Presence of a small plasmid in clinical isolates of Streptococcus pneumoniae

Twenty-four of 63 enteric Gram-negative organisms (38.1%) which were isolated from 35 apparently healthy Nigerian students were found to have low trimethoprim resistance (MIC less than 1000 mg/l). These isolates were also found to be resistant to several other antibiotics and trimethoprim resistance was found to be transferable from 15 (62.5%) of the trimethoprim resistant organisms into E. coli EC 1005. It is likely that the high percentage of trimethoprim resistance encountered in this study is related to the high rate of resistance transfer which was observed.     

Curing of the killer character of Saccharomyces cerevisiae with acridine orange

Acridine orange, an intercalating dye usually employed in the curing of bacterial plasmids, was tested for its ability to cure K1 and K2 killer strains (laboratory and wine strains). The results showed a high curing percentage of the killer character. This was demonstrated by the loss of M1 or M2 dsRNAs (responsible for toxin production and resistance to it) and because the meiotic products exhibited non-Mendelian segregation. The curing percentages varied, depending on the strain but not on the killer type, and showed similar efficiency as compared with other known curing agents.     

细菌质粒的消除

利用化学消除剂或改变生长条件可以消除细菌中的质粒,除宿主菌的特性及其所含质粒分子量大小之外,消除率还与消除剂浓度,作用时间有关,嵌合染料适用于消除大肠杆菌中的质粒,十二烷基硫酸钠对具有性纤毛的细菌作用效果较好,适当提高培养温度可消除一些细菌中的质粒,胸腺嘧啶限量法仅适用于其营养缺陷菌株的质粒消除,利用原生质体的形成与再生及反复冻融菌体均可消除细菌中的质粒。     

Enhancement of nodulation by some arid climate strains of <Emphasis Type="Italic">Rhizobium leguminosarum</Emphasis> biovar <Emphasis Type="Italic">trifolii</Emphasis> using protoplast fusion

Fourteen randomly clover indigenous nodulated Rhizobium strains were isolated from different locations in Saudi Arabia. They were identified as different strains of the genus Rhizobium leguminosarum biovar trifolii and characterized for their intrinsic antibiotic resistance against a range of antibiotics, nodulation capability and plasmid profiles. Results revealed the presence of high molecular weight plasmids (megaplasmids) in all the selected strains. Based on the ability for nodulation production, two weak strains (RtI1 and RtI2) and one efficient strain (RtA1) were selected for protoplast fusion and the numbers of nodules produced by the intra-specific protoplast fusion strains were investigated. Results clearly confirmed the effective role of the protoplast fusion in enhancing both nodulation production capacity of Rhizobium species and their range of antibiotic resistance. Protoplast fusion of the local Rhizobium species resulted in 1.93- to 5.67-fold increase in nodulation number compared to their parental strains, which was considered an excellent result concerning agricultural practices, especially the formation of nitrogen-fixing root nodules on legume crop plants. Protoplast fusion also produced fusants with a wide range of antibiotic resistance, another advantage added to the new strains against environmental stresses. In conclusion, protoplast fusion proved its efficiency as a tool for constructing a second generation of Rhizobia with much better characteristics for efficient applications in arid land.     

Sensitivity to detergents and plasmid curing in Enterococcus faecalis

This research reports the sensitivity of a clinical isolate of Enterococcus faecalis to sodium N-lauroylsarcosinate (sarkosyl) and sodium dodecyl sulfate (SDS), as well as the efficiency of these detergents in curing the strain. Compared to Escherichia coli, Enterococcus faecalis was very sensitive to both detergents, with minimum inhibitory concentrations (MIC) for the latter being 100 times lower than for Escherichia coli. The clinical isolate of Enterococcus faecalis used in this study exhibited plasmid-borne resistance to kanamycin (MIC 2 mg/ml) and tetracycline (MIC 50 μg/ml); 3% curing was observed after growth in the presence of sarkosyl but no curing was observed after growth in the presence of either SDS or acridine orange. In contrast, 35% curing of plasmid-bearing Escherichia coli was observed after growth in the presence of either SDS or acridine orange, but none was observed after growth in the presence of sarkosyl.     

