Restoration of mountain big sagebrush steppe following prescribed burning to control western juniper.

Western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroachment into mountain big sagebrush (Artemisia tridentata spp. vaseyana (Rydb.) Beetle) steppe has reduced livestock forage production, increased erosion risk, and degraded sagebrush-associated wildlife habitat. Western juniper has been successfully controlled with partial cutting followed by prescribed burning the next fall, but the herbaceous understory and sagebrush may be slow to recover. We evaluated the effectiveness of seeding perennial herbaceous vegetation and sagebrush at five sites where juniper was controlled by partially cutting and prescribed burning. Treatments tested at each site included an unseeded control, herbaceous seed mix (aerially seeded), and the herbaceous seed mix plus sagebrush seed. In the third year post-treatment, perennial grass cover and density were twice as high in plots receiving the herbaceous seed mix compared to the control plots. Sagebrush cover and density in the sagebrush seeded plots were between 74- and 290-fold and 62- and 155-fold greater than the other treatments. By the third year after treatment, sagebrush cover was as high as 12 % in the sagebrush seeded plots and between 0 % and 0.4 % where it was not seeded. These results indicate that aerial seeding perennial herbaceous vegetation can accelerate the recovery of perennial grasses which likely stabilize the site. Our results also suggest that seeding mountain big sagebrush after prescribed burning encroaching juniper can rapidly recover sagebrush cover and density. In areas where sagebrush habitat is limited, seeding sagebrush after juniper control may increase sagebrush habitat and decrease the risks to sagebrush-associated species.

Comparing burned and mowed treatments in mountain big sagebrush steppe.

Fires in mountain big sagebrush [Artemisia tridentata spp. vaseyana (Rydb.) Beetle] plant communities historically shifted dominance from woody to herbaceous vegetation. However, fire return intervals have lengthened with European settlement, and sagebrush dominance has increased at the expense of herbaceous vegetation in some plant communities. Management actions may be needed to decrease sagebrush in dense sagebrush stands to increase herbaceous vegetation. Prescribed fire is often used to remove sagebrush; however, mechanical treatments, such as mowing, are increasingly used because they are more controllable and do not pose an inherent risk of escape compared with fire. However, information on the effects of burned and mowed treatments on herbaceous vegetation and whether fire and mowed applications elicit similar vegetation responses are limited. We evaluated the effects of prescribed burning and mowing for 3 years after treatment in mountain big sagebrush plant communities. The burned and mowed treatments generally increased herbaceous cover, density, and production compared with untreated controls (P < 0.05). However, neither treatment induced a response in native perennial forb cover, density, or biomass (P > 0.05). In contrast, annual forb (predominately natives) cover, density, and biomass increased with mowing and burning (P < 0.05). Vegetation generally responded similarly in burned and mowed treatments; however, the burned treatment had less sagebrush, greater herbaceous vegetation production, and more bare ground than the mowed treatment (P < 0.05). These differences should be considered when selecting treatments to decrease sagebrush.

Interaction of historical and nonhistorical disturbances maintains native plant communities.

Historical disturbance regimes are often considered a critical element in maintaining native plant communities. However, the response of plant communities to disturbance may be fundamentally altered as a consequence of invasive plants, climate change, or prior disturbances. The appropriateness of historical disturbance patterns under modern conditions and the interactions among disturbances are issues that ecologists must address to protect and restore native plant communities. We evaluated the response of Artemisia tridentata ssp. wyomingensis (Beetle & A. Young) S.L. Welsh plant communities to their historical disturbance regime compared to other disturbance regimes. The historical disturbance regime of these plant communities was periodic fires with minimal grazing by large herbivores. We also investigated the influence of prior disturbance (grazing) on the response of these communities to subsequent disturbance (burning). Treatments were: (1) ungrazed (livestock grazing excluded since 1936) and unburned, (2) grazed and unburned, (3) ungrazed and burned (burned in 1993), and (4) grazed and burned. The ungrazed-burned treatment emulated the historical disturbance regime. Vegetation cover, density, and biomass production were measured the 12th, 13th, and 14th year post-burning. Prior to burning the presence of Bromus tectorum L., an exotic annual grass, was minimal (<0.5% cover), and vegetation characteristics were similar between grazed and ungrazed treatments. However, litter accumulation was almost twofold greater in ungrazed than in grazed treatments. Long-term grazing exclusion followed by burning resulted in a substantial B. tectorum invasion, but burning the grazed areas did not produce an invasion. The ungrazed-burned treatment also had less perennial vegetation than other treatments. The accumulation of litter (fuel) in ungrazed treatments may have resulted in greater fire-induced mortality of perennial vegetation in ungrazed compared to grazed treatments. Our results demonstrate that prior disturbances exert a strong influence on the response of plant communities to subsequent disturbances and suggest that low-severity disturbances may be needed in some plant communities to increase their resilience to more severe disturbances. Modern deviations from historical conditions can alter ecosystem response to disturbances, thus restoring the historical disturbance regime may not be an appropriate strategy for all ecosystems.

