RESULTS
16S rDNA sequence analysis and bacterial identification.
An almost-complete 16S rDNA sequence containing fewer
than 1% undetermined positions was obtained for all of the
isolates included in the study; thus, 177 query sequences were
available for comparison (Table 1). For three isolates (1.7%)
belonging to the genus Corynebacterium and an unidentified
species, DNA extraction had to be repeated after initial 16S
rDNA amplification attempts failed. For two isolates (1.1%),
the 16S rDNA sequencing procedure had to be carried out
twice after the first analysis demonstrated probable mixed sequences.
16S rDNA-based analysis resulted in the classification
of the isolates into three categories (Table 1 and Fig. 1). A
total of 139 of 177 isolates (78.5%) possessed a 16S rDNA
sequence with $99% similarity to that of a previously characterized
bacterial species. A total of 159 of 177 (89.8%) possessed
a 16S rDNA sequence with $97% similarity to that of
a genus member. Among these 159 isolates, Enterobacter and
Pantoea exhibited a 99% 16S rDNA sequence similarity with
GenBank sequences significantly less frequently than isolates
belonging to the other genera (P 5 0.04; odds ratio 5 0.32
[95% confidence interval, 0.10 to 1.14] [Fischer’s exact test]). A
total of 18 of 177 isolates (10.2%) had a 16S rDNA sequence
with ,97% similarity with the closest sequence in GenBank.
The efficiency in achieving a 99% 16S rDNA similarity level
was not significantly different between isolates obtained from
clinical or environmental sources. However, 12 of 18 isolates
with ,97% similarity to other GenBank sequences originated
from environmental sources (P 5 0.07 by the Mantel-Haenszel
test). A total of 41 original 16S rDNA sequences corresponding
to new species and new genera have been deposited in
public databases (Table 1).
Taxonomic relationships of unidentified isolates. 16S rDNA
analysis determined that 6 of 18 unidentified isolates belonged
to low-percent-G1C-content gram-positive bacteria, 4 of 18
belonged to high-percent-G1C-content gram-positive bacteria,
6 of 18 belonged to gamma subgroup Proteobacteria, and 2
of 18 belonged to the Bacteroides-Cytophaga phylum. The phylogenetic
relationships of these isolates as inferred by neighbor-joining
analysis are presented in Fig. 2.
Analysis of conventional-identification failures. Failures in
appropriate conventional identification are presented in Fig. 1.
Among 16 Bacillus isolates analyzed, 16S rDNA-based identi-
fication confirmed conventional identification for 3 isolates;
inaccurate conventional identification was a result of unmatched
Gram determination for 7 isolates, unmatched biochemical
profile determination for 5 isolates, and unmatched
growth requirement determination for 1 isolate. Failure to
accurately identify Escherichia coli isolates when using conventional
methods was a result of unmatched biochemical profile
determination for eight of nine isolates and of inaccurate oxidase
activity determination for one of nine isolates. Failure of
conventional identification of Staphylococcus spp. resulted
from inaccurate biochemical profile determination for all nine
isolates examined.