In this study, we analyzed the molecular typing and characteristics of A. baumannii
isolates in our ICU. From the MLST result, the predominant clone is ST92 in CNSAb isolates in our hospital, and this observation is also consistent with previous studies [14
], which revealed that ST92 has spread worldwide. However, limited diversity was observed in CNSAb isolates in our ICU, while there was a higher diversity in CSAb isolates. Interestingly, there were still 8 CSAb isolates being assigned as ST92, but none of the CSAb isolates belonged to ST75. This characteristic indicates that these 2 types might exhibit differences in clinical outcomes or have antibiotic resistance patterns. Although there were no significant differences in antibiotic resistance rates for the ST92 and ST75 CNSAb isolates, they still exhibited slight differences on their MIC ranges. As shown in , all ST92 CNSAb isolates were resistant to imipenem, with MICs ranging from 32-128 mg/L; most of them (71.4%, 10/14) were also resistant to meropenem with the same MIC range. However, the MIC of the imipenem-resistant ST75 isolates ranged from 64 to 128 mg/L, which is a more narrow resistance profile than that of ST92 (). However, there was no difference in the MIC range of meropenem for these 2 types. These results indicate that ST75 might have a more severe imipenem resistance range than ST92.
Genetic analysis showed that ST75 is the single locus variant (SLV) of ST92, which differs in its gpi
loci. While ST92 isolates show moderate carbapenem resistance rates, all ST75 types are resistant to carbapenem with a higher MIC range. Therefore, we hypothesized that mutations of the gpi
loci in ST92, e.g., ST75, might benefit its survival when encountering carbapenem selection. In contrast, in our collection, there were some ST92-type CSAb isolates, but none was ST75, which also inferred that ST75 CNSAb isolates might represent a severe epidemic marker in nosocomial infections in our area. Moreover, a study by He et al. [15
] showed that more ST138-CNSAb isolates are associated with ST92 in western China, which could give rise to the interesting assumption that the composition of CNSAb STs could exhibit geographic differences. These differences might be caused by different antibiotic usage habits, which may influence the evolution direction of ST92. Therefore, ST75 accompanied by ST92 might be the epidemical feature in eastern China.
In this study, we observed more diversity in CSAb isolates (5 ST types) than in CNSAb isolates (only 2 ST types). Interestingly, we observed some of CSAb isolates is ST92-CSAb type, and this type strain is the majority component in the epidemic CNSAb clonal CC92. While the other 4 STs, including ST112, ST145, ST345, and STn are singleton in the A. baumannii population, which shows no relation with epidemic strain CC92. As is known, natural selection is the major force that shapes the population structure of microbial communities in certain environments. A higher diversity in CSAb isolates might be due to fewer encounters with natural selection forces, e.g., antibiotics. Antibiotic resistance profiles showed that most antibiotics caused more resistance in the ST92-CNSAb group than in the ST92-CSAb group (). However, ST92-CSAb strains could potentially be a source of CNSAb strains, having acquired carbapenem resistance. Therefore, ST92-CSAb strains should be considered as important along with CNSAb isolates for future nosocomial infection control.
Genetic determinants analysis shows MBLs are not the major resistant determinants in CNSAb isolates in our hosptial, which is consistent with previous study [26
]. However, in some distinct areas of China, the MBL plays a more important role than the OXA-type carbapenemase in CNSAb [27
]. This implies that a geographical characteristic plays an important role in CNSAb isolates formation, and it could be due to different antibiotic usage in distinct areas. Integron analysis shows a high prevalence of this gene capture machinery. Although in this study, we only recovered 3 type of class 1 integron cassette, there still shows integron is highly associated with antibiotic resistance. However, there are some limitations in this integron cassette analysis method, e.g., some gene cassettes failed to be recovered due to the lack of a conserved terminal [28
]. However, there are several studies [29
] showing that there are multiple gene cassettes that contain the OXA
gene in A. baumannii
isolates. This might indicate that transmission of the carbapenemase resistant gene might be mediated by a very complex mechanism. Notably, whole genome sequencing of MDRAbs [31
] revealed a correlation between carbapenem resistance and the integron component. Most MDRAb strains harbor an antibiotic resistance island (Aba
R) in their genome, which provides great potential for antibiotic resistance. Moreover, in the present study, we observed that these gene cassettes are shared among these strains and other species from the integron database [29
], which provides a clue for the transmission of integrons among inner- and interspecies. Interestingly, several environmental isolates in our ICU possessed a class 1 integrase from a previous collection, while 1 isolate was detected with an empty gene cassette (data not shown). Similarly, there is a recent report that shows 1 environmental isolate of P. aeruginosa
with an empty cassette [35
]. This empty cassette potentially represents a reservoir with an increased capacity to adapt to the environment.
In conclusion, we found that there are 2 predominant distinct clones (ST92 and ST75) of CNSAb isolates, which clustered to CC92 in our medical ICU. ST92-A. baumannii is considered to be the clone with a worldwide dissemination, while ST75 could be another major clone accompanying the CNSAb isolates in our region. Moreover, the ST75 clone might act as a severe epidemic marker in the carbapenem-resistant A. baumannii isolates in our area. Therefore, CC92-CNSAb plays an important role in causing nosocomial infections in our medical ICU and thus, should be investigated as part of hospital infection control in the future.