Inhibitory effect of garlic on bacterial pathogens from spices

An unconventional technique for primary screening of bacterial susceptibility to garlic (Allium sativum Linn.), using a slice from its clove, was described. Aqueous extracts of garlic were found to possess a potent bacteriostatic principle against Gram-positive as well as Gram-negative foodborne bacterial pathogens. In agar medium, the minimum inhibitory concentrations (MICs) of garlic were 6–10 mg ml^–1 for Bacillus cereus, 30–40 mg ml^–1 for Staphylococcus aureus (excepting the isolate from garlic, where the MIC was 100 mg ml^–1), 20–30 mg ml^–1 for Clostridium perfringens, 10 mg ml^–1 for Escherichia coli (30 mg ml^–1 for the garlic isolate), 40–100 mg ml^–1 for Salmonella, and 10–40 mg ml^–1 for Shigella. It inhibited the growth of all these strains, which were resistant to some commonly used antibiotics. Most of the tested strains were resistant to penicillins, although sensitive to garlic. While the growth of B. cereus and Cl. perfringens was completely inhibited at 10 and 70 mg garlic, respectively, ml^–1 test broth, their respective enterotoxin production ceased at 10 and 50 mg garlic ml^–1.     

Antimicrobial activity of Coniothyrium minitans and its macrolide antibiotic macrosphelide A

Aims: Assessment of antimicrobial activity of the mycoparasite Coniothyrium minitans and its macrolide antibiotic macrosphelide A. Methods and Results: Thirteen isolates of C. minitans were tested for ability to inhibit a number of filamentous fungi, yeasts, oomycetes and bacteria in agar based tests. Activity was found against some ascomycetes, basidiomycetes, oomycetes and Gram‐positive bacteria, but not against zygomycetes, yeasts or Gram‐negative bacteria tested. Six C. minitans isolates (Conio, Contans, IVT1, CM/AP/3118, B279/1, A1/327/1) were found to produce macrosphelide A in liquid culture and no other antibiotics were detected. On agar, macrosphelide A inhibited growth of some ascomycetes, basidiomycetes, oomycetes and all four Gram‐positive bacteria tested, including the medically important Staphylococcus aureus with a minimum inhibitory concentration of ≤500 μg ml^?1. There was no inhibition observed against the yeasts and Gram‐negative bacteria when macrosphelide A was tested at 700 μg ml^?1. Conclusions: The spectrum and level of activity of macrosphelide A produced by C. minitans against micro‐organisms are extended markedly compared to previous reports. Significance and Impact of the Study: Macrosphelide A was effective against Staph. aureus. Further study on the control of this bacterium is merited in view of the development of antibiotic resistance.     

Antimicrobial mechanism of β-glycyrrhetinic acid isolated from licorice, Glycyrrhiza glabra

-Glycyrrhetinic acid isolated from Glycyrrhiza glabra had an antibacterial activity of 7.6 and 12.5 g ml^–1 against Bacillus subtilis and Staphylococcus epidermidis without causing hemolysis of human erythrocytes, whereas it was not inhibitory against Escherichia coli, Proteus vulgaris and various fungi. Confocal microscopy showed that -glycyrrhetinic acid was located within the bacteria but had not caused membrane disruption. It then inhibited synthesis of DNA, RNA and protein.     

Antimicrobial activity of crude extracts from mangrove fungal endophytes

The aim of this work was to select endophytic fungi from mangrove plants that produced antimicrobial substances. Minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) or minimal fungicidal concentrations (MFC) of crude extracts from 150 isolates were determined against potential human pathogens by a colorimetric microdilution method. Ninety-two isolates (61.3%) produced inhibitory compounds. Most of the extracts (28–32%) inhibited Staphylococcus aureus (MIC/MBC 4–200/64–200 μg ml⁻¹). Only two extracts inhibited Pseudomonas aeruginosa (MIC/MBC 200/>200 μg ml⁻¹). 25.5 and 11.7% inhibited Microsporum gypseum and Cryptococcus neoformans (MIC/MFC 4–200/8–200 μg ml⁻¹ and 8–200/8–200 μg ml⁻¹, respectively), while 7.5% were active against Candida albicans (MIC/MFC 32–200/32–200 μg ml⁻¹). None of the extracts inhibited Escherichia coli. The most active fungal extracts were from six genera, Acremonium, Diaporthe, Hypoxylon, Pestalotiopsis, Phomopsis, and Xylaria as identified using morphological and molecular methods. Phomopsis sp. MA194 (GU592007, GU592018) isolated from Rhizophora apiculata showed the broadest antimicrobial spectrum with low MIC values of 8–32 μg ml⁻¹against Gram-positive bacteria, yeasts and M. gypseum. It was concluded that endophytic fungi from mangrove plants are diverse, many produce compounds with antimicrobial activity and could be suitable sources of new antimicrobial natural products.     

