A 2-0-methylfucose moiety is present in the lipo-oligosaccharide ABSTRACTBradyrhizobiumjaponicum is a soil bacterium that forms nitrogen-fixing nodules on the roots of the agronomically important legume soybean. Microscopic observation of plant roots showed that butanol extract of B. japonicum strain USDA110 cultures induced for nod gene expression elicited root hair deformation, an early event in the modulation process. The metabolite produced by B. japonicum responsible for root hair deformation activity was purified. Chemical analysis of the compound revealed it to be a pentasaccharide of N-acetylglucosamine modified by a Ci8:, fatty acyl chain at the

The chemotactic response of Bradyrhizobium japonicum toward soybean seed and root exudates was examined. Assays using various isoflavones and fractionated exudate indicated that isoflavones are not the principal attractants in exudates. Likewise, induction of nod genes with isoflavones or seed exudate before assay did not enhance chemotaxis. Screening of numerous compounds revealed that only dicarboxylic acids and the amino acids glutamate and aspartate were strong attractants. The presence of glutamate, aspartate, and dicarboxylic acids in appreciable concentrations in soybean seed and root exudates indicates that these compounds likely represent natural chemoattractants for B. japonicum.

Rhizobium fredii USDA 206 harbors four large plasmids, one of which carries nodulation and nitrogen fixation genes. Previously isolated groups of plasmid-cured derivatives of strain USDA 206 were compared with each other to determine possible plasmid functions. Mutant strain 206CANS was isolated as a nonmucoid (Muc-) derivative of strain 206CA, a mutant that was cured of two plasmids. The Mucphenotype of 206CANS was only expressed when the strain was grown on certain media, particularly those with polyols as carbon sources. Plasmid pRj2O6b of strain 206CANS was previously shown to have a higher copy number than the same plasmid in strains USDA 206 and 206CA. When this plasmid was transferred to Muc+ strains, it conferred a nonmucoid phenotype on recipient strains. The symbiotic effectiveness of the wild-type and cured strains was compared. Overall, few differences were shown, but strains 206CA and 206CANS were found to have higher nitrogenase activities than the other strains. Thus, there appeared to be a possible relationship among exopolysaccharide synthesis, plasmid copy number, and symbiotic effectiveness.

Escherichia coli O157:H7 strains ATCC 35150 and ATCC 43894 and five pooled isolates from beef and pork freeze injured at -25 degrees C in beef infusion were used to inoculate ground beef. Samples (25 g each) were added to 225 ml of buffered peptone water with vancomycin, cefsulodin, and cefixime (BPW-VCC), 225 ml of modified EC broth plus novobiocin (mEC+n), and 225 ml of R&F enrichment broth (R&F-EB) and aerobically incubated at 41 to 42 degrees C. After 6, 7, 8, and 24 h of incubation, levels of E. coli O157:H7 recovered from each broth by a PCR assay with the BAX automated system as well as by conventional enrichment with the use of nonaerated mEC+n incubated at 35 degrees C for 24 h were compared with levels recovered by cultural isolation with immunomagnetic separation and plating on BCM E. coli O157:H7 chromogenic agar. For ground beef inoculated with a mean of 4.23 +/- 1.00 total cells (74% freeze injured) per 25 g, after 6 h the PCR assay identified 72.7, 57.6, and 66% of the samples for R&F-EB, BPW-VCC, and mEC+n, respectively, as presumptive positive, whereas the recovery rates after 7 and 8 h exceeded 90%, with the rate for R&F-EB being 100%. For ground beef inoculated with a mean of 1.50 +/- 0.56 total cells (80% freeze injured) per 25 g, after 6 h the PCR assay identified 47.6, 19.1, and 9.5% of the samples for R&F-EB, BPW-VCC, and mEC+n, respectively, as presumptive positive. These values increased to 81.0, 61.9, and 52.4% after 7 h and to 95.2, 61.9, and 71.4% after 8 h. After 24 h, only 55 to 60% of the samples at both inoculum levels tested positive by PCR with conventional enrichment and incubation, whereas >95% of the samples tested positive with R&F-EB aerated at 41 to 42 degrees C. Culture results for R&F-EB and mEC+n after 7 and 8 h of incubation were closely correlated with presumptive positive PCR results.

Nodulation, acetylene reduction activity, dry matter accumulation, and total nitrogen accumulation by nodulated plants growing in a nitrogen-free culture system were used to compare the symbiotic effectiveness of the fast-growing Rhizobium fredii USDA 191 with that of the slow-growing Bradyrhizobium japonicum USDA 110 in symbiosis with five soybean (Glycine max (L.) Merr.) cultivars. Measurement of the amount of nitrogen accumulated during a 20-day period of vegetative growth (28 to 48 days after transplanting) showed that USDA 110 fixed 3.7, 39.1, 4.6, and 57.3 times more N2 than did USDA 191 with cultivars Pickett 71, Harosoy 63, Lee, and Ransom as host plants, respectively. With the unimproved Peking cultivar as the host plant, USDA 191 fixed 3.3 times more N2 than did the USDA 110 during the 20-day period. The superior N2 fixation capability of USDA 110 wlth the four North American cultivars as hosts resulted primarily from higher nitrogenase activity per unit nodule mass (specific acetylene reduction activity) and higher nodule mass per plant. The higher N2-fixation capability of USDA 191 with the Peking cultivar as host resulted primarily from higher nodule mass per plant, which was associated with higher nodule numbers. There was significant variation in the N2-fixation capabilities of the four North American cultivar-USDA 191 symbioses. Pickett 71 and Lee cultivars fixed significantly more N2 in symbiosis with USDA 191 than did the Harosoy 63 and Ransom cultivars. This quantitative variation in N2-fixation capability suggests that the total incompatibility (effectiveness of nodulation and efficiency of N2 fixation) of host soybean plants and R. fredii strains is regulated by more than one host plant gene. These results indicate that it would not be prudent to introduce R. fredii strains into North American agritultural systems until more efficient N2-fixing symbioses between North American cultivars and these fast-growing strains can be developed. When inoculum containing equal numbers of USDA 191 and of strain USDA 110 was applied to the unimproved Peking cultivar in Perlite pot culture, 85% of the 160 nodules tested were occupied by USDA 191. With Lee and Ransom cultivars, 99 and 85% of 140 and 96 nodules tested, respectively, were occupied by USDA 110. Bradyrhizobium japonicum nodulates and fixes nitrogen on soybean roots. It grows slowly, with a generation time of 8 to 20 h. In 1982, Keyser et al. (9) reported a number of strains isolated from a Chinese soybean cultivar Peking (Glycine max (L.) Merr.) and from Glycine soja (L.) Sieb. and Zucc., the wild soybean. These strains grow more rapidly, with a generation time of 2 to 4 h, and have been classified as a new species, Rhizobiumfredii (18). Generally, R. fredii strains fail to form effective symbioses on American soybean cultivars (9). An exception is strain USDA 191 (24), which has been reported to be as effective as B. japonicum 61A76 on three soybean cultivars (Maple Arrow, Harosoy 63 and PI840-7-30) and G. soja (PI342-619B) by Hattori and Johns...