Linking nitrogen partitioning and species abundance to invasion resistance in the Great Basin.

Resource partitioning has been suggested as an important mechanism of invasion resistance. The relative importance of resource partitioning for invasion resistance, however, may depend on how species abundance is distributed in the plant community. This study had two objectives. First, we quantified the degree to which one resource, nitrogen (N), is partitioned by time, depth and chemical form among coexisting species from different functional groups by injecting (15)N into soils around the study species three times during the growing season, at two soil depths and as two chemical forms. A watering treatment also was applied to evaluate the impact of soil water content on N partitioning. Second, we examined the degree to which native functional groups contributed to invasion resistance by seeding a non-native annual grass into plots where bunchgrasses, perennial forbs or annual forbs had been removed. Bunchgrasses and forbs differed in timing, depth and chemical form of N capture, and these patterns of N partitioning were not affected by soil water content. However, when we incorporated abundance (biomass) with these relative measures of N capture to determine N sequestration by the community there was no evidence suggesting that functional groups partitioned different soil N pools. Instead, dominant bunchgrasses acquired the most N from all soil N pools. Consistent with these findings we also found that bunchgrasses were the only functional group that inhibited annual grass establishment. At natural levels of species abundance, N partitioning may facilitate coexistence but may not necessarily contribute to N sequestration and invasion resistance by the plant community. This suggests that a general mechanism of invasion resistance may not be expected across systems. Instead, the key mechanism of invasion resistance within a system may depend on trait variation among coexisting species and on how species abundance is distributed in the system.

Sodium chloride enhances recovery and growth of acid-stressed E. coli O157:H7.

AIMS: Combinations of sodium chloride and acid are frequently used to inhibit growth of spoilage and pathogenic bacteria in food. The influence of differing sodium chloride, lactate and pH values on the growth of stressed and unstressed cells of a non-toxigenic strain of Escherichia coli O157:H7 was studied. METHODS AND RESULTS: At pH 5.5 or 6.0, there was little or no effect on the growth rate in the presence of lactate and/or sodium chloride, but the lag times were longer as the lactate concentration increased. At pH 5.0, in the absence of sodium chloride, increasing the lactate concentration increased the growth rate and the lag time; no growth occurred in the presence of 1.5 g 100 g(-1) lactate. In the presence of 4-6 g 100 g(-1) sodium chloride, growth occurred at 1.5 g 100 g(-1) lactate. The growth rate was similar at all lactate concentrations. CONCLUSION: The results demonstrate that the presence of sodium chloride promoted growth of E. coli O157:H7, especially under stressful conditions of low pH. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings could have implications for the use of acid and sodium chloride as a preservation treatment for the inhibition of E. coli O157:H7 in food.

Bacterial thermal death kinetics based on probability distributions: the heat destruction of Clostridium botulinum and Salmonella Bedford.