Investigation of enhancement of two processes, sedimentation and conjugation, when bacteria are concentrated in ultrasonic standing waves

Cells aggregate and can be recovered from suspension when exposed to an ultrasonic standing wave field. The acoustic force on individual cells in a standing wave decreases with particle volume. A plane ultrasonic field generated by a transducer driven at 3.3 MHz was used here to investigate the removal of Escherischia coli, cells with dimensions of the order of 1.0 m, from batch suspension by sedimentation over a range of concentrations (10³ to 10¹⁰ cells ml^–1). Cell removal efficiencies greater than 90% were achieved at initial concentrations of 10¹⁰ cells ml^–1. Removal efficiencies decreased gradually to zero, as initial bacterial concentration was reduced to 10⁷ cells ml^–1. It was found that, when low concentrations of E. coli (10³ to 10⁵ cells ml^–1) were added to suspensions of larger particles (i.e. yeast cells) that were of sufficient concentration to form aggregates in the sound field, E. coli could be harvested to an efficiency of 40%. The results imply that the E. coli became trapped and sediment with aggregates of larger particles. Some strains of bacteria are capable of DNA transfer by conjugation. The transfer rate of E. coli RP4 plasmid is order of magnitudes greater when conjugation occurs on solid medium rather than in liquid suspension. We have investigated whether the conjugation rate would also be higher in ultrasonically induced E. coli clumps than in free suspension. The donor strain was mixed with a recipient strain of E. coli, then sonicated in a capillary at 4.6 MHz in a tubular transducer for 5 min. The bacteria aggregated successfully. Results showed a three-fold increase in the rate of conjugation compared to a liquid mating control.     

Effect of garlic and allium-derived products on the growth and metabolism of Spironucleus vortens

Spironucleus is a genus of small, flagellated parasites, many of which can infect a wide range of vertebrates and are a significant problem in aquaculture. Following the ban on the use of metronidazole in food fish due to toxicity problems, no satisfactory chemotherapies for the treatment of spironucleosis are currently available. Using membrane inlet mass spectrometry and automated optical density monitoring of growth, we investigated in vitro the effect of Allium sativum (garlic), a herbal remedy known for its antimicrobial properties, on the growth and metabolism of Spironucleus vortens, a parasite of tropical fish and putative agent of hole-in-the-head disease. The allium-derived thiosulfinate compounds allicin and ajoene, as well as an ajoene-free mixture of thiosulfinates and vinyl-dithiins were also tested. Whole, freeze-dried garlic and allium-derived compounds had an inhibitory effect on gas metabolism, exponential growth rate and final growth yield of S. vortens in Keister’s modified, TY-I-S33 culture medium. Of all the allium-derived compounds tested, the ajoene-free mixture of dithiins and thiosulfinates was the most effective with a minimum inhibitory concentration (MIC) of 107 μg ml⁻¹ and an inhibitory concentration at 50% (IC_50%) of 58 μg ml⁻¹. It was followed by ajoene (MIC = 83 μg ml⁻¹, IC_50% = 56 μg ml⁻¹) and raw garlic (MIC >20 mg ml⁻¹, IC_50% = 7.9 mg ml⁻¹); allicin being significantly less potent with an MIC and IC_50% above 160 μg ml⁻¹. All these concentrations are much higher than those reported to be required for the inhibition of most bacteria, protozoa and fungi previously investigated, indicating an unusual level of tolerance for allium-derived products in S. vortens. However, chemically synthesized derivatives of garlic constituents might prove a useful avenue for future research.     

