DOI: 10.1128/msphere.00311-26 ISSN: 2379-5042
Strain-specific outer membrane vesicle-mediated transfer of a
bla
OXA-24
-like plasmid in
Acinetobacter baumannii
restricts c
Shengnan Jiang, Biyong Yan, Haiyang Liu, Long Sun, Yan Chen, Lilin Liu, Yunsong Yu ABSTRACT
The rapid rise of multidrug-resistant
Acinetobacter
spp. poses a major global health threat. In
Acinetobacter
spp., carbapenem resistance is frequently mediated by OXA-type carbapenemases encoded by
bla
OXA
genes, including
bla
OXA-24
-like genes such as
bla
OXA-72
. Outer membrane vesicles (OMVs) are nanoscale proteoliposomes secreted by Gram-negative bacteria that can facilitate plasmid transfer, yet the mechanisms limiting their dissemination remain unclear. Nationwide screening in China of 2,880
Acinetobacter
clinical isolates identified 10 strains carrying
bla
OXA-24
and
bla
OXA-72
plasmids across three species (
A. baumannii
,
A. pittii
, and
A. baylyi
). Sequencing revealed nine distinct plasmids, most bearing a Rep_3/OrfX C-module backbone with
blaOXA
-p
dif
resistance modules (9/10). Among clinical strains, A2485 was identified as a high-OMV producer (2.74 ± 0.20-fold relative to ATCC 17978,
P
< 0.001). OMVs from A2485 transferred the
bla
OXA-72
plasmid to ATCC 17978 at 1.9 × 10⁻⁷ transformants per CFU, exceeding direct supernatant transfer at 3.2 × 10⁻⁸ transformants per CFU, and conferred resistance to meropenem, ceftazidime, and cefoperazone/sulbactam. Transfer was strictly donor-specific, with no transmission observed across 13 non-
A
.
baumannii
strains, and recipient strains failed to retransmit the plasmid, indicating a strong cross-species barrier and a non-reciprocal, donor-dependent process. Although all strains produced intact OMVs and encapsulated plasmid DNA, packaging efficiency varied considerably by host (~4.5–28.6 ng/μg). Strains with DNA loading (17978E/17978V: 28.62 and 23.98 ng/μg) equal to or exceeding that of donor A2485 (13.62 ng/μg) remained incapable of transfer. These findings demonstrate that dissemination is constrained by host-specific, post-packaging barriers rather than packaging efficiency, limiting resistance spread to specific lineages while enabling localized adaptation.
IMPORTANCE
Acinetobacter baumannii
is a leading hospital-associated pathogen, and carbapenem resistance mediated by
bla
OXA-24
-like genes poses a serious clinical challenge. Through nationwide screening of 2,880 clinical
Acinetobacter
isolates in China, we found that strains carrying
bla
OXA-24
and
bla
OXA-72
plasmids are relatively rare, occurring in only 10 isolates across three species. We provide the first evidence that outer membrane vesicle (OMV)-mediated plasmid transfer is subject to strict, host-specific barriers: the
bla
OXA-72
plasmid was successfully transferred among
A. baumannii
strains via OMVs, yet no transfer was observed to any of the 13 non-
A
.
baumannii
species tested. Transfer was also non-reciprocal—recipient strains that acquired the plasmid were unable to further disseminate it, whether via OMVs or supernatants. Critically, this restriction was not explained by DNA packaging efficiency, as strains with equal or greater plasmid loading than the donor remained incapable of transmission, pointing instead to post-packaging, donor-specific factors, such as membrane composition or vesicle biogenesis machinery. These findings clarify the sporadic distribution of
bla
OXA-24
-like plasmids in China and challenge the conventional assumption that mobile genetic elements inherently carry broad dissemination potential, revealing that OMV-mediated resistance spread is constrained to specific bacterial lineages.