Description of the outbreak
From October 2005 to August 2006, 56 clinical isolates of A. baumannii were evidenced by the VIGI@ct®. The micro-organism first appeared in October 2005, and from that time it was responsible of an epidemic cluster consisting of 56 strains isolated from 13 patients admitted to the medical ICU. Our medical ICU is a seven-bed unit (five in an open-plan area and two in single rooms), in which are admitted 135 patients per year. The frequency of HAI ranges from 20 to 43%.
The ten most frequent pathogens isolated in the ICU, from October 2005 and October 2006 were: Pseudomonas aeruginosa (16%), Klebsiella pneumoniae (15%), Staphylococcus epidermidis (9%), Enterococcus faecalis (8%), Escherichia coli (7%), Staphylococcus aureus (7%), A. baumannii (7%), Candida albicans (7%), Enterococcus faecium (3%), Staphylococcus haemolyticus (2.5%).
The first patient, in whom A. baumannii was isolated, was admitted to the ICU coming from another hospital (in October). This patient, evidently imported the A. baumannii to our ICU ward, where the pathogen was cross-transmitted to other patients. Out of 13 patients, seven displayed simple colonization (based on clinical evidence) while the others developed infections (Table 1). In January the outbreak became evident and to reduce infection rate the environmental cleaning was enhanced and the hand hygiene procedure was re-enforced (providing staff with alcohol hand rub). Initially these measures appeared to be effective, but the pathogen diffusion re-started on May. In July, in two weeks, the A. baumannii was isolated in four different patients. The laboratory promptly documented the epidemic peak and the head of ICU prepared a detailed report for the Sanitary Direction (SD) describing the last infection cases. The SD decided to close the ICU for about one month and existing patients were moved to the ICUs of adjacent hospitals. The ward environment was thoroughly cleaned using detergent and disinfectant and all disposable materials were replaced. Since the ward was re-opened, A. baumannii has no longer been isolated.
Clinical isolates
Fifty-six isolates of A. baumannii obtained from 13 patients, were cultured from a variety of sites, including endothracheal aspirates (ASB), bronchoalveoar lavage (BAL), central venous device tips (CVC), blood, wound swab and urine samples (Table 1). Among the 56 isolates from 13 different patients we identified 13 sub clones. The results of the REP-PCR analysis (Pearson correlation coefficient) as well as that of f-AFLP (using the Dice's coefficient) demonstrate, in fact, that each patient was colonized/infected from a single sub clone (Table 1). To each sub clone was assigned a number that was the same used to identify the patient. Figure 1 reports the REP-PCR profiles of the 13 sub clones and their similarity matrix The REP-PCR analysis showed a clonal relationship among the isolates (being their similarity greater than the 97%).
All the clinical isolates had the same antibiograms, being susceptible to gentamicin and colistin, but resistant to imipimen, meropenem, 3rd generation cephalosporins, amikacin, ciprofloxacin, ampicillin, aztreonam, and piperacillin-tazobactam. All strains tested were positive for the bla
OXA-58 gene (data not shown).
Environmental screening
Out of 168 environmental sites investigated, 15 were found to be contaminated by A. baumannii. Figure 2 shows the similarity matrix and the REP-PCR profiles of the environmental isolates, which are clonally related, as their similarity ranged from 97 to 100%. Following cleaning and disinfection of the ICU ward, no A. baumannii isolates were grown from repeated environmental cultures.
Relationship among clinical and environmental isolates
As shown in Figure 3, the genetic relationship is consistently high between the 15 environmental isolates (named sub clones) and the 13 sub clones from patients (the percent of similarity was greater than 97.5%) demonstrating transmission from patient-to-patient as well as spreading of the pathogen in the ICU environment.
Comparison of REP-PCR and f-AFLP results
The results of f-AFLP assays (performed on patient – as well as on environmental-strains; data not shown) compared with those obtained using REP-PCR, showed to be very similar. Both exhibit high discriminatory power, but f-AFLP appeared to be more labour intensive than REP-PCR automated in the DiversiLab system. In fact, the cluster analysis using f-AFLP is completed in a time ranging from 72 h to 96 h, while the DiversiLab automation allows a complete microbial typing analysis in approximately 4 h.