Piperine,an active ingredient of white pepper,suppresses the growth of multidrug-resistant toxigenic Vibrio cholerae and other pathogenic bacteria

Emergence and rapid spread of multidrug-resistant (MDR) bacteria including Vibrio cholerae are a global public health issue. Much attention has been paid to natural compounds, such as spices and herbs to find novel antimicrobial compounds as they are considered to be cheaper alternatives to develop as a drug. Here, we show that methanol extract of white pepper could inhibit the growth of V. cholerae O1 El Tor variant, responsible for the recent outbreaks/epidemics. Furthermore, we demonstrate for the first time that piperine, the major component of white pepper, showed a dose-dependent bactericidal effect on V. cholerae growth irrespective of their biotypes and serogroups in the presence of 200 and 300 µg ml⁻¹ of piperine, respectively. Piperine also inhibited the growth of MDR strains of Pseudomonas aeruginosa, Escherichia coli isolated from poultry and enterohemorrhagic/enteroaggregative E. coli O104 in the presence of 200 µg ml⁻¹. Interestingly, we did not observe any significant inhibitory effect of piperine on E. coli strains isolated from healthy person even up to 200 µg ml⁻¹. Our data suggest that piperine could be a novel antimicrobial agent in therapeutic and preventive applications against infections caused by pathogenic bacteria including MDR strains.     

Synthesis of shorter active analogues of Bactenecin7: The effect of change of N-terminal configuration on antimicrobial activity

Bactenecin7, a cationic antibacterial peptide, contains a repeating region of Xaa-Pro-Arg-Pro (Xaa = hydrophobic residues). A series of peptides, Xaa-Pro-Arg-Pro (Xaa = D-Ala, D-Leu, D-Val, D-Phe and D-Lys) were synthesized to investigate the effect of change of N-terminal configuration on antimicrobial activity. The conformationalpreferences of these peptides in water and TFE were examined by circular dichroism. All the synthetic peptides with D-amino acid substitution at N-terminal showed potent antifungal activity against Aspergillus niger, Aspergillus flavus and Fusarium moniliforme at the concentration level of 8–10 g ml^-1. But the same tetrapeptides were unable to kill or suppress the growth of gram-negativeand gram-positive bacteria such as Escherichia coli HB101, Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus even at the concentration level of 400 g ml^-1. The present study reveals that the change of configuration at the N-terminal of tetrapeptide has negative impact on antibacterial activity but enhanced antifungal activity.     

Food dependence and energetics of freeliving nematodes

Summary The food dependence of larval duration, fecundity and the intrinsic rate of natural increase follow a hyperbolic form, which can for the former be described by the Michaelis-Menten function.Maximal larval duration at 20° C is 62 h, maximal fecundity is 153 eggs per female and r _max is 1.136 per day. The lower food threshold is 10⁸ E. coli cells·ml^-1 (=0.06 mg dry weight·ml^-1) for larval growth and 2·10⁸ cells·ml^-1 for reproduction and r. 50% of maximal performances (K _s ) are attained at 5·10⁸ and 7.5·10⁸ cells·ml^-1 respectively.Reproductive effort at dense food is highest immediately after maturation (e.g. 50% of the total eggs produced by a female are laid within 2 days after onset of egg production). At lower food densities the reproductive effort is delayed.Larval mortality increases strongly below 10⁹ cells·ml^-1.The results reported sofar were obtained with E. coli cells which were harvested at the phase of decreasing population growth in batch cultures. With cells from the exponential and the stationary phase, performances are increased and decreased respectively. This is partly due to differences in bacterial biomass per unit cell, partly an expression of the change of nutritive value of bacterial cells with growth phases.     

Overproduction and secretion of Bacillus circulans endo-β-1,3-1,4-glucanase gene (bglBC1) in B. subtilis and B. megaterium

A gene coding for endo--1,3-1,4-glucanase (lichenase) containing a recombinant plasmid, pLL200K, was transferred from Bacillus circulans into a new shuttle plasmid, pLLS920, by ligating linearized DNAs of pLL200K and pUB110. B. subtilis RM125 and B. megaterium ATCC14945 transformed with pLLS920 produced the endo--1,3-1,4-glucanase. The enzyme was produced during active growth with maximum activity. The B. subtilis (pLLS920) enzyme was 83 times (8522 mU ml^–1) more active than that of the gene donor cells (103 mU ml^–1). The B. megaterium (pLLS920) enzyme was 7 times (735 mU ml^–1) more active than that of the gene donor cells. While E. coli secreted only about 10% of the produced enzyme, B. subtilis excreted the enzyme completely into the medium and B. megaterium by about 98%. The plasmid pLLS920 was stable in B. megaterium (98%), and in B. subtilis (51%) but not in E. coli (29%).     

