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Background The emergence of polymyxin B resistance among carbapenem-resistant Klebsiella pneumoniae (CRKP) causes clinical treatment to be more difficult. We aimed to investigate the risk factors and resistance mechanisms in the polymyxin resistant CRKP (PR-CRKP) strains. Methods From January 2021 to January 2022, 239 CRKP strains were selected, all of which were analyzed using antimicrobial susceptibility testing and clinical data. Polymerase chain reaction (PCR) was performed for the detection of resistance genes. RT-qPCR was used to quantify transcriptional levels of polymyxin resistance genes. Risk factors for polymyxin B resistant isolates were identified by logistic regression analysis. Results The resistance rate of polymyxin B was 5.02%. In all CRKP strains, 41.84% came from the ICU. The percentage of carbapenemase producing strains was 93.72%. The main carbapenem resistance gene was bla KPC (90.79%). In the 12 strains of PR-CRKP screened, pmrB and pmrK were overexpressed in all samples which were linked with polymyxin B resistance. Multivariate analysis showed that coronary heart disease may be an independent risk factor predisposing patients to polymyxin B resistance. Conclusion We determine the multifaceted mechanism and risk factors of polymyxin B resistance in CRKP. Polymyxin resistance is a complex and changing problem, and more research is required.

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Freshwater components in the Southern Ocean, whether sea ice meltwater or meteoric water, influence the growth of phytoplankton by affecting water stability and supplying dissolved iron (DFe). In addition, melting sea ice stimulates phytoplankton blooms by providing ice algae. In this study, sea ice meltwater and meteoric water in the Amundsen Sea (AS) were differentiated by their stable oxygen isotopic compositions (δ18O), while the phytoplankton carbon fixation rate (CFR) and iron uptake rate (FeUR) values were determined using the 14C and 55Fe tracer assays, respectively. Our results showed that FeUR exhibits a significant positive response only to sea ice meltwater, suggesting that DFe and algae provided by sea ice melting may be the main cause. In addition, the CFR had a slightly positive response to the freshwater input and a stronger correlation with the phytoplankton biomass, suggesting that the freshwater input may have enhanced the CFR through the algae released from sea ice melting. The FeUR normalized to the phytoplankton biomass was significantly positively correlated with the mixed layer depth, suggesting that water stability regulates the phytoplankton growth and the resulting Fe demand. A higher Fe demand per unit of carbon fixation during sea ice formation leads to a higher Fe/C ratio in phytoplankton. Although no significant correlations were observed between the FeUR, CFR, and meteoric water, meteoric water may have an effect on larger phytoplankton sensitive to Fe deficiencies. The results of culture experiments with DFe addition showed that the added Fe significantly enhanced the Fe uptake, carbon fixation, and Fe/C ratio of the cells, especially for micro-phytoplankton. The more pronounced response of micro-phytoplankton means that the meteoric water input may affect the efficiency of carbon export. Our study provides the first measurements of phytoplankton Fe quotas in the AS in austral late summer and early autumn, providing insights into how meteoric water and sea ice meltwater affect seasonal changes in Antarctic ecosystems.