Binding between lipopolysaccharide and cecropin A

Multiple resistance factors collectively promote inoculum-dependent dynamic survival during antimicrobial peptide exposure in Enterobacter cloacae

Antimicrobial peptides (AMPs) are a promising tool with which to fight rising antibiotic resistance. However, pathogenic bacteria are equipped with several AMP defense mechanisms, whose contributions to AMP resistance are often poorly defined. Here, we evaluate the genetic determinants of resistance to an insect AMP, cecropin B, in the opportunistic pathogen Enterobacter cloacae. Single-cell analysis of E. cloacae's response to cecropin revealed marked heterogeneity in cell survival, phenotypically reminiscent of heteroresistance (the ability of a subpopulation to grow in the presence of supra-MIC concentration of antimicrobial). The magnitude of this response was highly dependent on initial E. cloacae inoculum. We identified 3 genetic factors which collectively contribute to E. cloacae resistance in response to the AMP cecropin: The PhoPQ-two-component system, OmpT-mediated proteolytic cleavage of cecropin, and Rcs-mediated membrane stress response. Altogether, our data suggest that multiple, independent mechanisms contribute to AMP resistance in E. cloacae.

Multiple resistance factors collectively promote inoculum-dependent dynamic survival during antimicrobial peptide exposure in Enterobacter cloacae

Antimicrobial peptides (AMPs) are a promising tool with which to fight rising antibiotic resistance. However, pathogenic bacteria are equipped with several AMP defense mechanisms, whose contributions to AMP resistance are often poorly defined. Here, we evaluate the genetic determinants of resistance to an insect AMP, cecropin B, in the opportunistic pathogen Enterobacter cloacae. Single-cell analysis of E. cloacae's response to cecropin revealed marked heterogeneity in cell survival, phenotypically reminiscent of heteroresistance (the ability of a subpopulation to grow in the presence of supra-MIC concentration of antimicrobial). The magnitude of this response was highly dependent on initial E. cloacae inoculum. We identified 3 genetic factors which collectively contribute to E. cloacae resistance in response to the AMP cecropin: The PhoPQ-two-component system, OmpT-mediated proteolytic cleavage of cecropin, and Rcs-mediated membrane stress response. Altogether, this evidence suggests that multiple, independent mechanisms contribute to AMP resistance in E. cloacae.

Invited review: Mechanisms of endotoxin neutralization by synthetic cationic compounds

A basic challenge in the treatment of septic patients in critical care units is the release of bacterial pathogenicity factors such as lipopolysaccharide (LPS, endotoxin) from the cell envelope of Gram-negative bacteria due to killing by antibiotics. LPS aggregates may interact with serum and membrane proteins such as LBP (lipopolysaccharide-binding protein) and CD14 leading to the observed strong reaction of the immune system. Thus, an effective treatment of patients infected by Gram-negative bacteria must comprise beside bacterial killing the neutralization of endotoxins. Here, data are summarized for synthetic compounds indicating the stepwise development to very effective LPS-neutralizing agents. These data include synthetic peptides, based on the endotoxin-binding domains of natural binding proteins such as lactoferrin, Limulus anti-LPS factor, NK-lysin, and cathelicidins or based on LPS sequestering polyamines. Many of these compounds could be shown to act not only in vitro, but also in vivo (e.g . in animal models of sepsis), and might be useful in future clinical trials and in sepsis therapy.

Application of circular dichroism for structural analysis of surface-immobilized cecropin A interacting with lipoteichoic acid

The development of biomaterials integrating antimicrobial peptides (AMPs) for improved pathogen detection or use as therapeutic agents requires an understanding of how a peptide may behave once immobilized. Here, we use a combination of circular dichroism and capture assays to assess the structure-function relationship of the cationic amphipathic AMP, cecropin A (cecA), upon interaction with Gram-positive lipoteichoic acids (LTAs). In solution, cecA peptides underwent a change from a largely unstructured conformation in water to structures with significant α-helical content in the presence of both Bacillus subtilis and Staphylococcus aureus LTAs. After surface immobilization, cecA peptides attached by either C- or N-terminus were able to capture both LTAs as well as to undergo conformational changes in the presence of SDS similar to those observed in solution. However, in spite of demonstrated LTA binding activity and the ability to undergo conformational changes (i.e., with SDS), no structural changes were observed when cecA immobilized by its N-terminus was treated with either LTA preparation. On the other hand, cecA immobilized by its C-terminus underwent a conformational change in the presence of S. aureus, but not B. subtilis, LTA. These results indicate that after immobilization recognition of different targets by cationic AMPs may occur by mechanisms quite different from those in solution and that selectivity of these mechanisms is further dependent on the orientation of the immobilized peptide.