Synthesis,characterization and evaluation of antimicrobial and cytotoxic activities of biogenic silver nanoparticles synthesized from <Emphasis Type="Italic">Streptomyces xinghaiensis</Emphasis> OF1 strain

We report synthesis of silver nanoparticles (AgNPs) from Streptomyces xinghaiensis OF1 strain, which were characterised by UV–Vis and Fourier transform infrared spectroscopy, Zeta sizer, Nano tracking analyser, and Transmission electron microscopy. The antimicrobial activity of AgNPs alone, and in combination with antibiotics was evaluated against bacteria, namely Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Bacillus subtilis, and yeasts viz., Candida albicans and Malassezia furfur by using micro-dilution method. The minimum inhibitory concentration (MIC) and minimum biocidal concentration of AgNPs against bacterial and yeast strains were determined. Synergistic effect of AgNPs in combination with antibacterial and antifungal antibiotics was determined by FIC index. In addition, MTT assay was performed to study cytotoxicity of AgNPs alone and in combination with antibiotics against mouse fibroblasts and HeLa cell line. Biogenic AgNPs were stable, spherical, small, polydispersed and capped with organic compounds. The variable antimicrobial activity of AgNPs was observed against tested bacteria and yeasts. The lowest MIC (16 µg ml^?1) of AgNPs was found against P. aeruginosa, followed by C. albicans and M. furfur (both 32 µg ml^?1), B. subtilis and E. coli (both 64 µg ml^?1), and then S. aureus and Klebsiella pneumoniae (256 µg ml^?1). The high synergistic effect of antibiotics in combination with AgNPs against tested strains was found. The in vitro cytotoxicity of AgNPs against mouse fibroblasts and cancer HeLa cell lines revealed a dose dependent potential. The IC₅₀ value of AgNPs was found in concentrations of 4 and 3.8 µg ml^?1, respectively. Combination of AgNPs and antibiotics significantly decreased concentrations of both antimicrobials used and retained their high antibacterial and antifungal activity. The synthesis of AgNPs using S. xinghaiensis OF1 strain is an eco-friendly, cheap and nontoxic method. The antimicrobial activity of AgNPs could result from their small size. Remarkable synergistic effect of antibiotics and AgNPs offer their valuable potential in nanomedicine for clinical application as a combined therapy in the future.     

Use of a microbial fuel cell for the rapid enumeration of bacteria

Summary The detection of bacteria using a thionine mediated microbial fuel cell was examined. On addition of bacteria to the anode compartment of a fuel cell, a rapid increase in the current output was observed. Both the total change in the steady state current (mA) and the initial rate of change of current were proportional to the numbers of bacteria added. Regression analysis of plots of log₁₀ mA against log₁₀ bacteria ml^-1 (final concentration) upon the addition of E. coli K12, Lactococcus lactis, coliform sp. A1, Micrococcus sp. M3 but not Pseudomonas sp. P5 gave reasonable correlation coefficients. Determination of the rates of respiration and thionine reduction by E. coli indicated that the transfer of metabolic electrons from the bacteria to the mediator was reasonably efficient (approx. 50%). These results are discussed with respect to the potential application of this technique for the rapid estimation of the bacterial contamination of foods.     

Antimycotic Activity Potentiation of Allium sativum Extract and Silver Nanoparticles against Trichophyton rubrum

A natural and biocompatible extract of garlic as a support, decorated with silver nanoparticles, is a proposal to generate an effective antifungal agent against dermatophytes at low concentrations. Silver nanoparticles (AgNPs) with a diameter of 26±7 nm were synthesized and their antimycotic activity was examined against Trichophyton rubrum (T. rubrum), inhibiting 94 % of growth at a concentration of 0.08 mg ml^?1. Allium sativum (garlic) extract was also obtained (AsExt), and its MIC was 0.04 mg ml^?1. To increase the antifungal capacity of those systems, AsExt was decorated with AgNPs, obtaining AsExt‐AgNPs. Using an AsExt concentration of 0.04 mg ml^?1 in independent experiments with concentrations from 0.01 to 0.08 mg ml^?1 of AgNPs, it was possible to inhibit T. rubrum at all AgNPs concentrations; it proves a synergistic effect between AgNPs and AsExt. Even if 1 % of the minimum inhibitory concentration of AsExt (0.0004 mg ml^?1) is used, it was possible to inhibit T. rubrum at all concentrations of AgNPs, demonstrating the successful antimycotic activity potentiation when combining AsExt and AgNPs.     

