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MotA contains a conserved C-terminal cluster of negatively charged residues, and MotB contains a conserved N-terminal cluster of positively charged residues. Charge-altering mutations affecting these residues impair motility but do not diminish Mot protein levels. The motility defects are reversed by second-site mutations targeting the same or partner protein.The MotA 4 MotB 2 complex of Escherichia coli is the stator of the rotary flagellar motor (5) and the H ϩ channel that couples the proton motive force to motility (1,9,(11)(12)(13)(14)23). At least 11 complexes can be accommodated per motor (17). MotA spans the cytoplasmic membrane four times and has short periplasmic loops between transmembrane segment 1 (TM1) and TM2 and between TM3 and TM4 (3, 21). Large cytoplasmic domains occur between TM2 and TM3 and following TM4. MotB has a cytoplasmic N terminus of 25 residues, a single TM domain, and a periplasmic domain with a conserved peptidoglycan-binding motif (2, 18) that anchors the Mot complex to the cell wall (4, 10).We asked whether electrostatic interactions between negatively charged residues near the C terminus of MotA and positively charged residues near the N terminus of MotB facilitate formation of the MotA 4 MotB 2 complex for three reasons. (i) MotB is unstable in the absence of MotA (20), so coinsertion into the membrane and rapid complex assembly might be advantageous. (ii) Translation of motA and motB is coupled (20). Thus, the initial proximity of the C terminus of MotB and the N terminus of MotB would facilitate their early interaction. (iii) Clusters of charged residues in MotA and MotB are conserved.

Background: NeoSeekTM STEC is a single-source, service-based method for detection and identification of select Shiga toxin–producing Escherichia coli (STEC), including E. coli O157:H7 and STEC of somatic groups O26, O45, O103, O111, O121, and O145. The method is a multiplex molecular method utilizing more than 80 genetic targets to identify STEC in complex matrices such as food enrichment cultures. Objective: A study was conducted to validate the NeoSeek method for detection of select STEC in raw beef trim. Methods: Performance of the NeoSeek STEC method was compared with that of the U.S. Department of Agriculture, Food Safety and Inspection Service reference methods for E. coli O157:H7 and non-O157 STEC for detection of E. coli O157:H7 and E. coli O26:H11 in raw beef trim. Additionally, inclusivity/exclusivity testing and method robustness testing were performed. Results: Results of raw beef trim testing showed no statistically significant differences in performance between the NeoSeek and reference methods in the ability to detect either E. coli O157:H7 or E.coli O26:H11, as determined by probability of detection analysis. Results of inclusivity and exclusivity testing showed 100% expected results with target and nontarget bacteria, with the exception of a single strain of E. coli O157:H7, which was subsequently verified to be stx-negative by PCR. Conclusions and Highlights: NeoSeek STEC is an accurate, reliable method for rapid detection and identification of select STEC in complex populations such as beef trim enrichment cultures.

ANSR Listeria is a new diagnostic assay for detection of Listeria spp. in sponge or swab samples taken from a variety of environmental surfaces. The method is an isothermal nucleic acid amplification assay based on the nicking enzyme amplification reaction technology. Following single-step sample enrichment for 16-24 h, the assay is completed in 40 min, requiring only simple instrumentation. In inclusivity testing, 48 of 51 Listeria strains tested positive, with only the three strains of L. grayi producing negative results. Further investigation showed that L. grayi is reactive in the ANSR assay, but its ability to grow under the selective enrichment conditions used in the method is variable. In exclusivity testing, 32 species of non-Listeria, Gram-positive bacteria all produced negative ANSR assay results. Performance of the ANSR method was compared to that of the U.S. Department of Agriculture-Food Safety and Inspection Service reference culture procedure for detection of Listeria spp. in sponge or swab samples taken from inoculated stainless steel, plastic, ceramic tile, sealed concrete, and rubber surfaces. Data were analyzed using Chi-square and probability of detection models. Only one surface, stainless steel, showed a significant difference in performance between the methods, with the ANSR method producing more positive results. Results of internal trials were supported by findings from independent laboratory testing. The ANSR Listeria method can be used as an accurate, rapid, and simple alternative to standard culture methods for detection of Listeria spp. in environmental samples.