Resistance ofBradyrhizobium japonicum strains to different fungicides and herbicides

Of 50 strains ofB. japonicum, 64%, 44% and 54% were resistant to the fungicides captan, pentachloronitrobenzene and tetramethylthiuram disulphate, respectively, at concentrations above those recommended for treatment of soybeans. 100% and 84% of the strains were resistant to the herbicides trifluralin and metribuzin, respectively, also at concentrations above those recommended for soybeans. No strain was resistant to alachlor above 3.5 g/ml but 40% were sensitive to 2.5 g/ml, these being, respectively, the maximum and minimum concentrations recommended for soybeans.

---

Résumé Parmi 50 souches deBradyrhizobium japonicum, 64%, 44% et 54% sont résistantes respectivement aux agents fongicides suivants: le captan, le pentachloronitrobenzyne et le disulfate de tetraméthylthiurame, à des concentrations supérleures à celles recommandées pour le traitement des sojas. Cent pour cent et 84% des souches sont résistantes respectivement aux herbicides sulvants: le trifluraline et le métribuzine, également à des concentrations supérleures à celles recommandées pour les sojas. Aucune souche ne présente de résistance à l'alachlore au delà de 3.5 g/ml mais 40% sont senslbles jusqu'à 2.5 l/ml, les concentrations maximum et minimum en alachlore, recommandées pour les sojas.

     

Symbiotic interactions between soybean and competing strains ofBradyrhizobium japonicum

Selected symbiotic characteristics of fiveBradyrhizobium japonicum strains were assessed in association with ‘Ransom’ soybean plants (Glycine max [L.] Merr.). In the first of two greenhouse experiments, relative nodulation competitiveness of the strains was examined. Strains were grouped into pairs, and corresponding cells were applied to surface-disinfected seeds so as to provide seven ratios of cell numbers between the two strains. Tap root nodules were harvested 28 days after sowing and serotyped by means of an enzyme-linked immunosorbent assay. Strains differed considerably in nodulation competitiveness, and these differences were successfully quantified using relationships previously proposed in the literature. A second experiment involved assessment of the reproducibility of this technique and characterization of the symbiotic response to single- and double-strain inocula. Differences in relative nodulating abilities of strains were apparent between experiments and were possibly related to observed variations in greenhouse temperatures. Plant shoot weight and total N content were not significantly correlated with nodule number or weight when evaluated across inoculation treatments, but these correlations were often significant within inoculation treatments. Certain double-strain inocula produced either positive or negative effects on shoot weight, N content, and nodulation, when compared with values predicted from corresponding controls receiving single-strain inocula. Paper No. 11741 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

The influence of the culture medium on the competitive abilities ofBradyrhizobium japonicum strains

Growth on culture media results in changes in Rhizobium competitivity:Bradyrhizobium japonicum strain G2, when mixed with the strain GMB 1 in the same ratio, formed 85% of the total nodule numbers when the strains were cultured on the YEM medium, whereas, it formed only 55% of the nodules when they were cultured on the MSY medium.     

Competition between strains ofBradyrhizobium japonicum for nodulation of soybeans at different nitrogen fertilizer levels

Competitive abilities of 3 strains ofBradyrhizobium japonicum (E104, E109, E110) for nodulation of soybean (Glycine max) at increasing nitrogen fertilizer levels were studied. Dry weight of plants nodulated by strain E110 were depressed at 10 g N·m^–2, the highest fertilizer level, even when mixed with strain E109. Strain E104 alone or mixed with E109 increased dry matter production. Strain E110 formed many dually infected nodules with strain E104 present but not with strain E109. However, strain E104 formed nodules containing strain E109. Neither strain E110 or E109 produced bacteriocin, so the incompatibility of these two strains had to be due to another reason. Strain E104 successfully competes with strain E109 but not with E110 at 10 g N·m^–2. It is concluded that strain E110 dominates the symbiotic relationships even if other strains are also present in the nodules. However, at a high N-fertilizer level strain E110 decreases the plant yield in contrast to E104, which could be recommended as inoculant at increased levels of soluble soil-N.     