Antialgal activity of a hepatotoxin-producing cyanobacterium,Microcystis aeruginosa

Antimicrobial activity of toxin produced by a freshwater bloom-forming cyanobacterium Microcystis aeruginosa has been studied. When tested against certain green algae, cyanobacteria, heterotrophic bacteria and fungi, the toxin inhibited growth of only green algae and cyanobacteria. The toxin has been partially purified employing Thin layer chromatography (TLC) and High-performance liquid chromatography (HPLC) techniques and appears to be microcystin-LR (leucine–arginine). Both crude and purified toxins showed toxicity to mice, the clinical symptoms in test mice being similar to those produced by hepatotoxin. Purified toxin at a concentration of 50 g ml^–1 caused complete inhibition of growth followed by cell lysis in Nostoc muscorum and Anabaena BT1 after 6 days of toxin addition. Addition of toxin (25 g ml^–1) to the culture suspensions of the Nostoc and Anabaena strains caused instant and drastic loss of O₂ evolution. Furthermore a marked reduction (about 87%) in the ¹⁴CO₂ uptake was also observed at a concentration of 50 g ml^–1. Besides its inhibitory effects on photosynthetic processes, M. aeruginosa toxin (50 g ml^–1) also caused 90% loss of nitrogenase activity after 8 h of its addition. Experiments performed with ¹⁴C-labelled toxin indicate that the toxin uptake by cyanobacterial cells occurs both in light and dark. These results demonstrate that the toxin is strongly algicidal and point to the possibility that it may have an important role in establishment and maintenance of toxic blooms of M. aeruginosa in freshwater ecosystems. The relative significance of the hepatotoxic effect and the algicidal effect of the toxin is discussed with reference both to survival and dominance of M. aeruginosa in nature.     

Activity of cyanobacterial and algal odor compounds found in lake waters on green alga Chlorella pyrenoidosa growth

Volatile organic compounds produced by cyanobacteria and algae in freshwater lakes and contributing to the odour of lakes were tested for their ability to inhibit the growth of the green alga Chlorella pyrenoidosa by the paper disk-agar plate method. Geosmin, -cyclocitral, - and -ionones, and geranylacetone exhibited inhibitory activity by diffusion in the 2-5 mg ml^–1 range. - and -Ionones and geranylacetone showed inhibition through the vapor phase at 10 mg ml^–1. Dimethyl disulfide showed no inhibition at 10 mg ml^–1. The norcarotenoids, which are prominent contributors to the odour of lake waters, were not significantly active in inhibiting the growth of Chlorella, with activity in the same range as monoterpene alcohols in general.     

Rapid immunodetection of Escherichia coli

A filtration flow-through design was used to develop the rapid immunodetection of Escherichia coli. Polyclonal anti-E. coli IgG was conjugated to small, 0.8 Blue latex beads. Cells were mixed with conjugated beads in the presence of anti-E. coli monoclonal IgM. The suspension was then filtered through a 5 nitrocellulose membrane. The cell-containing complexes were effectively collected on the filter, forming a blue spot. The method produced reliable detection of E. coli at a concentration of 10⁵ cells ml^–1, which is a current benchmark figure for urinary tract infection (UTI) diagnosis.     

Glucosamine-mediated detoxification of p-benzoquinone and its removal by chitosan

p-Benzoquinone non-enzymatically reacted with d-glucosamine at physiological pH and moderate temperature. The reaction of p-benzoquinone with glucosamine was signaled by changes in the UV and visible spectra. The reactivity proceeded fastest at pH values above 7, with a sharp drop from pH 6.5 to 7.0, and the reaction was negligible in acidic conditions. The order of reactivity of amino sugars was d-mannosamine > d-glucosamine > d-galactosamine. From the reaction mixture, four conversion products were isolated and none was toxic to Escherichia coli even at 500–700 g ml^–1, while p-benzoquinone was cytotoxic to E. coli at 20 g ml^–1. Chitosan could react with p-benzoquinone efficiently and remove this toxicant in aqueous solution.     

