Nosocomial infections caused by gram-negative opportunistic pathogens, including Acinetobacter baumannii ( A. baumannii) and Klebsiella pneumoniae ( K. pneumoniae), pose a great challenge to health care management and human health. New treatment strategies are urgently needed to tackle with the spread of multi-drug resistant strains and the increase in bacterial virulence. Type Ⅵ secretion system (T6SS), a conservative secretory apparatus encoded in a variety of gram-negative bacteria, can inject effectors into other prokaryotic or eukaryotic cells in a contact-dependent manner to achieve antibacterial and anti-host properties. It is closely related to the environmental adaptability, competitiveness and colonization ability of bacteria in hosts. The T6SS gene cluster is composed of core genes, effector genes and associated genes, and the effectors encoded by it are highly diverse and play an important role in pathogen infection. This review summarized the advances in A. baumannii and K. pneumoniae T6SS in terms of competition, host colonization, interaction between conjugative plasmids and expression regulation, aiming to provide reference for future study on T6SS-related antimicrobial activity, virulence and resistance.

Objective:To predict the pathogens of bloodstream infection (BSI) in hematopoietic stem cell transplantation (HSCT) patients by plasma microbial cell-free DNA (mcfDNA) sequencing with and without additional amplification.Methods:A total of 978 HSCT patients were enrolled in Peking University People′s Hospital from March to July 2021, and the 7 428 blood samples were prospectively collected from pretransplant conditioning period to 4 months after transplantation. The plasma samples were separated and then cryopreserved. According to blood culture results and whether there were plasma samples before BSI onset, twenty-eight HSCT patients with positive blood culture (39 plasma samples within 1-8 days before BSI onset) and 9 HSCT patients with negative blood culture (9 plasma samples) were filtered. The 39 samples were performed with mcfDNA additional and non-additional amplification sequencing, and the 9 samples were only performed with additional amplification sequencing. With the blood culture results as the gold standard, the consistency between the sequencing and the blood culture results was observed. Student t test and Wilcoxon test were used for statistical analysis. Results:Without additional amplification sequencing, only 7 samples sequencing results were consistent with the blood culture results, and the total pathogen detection rate was 17.95% (7/39). The rates within 3 days and 4-8 days were 23.81% (5/21) and 2/18, respectively. The main pathogenic type detected was gram-negative bacteria (5/7). With additional amplification sequencing, the total pathogen detection rate was 59.26% (16/27) and the rate within 3 days was 8/13. The number of gram-positive bacteria detected was elevated (13/16) and the number of additional microorganisms in additional amplification sequencing was increased significantly ( P=0.001 0), compared with non-additional amplification sequencing. Moreover, additional sequencing analysis of 9 samples from patients with negative culture result showed that no pathogen was detected in six samples, and the common Torque teno virus in HSCT patients was detected in only three samples. Conclusion:The pathogen detection rate of plasma mcfDNA additional amplification sequencing was better than that of non-additional amplification sequencing in HSCT patients before BSI onset, especially in the first three days, which has the potential to predict BSI pathogens.