Despite the long history and excellent record of inactivation models used in thermal processing, there are relatively few approaches that attempt to describe the kinetics commonly observed. There are even fewer examples of models that allow the user to deal with the environmental conditions that influence these kinetics. We describe an approach that assumes a distribution of inactivation times within a population of bacterial cells. The concept allows for alternative interpretations of death kinetics and provides excellent descriptions of data generated with two important foodborne pathogens, Clostridium botulinum and Salmonella Bedford. The Salmonella Bedford data set used is unusual and perhaps unique in that it provides information where more than 50% of the population survival has been measured. These measurements are often overlooked or missed in experimental work but are essential when using a vitalistic approach, enabling calculation of a 50% lethal dose for destruction of bacteria. Use of the normal or Prentice distribution provided better fits to the data than other models commonly used to describe thermal death. There was no obvious bias in the fits even though significant tailing was evident. In addition, the procedure described allows data from all the conditions to be fitted rather than individual independent series. This enables a single equation to be derived that can be judged against the whole domain of the data. Approaches that provide accurate and unbiased descriptions of thermal death are likely to become increasingly important to ensure the safety of more marginal heat processes.

Survey of serum potassium reference measurements.

We compared the quality of reference measurements for serum potassium in four reference laboratories from three different European countries, using a panel of 60 native patients' samples. The reference methods were based on either ion chromatography (one laboratory) or flame atomic emission spectrometry (three laboratories). Performance specifications for serum potassium measurements were defined as a maximum overall coefficient of variation (CV) of 1.5%, a maximum bias of 0.65% and a maximum total error of 3.0%. The overall imprecision for all laboratories was in the range of 0.7 to 1.3%, and was thus below the proposed specification of 1.5%. However, two laboratories reported 12 and 13 quadruplicates with CVs exceeding this limit. The mean bias (expressed as deviation from the overall mean of all laboratories) for all reference laboratories was < 0.65%. In the lower concentration range, however, one laboratory exceeded this limit. No laboratory measured samples with a total error above 3.0%. From these results, it can be concluded that the reference measurements, and, thus, also the reference methodologies, based on ion chromatography and flame atomic emission spectrometry were equivalent, and able to satisfy current analytical specifications for serum potassium measurements.

Quantitative microbiological risk assessment: principles applied to determining the comparative risk of salmonellosis from chicken products.

Ensuring microbiological safety requires identification of realistic hazards and the means of controlling them. The risk assessment framework proposed by Codex Alimentarius allows the impact of raw materials and processes to be appreciated, and the output can be used for risk management and communication. Mathematical models allow numerical information to be processed by a computer and interpreted to give quantitative or comparative risk assessments. In this example, models have been put together according to the Codex. Alimentarius principles, providing a quantitative risk assessment (QRA) of salmonellosis from frozen poultry products. This model-based QRA takes into account three types of information: occurrence and distribution of the agent, sensitivity of populations to infection (e.g., normal or susceptible), and the effect of cooking (in the factory or home) on concentration of the agent and hence risks of infection after product consumption. It only demonstrates the impact of a single-process step (heating) and the effect of changes in population sensitivity, raw material quality, and cooking regime on the final risk. The effects of growth and recontamination are not considered. To aid risk communication, the models have been visualized by means of displays and slider controls on a computer screen because effective communication is essential to encourage manufacturers and their product designers to assess the effect of changes in processing or materials on risk.

The use of an automated growth analyser to measure recovery times of single heat-injured Salmonella cells.

A new approach to the study of recovery times of single heat-injured Salmonella cells is described. It comprises the generation of a standard heat-injured culture, serial dilution of this culture to near extinction, inoculation of the serial dilutions across many microtitre plates and measurement of the subsequent recovery and growth using an automated turbidometric analyser. Lag times for individual cells were estimated from turbidity data using a model that accurately extrapolated the growth curve back to the starting inoculum level. Lag times were compared using a number of different commercially available pre-enrichment media. The most typical result was a very broad distribution of lag times at the single cell inoculum level, with many values in excess of 20 h. Even at an inoculum level 10-fold higher, lag times for some injured cells were estimated to be > 10 h. More significantly, it was found that some media recovered more injured cells than others and vice versa. Between the worst and best media there were as many as 3 log10 cycles difference in the number of cells recoverable. No trends were apparent linking choice of medium with performance. The implications of these findings, in relation to traditional and rapid methodology, are discussed.