Demethylation and degradation of phenylmethylethers by the sulfide-methylating homoacetogenic bacterium strain TMBS 4

Biochemical studies on anaerobic phenylme-thylether cleavage by homoacetogenic bacteria have been hampered so far by the complexity of the reaction chain involving methyl transfer to acetyl-CoA synthase and subsequent methyl group carbonylation to acetyl-CoA. Strain TMBS 4 differs from other demethylating homoacetogenic bacteria in using sulfide as a methyl acceptor, thereby forming methanethiol and dimethylsulfide. Growing and resting cells of strain TMBS 4 used alternatitively CO₂ as a precursor of the methyl acceptor CO for homoacetogenic acetate formation. Demethylation was inhibited by propyl iodide and reactivated by light, indicating involvement of a corrinoid-dependent methyltransferase. Strain TMBS 4 contained ca. 750 nmol g dry mass^-1 of a corrinoid tentatively identified as 5-hydroxybenzimidazolyl cobamide. A photometric assay for measuring the demethylation activity in cell extracts was developed based on the formation of a yellow complex of Ti³⁺ with 5-hydroxyvanillate produced from syringate by demethylation. In cell extracts, the methyltransfer reaction from methoxylated aromatic compounds to sulfide or methanethiol depended on reductive activation by Ti³⁺. ATP and Mg²⁺ together greatly stimulated this reductive activation without being necessary for the demethylation reaction itself. The specific activity of the transmethylating enzyme system increased proportionally with protein concentration up to 3 mg ml^-1 reaching a constant level of 20 nmol min^-1 mg^-1 at protein concentrations 10 mg ml^-1. The specific rate of activation increased in a non-linear manner with protein concentration. Strain TMBS 4 degraded gallate, the product of sequential demethylations, to 3 acetate through the phloroglucinol pathway as found earlier with Pelobacter acidigallici.Abbreviations BV benzyl viologen - CTAB cetyltrimethylammonium bromide - H₄folate tetrahydrofolate - MOPS 3-[N-morpholino]propanesulfonic acid - MV methyl viologen - NTA nitrilotriacetate - t_d doubling time - TMB 3,4,5-trimethoxybenzoate     

The influence of pH and external H+ concentration on caesium toxicity and accumulation inEscherichia coli andBacillus subtilis

Summary Toxicity screening ofEscherichia coli NCIB 9484 andBacillus subtilis 007, NCIB 168 and NCIB 1650 has shown Cs⁺ to be the most toxic Group 1 metal cation. However, toxicity and accumulation of Cs⁺ by the bacteria was affected by two main external factors; pH and the presence of other monovalent cations, particularly K⁺. Over the pH range 6–9 bothE. coli andB. subtilis showed increasing sensitivity towards caesium as the pH was raised. The presence of K⁺ and Na⁺ in the laboratory media used lowered caesium toxicity and lowered acumulation of the metal. In order to assess accurately Cs⁺ toxicity towards the bacterial strains it was therefore necessary to define the K⁺:Cs⁺ ratio in the external medium. The minimum inhibitory K⁺:Cs⁺ concentration ratio for theBacillus strains tested was in the range 12–13 whileE. coli had a minimum inhibitory K⁺:Cs⁺ concentration ratio of 16.     

Effects of water extracts of a composted manure-straw mixture on the plant pathogen Botrytis cinerea

Manure-straw mixtures were composted and water extracts, made by incubating compost in water for 3 to 18 days, were assessed for antagonistic activity against Botrytis cinerea, using a range of tests. Extracts of all ages inhibited conidial germination on glass slides and reduced mycelial growth on agar. Mixing extracts of all ages with droplets of suspensions of B. cinerea conidia on detached Phaseolus bean leaves suppressed lesion development, but only 3- to 8-day-old extracts had an effect when sprayed onto leaves 2 days before inoculation. Extracts contained a large and varied microbial population of actinomycetes (0.3 to 2.4×10⁵ c.f.u.ml^–1), bacteria (1.5 to 5.6×10¹⁰ c.f.u.ml^–1), filamentous fungi (25.0 to 45.5 c.f.u. ml^–1) and yeasts (26.1 to 62.6 c.f.u.ml^–1). Eight- and 18-day-old extracts lost activity completely on filter sterilization or autoclaving. Weekly sprays of 8-day-old extracts onto lettuce in the glasshouse had no effect on the incidence of grey mould, but significantly reduced its severity and increased marketable yield. The use of compost extracts in biocontrol of plant diseases and their possible mode of action is discussed.M.P. McQuilken and J.M. Whipps are and J.M. Lynch was with the Microbiology and Crop Protection Department, Horticulture Research International, Littlehampton BN17 6LP, UK; J.M. Lynch is now with the School of Biological Sciences, University of Surrey, Guildford GU2 5XH, UK.