A total of 15 strains of Mycoplasmawere examined for in vitro sensitivity to 22 commonly used antibiotics and 9 nitrofurans. They were strains of Mycoplasma mycoides var. mycoides, M. mycoides var. capriy M. hyorhinis, M. suipneumoniae (hyopneumoniae), M. granularum, M. cams, M. pulmonis, M. arthritidiSy M. neurolyticum, M. gallisepticum, and M. laidlawii, all of which were isolated from various animals, except for one strain of M. laidlawii which was isolated from sewage. The sensitivity was determined by observing inhibition of growth in the agar and broth dilution systems. Amongall the mycoplasmasexamined, there were no markeddifferences in susceptibility to these drugs, with the exception of erythromycin and oleandomycin. Anti-tumor antibiotics, i. e., actinomycin D and mitomycin C, were the most active of all the agents. Tylosin, bottromycin, spiramycin and tetracycline followed them in activity. Kasugamycin, polymyxin B and colistin were noninhibitory. M.suipneumoniaeå , which is known as the etiological agent of swine enzootic pneumonia (SEP), and other species of respiratory mycoplasmas of swine were compared with regard to minimuminhibitory concentrations (MIC) upon these drugs. The sensitivity of M. suipneumoniae to the drugs used was similar to that of other mycoplasma. Amongthe new nitrofurans tested, drugs with high activity against the mycoplasma were discovered. Manystudies have been made previously on the sensitivity of various mycoplasmas to antimicrobial drugs. Because of the high host specificity of mycoplasmas, strains derived from the same individual host species were chiefly used in such studies.

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Four glucose-nonfermenting Acholeplasma strains were isolated from oral cavities of horses and a horse vagina. The biological and serological properties of these isolates were distinct from those of the eight currently recognized Acholeplasma species. These strains were regarded as belonging to a new species, which was given the name Acholeplasma parvum. Strain H23M was designated the type strain of A. parvum, and a culture of this strain has been deposited in the American Type Culture Collection as strain ATCC 29892.Previously, we reported (14, 24) the isolation of four strains of Acholeplasmu from horses; these strains differed from the previously described species of Acholeplusma by lacking the ability to ferment glucose. This study was undertaken to determine whether these strains constitute a new species. MATERIALS AND METHODSBacterial strains. Strains H7M, H15M, and H23MT (type strain) were isolated from the oral cavities of healthy horses, and strain H23V was isolated from the vagina of a healthy horse. Each strain was purified by picking growth from a well-isolated single colony, inoculating this growth into broth, and, after 4 days of incubation, filtering the resulting broth culture through a 450-nm filter; the filtrate was plated onto a solid medium to obtain isolated colonies. This procedure was repeated at least three times (21). The following type strains of Acholeplusma and Mycoplasma species were used for comparative purposes: Media. The acholeplasmas were cultivated in a medium containing seven parts of PPLO broth without crystal violet (Difco Laboratories, Detroit, Mich.), two parts of unheated horse serum, and one part of 25% (wt/wt) fresh yeast extract; this medium was supplemented with 1% Phytone (BBL Microbiology Systems, Cockeysville, Md.) and 1% penicillin G (100,000 U/ml). A serum-free medium (the medium described above but without serum) was also used. Solid medium was prepared by adding 1.2% granulated agar (catalog no. 11849; BBL) to the liquid medium.Morphological studies. Agar plates were incubated at both 30 and 37°C under aerobic conditions; 5-day-old colonies were observed with a stereomicroscope.For electron microscopy, cells collected by centrifugation from 24-h-old serum-free broth cultures were fixed in a mixture containing 5% glutaraldehyde and 1% paraformaldehyde, postfixed in 1% osmium tetraoxide, and embedded in Epon 812. Ultrathin sections were stained with uranyl acetate and lead nitrate and were examined with a Hitachi HU-12 electron microscope.Reversion experiments. The strains were subcultured five times in liquid medium without penicillin, and each culture was inoculated onto solid medium without bacterial inhibitors.Filtration studies. Filtration tests were performed with 24-h-old cultures of strains H7M and H23MT grown in the serum-free medium. Filterability was determined by using a Swinny hypodermic adapter and membrane filters (Millipore Corp., Bedford, Mass.) with average pore diameters of 800, 450, 220, and 100 nm.Sterol requirement. To determine the sterol r...

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