The design and usage of a portable incubator for inexpensive in-field water analysis

Abstract

Humanitarian engineers need an inexpensive, fast, visually compelling way to assess bacterial water quality in remote locations. One way to do this is with 3M Petrifilm E. coli/Coliform (EC) Count Plates to detect E. coli in water samples. These require incubation at close to body temperature. To meet this need, we provide a free, open-source design of a battery-powered incubator capable of maintaining 35°C ± 1°C for up to 65 hours in ambient temperature of 25°C. Our incubator, called the Armadillo, can be replicated by an ordinarily skilled person in five hours for under USD $200.00 in materials cost. This paper summarises the reference documentation for construction, sample handling, inoculation, and incubation using Petrifilms and the Armadillo. Colony-forming unit (CFU) counts generated by the Armadillo are compared side-by-side with a laboratory-grade incubator. Incubation performance at ambient temperatures of 25°C and 4°C shows that a single battery charge reliably powers a full incubation period of 48 hours under normal ambient temperatures.

Microbiology in the Field: Construction and Validation of a Portable Incubator for Real-Time Quantification of Coliforms and Other Bacteria

“…To minimize temporal degradation, environmental samples would ideally be processed as soon as they are collected. Several studies have investigated the use of microbiological testing during field assessments and in low-resource settings (10)(11)(12)(13)(14), employing products such as Petrifilms TM (3M TM ). However, these studies often lacked a strong microbiological foundation and comparisons to standard techniques.…”

“…Both studies found high rates of variability within and between the methods leading to low confidence in the results (12,15). There is a clear need for the development of robust methods to incubate bacteria in the field leading to timely, reliable and reproducible quantification of bacteria in water samples (11,14,(16)(17)(18)(19)(20).…”

“…A humanitarian outreach program by Engineers Without Borders (Austin, Texas) developed a low-cost, battery-operated incubator, the Armadillo, for use with Petrifilms TM , with education about water quality in remote environments via the visually striking results being its primary use (14,21). Similarly, a low-cost field incubator was used in an assessment of bacterial contamination of groundwater in Malawi (19).…”

“…Although these designs improved greatly upon previous studies where samples were incubated at ambient temperature or by using body heat and securing them to their body, the lack of several important microbiological principals, measures of reliability and reproducibility of results, and comparisons with standard methods currently preclude these from being used for widespread research and monitoring activities. Although multiple previous studies have used portable incubators for sampling in remote locations, samples could only be incubated when the incubator was stationary, posing further limitations on their use (14,(17)(18)(19)(20). These previous efforts have shown proof of concept for fieldbased microbiological testing using Petrifilms TM and highlight the demand for affordable and versatile portable incubators for field projects globally (14,17,19,21,22).…”

“…Although multiple previous studies have used portable incubators for sampling in remote locations, samples could only be incubated when the incubator was stationary, posing further limitations on their use (14,(17)(18)(19)(20). These previous efforts have shown proof of concept for fieldbased microbiological testing using Petrifilms TM and highlight the demand for affordable and versatile portable incubators for field projects globally (14,17,19,21,22).…”