两种牙体充填材料对乳牙边缘微渗漏及固化效率的体外研究

目的:比较纳米瓷充填材料和复合树脂充填乳牙V类洞边缘微渗漏的影响以及固化效率的差异。方法:第一部分为微渗漏实验,选取滞留拔除的上颌乳前牙20颗,唇侧制备V类洞,随机分为A、B两组,分别用Composan LCM和Filtek~(TM) Z350充填,亚甲基蓝染色处理,24 h后使用体式显微镜观察,测量染料渗入深度为微渗漏值,每组各选2个样本扫描电镜观察充填体与牙体组织间粘结情况。第二部分为显微硬度实验,分别用两种充填材料制作模块各9个,记为A、B两组,材料模块光固化24 h后使用显微硬度仪检测充填材料的固化效率。结果:Composan LCM组微渗漏值明显低于Filtek~(TM)Z350组(P0.05);扫描电镜下观察Filtek~(TM) Z350组充填材料与牙体组织间可见裂缝,界面参差不齐,有不连续的小断裂,Composan LCM组界面连续均匀,未见明显裂缝;两组充填材料的固化效率无显著差异(P0.05)。结论:纳米瓷充填材料和复合树脂均可用于乳牙龋洞充填,使用纳米瓷充填材料可减少微渗漏。     

Regulation of cell-wall morphology and growth encoded by plasmids in Shigella dysenteriae type 1

Shigella dysenteriae type 1, isolated locally, was found to contain six plasmids and these could be eliminated using SDS. A 70-kb plasmid was necessary to maintain the normal cell-wall morphology. Synthesis of nine major membrane proteins (90 to 40 kDa) was severely impaired in all-plasmid cured strains. Electron microscopy revealed a prominent separation between the outer and inner membranes of the cured strains, indicating that plasmid loss led to defects in the cell envelope. The growth rates of the strains having only the 70-kb plasmid and the plasmidless strain were 3- to 30-fold less than in the wild type strain.The authors are with the National Institute of Cholera and Enteric Diseases, P-33, C.I.T. Road, Scheme XM, Beliaghata, Calcutta-700 010, India     

Octosporea bayeri: fumidil B inhibits vertical transmission in Daphnia magna

Microsporidia are a highly successful and ecologically diverse group of parasites, and thus represent interesting model systems for research on host-parasite interactions. However, such research often requires the ability to cure hosts of infections, a difficult task, given the short lifespan of most invertebrates and the efficient vertical transmission of some parasites. To our knowledge, few treatments are available to cure microsporidiosis in invertebrate hosts, and protocols have not yet been developed to inhibit vertical transmission and thereby cure host lines. We present a protocol for inhibiting vertical transmission of the microsporidian parasite Octosporea bayeri in the freshwater crustacean Daphnia magna. We used 100 mg/L Fumidil B dissolved in the culture medium of the host. This technique allowed Daphnia to survive and reproduce and inhibited vertical transmission of the parasite. The method presented here may be of general interest for other aquatic host-parasite systems involving microsporidia.     

Pseudomonas C12B, an SDS degrading strain, harbours a plasmid coding for degradation of medium chain length n-alkanes

Pseudomonas C12B is able to degrade alkyl sulfates, alkylbenzene sulfonates, and linear alkanes and alkenes. Mitomycin C curing experiments and conjugation experiments demonstrated that the ability to utilize n-alkanes (C₉–C₁₂) and n-alkenes (C₁₀ and C₁₂) of medium chain length was plasmid-encoded. The plasmid was designated pDEC. Its size was estimated at several hundreds kb according to mobility in agarose gels. The plasmid did not confer resistance to the antibiotics tested. Analysis of alkylsulfatases P1 and P2 in original and cured strains confirmed that both enzymes are encoded by the chromosome. The ability of Pseudomonas C12B to utilize alkylbenzene sulfonates also appears to be encoded by the chromosome. pDEC could be transferred only to cured derivatives of Pseudomonas C12B, but not to strains of P. aeruginosa, P. putida, or Acinetobacter sp. Cured derivatives of Pseudomonas C12B could not serve as hosts for the broad host range plasmid CAM–OCT. The enzyme system encoded by the putative dec genes present on plasmid pDEC differs from the system coded by the alk genes of plasmid OCT in the size range of hydrocarbons preferentially used.