There have been reports of clonal outbreaks related to environmental reservoirs before [
16], nevertheless, the setting in many hospitals is complicated and unpredictable. In these cases, sporadic and epidemic clones, along with different reservoirs such as environmental reservoirs or colonized and infected patients could coexist. Thus, the epidemiology of
A. baumannii is difficult to explain. An important limitation of the presented research is that environmental strains were not collected from the ICU; therefore, this study lacks surveillance for source identification. Another limitation was the lack of routine culture monitoring for all patients that were newly admitted to the ICU. Therefore, it was impossible to distinguish between new cases which acquired CRAB in the ICU and imported cases which acquired CRAB before admission to the ICU. Yamada et al. reported that invasive procedures, ICU duration, recent surgery, and exposure to broad-spectrum antibiotics are risk factors for colonization or infection by MDR
A. baumannii [
17]. The ICU have been seen as epicenters of the epidemiology of
A. baumannii [
18]. In several previous studies, the unit had to be completely shut down to control the epidemic [
19,
20]. However, the ICU in this study could not be closed during hospital outbreak because it was the only referral agency in the area. Furthermore, since there is no single room in the ICU of this hospital, it cannot be completely isolated and disinfected, which also introduces difficulties for infection control. Fortunately, there has been a downward trend since 2015. This outbreak demonstrated that it is effective to implement strict contact precautions and to decrease environmental contamination to control the outbreak of CRAB.
The clinical isolates of
A. baumannii are mainly composed of three international clone lineages called European clones I, II, and III. In the MLST scheme, most outbreak strains belong to CC1 and CC2, corresponding to European clones I and II, respectively [
21]. So far, ST2 type as the most common ST in CC2 has been reported in Australia, Russia, Italy, and China [
22‐
25]. The results of this study show that all strains belonged to ST2 except for one isolate, which belonged to the new genotype ST1199. Furthermore, this study showed that in the clonal isolate, ERIC-PCR was capable to accurately cluster CC2 isolates. Compared to MLST, ERIC-PCR has a cost advantage. These results indicated that MLST combined with ERIC-PCR may be an economical method to solve the hospital epidemiology of
A. baumannii.
Our results suggest that carrying the
blaOXA-23 gene is the main cause of the CRAB resistance phenotype, which is consistent with previous reports [
26,
27]. Interestingly,
blaOXA-24 was also discovered in the presented experiments, and it was also found in Spain, Georgia, France, and the US [
28]. This is the first time that
blaOXA-24 has been detected in eastern China. The
blaADC was also found in the current study, which is intrinsic in
A. baumannii. It was found that the insertion sequences(ISs) can activate
blaOXA-23,
blaOXA-51 genes, and
AmpC [
22,
29]. Resistance can arise by mutation and transmission is divided into vertical transmission and horizontal gene transfer (resistance genes are obtained from other bacteria). Intercellular transfer mainly includes three mechanisms: natural transformation (direct DNA transfer), transduction (by phage transfer), and conjugation(by plasmids transfer). Conjugation is the most common drug resistance transmission mode [
30]. Therefore, a conjugation experiment was performed to verify the horizontal transfer of drug resistance genes. The
blaOXA-51-like gene is inherent in
A. baumannii and initially located on chromosomes. However, studies have shown that the
blaOXA-51-like gene and its upstream ISAba1 sequence have been together transmitted via plasmid in Taiwan [
31]. In this study, a
blaOXA-51 carrying plasmid was transferred by conjugation to non-
A. baumannii species (
E. coli J53). This phenomenon not only further complicates the management of
A. baumannii infections but also affects the accuracy of
blaOXA-51-like as a standard for distinguishing
A. baumannii.
blaOXA-23-producing isolates were prevalent in ICU in the First Affiliated Hospital of Jiamusi University hospital, and
blaOXA-23 can be inserted into chromosomes and plasmids and encircled by various genetic contexts. The main reported genetic background includes three categories, including genomic islands, transposons, and ISs [
32]. In the present study, all conjugated plasmids contained the
blaOXA-23 gene, but only the type B of ERIC-PCR isolate successfully transferred the
ISAbal-
blaOXA-23 structure in conjugation, indicating that typeA and typeB have different transfer systems and the mechanism needs further verification. The
blaOXA-24 gene found in
A. baumannii does not have the typical structure associated with DNA mobilization, such as ISs. However, this gene is flanked by XerC/XerD binding sites and may be involved in gene mobilization [
32,
33]. This indicates that
blaOXA-24 differs from
blaOXA-23 in both genetic environment and plasmid type. Attempts to obtain
blaOXA-24-producing transconjugants were unsuccessful. This is consistent with previous research [
33]. The possible reason is that
blaOXA-24 genes were located in the genome or the plasmid containing the gene cannot be replicated in
E. coli J53. However, the spread of
blaOXA-24 should still receive attention.