In order to isolate, study and efficiently treat a bacterial outbreak, it is vital to be able to grow, store and identify the particular strains of bacteria that cause the disease. While this can be a fairly simple task in a well stocked laboratory, it’s a lot harder to achieve out in the field, in tropical or rural areas without access to much laboratory equipment or a reliable electricity supply. New techniques for working in an electricity-free environment are therefore both interesting and very important for the treatment of tropical bacterial diseases.
A recent paper looks at how the bacteria Salmonella enterica can be cultured and strain-identified in an environment with limited resources. Rather than using a conventional incubator, the researchers used an insulated container (previously used as a vaccine storage box!) and little “phase-change packets” to control the temperature. These packets contain a substance that maintains a temperature of 38°C while changing from its liquid form back to a solid.
Once incubated, the culture bottles were checked every day for the discolouration of a carbon dioxide indicator placed on the bottom of the bottle, to check that they were still alive and growing. The incubator was then re-filled with fresh packets and all growing cultures placed back inside. This process was repeated for seven days.
In the comparison group the cultures were placed in an electrical incubator set to thirty seven degrees. After the seven days, samples from both methods were tested using standard biochemical methods. To further explore just how long these cultured bacteria would survive at room temperature, samples from the bottles were re-grown for a week under both conditions (electrical incubator and phase-change packets) and then left at room temperature, tested every week for bacterial survival.
From the 65 individuals with confirmed enteric fever, 55 (84.6%) were identified by the conventional blood culture and 60 (92.3%) were identified by the experimental method. The overall percentage of agreement between the two methods was 94.4% (for the statistically minded the 95% confidence intervals were 91.2%–96.7%). The bacteria left at room temperature were found to survive for at least six months without the addition of any further nutrients or media.
The phase-change packet incubators therefore seem to work just as well as conventional electrical ones, despite the slight temperature decreases caused by change-over of the packets. Anyone who has worked in a lab will have had nightmares about power-cuts to the incubator (and fridges left open!) but in the front lines of tropical disease research this is an everyday reality in rural or isolated areas.
As well as electricity supply, the other consideration for working in such areas is of course the cost. The phase-change incubator came out at $50 for the incubator / packets and $2.30 per bottle (including carbon-dioxide sensor). The researchers are therefore currently working on the production of a lower cost bottle which incorporates a colour-changing growth incubator. They also point out that cheaper or locally produced versions of the phase-change packets could be sought, and the practice of putting them into an empty box has not been patented.
Andrews JR, Prajapati KG, Eypper E, Shrestha P, Shakya M, et al. (2013) Evaluation of an Electricity-free, Culture-based Approach for Detecting Typhoidal SalmonellaBacteremia during Enteric Fever in a High Burden, Resource-limited Setting. PLoS Negl Trop Dis 7(6): e2292. doi:10.1371/journal.pntd.0002292