Agar Growth Medium Research Articles

Distribution and Exudation of Allelochemicals in Wheat Triticum aestivum

Wheat allelopathy has potential for weed suppression. Allelochemicals were identified in wheat seedlings, and they were exuded from seedlings into agar growth medium. p-Hydroxybenzoic, trans-p-coumaric, cis-p-coumaric, syringic, vanillic, trans-ferulic, and cis-ferulic acids and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) were identified in both the shoots and roots of 17-day-old wheat seedlings and their associated agar growth medium. Wheat accessions with previously identified allelopathic activity tended to contain higher levels of allelochemicals than poorly allelopathic ones. The allelopathic compounds present in the shoots generally also were identified in the roots and in the agar medium. Allelochemicals were distributed differentially in wheat, with roots normally containing higher levels of allelochemicals than the shoots. When the eight allelochemicals were grouped into benzoic acid and cinnamic acid derivatives, DIMBOA, total coumaric, and total ferulic acids, the amount of each group of allelochemicals was correlated between the roots and the shoots. Most of the allelochemicals identified in the shoots and roots could be exuded by the living roots of wheat seedling into the agar growth medium. However, the amounts of allelochemicals in the agar growth medium were not proportional to those in the roots. Results suggest that wheat plants may retain allelochemicals once synthesized. The presence of allelochemicals in the agar growth medium demonstrated that wheat seedlings were able to synthesize and to exude phytotoxic compounds through their root system that could inhibit the root growth of annual ryegrass.

Assessment of the Suitability of Different Substrate Materials for Bioaerosol Sampling

Bioaerosols may comprise infectious, allergenic and microbial particles. A range of static bioaerosol sampling methods currently exist and are described in detail by Crook (1996). Most of these aim to collect the sample in a viable, culturable state so that an estimate of the bioaerosol concentration may be made by counting the number of colonies that develop on agar growth media. The majority of these methods are poorly characterised, either in terms of their physical sampling efficiency, or in terms of the recovery and survival rates of collected microorganisms. Personal sampling for bioaerosols is generally carried out using standard aerosol samplers that collect the aerosol onto a filter. The dehydration effects caused by this method can greatly reduce the viability of the more delicate micro-organisms, especially bacteria. This is not problematic where the analysis of bioaerosol concentrations is carried out on both living and dead cells using direct counting by microscopy, biochemical or molecular detection methods. In some instances however, culturing is necessary, either to aid species identification (in the case of allergenic organisms) or to demonstrate the viability of pathogens. Existing samplers therefore have a number of disadvantages when used to assess health risks to workers arising from exposure to airborne micro-organisms. A rational, scientifically-based scheme for assessing bioaerosol exposures requires a personal sampler with the following characteristics:

Influence of oleoresin constituents fromPinus ponderosa andPinus jeffreyi on growth of mycangial fungi fromDendroctonus ponderosae andDendroctonus jeffreyi

Dendroctonus jeffreyi andD. ponderosae are morphologically similar sympatric species of pine bark beetles over portions of their geographic ranges; however,D. jeffreyi is monophagous onP. jeffreyi whileD. ponderosae is highly polyphagous. Both species carry a species of mycangial fungi that are also very similar in appearance. Growth of the two mycangial fungi and of the fungusLeptographium terebrantis (associated with the polyphagous and non-tree-killingDendroctonus valens) in the presence of oleoresin constituents of host and nonhost conifers was tested by placing individual chemicals on agar growth medium and by growing the cultures in saturated atmospheres of the chemicals. The fungus associated withD. jeffreyi showed greater tolerance for chemical constituents placed on the growth medium than the other two fungi, and growth after three days was enhanced by heptane, the dominant constituent ofP. jeffreyi oleoresin. Growth of all three species of fungi was reduced by the resin constituents when the chemicals were presented as saturated atmospheres. The results suggest that the influence of the tree on growth of the symbiotic fungi of the bark beetles during the initial attack process may be different than after colonization by the beetles is complete. The difference in the responses of the apparently related species of mycangial fungi may provide some new insight into the evolutionary history of these beetle/mycangial fungus/host tree systems.

