Shiga toxin-producing Escherichia coli in sheep and pre-slaughter lambs in eastern Australia

Sheep and lambs from 14 farms in southern Queensland and one from central New South Wales were surveyed to determine the prevalence of Shiga toxin-producing Escherichia coli (STEC). STEC, isolated from 45% of 144 sheep faeces collected on the farms and 36% of 72 lamb faeces from abattoir yards, were tested for the presence of genes encoding virulence factors. Most (64%) of the 117 STEC isolates contained Shiga toxin 1 and 2 genes, 22% contained those encoding Shiga toxin 1, and 14% contained genes encoding Shiga toxin 2. The genes encoding the E. coli attaching and effacing factor were present in 2.6% of STEC and 26% contained the enterohaemolysin gene. The isolates that contained the E. coli attaching and effacing gene were serotype O157:H. This study has shown that STEC are widely distributed in eastern Australian sheep and lambs and are shed in their faeces prior to slaughter. Thus, there is potential for contamination of carcasses and entry of STEC into the human food chain.     

Complex binding interactions between multicomponent mixtures and odorant receptors in the olfactory organ of the Caribbean spiny lobster Panulirus argus

Our study was designed to examine how components of complex mixtures can inhibit the binding of other components to receptor sites in the olfactory system of the spiny lobster Panulirus argus. Biochemical binding assays were used to study how two- to six-component mixtures inhibit binding of the radiolabeled odorants taurine, L-glutamate and adenosine-5'-monophosphate to a tissue fraction rich in dendritic membrane of olfactory receptor neurons. Our results indicate that binding inhibition by mixtures can be large and is dependent on the nature of the odorant ligand and on the concentration and composition of the mixture. The binding inhibition by mixtures of structurally related components was generally predicted using a competitive binding model and binding inhibition data for the individual components. This was not the case for binding inhibition by most mixtures of structurally unrelated odorants. The binding inhibition for these mixtures was generally smaller than that for one or more of their components, indicating that complex binding interactions between components can reduce their ability to inhibit binding. The magnitude of binding inhibition was influenced more by the mixture's precise composition than by the number of components in it, since mixtures with few components were sometimes more inhibitory than mixtures with more components. These findings raise the possibility that complex binding interactions between components of a mixture and their receptors may shape the output of olfactory receptor neurons to complex mixtures.      

Nucleotide sequence of the nifH gene coding for nitrogen reductase in the acetic acid bacterium Acetobacter diazotrophicus

The nifH gene sequence of the nitrogen-fixing bacterium Acetobacter diazotrophicus was determined with the use of the polymerase chain reaction and universal degenerate oligonucleotide primers. The gene shows highest pair-wise similarity to the nifH gene of Azospirillum brasilense . The phylogenetic relationships of the nifH gene sequences were compared with those inferred from 16S rRNA gene sequences. Knowledge of the sequence of the nifH gene contributes to the growing database of nifH gene sequences, and will allow the detection of Acet. diazotrophicus from environmental samples with nifH gene-based primers.     

Dynamics of Shiga toxin-producing Escherichia coli (STEC) in feedlot cattle

Feedlot cattle were monitored during fattening to determine changes in faecal shedding of Shiga toxin-producing Escherichia coli (STEC) and their relation to the coliform population. Faecal samples were enriched, screened for Shiga toxin genes (stx) by a polymerase chain reaction test and isolated using colony hybridization. During 117 d in the feedlot, there were differences in the numbers of coliforms shed and in the percentage of samples positive for stx. These fluctuations did not appear to be consistently related to changes in feed or time in the feedlot. The mean log coliform count for stx-positive samples (log 5.85 g-1) was similar to that for stx-negative samples (log 6.00 g-1). The STEC isolates obtained from the first 5 d in the feedlot belonged to eight serotypes. Later, one serotype (O136:H16) became the predominant STEC which appeared to be one clone as characterized by virulence determinants and pulsed-field gel electrophoresis.     

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.