Journal of Medical Microbiology - Current Issue

Introduction. Gram-negative bloodstream infections (GNBSIs) carry a significant global health burden. Escherichia coli and Klebsiella pneumoniae are the two most common causes of healthcare-associated GNBSI, which may arise from gastrointestinal tract (GIT) colonization.

Gap Statement. We do not fully understand how GNBSIs arise from GIT colonization.

Aim. To understand E. coli and K. pneumoniae genomic and phenotypic adaptations that underpin transition from GIT colonization to invasive bloodstream infection.

Methodology. This study identified ‘linked’ faecal and blood isolates from children with healthcare-associated GNBSI caused by E. coli and K. pneumoniae. Linked pairs were compared for antimicrobial resistance and biofilm formation and underwent comparative genomic analysis via whole-genome sequencing, comparative average nucleotide identity and core genome single nucleotide polymorphism (SNP) analysis.

Results. Five isolate pairs (three E. coli, two K. pneumoniae) showed high relatedness, supporting the GIT origin of bloodstream infection. Isolates within pairs had identical virulence genes, whereas phenotypic assays revealed changes in antimicrobial susceptibility, with one pair undergoing changes in resistance gene profiles and increased biofilm formation in four out of five isolates.

Conclusion. This study provides insight into within-host evolution from gastrointestinal colonization to bloodstream invasion in Gram-negative pathogens. Convergence on metabolic adaptation and biofilm formation suggests that these traits may be advantageous in healthcare-associated GNBSI. Further studies involving larger cohorts alongside functional validation of mutations are needed to better understand GNBSI pathogenesis.

Introduction. Sepsis is a major contributor to the global burden of disease. Its effective treatment is time-critical and relies on timely access to significant blood culture (BC) results.

Hypothesis/gap statement. The value of BC results to the requesting service is reduced by delays in report completion. Centralized clinical laboratory networks have little insight into pre-analytical causes of delayed BC results.

Aim. We aimed to assess variations in laboratory investigation of bloodstream infection and sepsis throughout a state-wide laboratory network, with the goal to design suitable remedial action.

Methodology. We analysed BCs from collection to final reporting in a public pathology service by univariate analysis and supervised machine learning.

Results. Of the 5,436 first-positive BCs from all Western Australian (WA) public laboratories in 2023, 1,343 (24.7%) came from regional sources. A total of 1,052 (78.3%) regional BCs were from emergency departments, and 831 (64.5%) of these were collected out of hours, rising during the 24 h cycle. Regional BCs took 33 h more than urban area cultures to reach a final report (103 compared with 70 h). Regional BC Gram stains were delayed by 31 h (69 compared with 38 h) and took over 97 h from collection to report in 25% regional Gram-stain results. Regional BCs added a 15 h delay to first results when significant species were mixed with potential contaminants, and 23 h when mixed with other significant species.

Conclusion. In WA, substantial delays to actionable BC results were common. The time taken to transport specimens to a laboratory was a small fraction of these delays. Monitoring of the steps in BC workflow completion can be used to improve the quality and safety of BC service provision within the limits of current technology, though solutions to this critical capability gap vary with location.

Introduction. Clinical trials and observational studies of co-formulated regimens bictegravir/emtricitabine/tenofovir alafenamide (BIC/F/TAF) and doravirine/lamivudine, tenofovir disoproxil fumarate (DOR/3TC/TDF) demonstrated potent efficacy in both antiretroviral therapy-naive and -experienced patients, but data on efficacy in naive people with high-level Human Immunodeficiency Virus (HIV) viraemia are still lacking.

Hypothesis. BIC/F/TAF and DOR/3TC/TDF have comparable effectiveness in naive people with high-level viraemia.

Aim. To compare the effectiveness of BIC/F/TAF and DOR/3TC/TDF in naive people with high-level viraemia.