Stimulation ofBradyrhizobium japonicum by allelochemicals from green plants

Yeast extract-mannitol broth supplemented with 3 mg.ml⁻¹ extract from aerial parts of plants was noculated withB. japonicum 3I1b110. Filter-sterilized water extracts of foxtail (Setaria viridis), lambsquarters (Chenopodium album), Johnsongrass (Sorghum halepense) and Pennsylvania smartweed (Polygonum pensylvanicum) stimulated growth of the bacterium by 3–4 times after 48 h incubation; however, similar extracts from several other plants were marginally stimulatory. Autoclaved water extracts and methanol (75%) extracts were less stimulatory than filter-sterilized water extracts. Autoclaved water extracts of nutsedge (Cyperus esculentus) and ragweed (Ambrosia psilostachya), and methanol extracts of smartweed, peat and ragweed depressed growth of the bacterium. Filter-sterilized water extracts from shoots of foxtail and lambsquarters increased growth of the bacterium linearly up to 6 mg.ml⁻¹. At that level growth in the presence of lambsquarters shoot extract was almost 20 times that of the control. Comparable growth stimulation by this extract was also found in four other strains ofB. japonicum. The water extracts from roots and stems of lambsquarters, and roots, shoots and inflorescences of foxtail at 3 mg.ml⁻¹ stimulated growth of the bacterium by more than 40 times the control.     

Resistance of hospital strains of microorganisms to antibiotics and antiseptics]

The development of resistance of collection and freshly isolated S. aureus and C. albicans strains to the antiseptic decamethoxim and an original diarylcyclohexane derivative was studied comparatively in vitro. It was shown that the rate of the resistance development was low. After 20 subcultures in the presence of increasing concentrations of decamethoxin, its sensitivity of S. aureus and C. albicans decreased 16-32 and 16-fold respectively. After 20 subcultures in the presence of increasing doses of the diarylcyclohexane derivative, its sensitivity S. aureus and C. albicans decreased 4- and 4-8-fold, respectively. It was found that in the hospital strains of S. aureus and C. albicans, the antibiotic resistance and sensitivity to decamethoxin and the diarylcyclohexane remained high, which indicated that there was no cross resistance to these compounds in the strains studied.     

Interaction of neomycin with other antibiotics on selected bacterial strains

Antimicrobial combinations are used most frequently to provide broad-spectrum empirical coverage in the treatment of bacterial infections. However, combination of two antibiotics may not influence their activity, may lead to synergy or antagonism in the activity. Neomycin may be combined with one of the following antibiotics: ampicillin, procaine penicillin, gramicidin, bacitracin, polymyxin B, lincomycin, oxytetracycline, and erythromycin in some human and veterinary multiantibiotic drugs distributed in Poland. The checkerboard method has been one of the traditional assays for the measurement of antibiotic interactions. The aim of this study was to analyse the activity interaction of neomycin with second antibiotic in multiantibiotic drugs distributed in Poland on standards and clinical bacterial strains. Checkerboard results for all strains demonstrated synergism for 2.5% of combinations, only for standards strains. In one case Salmonella Enteritidis, in combination of neomycin with bacitracin, inhibition effect was observed. Additive effects were predominant--49%. In 18% neutral effects were shown, but in 26% of combinations FIC indexes were not possible to calculate, because of the resistance of clinical strains to the highest concentration of at least one antibiotic. In combination of aminoglycoside (neomycin) with beta-lactams antibiotics (ampicillin, procaine penicillin) in vitro, no synergy was observed for all examined strains. The best results were achieved for combinations of neomycin with peptide antibiotics (polymyxin, gramicidin and bacitracin)--5 for all 6 synergy effect observed.     

Resistance of selected strains of Pseudomonas aeruginosa to low-intensity ultraviolet radiation.

