The Journal of Medical Microbiology, Vol 27, Issue 3 191-198, Copyright © 1988 by Society for General Microbiology
Pathogenicity of capsulate and non-capsulate members of Bacteroides fragilis and B. melaninogenicus groups in mixed infection with Escherichia coli and Streptococcus pyogenes
I. Brook
Department of Pediatrics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814-5145.
The relationships between capsulate and non-capsulate Bacteroides fragilis
strains and Escherichia coli, and between capsulate and non-capsulate
strains of the B. melaninogenicus group and Streptococcus pyogenes, were
studied in a subcutaneous abscess model in mice. Selective antimicrobial
agents directed against either aerobic or anaerobic bacteria were used
alone or in combination to explore the effect of eradication of one
component of the mixed infection. Single agent therapy effective against
both aerobic and anaerobic flora was also employed. Single therapy of mixed
infection directed at the elimination of only one organism (S. pyogenes, E.
coli or Bacteroides sp.) caused significant reductions in the numbers of
sensitive organisms and also smaller yet significant decreases in the
numbers of insensitive organisms. However, the abscesses were not
eliminated after such therapy. Combination therapy or use of a single agent
(cefoxitin) directed against the aerobic and anaerobic components of the
infection was more effective. Non-capsulate Bacteroides spp. became
capsulate after passage in mice mixed with either S. pyogenes or E. coli.
Therapy directed at the elimination of S. pyogenes and E. coli did not
prevent the emergence of capsulate Bacteroides spp. These data demonstrate
the synergy between all members of the B. fragilis group and E. coli and
between the B. melaninogenicus group and S. pyogenes, and reiterate the
need to direct antimicrobial therapy at the eradication of the aerobic and
anaerobic components of mixed infections.
