Widespread carbapenem resistant Acinetobacter baumannii clones in Italian hospitals revealed by a multicenter study

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Abstract

Population diversity, susceptibility to antibiotics including carbapenems of 277 Acinetobacter baumannii strains collected in 17 Italian hospitals over a 6-months’ period was assessed. Semi-automated rep-PCR was used for screening strains for genotypic relatedness. AFLP analysis and MLST were used as definitive methods for strain, species and/or clone identification.

Among the 277 strains, 49 rep-PCR types were distinguished with four types (1–4) predominant, indicating both intra- and interhospital spread. AFLP analysis allowed to distinguish 51 types and largely confirmed rep-typing results. Isolates with predominant rep-types 1 and 2 (in 3 and 9 hospitals) were allocated to EU clones I and II, respectively. Rep-type 3 (8 hospitals) belonged to a new clone (“Italian clone”). Rep-type 4 was found in 2 neighbouring hospitals. Two isolates from 2 locations belonged to EU clone III. Twenty-five isolates were identified by AFLP-analysis to A. pittii, emphasizing misidentification by phenotypic methods. MLST confirmed clone identification by AFLP; demonstrating also that the “Italian clone” was ST78, recently detected in different Mediterranean countries.

Multidrug resistance, defined as resistance to 9 out of the 11 drugs tested, was common in 10 out of 17 hospitals. The high prevalence of carbapenem resistance was associated with OXA-58 found in 9 out of the 10 hospitals. A high percentage of noted very major errors in susceptibility testing, especially for amikacin and meropenem, was probably due to heteroresistant strains. The occurrence of carbapenem and multidrug resistance in A. baumannii was mainly confined to a limited number of clonal lineages of A. baumannii.

Introduction

Acinetobacter baumannii and the genetically related Acinetobacter genomic species (gen. sp.) 3 and 13TU (recently named, respectively, as “Acinetobacter pittii” and “Acinetobacter nosocomialis”) (Nemec et al., 2011) are the Acinetobacter species most commonly involved in human infections and hospitals outbreaks. A. pittii and A. nosocomialis will be validly published by citation on Validation List 140 (publication in the July 2011 issue of the IJSEM, J. Euzeby, personal comunication). A. baumannii in particular is notorious for its resistance to antibiotics and ability to spread (Dijkshoorn et al., 2007, Peleg et al., 2008, Towner, 2009). Many outbreaks have been associated with three major international clones, European (EU) clones I–III (Dijkshoorn et al., 1996, van Dessel et al., 2004). Until recently, carbapenems were among the last options for treatment but resistance to carbapenem in A. baumannii is now frequent (Higgins et al., 2010, Poirel and Nordmann, 2006). Resistance to other potentially useful drugs (i.e. colistin and tigecyclin) has also been described (Al-Sweih et al., 2011, Navon-Venezia et al., 2007).

Few reports have focused on A. baumannii in Italy, mainly descriptions of local outbreaks (D’Arezzo et al., 2009, Giannouli et al., 2010). In 2006, a study group of the Association for Prevention and Study of Infections (APSI) evaluated the usefulness of the semi-automated repetitive-sequence-based PCR (rep-PCR) (Healy et al., 2005) for typing A. baumannii isolated over a 4-week period in 13 Italian hospitals (Carretto et al., 2008). That study also provided a diverse picture of incidence, clinical significance and antibiotic susceptibility of A. baumannii in Italy. To obtain a more detailed view of the epidemiology and antibiotic resistance of A. baumannii, a new prospective study was proposed by the APSI “Acinetobacter Study Group” (AASG). The aim was to assess the population diversity and antibiotic susceptibility of A. baumannii in 17 Italian hospitals over a 6-months’ period in 2007. Semi-automated rep-PCR method was used for initial typing, while AFLP™ analysis and multi-locus sequence typing MLST (Diancourt et al., 2010) were used for definitive characterization of isolates at species, strain and clonal level. Furthermore, antibiotic susceptibilities and presence of carbapenem resistance-related genes were determined.

Section snippets

Hospitals and bacteria

The AASG, representing 17 hospitals scattered over Italy (Fig. 1) of different size and classification, collected A. baumannii isolates from 1 February till 31 July 2007. The number of beds of these hospitals ranged from 300 to 3500 (median 1000), with a total study population of 18,000 beds. Only the first isolate from each patient was included in the study. Strains were sent to the reference laboratory (Pavia) and stored at –80 °C until testing. For each strain, a report containing presumptive

Bacterial strains

A total of 320 putative A. baumannii isolates were recovered by 16 out of the 17 Centers. Of these, 311 isolates were received by the reference laboratory for phenotypic characterization (Gerner-Smidt et al., 1991). Two hundred and eighty isolates from 15 hospitals grew in a static BHI broth culture at 44 °C which identified them to A. baumannii or Acinetobacter nosocomialis (Gerner-Smidt et al., 1991). The number of A. baumannii isolates from the 15 hospitals ranged from 1 to 54 (median 14).

Molecular analysis

Discussion and conclusions

The aim of the present study was to assess, further to a previous study, the prevalence, type diversity, and antimicrobial resistance of A. baumannii in a representative group of Italian hospitals. Multidrug resistance was noticed in most but not all hospitals. Carbapenem resistance was nearly exclusively associated with OXA-58, while OXA-23 or OXA-24 was not detected, although OXA-23 has been reported for other parts of the country and, recently, in some hospitals that took part in this study (

Transparency declaration

BioMérieux Italia kindly provided to APSI the materials used for the rep-PCR experiments.

Acknowledgments

This work was supported by grants to Piero Marone (Ricerca Corrente 2007–2011: “Sorveglianza delle infezioni associate all’assistenza nei pazienti immunodepressi: tipizzazione epidemiologica e studio delle resistenze dei patogeni ospedalieri”).

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