Some modification to the media for rapid automated detection of salmonellas by conductance measurement

A selenite medium for the automated detection of salmonellas by conductance measurements has been modified to eliminate the negative results given by some dulcitol-negative strains. The dulcitol is replaced with mannitol and pre-enrichment is best done in buffered peptone water containing mannitol and dimethylsulphoxide. It is suggested that both versions of the selenite medium be used initially.     

Detection of salmonellas in confectionery products by conductance

A modified lysine decarboxylase broth has been developed which could be used with a Bactometer M123 to differentiate salmonellas from other bacteria by the characteristics of the conductance detection curve. The medium was used in combination with a selenite cystine trimethylamine oxide dulcitol medium to screen 50 strains of salmonellas and 42 strains of other organisms to establish detection curve magnitude and rate values which could be used to identify curves specific to salmonellas. The combination of media detected all salmonellas tested except Salmonella pullorum. The two media were used to screen 100 inoculated product samples with the Bactometer instrument, in parallel with traditional plating procedures, and using various combinations of pre-enrichment and selective enrichment incubation periods. After 24 h pre-enrichment, the Bactometer system detected more positive samples than the conventional plating procedures after pre-enrichment and selective enrichment. It is considered that these media used in parallel in the Bactometer after conventional pre-enrichment could provide a 48 h screening procedure for salmonellas with a sensitivity comparable to present plating procedures.     

Detection of salmonellas in confectionery products by conductance

A modified lysine decarboxylase broth has been developed which could be used with a Bactometer M123 to differentiate salmonellas from other bacteria by the characteristics of the conductance detection curve. The medium was used in combination with a selenite cystine trimethylamine oxide dulcitol medium to screen 50 strains of salmonellas and 42 strains of other organisms to establish detection curve magnitude and rate values which could be used to identify curves specific to salmonellas. The combination of media detected all salmonellas tested except Salmonella pullorum . The two media were used to screen 100 inoculated product samples with the Bactometer instrument, in parallel with traditional plating procedures, and using various combinations of pre-enrichment and selective enrichment incubation periods. After 24 h pre-enrichment, the Bactometer system detected more positive samples than the conventional plating procedures after pre-enrichment and selective enrichment. It is considered that these media used in parallel in the Bactometer after conventional pre-enrichment could provide a 48 h screening procedure for salmonellas with a sensitivity comparable to present plating procedures.     

Detection of salmonellas in animal feeds by electrical conductance

A comparison was made between standard culture methods and electrical conductance using a Malthus AT Microbiological Analyser for the examination of animal feeds for salmonella. Conductance testing with a selenite cystine/trimethylamine-N-oxide/dulcitol medium resulted in the detection of salmonellas in 49 of 55 known positive animal feeds, 13 of 19 spiked feed samples and 36 of 47 salmonella cultures. Testing with a lysine decarboxylase/glucose medium gave significantly better results (P less than 0.05) than with selenite cystine medium but five lysine decarboxylase negative strains of salmonella were undetected. When both media were used in parallel all salmonella positive samples were detected. No difference was found between preenrichment in buffered peptone water containing trimethylamine/mannitol and that containing lysine/glucose. Positive detection criteria for selenite medium of conductance peak at greater than or equal to 500 microsiemens (microS) with a rate of change of greater than or equal to 60 microS/h or 400-499 microS with a rate of change of greater than or equal to 40 microS/h and for lysine medium with a peak of greater than or equal to 100 microS have been established. The method offers savings in media and operating costs over conventional standard culture methods, provides results within 48 h and is recommended for statutory feed monitoring purposes.     

Growth of salmonellas in different enrichment media

The usefulness of selenite-F (S-F), tetrathionate (MKT) and Rappaport-10 (R-10) broths as enrichment media to support growth of salmonellas either alone or in the presence of other competing organisms was studied. Their ability to support the growth of stressed salmonellas from water was also investigated. It was observed that R-10 was more inhibitory to competing organisms than MKT and S-F. It strongly inhibited the growth of Pseudomonas aeruginosa, Citrobacter freundii and Proteus vulgaris though not of Escherichia coli and Enterobacter aerogenes. It was more toxic, however, to small numbers of salmonellas than MKT and S-F. Tetrathionate was strongly inhibitory for E. coli and Ent. aerogenes but much less so for Proteus and Pseudomonas species. Selenite-F was much less inhibitory than MKT to Ps. aeruginosa and it did not inhibit growth of E. coli and Ent. aerogenes as much as MKT. Salmonellas were inhibited by all three enrichment media and none of them is ideally suited for direct use. Of the three media, R-10 was much more inhibitory to stressed organisms than S-F or MKT.     

