Eighty-four consecutive CRAB isolates were recovered from 36 patients, who were admitted to the ICU under study between October 1, 2010 and March 31, 2011.
Based upon the inclusion criteria, 61 isolates were submitted to testing by PCR analysis to confirm the presence of carbapenemase genetic determinants and molecular typing by rep-PCR and, for representative isolates, by MLST. A number of isolates from each patient ranging between one and six was analyzed. The clinical samples more prevalent were bronchial aspirates (n. 29), followed by swabs from wound or tracheostomy (n. 8), blood (n. 7), drainage fluids and tips of central venous catheter (n. 6 each).
Among the 61 isolates, all (100%) were resistant to penicillins (ampicillin, carbenicillin), cephalosporins (cefepime, cefotaxime, ceftazidime), β-lactam-β-lactamase inhibitor combinations and fluoroquinolones, whereas susceptibility to aminoglycosides [amikacin, four (6.5%) susceptible and two (3.3%) intermediate; gentamicin, one (1.6%) susceptible and nine (14.7%) intermediate; tobramicin, six (9.8%) susceptible and 12 (19.7%) intermediate) and sulfamethoxazole-trimethoprim, nine (14.7%) susceptible] was variable. Moreover, 50 (82.0%) and 11 isolates (18.0%) tested, respectively, susceptible (MICs 0.5 – 2
mg/L) and intermediate (MIC 4
mg/L) to tigecycline. Only colistin showed 100% susceptibility with MICs ranging between 0.5 and 2
mg/L. Three strains only had a MIC for imipenem of 8
mg/L, whilst all the remaining isolates exhibited MICs
mg/L. Moreover, MICs for meropenem were
mg/L for all, but three strains that showed MICs of 4 and 8
mg/L in one and two cases, respectively.
The 61 CRAB strains were investigated for the presence of carbapenemase genes. All isolates harboured a blaOXA-51-like
sequence. Forty-nine isolates had, in addition, blaOXA-23
and two only blaOXA-58
genes. No MBL (IMP and VIM) gene sequence was detected. The two OXA-58 isolates had MICs for imipenem
mg/L, whereas for meropenem of 4 and 8
mg/L, respectively. Ten isolates for which the imipenem MIC was
mg/L had an unidentified carbapenem resistance mechanism. All tested negative for the blaOXA-143
sequence. Presence of ISAba
1 in the promoter region of the blaOXA-51-like
gene was not investigated because of the inconsistent literature findings about its correlation with carbapenem resistance and the possible involvement of alternative mechanisms [1
By adopting a similarity coefficient of ≥95% as the threshold, all isolates, but one, clustered into five distinct groups named A to E (Fig.). Thirty-one out of 61 clustered in a large group that included 29 blaOXA-23 and the two blaOXA58 isolates. Two further subtype clusters, B and C, included 6 and 13 OXA-23 producing isolates, respectively. Strains belonging to the subtype clusters A, B and C clustered at a 94% similarity level. Cluster D and cluster E grouped eight and two isolates, respectively, that tested negative for the carbapenem resistance genes under investigation. One isolate only did not cluster.
Figure 1 Dendrogram and computer-generated image of rep-PCR banding patterns of the 61 CRAB isolates under study. Similarity calculation is based upon the Kullback–Leibler method, clustering is based upon the Unweighted Pair Group Method with Arithmetic (more ...)
MLST attributed all isolates, but two, with sequence type (ST)2, whereas the two remaining isolates belonged to ST78.
There was not apparent relationship between subtype clusters, carbapenem resistance genes and the whole resistance patterns or carbapenem resistance phenotype. Twelve out of 14 isolates showing susceptibility or intermediate susceptibility to at least one aminoglycoside were isolated in the first three months of the investigation.
The main demographic and clinic characteristics of the 36 patients are summarized in Table
. The respiratory tract was the most common site of infection (26 out of 36 cases, 72.2%). Moreover, a high infection related mortality rate was observed (18 out of 35 patients with a known outcome, 51.4%). From 19 patients other MDR organisms in addition to CRAB were isolated from at least one clinical sample (Table
). Of special interest, eight patients were co-colonized or co-infected at the same site of CRAB by Klebsiella pneumoniae
carbapenemase-producing Klebsiella pneumoniae
Demographic and clinical characteristics of the patients admitted in the period October 1, 2010 – March 31, 2011 and infected by CRAB
From 19 out of 36 patients two to six CRAB isolates were consecutively identified in the period under study. In 11 patients an indistinguishable CRAB isolate was repeatedly identified, unlike from the remaining eight patients. In particular, three out of these patients yielded isolates belonging to distinct subtype clusters and/or with distinct carbapenemase profiles in different clinical samples obtained up to 24 hours apart from each other; alternatively, distinct isolates were sequentially identified from the same or different clinical samples from five further patients within intervals of time ranging between four and seventy days (Table
Source, time of isolation, carbapenem MICs and molecular characteristics of the replicate CRAB isolates from the eight patients yielding distinct isolates by subtype cluster and/or carbapenemase profile