The cause of cellulitis is incompletely understood,
because a causative pathogen is not identified
in most cases20; this is consistent with our
study, in which 80% of cellulitis lesions could
not be cultured because skin was intact. Expert
opinion8 and empirical data21,22 suggest that cellulitis
is most commonly caused by Streptococcus
pyogenes. Our findings are provocative, because
TMP-SMX has been considered a poor empirical
choice for the treatment of cellulitis. Recent data
show that S. pyogenes strains may be TMP-SMX–
susceptible if low-concentration thymidine agar
is used for testing.23 Our results showing that
TMP-SMX and clindamycin have similar efficacy
in patients with cellulitis are consistent with these
in vitro data.