One hundred and fifty -one Bacteroides isolates were collected
from a variety of clinical samples (wound, swabs, vaginal swabs and
blood cultures) and from the cervix of women attending a colposcopy
A method for the isolation of plasmid DNA from Bacteroides
species was developed and provided a simple and reproducible
technique for the analysis of the plasmid content of the isolates.
The strains were divided into three groups, the B. fragilis group,
the melaninogenicus /oralis group and the asaccharolytic group. The
incidence of plasmids in each group was determined.
Forty per cent of the strains were found to have plasmids but
there was great variation in the plasmid distribution within each
group. Sixy -one percent of the B. fragilis strains had plasmids as opposed to 23% of the asaccharolytic Bacteroides, and 10% of the
melaninogenicus /oralis strains. Plasmids ranging in size from 1.1 to
108 MegaDaltons (MDa) were detected but most of the plasmids were
less than 10 MDa. Seventy -nine percent of the plasmid- containing
strains had more than one plasmid.
The susceptibility of the isolates to chloramphenicol,
clindamycin, metronidazole, erythromycin, imipenem, moxalactam,
cefoxitin, cefotetan, cefotaxime, cefuroxime, penicillin and
nalidixic acid was investigated. Chloramphenicol was the most active
non -beta -lactam antibiotic. Six percent of the strains were
resistant to clindamycin and metronidazole resistance, though rare,
was observed in 2% of the strains. Among the beta -lactam
antibiotics, imipenem, though not in use in the United Kingdom was
found to have excellent activity against the Bacteroides. Moxalactam
and cefoxitin were also very active with less than 1% of the strains
displaying resistance. Various levels of resistance, ranging from
4 to 50% were found amongst the three Bacteroides groups. There was
evidence of species variation in antimicrobial susceptibility.
Attempts were made to correlate plasmid presence with observed
antibiotic resistance and capsule variation within the strains.
Transferable macrolide -lincosamide -streptogramin (MLS) resistance
determinants were not identified and observed MLS resistance could
not be cured. However, a possible correlation was found between
plasmid content and resistance to cefoxitin but plasmid content was
not found to alter the resistance pattern of strains to nalidixic
acid. Capsule variation was found to depend on altered gene
expression and not on the presence or absence of plasmids.