DEVELOPMENT OF AN ANTIMICROBIAL SUSCEPTIBILITY TESTING METHOD SUITABLE FOR PERFORMANCE DURING SPACE-FLIGHT

Very little is known regarding the effects of the microgravity environ ment of space flight upon the action of antimicrobial agents on bacter ial pathogens, This study was undertaken to develop a simple method fo r conducting antibacterial susceptibility tests during a space shuttle mission, Specially prepared susceptibility test research cards (bioMe rieux Vitek, Hazelwood, Mo.) were designed to include 6 to 11 serial t wofold dilutions of 14 antimicrobial agents, including penicillins, ce phalosporins, a beta-lactamase inhibitor, vancomycin, erythromycin, te tracycline, gentamicin, ciprofloxacin, and trimethoprim-sulfamethoxazo le. MICs of the drugs were determined by visual reading of color end p oints in the Vitek research cards made possible by incorporation of a colorimetric growth indicator (alamarBlue; Accumed International, West lake, Ohio), This study has demonstrated reproducible susceptibility r esults in the testing of isolates of Staphylococcus aureus, group A St reptococcus species, Enterococcus faecalis, Escherichia coli (beta-lac tamase-positive and -negative strains), Klebsiella pneumoniae, Enterob acter cloacae, and Pseudomonas aeruginosa, In some instances, the MICs were comparable to those determined by a standard broth microdilution method, while in some cases the unique test media and format yielded slightly different values that were themselves reproducible, The propo sed in-flight experiment will include inoculation of the Vitek cards o n the ground prior to launch of the space shuttle, storage of inoculat ed cards at refrigeration temperature aboard the space shuttle until e xperiment initiation, and then incubation of the cards for 18 to 48 h prior to visual interpretation of MICs by the mission's astronauts, Gr ound-based studies have shown reproducible MICs following storage of i noculated cards for 7 days at 4 to 8 degrees C to accommodate the miss ion's time schedule and the astronaut's activities, For comparison, gr ound-based control (normal gravity) MIC values will be generated by si multaneous inoculation and incubation of a second set of test cards in a laboratory at the launch site, This procedure can provide for a saf e and compact experiment that should yield new information on the effe cts of microgravity on the biological activities of various classes of antibiotics.