Construction of supported lipid membrane modified piezoelectric biosensor for sensitive assay of cholera toxin based on surface-agglutination of ganglioside-bearing liposomes

A novel piezoelelctric biosensor has been developed for cholera toxin (CT) detection based on the analyte-mediated surface-agglutination of ganglioside (GM1)-functionalized liposomes. To achieve a CT-specific agglutination at the surface, the gold electrode is modified by a GM1-functionalized supported lipid membrane via spontaneous spread of the liposomes on a self-assembled monolayer of a long-chain alkanethiol. In the presence of CT, the GM1-incorporated liposomes in assay medium will rapidly specifically agglutinate at the electrode surface through the binding of CT to GM1 on the electrode surface and the liposome interface. This results in an enormous mass loading on the piezoelelctric crystal as well as a significant increase of density and viscosity at the interface, thereby generating a decrease in frequency of the piezoelelctric crystal. The combination of mass loading with interfacial change in the surface-agglutination reaction allows the developed piezoelelctric biosensor to show substantial signal amplification in response to the analyte CT. The detection limit can be achieved as low as 25 ng mL(-1) CT. This is the first demonstration on CT detection based on specific surface-agglutination of GM1-modified liposomes. The supported lipid layer based sensing interface can be prepared readily and renewably, making the developed technique especially useful for simple, reusable and sensitive determination of proteins.

The role of endotoxin in infection: Helicobacter pylori and Campylobacter jejuni

Both Helicobacter pylori and Campylobacter jejuni are highly prevalent Gram-negative microaerophilic bacteria which are gastrointestinal pathogens of humans; H. pylori colonizes the gastroduodenal compartment and C. jejuni the intestinal mucosa. Although H. pylori causes chronic gastric infection leading to gastritis, peptic ulcers and eventually gastric cancer while C. jejuni causes acute infection inducing diarrhoeal disease, the endotoxin molecules of both bacterial species contrastingly contribute to their pathogenesis and the autoimmune sequelae each induces. Compared with enterobacterial endotoxin, that of H. pylori has significantly lower endotoxic and immuno-activities, the molecular basis for which is the underphosphorylation and underacylation of the lipid A component that interacts with immune receptors. This induction of low immunological responsiveness by endotoxin may aid the prolongation of H. pylori infection and therefore infection chronicity. On the other hand, this contrasts with acute infection-causing C. jejuni where overt inflammation contributes to pathology and diarrhoea production, and whose endotoxin is immunologically and endotoxically active. Futhermore, both H. pylori and C. jejuni exhibit molecular mimicry in the saccharide components of their endotoxins which can induce autoreactive antibodies; H. pylori expresses mimicry of Lewis and some ABO blood group antigens, C. jejuni mimicry of gangliosides. The former has been implicated in influencing the development of inflammation and gastric atrophy (a precursor of gastic cancer), the latter is central to the development of the neurological disorder Guillain-Barré syndrome. Both diseases raise important questions concerning infection-induced autoimmunity awaiting to be addressed.

An ultrasensitive chemiluminescence biosensor for cholera toxin based on ganglioside-functionalized supported lipid membrane and liposome

A novel chemiluminescence biosensor based on a supported lipid layer incorporated with ganglioside GM1 was developed for the detection of cholera toxin. The planar supported lipid membrane was prepared as biosensing interface via spontaneous spread of ganglioside-incorporated phospholipid vesicles on the octadecanethiol-coated gold surface. The specific interaction of multivalent CT by ganglioside GM1 molecules enables the biosensor to be implemented via a sandwiched format using a liposome probe functionalized with GM1 and horseradish peroxidase (HRP). Then, the presence of the target CT could be determined via the HRP-catalyzed enhanced chemiluminescence reaction. The developed strategy offers several unique advantages over conventional biosensors in that it allows for an easy construction and renewal of the sensing interface, a small background signal due to low non-specific adsorption of serum constituents on the lipid membrane, and effective immobilization of multiple biocatalytic amplifiers and recognition components via common phospholipid reagents. The developed biosensor was shown to give chemiluminescence signal in linear correlation to CT concentration within the range from 1pgmL(-1) to 1ngmL(-1) with readily achievable detection limit of 0.8pgmL(-1).

