Antimicrobial susceptibility
Disk diffusion showed that all isolates were resistant to cefotaxime and positive for ESBL production by disk confirmatory test using cefotaxime/clavulanate and ceftazidime/clavulanate (Table
1). The MICs of β-lactams and β-lactam/inhibitor combinations were determined by broth microdilution technique. All clinical isolates were resistant to cefpodoxime, cefepime and resistant or intermediately resistant to aztreonam. The phenotypic ESBL microdilution confirmatory test was positive, showing a decrease by 7 doubling dilutions in the presence of clavulanic acid (Table
2). The
K. pneumoniae clinical isolates were also resistant to other non-β-lactam antibiotics such as tetracycline, gentamicin and fluoroquinolones (Table
1).
Table 1
Susceptibility data of the clinical isolates and the transconjugant
Kp4 | 6 | 18 | 18 | 25 | 6 | 8 | 20 | 6 | ND |
Kp8 | 6 | 16 | 19 | 26 | 6 | 8 | 20 | 6 | ND |
Kp15 | 6 | 16 | 18 | 25 | 6 | 8 | 20 | 6 | ND |
ENT | 10 | 20 | 21 | 25 | 20 | 17 | 21 | 30 | ND |
EC | 9 | 22 | 15 | 26 | 19 | 19 | 20 | 32 | 6 |
TcEC
a
| 11 | 31 | 20 | 32 | 25 | ND | ND | ND | 33 |
EC Na azR
| 34 | 32 | 29 | 29 | 22 | 23 | ND | 28 | 30 |
Table 2
MIC data of selected β-lactams and characteristics of β-lactamases produced by clinical isolates of Enterobacteriaceae from Egypt
| | | Clox | CLA | | | | | | | | |
Kp4 | 5.4 | No | No | Yes |
bla
TEM
| >128 | 1 | >128 | 0.06 | 8 | 128 | 0.12 |
Kp8 | 6.3 | No | No | Yes | -
c
| >128 | 2 | >128 | 0.06 | 8 | 128 | 0.12 |
Kp15 | 7.6 | No | No | Yes |
bla
SHV
| >128 | 2 | >128 | <0.03 | 8 | 128 | 0.12 |
| 8.0 | Yes | No | Yes |
bla
CTX-M-14
| | | | | | | |
ENT | 6.3 | No | No | Yes | -
c
| >128 | 2 | >128 | <0.03 | >16 | 32 | 0.25 |
| 7.6 | No | No | Yes | -
e
| | | | | | | |
| 8.0 | Yes | No | Yes |
bla
CTX-M-14
| | | | | | | |
| 8.9 | No | Yes | No | -
d
| | | | | | | |
EC | 5.4 | No | No | Yes |
bla
TEM
| >128 | 0.25 | >128 | <0.03 | <4 | 16 | 0.12 |
| 6.0 | No | No | Yes | -
c
| | | | | | | |
| 6.6 | No | No | Yes | -
c
| | | | | | | |
| 9.0 | Yes | No | Yes |
bla
CTX-M-15
| | | | | | | |
Isolates of the Enterobacteriaceae producing CTX-M ESBLs are resistant to cefotaxime (MICs ≥ 64 mg/L) [
9] and cefepime (MICs ≥ 32 mg/L) [
19‐
22], but are susceptible or intermediate to ceftazidime [
9]. The phenotypic characteristics of the clinical isolates in this study suggested the presence of CTX-M ESBLs. Screening using ceftazidime alone is not sufficient for organisms producing CTX-M ESBLs [
16]. However, CTX-M-15 has been reported to possess some hydrolytic activity against ceftazidime [
10]. The
E. coli isolate producing CTX-M-15 was intermediate to ceftazidime using disk susceptibility test (Table
1).
Characterization of β-lactamases
Isoelectric focusing (IEF) of crude sonicates of the clinical isolates was done by a cefotaxime/β-lactamase inhibitor overlay technique. Two enzymes focused at pI values of 8.0 and 9.0, were inhibited by clavulanic acid, and showed an extended spectrum of activity by hydrolyzing cefotaxime (Table
2). PCR and sequence analysis identified
bla
CTX-M-14 in one isolate of
K. pneumoniae (KP 4) and the
E. cloacae isolate, and
bla
CTX-M-15 in the
E. coli clinical isolate (Table
2). Only one
K. pneumoniae isolate was evaluated by sequence analysis because all three of the
K. pneumoniae isolates showed the same enzymes on the IEF gel (Table
2).
All isolates produced multiple β-lactamases that were inhibited by clavulanate:
K. pneumoniae (pI values 5.4, 6.3, 7.6, and 8.0),
E. cloacae (pI values 6.3, 7.6, 8.0), and
E. coli (pI values, 5.4, 6.0, 6.6 and 9.0) (Table
2). The
bla
TEM gene was detected in all
K. pneumoniae and
E. coli isolates, and corresponded to the β-lactamase band that focused at pI value of 5.4 when evaluated by IEF (Table
2). The
bla
SHV gene was present in the
K. pneumoniae isolates (β-lactamase band focusing at pI value, 7.6-Table
2). The β-lactamase band that focused at pI value of 7.6 in the
E. cloacae isolate was most likely not an SHV-enzyme since SHV-specific PCR was negative. Further sequencing experiments for the
bla
TEM and
bla
SHV genes were not done.
Analysis of the upstream sequence of
bla
CTX-M-14 and of
bla
CTX-M-15 revealed the presence of the right terminal inverted repeat of the insertion sequence IS
Ecp1 and the putative promoter region (-10 and -35) associated with this element [
23].
The results of the conjugation experiment showed that
bla
CTX-M-15 was carried on a conjugative plasmid (Table
1). The movement of
bla
CTX-M-15 was verified in the transconjugant using CTX-M-group 1-specific PCR [
16]. The
bla
CTX-M-14 gene was not mobilized by conjugation.
A surveillance report on antibiotic resistance in the Southeastern Mediterranean region screened only
E. coli isolates from different medical centers in Egypt [
2]. Other important nosocomial isolates such as
K. pneumoniae and
E. cloacae were not evaluated in that study [
2]. A recent outbreak was reported in a neonatal intensive care unit in Cairo, Egypt, in which 80% of the isolates were
K. pneumoniae, of which 58% were ESBL producers [
24]. Therefore, it is important not to limit extended-spectrum cephalosporin susceptibility screening in Egypt to
E. coli but to include
K. pneumoniae as well as other Enterobacteriaceae such as
E. cloacae.
It is important for clinical microbiologists in Egyptian hospitals to screen for CTX-M ESBL producers. In addition, clinical microbiologists and physicians need to be aware that these enzymes are present in many different types of Enterobacteriaceae. This information is essential for determining the most appropriate empirical antibiotic therapy.