Inoculation effects on legumes grown in soil previously treated with sewage sludge

The effects on legumes of the long-term land application of sewage sludge are not clear. Sludge-bome toxic elements, as well as essential nutrients and organic matter, complicate the response of legumes in association with their symbionts. To examine this problem, several strains and species of Rhizobium and Bradyrhizobium were studied for their response to the presence of heavy metals in agar growth media. Bradyrhizobium japonicum was by far the most metal-tolerant of the organisms examined, often able to tolerate several times more Zn and Cd in agar media than more metal-sensitive organisms, such as Rhizobium leguminosarum bv. trifolii. Soils were subsequently collected from metal-contaminated plots located at the Woburn Market Garden Experiment, and rhizobia were inoculated into these soils and sown with the appropriate homologous legume. Uninoculated controls and unamended soils were also sown. White clover ( Trifolium repens) in the uninoculated, sludge-amended treatment contained numerous, small white and ineffective nodules. Inoculation enhanced nodulation and plant growth, but not to levels observed for plants grown on soil not amended with sludge. Inoculation with effective strains of rhizobia had little effect on plants grown in control soils since an indigenous, effective population compatible with white clover was present in adequate numbers. Only soybeans ( Glycine max) responded to inoculation since soybeans had never been cultivated in these soils. These results confirm earlier observations that R. leguminosarum bv. trifolii in soils exposed to sludge-borne metals for many years are ineffective. Microbial species, chemical characteristics of the soil into which the sludge was added and the length of time the microbes were exposed to the metals each affected the response of the macro- and microsymbionts.

Effects of selected fungicides on in vitro spore germination and vegetative growth of Ascosphaera aggregata

tive growth of Ascosphaera aggregata, the causative organism of chalkbrood in the alfalfa leafcutter bee, Megachile rotundata. The compounds were incorporated directly into liquid or agar growth medium. EC for inhibition of vegetative growth on an agar surface were estimated as follows: enilconazole, 0.034; Rovral (iprodione), 1.10; Benlate (benomyl), 1.16; and Ascocidin (N-glycosylpolifungin), 135.6 ppm (m/v). DFMO (α-DL-difluoromethyl omithine) stimulated growth resulting in an approximately 50% increased radius at 10 000 ppm compared to the control. Benlate inhibited spore germination; at the highest concentration only, Rovral inhibited germination while Ascocidin and enilconazole exhibited a slight effect. DFMO did not affect spore germination. Ascocidin and Benlate exhibited most effect on germling viability at 48 h followed by Rovral and enilconazole. DFMO had no effect on germling viability. Benlate, Rovral and enilconazole had the most pronounced effect on germ tube growth at 96 h. Ascocidin affected germ tube growth at the higest concentration; DFMO exhibited little to no effect. Comparisons between in vitro results obtained in the present study with previously published in vivo results indicate that in vitro effects on spore germination most closely correlated with efficacy against chalkbrood in vivo, suggesting that compounds with activity against spore germination or germling viability may be the most promising candidates for use against chalkbrood.

Neutralization efficacy of Dey‐Engley medium in testing of contact lens disinfecting solutions

A quantitative assay for the demonstration of neutralizer efficacy was developed to monitor contact lens disinfecting solutions. Adequate neutralization of disinfecting agents is essential to the accurate determination of disinfecting activity with time. This method employed the recovery of small numbers of micro-organisms from neutralizing medium containing a disinfectant. A statistical estimation of significance between treatments demonstrated that Dey-Engley medium (DE; Difco) was generally effective when tested as an agar growth medium with several bacterial test organisms. DE medium from another vendor was less effective, underscoring the need for laboratory quality control and monitoring. DE agar (Difco) adequately neutralized all solutions tested at a 1:20 dilution. The solutions included those containing Dymed (polyaminopropyl biguanide, 0.00005%), chlorhexidine (0.005%), Polyquad (0.001%), chlorhexidine (0.005%) and thimerosal (BP, 0.001%), thimerosal (BP, 0.002%) and Tris(2-hydroxyethyl) tallow ammonium chloride (0.013%), and a solution preserved with 115 ppm benzalkonium chloride (BAK). A modification of this medium was developed which retained virtually all of the neutralizing efficacy for the solutions tested while allowing the use of automated testing procedures.