Growth of salmonellas in different enrichment media

The usefulness-of selenite-F (S-F), tetrathionate (MKT) and Rappaport-10 (R-10) broths as enrichment media to support growth of salmonellas either alone or in the presence of other competing organisms was studied. Their ability to support the growth of stressed salmonellas from water was also investigated. It was observed that R-10 was more inhibitory to competing organisms than MKT and S-F. It strongly inhibited the growth of Pseudomonas aeruginosa, Citrobacter freundii and Proteus vulgaris though not of Escherichia coli and Enterobacter aerogenes. It was more toxic, however, to small numbers of salmonellas than MKT and S-F. Tetrathionate was strongly inhibitory for E. coli and Ent. aerogenes but much less so for Proteus and Pseudomonas species. Selenite-F was much less inhibitory than MKT to Ps. aeruginosa and it did not inhibit growth of E. coli and Ent. aerogenes as much as MKT. Salmonellas were inhibited by all three enrichment media and none of them is ideally suited for direct use. Of the three media, R-10 was much more inhibitory to stressed organisms than S-F or MKT.     

A complete protocol using conductance for rapid detection of salmonellas in confectionery materials

Increased confidence in conductimetric detection of salmonellas was achieved by combining a bacteriophage-based test with use of a selenite cystine trimethylamine oxide dulcitol medium and a modified lysine decarboxylase broth. All 81 Salmonella isolates tested were detected and few of the 39 non-salmonellas gave false positives. Results from the screening of 43 inoculated product samples further support the use of this simple, rapid method for routine salmonella testing in the food industry.     

Improvements to a lysine medium for detection of salmonellas by elctrical conductance

Improvements in performance of a lysine conductance medium for the detection of salmonellas were achieved from a study of the effects of its various components. When sodium biselenite was included as an inhibitor for non-salmonella organisms conductance signals were depressed. The inclusion of sodium chloride reduced this toxicity and improved conductance responses. Increasing the pH to pH 7.0 prevented the medium becoming too acidic and inhibitory to salmonellas. The new medium detected 70-0% of salmonella-positive animal protein samples.     

Evaluation of different plating media used in the isolation of salmonellas from environmental samples

Different serotypes of salmonellas were compared for selectivity and efficiency of recovery using 11 plating media. No optimal growth was obtained after 24 h incubation in any of the media, but after 48 h, brilliant green, brilliant green-phenol red-lactose-sucrose, bismuth sulphite, xylose-lysine-deoxycholate and Hektoen enteric agars showed optimal recovery of all the salmonella serotypes. Xylose-lysine-deoxycholate and brilliant green-phenol red-lactose-sucrose agars were the most selective media for all salmonella serotypes. Addition of 10 micrograms/ml of sodium novobiocin to the tryptic soy-xylose-lysine and tryptic soy-brilliant green agars significantly improved their selectivity but reduced or inhibited the growth of some salmonella serotypes, including Salmonella typhi. Xylose-lysine-deoxycholate agar gave the highest recovery percentage of stressed salmonellas with a double-agar layer technique. Good recovery was also obtained on brilliant green-phenol red-lactose-sucrose, tryptic soy-brilliant green, tryptic soy-brilliant green-novobiocin, tryptic, soy-xylose-lysine and tryptic soy-xylose-lysine-novobiocin agars. Salmonella-shigella agar was the least efficient medium for the recovery of salmonellas under stress-induced or non-stressed conditions.     

Evaluation of different plating media used in the isolation of salmonellas from environmental samples

Different serotypes of salmonellas were compared for selectivity and efficiency of recovery using 11 plating media. No optimal growth was obtained after 24h incubation in any of the media, but after 48h, brilliant green, brilliant green-phenol red-lactose-sucrose, bismuth sulphite, xylose-lysine-deoxycholate and Hektoen enteric agars showed optimal recovery of all the salmonella serotypes.
Xylose-lysine-deoxycholate and brilliant green-phenol red-lactose-sucrose agars were the most selective media for all salmonella serotypes. Addition of 10 μg/ml of sodium novobiocin to the tryptic soy-xylose-lysine and tryptic soy-brilliant green agars significantly improved their selectivity but reduced or inhibited the growth of some salmonella serotypes, including Salmonella typhi. Xylose-lysine-deoxycholate agar gave the highest recovery percentage of stressed salmonellas with a double-agar layer technique. Good recovery was also obtained on brilliant green-phenol red-lactose-sucrose, tryptic soy-brilliant green, tryptic soy-brilliant green-novobiocin, tryptic, soy-xylose-lysine and tryptic soy-xylose-lysine-novobiocin agars. Salmonella-shigella agar was the least efficient medium for the recovery of salmonellas under stress-induced or non-stressed conditions.     