The resistances of 10 strains of Pseudomonas aeruginosa and other microorganisms to an ultraviolet (UV) intensity of 100 muW/cm2 were determined. Organisms were exposed in 2- or 15-ml saline suspensions contained in uncapped polyethylene bottles for increasing periods of time, and the surviving fractions were enumerated. Decimal reduction times were calculated by regression analysis, using the least-squares method. The 10 strains of P. aeruginosa, compared with Micrococcus radiodurans and Candida albicans, were very susceptible to low-intensity UV radiation. Results from this study showed that a UV intensity of 100 muW/cm2 penetrated saline suspensions up to 40 mm deep sufficiently to kill high levels of microbial cells, especially P. aeruginosa cells. These results allowed us to design a system for determining and monitoring the sterilization capability of low-intensity UV radiation. In our particular case, UV proved to be an efficient mode for sterilizing saline suspensions of P. aeruginosa in polyethylene bottles. The significance and application of these findings with regard to supporting UV as a sterilant are discussed.     

Sensitivity to selected beta-lactam antibiotics of clinical strains of Escherichia coli and Klebsiella pneumoniae

The studies aimed at analysing the resistance to some beta-lactam antibiotics among E. coli and K. pneumoniae clinical isolates and at evaluating. The extended spectrum of beta-lactamases (ESBL) production in the isolates. The analysis included 137 E. coli strains and 52 K. pneumoniae strains, isolated from hospitalized patients and out-patients treated in the first trimester of 1998. The strains were identified using the ATB computer system. Antibiotic sensitivity of the isolates was determined by disc-diffusion tests. ESBL production capacity of E. coli and K. pneumoniae strains was estimated by double-disc and ATB BLSA tests. Most of the analysed E. coli strains were found to exhibit significant sensitivity to compound penicillin preparations containing beta-lactam inhibitor (Augmentin, Tazocin) and to the third generation cefalosporins, in contrast, K. pneumoniae strains much more frequently were resistant to the drugs. Among the obtained isolates, 3 (2.2%) E. coli strains and 21 (40.4%) K. pneumoniae strains produced ESBL but all the isolates proved sensitive to imipenem. In evaluation of ESBL production-detecting tests, the double-disc test was found to be more reliable than ATB BLSA test.     

Use of central composite experimental design for the development ofBradyrhizobium japonicum liquid inoculants

Central composite experimental designs realized with flask cultures and completed by fermentor studies were used to developB. japonicum liquid inoculants. Two different media are described, using glycerol or glucose, in the presence of only 1 g/l yeast extract. They allow the production of cultures containing more than 10¹⁰ viable cell/ml, able to nodulate soybean efficiently.     

Growth and respiration ofBradyrhizobium japonicum USDA 143 with nitrous oxide as the terminal electron acceptor

Bradyrhizobium japonicum USDA 143 grew chemoorganotrophically under anoxic conditions with exogenous N₂O as the sole terminal electron acceptor. Cell growth and dissimilatory N₂O reduction were significantly inhibited by C₂H₂ when either N₂O or N₂O plus NO ₃ ^– served as terminal electron acceptor(s). Reduction of N₂O accounted for 20% of the energy for cell growth in cultures supplied with NO ₃ ^– as the terminal electron acceptor. Nitrous oxide was produced stoichiometrically in cultures containing NO ₃ ^– and C₂H₂, but cell growth was proportionately reduced when compared with cultures supplied with an equal amount of NO ₃ ^– . Exogenous N₂O delayed the reduction of NO ₃ ^– in cultures supplied with both electron acceptors. Direct amperometric monitoring of N₂O respiration showed a specific activity of 0.082±0.004 moles N₂O/min/mg cell protein, and azide inhibited cell respiration.     

