Background: Organisms producing
CTX-M-ß-lactamase are emerging around the world as a source of resistance to
oxyiminocephalosporins such as cefotaxime. However, the laboratory detection of
these strains is not well defined. The aim of this study was to determine the
presence and prevalence of known CTX-M-ß-lactamases genes in clinical isolates
of  Escherichia coli from hospitals of
Tehran.Methods: During six months (September to February,
2006), 160 clinical isolates of Escherichia coli collected from three
university hospitals of Tehran.  Phenotypic screening and confirmation
tests for ESBL detection was according to CLSI advised. All of the
ESBL-producing isolates were examined by PCR for presence of bla
_CTX-M genes.Results: Primary phenptypic tests
revealed that %56.69 (n=89) of E. coli isolates produced ESBLs. In
confirmatory tests by use of clavulanic acid, ESBL production were confirmed
(P+C+) in %96.7 (n=86) of isolates with primary positive test. The presence of
an ESBL was not confirmed (P+C-) in 3.3% (n=3) of the screen positive. Of all
screen positive isolates, 34 (35.78%) were positive for bla
_CTX-M genes from the CTX-M-I group, indicating CTX-M-1-like ß-lactamases
and Two (2.1%) were positive for bla _CTX-M genes from the
CTX-M-III group, indicating CTX-M-3-like ß-lactamases. The remainder 59 (62.2%)
were negative for bla _CTX-M
genes.Conclusions: The levels of resistance to ceftazidim
were remarkably varible among CTX-M producers. This study provides futher
evidence of the global dissemination of  CTX-M type ESBLs and emphasize the
need for their epidemiological monitoring.