The effect of sodium sulphacetamide and sodium mandelate in brilliant green agar on the growth of salmonellas

An assessment was made of the ability of 425 isolates of salmonellas, belonging to 54 serotypes, to grow on seven selective media. Several isolates of Salmonella dublin and Salm. paratyphi B failed to grow on brilliant green agar supplemented with sodium sulphacetamide and sodium mandelate. On this medium the colonies of 30 isolates which were able to grow were extremely small after 20 h incubation, and consequently their recognition was difficult. The sodium sulphacetamide was responsible for the reduction in colony size.     

The effect of sodium sulphacetamide and sodium mandelate in brilliant green agar on the growth of salmonellas

An assessment was made of the ability of 425 isolates of salmonellas, belonging to 54 serotypes, to grow on seven selective media. Several isolates of Salmonella dublin and Salm. paratyphi B failed to grow on brilliant green agar supplemented with sodium sulphacetamide and sodium mandelate. On this medium the colonies of 30 isolates which were able to grow were extremely small after 20 h incubation, and consequently their recognition was difficult. The sodium sulphacetamide was responsible for the reduction in colony size.     

A note on the effect of the lactoperoxidase systems on salmonellas in vitro and in vivo

The lactoperoxidase system (LPS), a natural bactericidal system in milk, was investigated for its activity against salmonellas in vivo and in vitro. In acidified raw milk, in which the LPS was supplemented with an exogenous supply of H2O2, the numbers of salmonellas decreased rapidly. Different salmonella serotypes were affected to the same extent; rough strains, however, were more susceptible than smooth strains. When calves were fed on fresh milk, containing the LPS, and challenged with Salmonella typhimurium in doses of either 10(9) or 10(10), the clinical findings and salmonella excretion patterns were similar to those of control calves fed on heated milk. It was concluded that further studies, perhaps in the field, are necessary to evaluate LPS as a possible non-antibiotic system to control salmonellosis.     

A note on the effect of the lactoperoxidase systems on salmonellas in vitro and in vivo

The lactoperoxidase system (LPS), a natural bactericidal system in milk, was investigated for its activity against salmonellas in vivo and in vitro. In acidified raw milk, in which the LPS was supplemented with an exogenous supply of H₂O₂, the numbers of salmonellas decreased rapidly. Different salmonella serotypes were affected to the same extent; rough strains, however, were more susceptible than smooth strains. When calves were fed on fresh milk, containing the LPS, and challenged with Salmonella typhimurium in doses of either 10⁹ or 10¹⁰, the clinical findings and salmonella excretion patterns were similar to those of control calves fed on heated milk. It was concluded that further studies, perhaps in the field, are necessary to evaluate LPS as a possible non-antibiotic system to control salmonellosis.     

The effect of illumination on the electrical conductance of rhodopsin solutions

An apparatus was constructed in order to record continuously and simultaneously changes in extinction and electrical conductance of rhodopsin solutions. With this apparatus, changes in electrical conductance on exposing rhodopsin to light were investigated. On illumination solutions of rhodopsin revealed a conductance change so long as they preserved their photosensitivity. The conductance change begins almost immediately upon illumination and is almost proportional to the amount of rhodopsin decomposed, continuing until rhodopsin is converted to indicator yellow. Near pH 7 the conductance is apt to increase slightly, while it decreases considerably outside the range of pH 6–9, being accompanied by a pH change towards neutrality. The conductance change is regarded as an essential property of rhodopsin, because it occurs in aqueous suspension as well as in digitonin solution; it may be caused by hydrogen or hydroxyl ions and some other conductive substances. It is also noteworthy that the petroleum ether-soluble component of the rod outer segments—presumably the lipide—tends to increase the conductance change. In suspensions of rod outer segments and retinal homogenates, the conductance increases on illumination irrespective of pH: this may be due to secondary reactions following the photic reaction of rhodopsin. We shall discuss the significance of the conductance change in relation to the initiation of visual excitation.     

The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G_Si) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G_Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G_Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q_o). Under conditions of D = 1 kPa and saturating Q_o, G_Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood‐to‐leaf area ratio (A_S:A_L) doubled. A simple hydraulic model explained the variation in G_Si based on an inverse relationship with height, and a linear relationship with A_S:A_L. Thus, in F. sylvatica, adjustments in A_S:A_L partially compensate for the negative effect of increased flow‐path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.