A compact, lightweight, device makes it possible to assess the effects of antibiotics on bacteria. The device safely contains the antibiotic/bacteria mixtures, and its operation involves minimal ancillary equipment and minimal expense of time and effort by technicians. Originally designed for use aboard spacecraft, the device could also be mass-produced for use on Earth in isolated, remote environments and in situations in which automated instrumentation is inaccessible.

The Reaction Wells of This Device exhibit colors that were developed in a test of antibiotic substrates with Staphylococcus aureus.

Testing to assess the effectiveness of antibiotics has been made necessary by the emergence of resistance to antibiotics in several common bacterial pathogens. Such testing is routinely performed in clinical laboratories by traditional manual and automated methods. An important measure of antibiotic effectiveness in laboratory testing is the highest dilution (minimum concentration) at which the drug under test inhibits the growth of bacteria; this concentration is known as the minimal inhibitory concentration (MIC). Using the present device, one can determine the MIC of an antibiotic with respect to the bacteria of interest, without the complex fluid-handling procedures and without the relatively bulky, expensive, power-consuming equipment of traditional laboratory testing.

The device is a modified version of a commercial test card that provides visual indications of MIC values. The card is credit-card-sized, weighs less than 15 grams, and contains 30 small reaction wells. The reaction wells are filled with dehydrated antibiotic substrates mixed with the antibiotics at various levels of dilution. The modification consists of the addition of Alamar Blue (a redox indicator) to the mixture in the reaction wells. This redox indicator serves as a visual indicator of the growth of bacteria in the wells. Resistance of microbes to the antibiotic in a given well is manifested by a distinctive bright pink color in the wells, whereas susceptibility of the microbes to the antibiotic is indicated by a blue color in that well.

To initiate a test, a technician inoculates the wells with a suspension of microbes that has been prepared from a 24-hour culture of the species of interest. The inoculated device is placed in a Ziploc (or equivalent) edge-sealing plastic bag and incubated at a temperature of 35° C for 18 to 24 hours. After incubation, the device (see figure) is inspected and the colors in the wells are recorded.

In the original space-flight application, two devices are inoculated and bagged in a terrestrial laboratory. One device is transported under refrigeration at 4° C, then incubated in the low-gravity test environment; another device that serves as a control is similarly refrigerated and incubated on Earth.

This work was done by Duane L. Pierson of Johnson Space Center, Joyce A. Skweres and Saroj K. Mishra of KRUG Life Sciences, and James H. Jorgensen of UT-Health Science. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Bio-Medical category. MSC-22658

NASA Tech Briefs Magazine

This article first appeared in the August, 2000 issue of NASA Tech Briefs Magazine.

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