total number of isolates was determined in 711 plaque samples. The colony lift method and DNA probes provided a sensitive, economical and quantitative method for enumerating cultivable microbial species in subgingival plaque samples. In addition, the amplification provided by growing the organisms on agar plates facilitated determination of numbers of organisms in small plaque samples, such as those from healthy sites.