Molecular and clinical epidemiology of carbapenem resistant Acinetobacter baumannii ST2 in Oceania: a multicounty cohort study

Carbapenem resistant Acinetobacter baumannii (CRAb) is categorised by the World Health Organization
(WHO) as a pathogen of critical concern. However, little is known about CRAb transmission within the Oceania.
region. This study addresses this knowledge gap by using molecular epidemiology to characterise the phylogenetic
relationships of CRAb isolated in hospitals in Fiji, Samoa, and other countries within the Oceania region including
Australia and New Zealand, and India from South Asia. In this multicounty cohort study, we analyzed clinical isolates of CRAb collected from the Colonial War
Memorial Hospital (CWMH) in Fiji from January through December 2019 (n = 64) and Tupua Tamasese Mea’ole
Hospital (TTMH) in Samoa from November 2017 through June 2021 (n = 32). All isolates were characterized using
mass spectrometry, antimicrobial susceptibility testing, and whole-genome sequencing. For CWMH, data were
collected on clinical and demographic characteristics of patients with CRAb, duration of hospital stay, mortality
and assessing the appropriateness of meropenem use from the treated patients who had CRAb infections. To
provide a broader geographical context, CRAb strains from Fiji and Samoa were compared with CRAb sequences
from Australia collected in 2016–2018 (n = 22), New Zealand in 2018–2021 (n = 13), and India in 2019 (n = 58), a
country which has close medical links with Fiji. Phylogenetic relationships of all these CRAb isolates were
determined using differences in core genome SNPs.
Findings Of CRAb isolates, 49 (77%) of 64 from Fiji and all 32 (100%) from Samoa belonged to CRAb sequence type 2
(ST2). All ST2 isolates from both countries harboured blaOXA-23, blaOXA-66 and ampC-2 genes, mediating resistance to
β-lactam antimicrobials, including cephalosporins and carbapenems. The blaOXA-23 gene was associated with two
copies of ISAba1 insertion element, forming the composite transposon Tn2006, on the chromosome. Two distinct
clusters (group 1 and group 2) of CRAb ST2 were detected in Fiji. The first group shared common ancestral linkage to
all CRAb ST2 collected from Fiji’s historic outbreak in 2016/2017, Samoa, Australia and 54% of total New Zealand
isolates; they formed a single cluster with a median (range) SNP difference of 13 (0–102). The second group shared
common ancestral linkage to 3% of the total CRAb ST2 isolated from India. Fifty eight of the 64 patients with CRAb
infections at the CWMH had their first positive CRAb sample collected 72 h or more following admission. Mer�openem use was deemed inappropriate in 15 (48%) of the 31 patients that received treatment with meropenem in
Fiji. Other strains of CRAb ST1, ST25, ST107, and ST1112 were also detected in Fiji.