Broad-spectrum antibiotic resistance of Planctomycetes organisms determined by Etest

The in vitro susceptibility of Planctomycetes organisms to antibiotics has seldom been studied and when it has, a variety of methods have been used. The objective of the study was to expand the knowledge of Planctomycetes antibiotic susceptibility patterns. Planctomyces maris, Planctomyces brasiliensis, Blastopirellula marina, Planctomyces limnophilus, Gemmata obscuriglobus and Rhodopirellula baltica reference strains were tested for in vitro susceptibility to 18 antibiotics, representing 11 antibiotic families, using the Etest method. All Planctomycetes organisms were found to be resistant to beta-lactams, with MICs of > 32 mg/L for penicillin G and imipenem, and MICs of > 256 mg/L for ampicillin, cefalotin and ceftriaxone. The organisms were resistant to nalidixic acid and vancomycin (MIC > 256 mg/L), but susceptible to tetracycline (MICs < 0.016-0.5 mg/L) and doxycycline (MICs < 0.016-1 mg/L). The MIC of gentamicin ranged from 1 mg/L (P. limnophilus) to > 256 mg/L (B. marina and P. brasiliensis); the MIC of erythromycin ranged from 0.032 mg/L (P. limnophilus) to 2 mg/L (P. brasiliensis); the MIC for ciprofloxacin ranged from 0.008 mg/L (R. baltica) to > 32 mg/L (P. brasiliensis); and the MIC for colistin ranged from 0.125 mg/L (P. limnophilus) to 96 mg/L (B. marina). In addition to shedding new light on the biology of Planctomycetes organisms, these data could be used for the further phenotypic characterization of Planctomycetes organisms, and for the optimization of culture media for the primary isolation and growth of Planctomycetes organisms from contaminated specimens.