Application of Ganglioside-Sensitized Liposomes in a Flow Injection Immunoanalytical System for the Determination of Cholera Toxin

Cholera, an acute infectious disease associated with water and seafood contamination, is caused by the bacterium Vibrio cholerae, which lives and colonizes in the small intestine and secretes cholera toxin (CT), a causative agent for diarrhea in humans. Based on earlier lateral flow assays, a flow injection liposome immunoanalysis (FILIA) system with excellent sensitivity was developed in this study for the determination of CT at zeptomole levels. Ganglioside (GM1), found to have specific affinity toward CT, was inserted into the phospholipid bilayer during the liposome synthesis. These GM1-sensitized, sulforhodamine B (SRB) dye-entrapping liposomes were used as probes in the FILIA system. Anti-CT antibodies were immobilized in its microcapillary. CT was detected by the formation of a sandwich complex between the immobilized antibody and GM1 liposomes. During the assay, the sample was introduced first into the column, and then liposomes were injected to bind to all CT captured by the antibody in the microcapillary. Subsequently, the SRB dye molecules were released from the bound liposomes via the addition of the detergent octyl glucopyranoside. The released dye molecules were transported to a flow-through fluorescence detector for quantification. The FILIA system was optimized with respect to flow rate, antibody concentration, liposome concentration, and injected sample volume. The calibration curve for CT had a linear range of 10-16 to 10-14 g mL-1. The detection limit of this immunosensor was 6.6 x 10(-17) g mL-1 in 200-microL samples (equivalent to 13 ag or 1.1 zmol).

The lipid A 1-phosphatase of Helicobacter pylori is required for resistance to the antimicrobial peptide polymyxin

Modification of the phosphate groups of lipid A with amine-containing substituents, such as phosphoethanolamine, reduces the overall net negative charge of gram-negative bacterial lipopolysaccharide, thereby lowering its affinity to cationic antimicrobial peptides. Modification of the 1 position of Helicobacter pylori lipid A is a two-step process involving the removal of the 1-phosphate group by a lipid A phosphatase, LpxEHP (Hp0021), followed by the addition of a phosphoethanolamine residue catalyzed by EptAHP (Hp0022). To demonstrate the importance of modifying the 1 position of H. pylori lipid A, we generated LpxEHP-deficient mutants in various H. pylori strains by insertion of a chloramphenicol resistance cassette into lpxEHP and examined the significance of LpxE with respect to cationic antimicrobial peptide resistance. Using both mass spectrometry analysis and an in vitro assay system, we showed that the loss of LpxEHP activity in various H. pylori strains resulted in the loss of modification of the 1 position of H. pylori lipid A, thus confirming the function of LpxEHP. Due to its unique lipid A structure, H. pylori is highly resistant to the antimicrobial peptide polymyxin (MIC > 250 microg/ml). However, disruption of lpxEHP in H. pylori results in a dramatic decrease in polymyxin resistance (MIC, 10 microg/ml). In conclusion, we have characterized the first gram-negative LpxE-deficient mutant and have shown the importance of modifying the 1 position of H. pylori lipid A for resistance to polymyxin.