An example of the stages in the development of a predictive mathematical model for microbial growth: the effects of NaCl, pH and temperature on the growth of Aeromonas hydrophila.

The stages involved in developing a predictive model are illustrated using data describing the effects of temperature (3-20 degrees C), NaCl concentration (0.5-4.5% w/v) and pH (4.6-7.0) on the aerobic growth of Aeromonas hydrophila (cocktail of 6 strains). Optical density measurements using micro-titre plates were used as an initial screen, to determine the appropriate sampling times for viable counts to be made and to determine the approximate boundaries for growth. Growth curves were generated from viable counts and fitted using a modified Gompertz equation. Quadratic response surface equations were fitted to the log of lag and generation times, in response to the variables of temperature, NaCl and pH (in terms of hydrogen ion concentration). The effects of various combinations of these controlling factors are described. Comparisons between predicted growth rates and lag times from our response surface equations and other models for growth of A. hydrophila, developed with viable count data and optical density measurements, are made, together with comparisons with data from the literature on the growth of this bacterium in foods.

A comparison of Agiolax and lactulose in elderly patients with chronic constipation.

In a double-blind crossover study the efficacies of Agiolax, a combination of fibre and senna pod, and lactulose were compared in 77 long-stay elderly patients with chronic constipation. Mean daily bowel frequency, stool consistency and ease of evacuation were significantly greater with Agiolax than lactulose. The recommended dose was exceeded more frequently with lactulose than Agiolax (chi 2 = 8.38, p < 0.01). Adverse effects were not different for the 2 treatments. In long-stay elderly patients with chronic constipation Agiolax and lactulose were well tolerated, but Agiolax proved a more effective treatment.

A classification of residential neighbourhoods (ACORN) in relation to dental health and dental health behaviours.

Five-year-old children in two district health authorities in the North Western Region of England were examined during 1989-90. Questionnaires were issued to parents to determine whether dental health and related behaviours were associated with socio-economic status. Parents of 1858 children in Salford (response rate = 78 per cent) and 1691 in Trafford (response rate = 81 per cent) returned the questionnaires. Their replies were analysed by dividing the children into a hierarchy according to a classification of residential neighbourhood (ACORN). Significantly fewer in the highest of three Divisions in both districts had suffered from toothache and had undergone a general anaesthetic to have teeth extracted. They were also more likely to have had their teeth brushed with toothpaste by their first birthday and to have visited the dentist by their second. Because ACORN had detected these differences in dental health related behaviour it was possible to identify localities towards which dental health promotion activities can be directed.

Dental caries in 12-year-old children in Salford, UK: changes over 28 years.

A sample of 111 12-year-old children in Salford, UK was examined in 1988, and the findings compared with those of a survey carried out in 1960 of 12-year-old children in the same city. Between 1960 and 1988, the mean DMFT of the boys fell from 6.04 to 2.34 and that of the girls from 6.54 to 3.40. All types of teeth were affected by caries in 1960 but in 1988 none of the canines and mandibular lateral incisors were affected. In 1960 the permanent first molars contributed 57% of the boys' and 54% of the girls' total DMFT while in 1988 they contributed 87% of the boys' and 73% of the girls' DMFT. Jackson's Treatment Index showed that the dental services were making a greater impact in 1988; for the boys it rose from 29.5 to 63.2 and for the girls from 27.8 to 62.3.

Dental health education: training of homecarers of mentally handicapped adults.

A seven week induction course for homecarers of mentally handicapped adults, organised by the Social Services Department in Salford, UK, includes a dental health education session of one and a half hours. The session is divided into three parts; an illustrated discussion, a demonstration of tooth-brushing and a practical component session where carers are divided into pairs and carry out oral hygiene measures for each other. The effectiveness of the session has been assessed using a questionnaire. This was completed by 33 carers who attended the session and 31 carers who had not. Many members of both groups knew about the shape and colour of healthy gums and what plaque was. However significantly fewer of the control group knew that gum disease resulted from a lack of toothbrushing, what to do with bleeding gums and the amount of toothpaste to use on the toothbrush.