Many bacteria use N-acyl homoserine lactone (AHL) signals to coordinate the behavior of individual cells in a local population. The successful infection of eukaryotic hosts by bacteria seems to depend particularly on such AHL-mediated ''quorum-sensing'' regulation. We have used proteome analysis to show that a eukaryotic host, the model legume Medicago truncatula, is able to detect nanomolar to micromolar concentrations of bacterial AHLs from both symbiotic (Sinorhizobium meliloti) and pathogenic (Pseudomonas aeruginosa) bacteria, and that it responds in a global manner by significant changes in the accumulation of over 150 proteins, 99 of which have been identified by peptide mass fingerprinting. The accumulation of specific proteins and isoforms depended on AHL structure, concentration, and time of exposure. AHLs were also found to induce tissue-specific activation of ␤-glucuronidase (GUS) reporter fusions to an auxin-responsive and three chalcone synthase promoters, consistent with AHL-induced changes in the accumulation of auxin-responsive and flavonoid synthesis proteins. In addition, exposure to AHLs was found to induce changes in the secretion of compounds by the plants that mimic quorum-sensing signals and thus have the potential to disrupt quorum sensing in associated bacteria. Our results indicate that eukaryotes have an extensive range of functional responses to AHLs that may play important roles in the beneficial or pathogenic outcomes of eukaryote-prokaryote interactions.

Methicillin resistance in Staphylococcus aureus can be difficult to recognize because it may be expressed in only a small portion of the population. The optimal reference to use in comparing different test systems is controversial. In this study MIC values of less than or equal to 4 mg/l as observed in two experiments, each including 12 test systems (24 h incubation), were used to define strains as methicillin-susceptible. This strict standard was used to compare 24 quantitative and 36 agar diffusion test systems applied to 61 strains. These test systems included six agar media (Mueller Hinton, Oxoid and BBL; and Iso-Sensitest, Oxoid; all with and without 5% NaCl), three broth media of the same type, two incubation temperatures (30 degrees C and 37 degrees C), two incubation times (24 h and 48 h) and five disc types [methicillin 10 micrograms (M 10), three types of methicillin 5 micrograms (M 5) and oxacillin 1 microgram (O 1)]. Standardized high inocula were used. It was concluded that 24 h incubation is preferable to 48 h; at 24 h, differences in recognition of methicillin-resistant Staphylococcus aureus (MRSA) among 12 quantitative test systems were not statistically significant; incubation at 30 degrees C gave slightly better results in diffusion tests. Reproducibility of quantitative data (Friedman test) was better on Mueller Hinton media (p less than 0.001) than on Iso-Sensitest medium in both dilution and agar diffusion test systems. The rank order of discs was M 5 greater than M 10 greater than O 1 (p less than 0.001). An inhibition zone of less than 13 mm with a M 5 micrograms (BBL) disc should be interpreted as MRSA.

Single and combined resistance to quinolones and beta-lactams was determined after serial transfers of 18 selected strains of Pseudomonas aeruginosa (5), Acinetobacter calcoaceticus (3) and beta-lactamase producing enterobacterial strains (10), in broth dilutions of 4 quinolones and 8 beta-lactams. Two definitions for resistance were used: (I) 8-fold MIC increase and stability of acquired resistance after five transfers in drug-free broth; and (II) 8-fold MIC increase over the breakpoint level. Using definition I, after transfers on a beta-lactam, resistance to one or more beta-lactams was noted in 45%, to one or more quinolones in 7% of strains. After transfers on a quinolone, the frequency of resistance to quinolones was 82%, to beta-lactams 26%. When all tests were counted separately, the resistance percentages were lower, but they showed the same trend. Calculation according to definition II showed quinolone resistance in 5.4% of all tests after transfer on a beta-lactam and beta-lactam resistance in 23% after transfer on a quinolone. Serial transfers on imipenem led to fewer cases of resistance (13%) to beta-lactams than transfers on any other beta-lactam (19–39%). There were no conclusive differences between the 7 other beta-lactams.

By means of a micro-dilution technique (10(5) cfu/cup) 30 strains of Pseudomonas spp. were found resistant to Sch 34343. The susceptibility of Acinetobacter spp. (20 strains) was greatest for imipenem, followed by Sch 34343 (MIC approximately 0.7 mg/l), ceftazidime and ceftriaxone. Ten Moraxella strains were very susceptible to Sch 34343, imipenem and ceftazidime. For 35 strains of Neisseria gonorrhoeae (ten beta-lactamase-positive) and ten strains of N. meningitidis we noted no resistance to Sch 34343 (MIC 0.015-0.06 mg/l), or to any of the other drugs tested. Sixty-two strains of Enterobacteriaceae producing various beta-lactamases were tested for their sensitivity to Sch 34343 and seven other beta-lactams. To an even greater extent than imipenem and Sch 34343, latamoxef and ceftazidime proved very active against all these strains. A PSE-4-producing Escherichia coli strain was resistant to Sch 34343. The inoculum effect (10(6) vs 10(4) cfu/ml) on Sch 34343 activity was usually small (1-4). Tests for the beta-lactamase susceptibility of Sch 34343 showed that only one PSE-3, one PSE-4 and one chromosomal beta-lactamase from a strain of Enterobacter cloacae were slightly active.

The ABAC system for antibiotics susceptibility testing was compared with an agar diffusion method in 14 960 tests, including 23 antibacterial agents. Identical breakpoints were used. Only 3% major discrepancies (M.d.; sensitive vs resistant) and 19% minor discrepancies (m.d.; intermediate vs sensitive or resistant) were noted. Major discrepancies were mainly found for methicillin (Staphylococcus aureus), netilmicin (Pseudomonas aeruginosa), chloramphenicol, sulphamethoxazole and trimethoprim (Proteus sp.) and were checked by quantitative susceptibility tests. These showed ABAC to be at fault in 41--47% of discrepancies, the diffusion test in 21--32% and 21--37% were intermediate. Half of the m.d. involved beta-lactams, which is explained by two low breakpoints. Except for methicillin and netilmicin the overall results showed ABAC to be equal to the agar diffusion method. Technical faults, like leakage and incorrect filling of cups in the plastic rotors of ABAC, occurred in 14% of the rotors.