SURVEY OF ANTIMICROBIAL SUSCEPTIBILITY OF MODERATELY HALOPHILIC EUBACTERIA AND EXTREMELY HALOPHILIC AEROBIC ARCHAEOBACTERIA - UTILIZATION OF ANTIMICROBIAL RESISTANCE AS A GENETIC-MARKER

The susceptibility patterns of 94 moderately halophilic eubacteria and 24 extremely halophilic archaeobacteria to 22 antimicrobials were sur veyed by using an agar dilution method. The moderate halophiles includ ed 16 culture collection strains and 78 fresh isolates, representative s of Vibrio costicola, Deleya halophila, Deleya salina, Volcaniella eu ryhalina, and Chromohalobacter marismortui. On the basis of their MICs , the moderately halophilic collection strains showed very heterogeneo us responses to the majority of antimicrobials tested and especially t o ampicillin, cephalotin, and penicillin G. However, they showed a sim ilar high tolerance to amynoglycosides, spectinomycin, and sulfonamide , and a high sensitivity to rifampicin, which was the most active inhi bitor. The responses of the fresh isolates were also very heterogeneou s and, in general, different susceptibility patterns to those showed b y the corresponding collection strains were found. A general sensitivi ty of the halobacteria towards anysomycin, aphidicolin, rifampicin, an d to novobiocin was found. This latter antibiotic showed the highest t oxicity since MIC values as low as 0.06 mu g/ml were obtained. In cont rast, the response to the other antibiotics was very heterogeneous. Th e frequencies of spontaneous mutation of resistance to different antib iotics for some selected collection strains have been determined for g enetic purposes. For moderate halophiles, optimal frequencies were fou nd with respect to streptomycin, rifampicin and chloramphenicol. On th e other hand, the resistances to anysomicin, josamycin, monensin, and rifampicin appeared as the most suitable for halobacteria.