Localization of genes conferring resistance to selected groups of antibiotics in methicillin-resistant strains of Staphylococcus aureus

Localization of genes conferring resistance to MLS, tetracyclines, chloramphenicol, gentamycin and neomycin in 80 MRSA strains isolated from hospital specimens was determined. The obtained results were compared to DNA patterns of the examined strains after digestion with SmaI and separation in pulsed field electrophoresis (PFGE). It was shown that genes of resistance to MLS (ErmI+) in the case of 13 strains were located on chromosome and in the case of 37 strains on plasmids (16 strains had ErmI+ and 21 strains had ErmI-). Genes determining resistance to tetracyclines were localised on chromosome in the case of 39 (23 strains possessed TetK, 11 strains had TetM and 5 strains possessed both TetK and TetM determinants) and in the case of 32 strains on plasmids. Chloramphenicol resistance genes were localised on plasmids in all 30 resistant strains. Genes conferring resistance to gentamycin were present in 31 of the investigated strains on chromosome and in two strains on plasmids. Neomycin resistance genes were plasmid in 34 strains. It was shown that the localization of the resistance genes and the PFGE patterns of the investigated strains were highly correlated.     

Susceptibility to selected antibiotics of Yersinia enterocolitica 03 strains, carrying and not carrying plasmid pYV

A total of 199 clinical strains of Yersinia enterocolitica serotype O3, biotype 4 were tested for their susceptibility to antibiotics (158 strains carried the virulence plasmid pYV and 41 strains did not). 114 isolates were tested by standard disk diffusion method for 21 antibiotics. Almost all tested strains were resistant to ampicillin and cefazolin and susceptible to amoxycillin/clavulanate, cefaclor, cefamandole, cefuroxime, cefotaxime, ceftriaxone, aztreonam, imipenem, gentamicin, amikacin, netilmicin, tetracycline, doxycycline, chloramphenicol, ciprofloxacin, sulphamethoxazole, co-trimoxazole, trimethoprim and furazolidone. In addition minimal inhibitory concentrations (MICs) of 15 antibiotics were determined by agar dilution method for all 199 strains (158 plasmid positive and 41 strains plasmid negative). Third-generation cephalosporins such as cefotaxime and ceftriaxone and a fluoroquinolone (ciprofloxacin) were the most active antimicrobial agents, tested followed by aztreonam, imipenem, trimethoprim, tetracycline, gentamicin, chloramphenicol, amoxycillin/clavulanate, cefaclor, cefuroxime, amikacin, furazolidone and sulphamethoxazole. The present study demonstrated a high susceptibility of clinical strains of Y. enterocolitica to most of the tested antibiotics. In general, there was no significant difference between susceptibility of virulence plasmid pYV positive and virulence plasmid negative strains to antibacterial agents.     

Resistance of Shigella to antibiotics

Antibiotic sensitivity of 104 Shigella clinical strains and 104 Escherichia coli strains isolated from patients with acute dysentery not treated with antibiotics in 1986-1987 was studied. It was shown that 100 per cent of the dysentery pathogens and colon bacilli were antibiotic resistant. Strains resistant simultaneously to chloramphenicol, ampicillin, streptomycin, tetracycline, monomycin and kanamycin were the most frequent among the dysentery pathogens. Colon bacilli and dysentery pathogens isolated from the same patient had specific sets of antibiotic resistance markers. Pathogenetic therapy of dysentery and exclusion of antibiotic use for several years did not result in lower numbers of Shigella antibiotic resistant strains.     

Susceptibility of Klebsiella oxytoca to selected antibiotics

Fifty two clinical isolates of K. oxytoca were included. All of analysed strains were isolated from wound swabs. The aim of this study was to evaluate MIC value of amoxicillin with clavulanic acid, tigecycline and ciprofloxacin. The susceptibility to amoxicillin with clavulanic acid and tigecycline was tested by the Etest. The susceptibility to ciprofloxacine was tested by the agar dilution method. Among of analysed K. oxytoca strains 44 (84.6%) were susceptible to tigecycline, 27 (51.9%) to amoxicilline with clavulanic acid and 21 (40.4%) to ciprofloxacine. These data suggest that tigecycline, may be an effective therapeutic option for the treatment infections caused by K. oxytoca strains.