Clinical outcomes of hospital-acquired infection with Acinetobacter nosocomialis and Acinetobacter pittii

Abstract

The role of Acinetobacter nosocomialis and Acinetobacter pittii, which belong to the A. calcoaceticus-A. baumannii complex, in hospital-acquired infections is increasingly recognized. Here we describe a retrospective cohort study of hospital-acquired A. calcoaceticus-A. baumannii complex infections at a university hospital in Thailand. A total of 222 unique cases were identified between January 2010 and December 2011. The genomospecies of the A. calcoaceticus-A. baumannii complex isolates were classified as follows: A. baumannii, 197 (89%); A. nosocomialis, 18 (8%); and A. pittii, 7 (3%). All A. nosocomialis and A. pittii isolates were susceptible to imipenem and meropenem. The patients infected with A. nosocomialis and A. pittii had lower 30-day mortality than those infected with carbapenem-susceptible A. baumannii (P = 0.025) and carbapenem-resistant A. baumannii (P = 0.013). The factors influencing 30-day mortality were infection with non-baumannii A. calcoaceticus-A. baumannii complex (hazard ratio [HR], 0.12; 95% confidence interval [CI], 0.03 to 0.51; P = 0.004), infection with carbapenem-resistant A. baumannii (HR, 1.57; 95% CI, 0.89 to 2.79; P = 0.105), appropriate empirical antimicrobial therapy (HR, 0.38; 95% CI, 0.23 to 0.61; P < 0.001), and higher acute physiology and chronic health evaluation II (APACHE II) score (HR, 1.15; 95% CI, 1.10 to 1.19; P < 0.001). In Galleria mellonella assays, the survival rates were significantly higher for the larvae infected with A. nosocomialis or A. pittii than for those infected with either carbapenem-susceptible A. baumannii or carbapenem-resistant A. baumannii, but no differences in survival rates were observed between carbapenem-susceptible A. baumannii and carbapenem-resistant A. baumannii. These findings suggest intrinsic differences in virulence between non-baumannii A. calcoaceticus-A. baumannii complex species and A. baumannii but not between carbapenem-susceptible and resistant A. baumannii.

FIG 1

Flowchart of the study enrollment. A total of 25, 58, and 139 patients were included in the non-baumannii Acinetobacter calcoaceticus-Acinetobacter baumannii (Acb) complex group, carbapenem-susceptible A. baumannii (CSAB) group, and carbapenem-intermediate or -resistant A. baumannii (CRAB) group.

FIG 2

Kaplan-Meier survival curves of the patients in the non-baumannii A. calcoaceticus-A. baumannii complex group (A. nosocomialis or A. pittii), carbapenem-susceptible A. baumannii (CSAB) group, and carbapenem-intermediate or -resistant A. baumannii (CRAB) group. Levels of survival among these three groups were significantly different, with non-baumannii A. calcoaceticus-A. baumannii complex infection and CRAB infection associated with the lowest and highest mortality levels, respectively (P < 0.001, log-rank test).

FIG 3

Survival of G. mellonella. The larvae were infected with 2 A. nosocomialis, 2 A. pittii, 2 carbapenem-susceptible A. baumannii (CSAB), and 2 carbapenem-resistant A. baumannii (CRAB) isolates and observed daily for 6 days. Survival rates were significantly higher for the larvae infected with non-baumannii A. calcoaceticus-A. baumannii complex species than for those infected with A. baumannii, either CSAB or CRAB (P < 0.05, Student's t test), whereas there was no significant difference in the levels of survival between the larvae infected with CSAB and those infected with CRAB.