Fungicide seed treatments increase growth of perennial ryegrass

Field, laboratory and glasshouse experiments were carried out to measure effects of seed treatments with captan or thiram on growth of perennial ryegrass (Lolium perenne L.). Field-sown captan- or thiram-treated seed gave twice as many seedlings as untreated seed. Spaced plants growing from fungicide-treated seed produced almost 6 times more dry matter 16 weeks after sowing than those from untreated seed. This effect, though diminishing with time, was still apparent more than a year after sowing. Fungicides in sterile agar growth medium were phytotoxic to seedlings at concentrations of 10μg/ml and greater. Seedlings grown from treated seeds sown from 5 to 15 mm away from developing colonies of the virulent seedling pathogenFusarium oxysporum Schlecht. were more than 4 times larger than those grown from untreated seeds. Captan-treated seed sown into pots containing field soil produced more and larger seedlings than untreated seed. Methyl bromide fumigation of the same soil also increased both number and size of seedlings. Fungicidal, rather than direct chemical effects, at early stages of seedling growth, account for increased growth of plants from fungicide-treated seed.

Fatty acid regulation of the germination of Erynia variabilis conidia on adults and puparia of the lesser housefly, Fannia canicularis

Conidia of the entomogenous fungus Erynia variabilis germinate vegetatively on the cuticular surface of adults of the lesser housefly, Fannia canicularis, and on a basal agar growth medium in which cuticular lipid extracts of adult flies have been incorporated. Formation and discharge of secondary spores occur on puparial cuticles and on media in which puparial cuticle lipid extracts have been included. Relative free fatty acid compositions of puparial and adult cuticle lipid extracts are nearly identical, but adults average ca. five times the concentration of free fatty acids on their cuticle surface as that found on puparia. A regulatory role for esterified fatty acids in conidial germination is suggested. Limitation of E. variabilis to adult dipteran hosts is postulated to be due in part to characteristics of the fatty acids of this order of insects: the presence of sufficient oleic acid to induce germination; high levels of palmitoleic acid to enhance mycelial growth; and low levels of linoleic and linolenic acids, which have deleterious effects on spore germination and mycelial proliferation.

Studies on the Fungal Pathogen of Dutch Elm Disease

The fungal pathogen of the Dutch elm disease (DED), Ceratocystis ulmi (Buisman) C. Moreau, has eluded effective control since its introduction in the United States more than sixty years ago. Our studies on DED include establishing biological control agents against C. ulmi. In this report we describe the inhibitory action of the antibiotic polymyxin B on the causal agent of DED.In screening a number of antibiotics against C. ulmi, we observed that filter paper discs containing 300 units (U) of polymyxin B (Difco Laboratories) per disc, produced zones of inhibition to the fungus grown on potato dextrose agar or Sabouraud agar plates (100mm x 15mm), Fig. 1a. Total inhibition of fungal growth on a plate occurred when agar overlays containing fungus and antibiotic (polymyxin B sulfate, ICN Pharmaceuticals, Inc.) were poured on the underlying agar growth medium. The agar overlays consisted of the following: 4.5 ml of 0.7% agar, 0.5 ml of fungus (control plate); 4.0 ml of 0.7% agar, 0.5 ml of fungus, 0.5 ml of polymyxin B sulfate (77,700 U). Fig. 1, b and c, compares a control plate and polymyxin plate after seven days.

Quantitative estimation of rhizobia in non-sterile soil using antibiotics and fungicides

Rapid and accurate soil population counts of antibiotic labelled slow-growing rhizobia have been obtained using the agar growth medium described.

Fluorophores as visualization aides in agar growth media

A survey of 26 fluorophores revealed 5 which were non-inhibitory to Staphylococcus aureus and Escherichia coli and produced low background and high colony fluorescence.