Molecular characterization of integrons in epidemiologically unrelated clinical isolates of Acinetobacter baumannii from Italian hospitals reveals a limited diversity of gene cassette arrays

Abstract

Integron carriage by 36 epidemiologically unrelated Acinetobacter baumannii isolates collected over an 11-year period from patients in six different Italian hospitals was investigated. Sixteen type 1 integron-positive isolates (44%) were found, 13 of which carried the same array of cassettes, i.e., aacC1, orfX, orfX', and aadA1a. As ribotype analysis of the isolates demonstrated a notable genetic diversity, horizontal transfer of the entire integron structure or ancient acquisition was hypothesized.

FIG. 1.

Amplification products obtained from the variable regions of type 1 integrons (A) and results of their restriction analysis (B). Panel A shows amplicons obtained with primers 5′CS and 3′CS from representative isolates. Lane 1, isolate AC-54/97 (amplicons of 2.5 and 2.4 kb); lane 2, isolate 7B (amplicon of 2.5 kb); lane 3, isolate 200A (amplicon of 2.2 kb); lane 4, isolate 204A (amplicon of 0.8 kb); M, kilobase pair ladder (Amersham Pharmacia Biotech, Milan, Italy). Panel B shows digestion with AluI (lanes 5 and 7) or MspI (lanes 6 and 8) of a representative amplicon of 2.5 kb (isolate 7B [lanes 5 and 6]) or 2.2 kb (isolate 200A [lanes 7 and 8]). DNA fragments were separated on a 2% agarose gel. Lane S, 100-bp ladder (Amersham Pharmacia Biotech).

FIG. 2.

Structure of the variable region of type 1 integrons detected in this study in A. baumannii and comparison with that of a similar integron previously characterized from other species. Coding sequences are indicated by arrows with the corresponding names; the attC and attI sites are indicated by black and gray filled rectangles, respectively. (A) Structure of the integron found in IncL/M plasmids from K. oxytoca (15) and S. enterica serotype Typhimurium (21, 24). The same sequence was also deposited in EMBL/GenBank under accession no. AF282595 (unpublished). (B through D) Amplicons of 2.5, 2.2, and 0.8 kb, respectively, found in this work.