Abstract: Objective To understand the distribution of pathogenic bacteria and changes in peripheral blood lymphocyte subpopulations in children with positive blood cultures in Hebei Children′s Hospital, and to provide a reference basis for the diagnosis and treatment of patients with bloodstream infections in clinical settings. 
Methods Nine hundred and seventy-four children with positive blood cultures were collected from Outpatient Department, Emergency Department and Inpatient Department of Hebei Children′s Hospital for bacterial culture and identification. Among them, 468 children were tested for lymphocyte subpopulations, and mixed infections and co-infections were excluded. Finally, 219 children with purely positive blood cultures were finally included as a case group, and 250 children undergoing physical examination were included as a healthy control group. A case-control study was used to analyse the changes in the peripheral blood lymphocyte subpopulations in children with bloodstream infections. 
Results In 2022, the top five strains isolated from blood cultures were Staphylococcus epidermidis (25.20%), Staphylococcus hominis (11.20%), Staphylococcus aureus (9.00%), Streptococcus pneumoniae (7.00%), and Klebsiella pneumoniae (6.40%). In 2023, the top five positive strains isolated from blood culture were Staphylococcus epidermidis (23.21%), Staphylococcus hominis (17.93%), Klebsiella pneumoniae (11.60%), Staphylococcus aureus (7.59%), and Escherichia coli (6.75%). Changes in peripheral blood lymphocyte subpopulations were consistent between children with Gram-positive bacterial infections and those with positive blood cultures, both of which showed that the percentages of peripheral blood total T cells, helper T cells, suppressor T cells, and double-negative T lymphocytes were significantly lower than those of healthy control group (P＜0.001), whereas the percentages of B-lymphocytes and NK cells were significantly higher than those of healthy control group (P＜0.001). Percentages of total peripheral blood T cells, suppressor T cells and double-negative T lymphocytes in children with Gram-negative bacterial infections were significantly lower than those in the healthy control group (P＜0.001), and the differences in percentages of peripheral blood helper and double-positive T lymphocytes, B lymphocytes and NK cells were not statistically significant when compared with those in the healthy control group (P＞0.05). 
Conclusion Both Gram-positive and Gram-negative bloodstream infections are associated with cellular immune suppression. However, unlike children with Gram-negative bloodstream infections, children with Gram-positive bloodstream infections also have abnormal activation of humoral and intrinsic immune functions. 

Key words: infections, blood culture, child, pathogens