Methodology. We performed a retrospective cohort study of adult people living with HIV, who were naive to antiretroviral therapy, had baseline HIV-1 RNA >500,000 copies ml−1 and initiated the co-formulated regimen BIC/F/TAF or DOR/3TC/TDF. Virological efficacy, changes in immunological parameters and safety profiles after 12 months of treatment were evaluated.

Results. Inclusion criteria were met by 78 patients: 43 in the BIC/F/TAF group and 35 in the DOR/3TC/TDF group. Baseline characteristics were similar in both groups: median age was 45.2 years, median HIV RNA was 5.95 log10, median CD4 T lymphocyte count was 383 cells mm−3 and 22 patients (28%) had HIV RNA >106 copies ml−1. After 12 months, virological efficacy was comparable: HIV RNA <20 copies ml−1 was obtained in 40 patients (93%) in the BIC/F/TAF group and in 31 (89 %) in the DOR/3TC/TDF group. The median increase in CD4 T lymphocyte count was comparable between groups (+139 and +117 cells mm−3, respectively), such as incidence of adverse events, while median increase in low-density lipoprotein cholesterol and weight gain was significantly greater in the BIC/F/TAF group (weight change, +1.64 kg vs. +0.85 kg; P=0.013).

Conclusion. In this real-life cohort, BIC/F/TAF and DOR/3TC/TDF showed high and comparable virological efficacy in naive patients with baseline HIV RNA above 500,000 copies ml−1.

Introduction . Burkholderia cenocepacia, a member of the Burkholderia cepacia complex (Bcc), causes severe, treatment-resistant lung infections in individuals with cystic fibrosis and chronic granulomatous disease.

Hypothesis. We hypothesized that the C3HeB/FeJ mouse strain, characterized by impaired macrophage-mediated immunity, would be susceptible to persistent infection by clinical Bcc isolates.

Aim. To evaluate C3HeB/FeJ susceptibility to various Bcc isolates and characterize the macrophage-intrinsic responses and antibiotic susceptibility within this model.

Methodology. C3HeB/FeJ and C57BL/6 mice were compared for susceptibility to pulmonary infection with B. cenocepacia AU0728 following intratracheal inoculation. Bacterial persistence and extrapulmonary dissemination were monitored for up to 42 days. Host immune responses were assessed through systemic leucocyte profiling and analysis of bone marrow–derived macrophage (BMDM) function, including intracellular bacterial control, cytokine production and time-resolved cell-death responses. Model breadth was evaluated using three additional clinical Bcc isolates. Finally, the in vivo efficacy of high-dose ceftazidime or meropenem was assessed against established pulmonary infection.

Results. C3HeB/FeJ mice sustained significantly higher pulmonary burdens of B. cenocepacia AU0728 than C57BL/6 mice, resulting in a stable, non-sterilizing infection with extrapulmonary dissemination lasting at least 42 days. In vitro, C3HeB/FeJ BMDMs supported higher intracellular bacterial loads and failed to mount effective IFN-γ–dependent intracellular control. Mixed-effects modelling of cell-death responses revealed that both strains initiated early apoptotic signalling following infection; however, this response was attenuated in C3HeB/FeJ macrophages and was not restored by IFN-γ pretreatment. In contrast, late-stage membrane permeabilization increased progressively over time and exhibited strain- and IFN-γ–dependent modulation at later stages of infection. Susceptibility was highly strain-specific: three additional clinical Bcc isolates were rapidly cleared from C3HeB/FeJ lungs. Treatment with ceftazidime or meropenem reduced established pulmonary bacterial burdens by ~1.4–1.5 log₁₀ c.f.u. but did not achieve sterilization.

Conclusion. These findings identify C3HeB/FeJ mice as a selectively susceptible host for sustained pulmonary infection by B. cenocepacia AU0728 and demonstrate that persistence in this model is strongly strain-dependent. Impaired macrophage-intrinsic IFN-γ–dependent control and stage-specific dysregulation of cell-death responses contribute to bacterial persistence. This immunocompetent mouse model provides a tractable platform for dissecting strain-level virulence mechanisms and for evaluating therapeutic strategies targeting chronic Bcc infection.