The molecular epidemiology of multidrug-resistant Acinetobacter baumannii was investigated in the medicalsurgical
intensive care unit (ICU) of a university hospital in Italy during two window periods in which two
sequential A. baumannii epidemics occurred. Genotype analysis by pulsed-field gel electrophoresis (PFGE) of
A. baumannii isolates from 131 patients identified nine distinct PFGE patterns. Of these, PFGE clones B and
I predominated and occurred sequentially during the two epidemics. A. baumannii epidemic clones showed a
multidrug-resistant antibiotype, being clone B resistant to all antimicrobials tested except the carbapenems
and clone I resistant to all antimicrobials except ampicillin-sulbactam and gentamicin. Type 1 integrons of 2.5
and 2.2 kb were amplified from the chromosomal DNA of epidemic PFGE clones B and I, respectively, but not
from the chromosomal DNA of the nonepidemic clones. Nucleotide analysis of clone B integron identified four
gene cassettes: aacC1, which confers resistance to gentamicin; two open reading frames (ORFs) coding for
unknown products; and aadA1a, which confers resistance to spectinomycin and streptomycin. The integron of
clone I contained three gene cassettes: aacA4, which confers resistance to amikacin, netilmicin, and tobramycin;
an unknown ORF; and blaOXA-20, which codes for a class D -lactamase that confers resistance to
amoxicillin, ticarcillin, oxacillin, and cloxacillin. Also, the blaIMP allele was amplified from chromosomal DNA
of A. baumannii strains of PFGE type I. Class 1 integrons carrying antimicrobial resistance genes and blaIMP
allele in A. baumannii epidemic strains correlated with the high use rates of broad-spectrum cephalosporins,
carbapenems, and aminoglycosides in the ICU during the study period
