Background In low-resource settings, blood culture bottles (BCBs) are often visually inspected for signs of growth. We developed a “Turbidimeter” to objectively assess growth based on the presence of broth turbidity. The first generation Turbidimeter detected growth in four out of ten bacterial species (i.e. three Enterobacterales and one Streptococcus sp.) in simulated blood cultures inoculated with spiked horse blood. Here, we present the results of the second generation Turbidimeter.

Methods Home-made BCBs (30 ml Tryptone Soy Broth with 0.3 mg/ml sodium-polyanethole sulphate) were inoculated with 2 ml of fresh human blood spiked with 20 different microorganisms (i.e. five Enterobacterales, three non-fermenting Gram-negative bacteria, two staphylococci, one Enterococcus sp., four streptococci, two fastidious organisms and three yeast spp). The BCBs were incubated in Turbidimeter modules inside a conventional incubator for 20–96 hours; measurements were done every 30 seconds. Growth detection was based on the decrease of transmitted and increase of scattered light. We compared growth detection between the first and second turbidimeter generation (improved design with new LED and detector).

Results Growth was detected in 80% of 118 BCBs tested, showing full detection of all enterococci, streptococci, and non-fermenting Gram-negative bacteria (100%), and partial detection of Enterobacterales (84%), staphylococci (88%), fastidious organisms (17%) and yeast (67%). Compared with the first prototype, growth detection improved considerably. Six microorganisms that remained undetected before, were now (partially) detected: 100% growth detection of BCBs with Staphylococcus aureus, Burkholderia cepacia and Pseudomonas aeruginosa and 17% of BCBs with Candida albicans, Haemophilus influenzae and Neisseria subflava.

Conclusion The second generation Turbidimeter showed improved growth detection (17–100% of BCBs) compared with the first prototype. Field testing will be conducted within the EDCTP2-funded SIMBLE project from May 2023 until end of 2024 in Benin and Burkina Faso, to evaluate performance and ease-of-use for future implementation in field laboratories.