A bactericidal cecropin-A peptide with a stabilized alpha-helical structure possess an increased killing capacity but no proinflammatory activity

Antibacterial peptides are part of the innate immune system in a variety of different species including humans. Some of these peptides have also been shown to have effects on immune competent cells such as professional phagocytes. We have recently shown that a cecropin-like peptide from Helicobacter pylori, Hp(2-20), in addition to being bactericidal possesses proinflammatory effects and can recruit and activate neutrophils as well as monocytes. It is well established that cecropins have the ability to adopt amphipathic alpha-helices, which is thought to be required for their bactericidal activity. In this study we show the same structural requirements for Hp(2-20). Breaking the helical structure of Hp(2-20) reduced the antibacterial effect and abolished its proinflammatory activity. A C-terminal truncated cecropin A peptide that highly resembles Hp(2-20) failed to activate neutrophils and computer-based structural simulations revealed a difference between the two peptides in the stability of their helical structures. A hybrid peptide with amino acid substitutions stabilizing the alpha-helical structure of the truncated cecropin A peptide did not introduce any proinflammatory activity; the bactericidal activity was, however, increased. We thus conclude that the proinflammatory effect of Hp(2-20) is a unique sequence-specific feature of the peptide and the ability to adopt a stable amphipathic helix is a necessary but not sufficient criterion for the functional dualism of the peptide.

The interaction of cationic antimicrobial peptides with vesicles containing synthetic glycolipids as models of the outer membrane of gram-negative bacteria

Two simple lipid A analogues methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside (GL1) and methyl 2,3-di-O-tetradecanoyl-alpha-D-glucopyranoside 4-O-phosphate (GL2) were synthesized and used for preparing mixed phosphocholine vesicles as models of the outer membrane of gram-negative bacteria. The interaction of these model membranes with magainin 2, a representative of the alpha-helical membrane active peptides, and apidaecin Ib and drosocin, two insect Pro-rich peptides which do not act at the level of the cellular membrane, were studied by CD and dye-releasing experiments. The CD spectra of apidaecin Ib and drosocin in the presence of GL1- or GL2-containing vesicles were consistent with largely unordered structures, whereas, according to the CD spectra, magainin 2 adopted an amphipathic alpha-helical conformation, particularly in the presence of negatively charged bilayers. The ability of the peptides to fold into amphipathic conformations was strictly correlated to their ability to bind and to permeabilize phospholipid as well as glycolipid membranes. Apidaecin Ib and drosocin, which are unable to adopt an amphipathic structure, showed negligible dye-leakage activity even in the presence of GL2-containing vesicles. It is reasonable to suppose that, as for the killing mechanism, the two classes of antimicrobial peptides follow different patterns to cross the bacterial outer membrane.

Specific interactions of the antimicrobial peptide cyclic beta-sheet tachyplesin I with lipopolysaccharides

The cyclic beta-sheet antimicrobial peptide tachyplesin I (T-SS) was found to show 280-fold higher affinity for lipopolysaccharides (LPS) compared with acidic phospholipids, whereas the linear alpha-helical peptide F5W-magainin 2 (MG2) could not discriminate between LPS and acidic phospholipids. The recognition site was the lipid A moiety and the cyclic structure was crucial to this specific binding. The cyclic structure also endowed the peptide with very rapid outer membrane (OM) permeabilization.

CAY-I, a fungicidal saponin from Capsicum sp. fruit

Saponins are steroidal or terpenoid-based glycosides with surface active properties. A steroidal saponin, CAY-1, with a molecular weight of 1243.35 Da, was isolated and purified to homogeneity from commercially available dry, ground fruit of Capsicum frutescens. CAY-1 was shown to be a potent fungicide for the germinating conidia of Aspergillus flavus, A. fumigatus, A. parasiticus and A. niger with species-dependent LD90 values between 3 and 20 microM. Activity against some Aspergillus species was affected by the test medium used. In vitro assays, CAY-1 was effective against Pneumocystis carinii (IC50): 9.5 microM) and Candida albicans (IC90: 6.2 microM). CAY-1 had no effect on the viability of the nongerminating conidia of the two filamentous fungi, P. carinii and C. albicans, nor on the conidial type of Fusarium oxysporum. It was ineffective against the bacteria Enterobacter agglomerans, Bacillus subtilis, Escherichia coli and Staphylococcus aureus. CAY-1 was not cytotoxic to A 549 lung carcinoma cells or HeLa cells at effective fungicidal concentrations. The results indicate that CAY-1 is an effective fungicide for Aspergillus species, C. albicans and P. carinii at concentrations below the threshold for mammalian cell toxicity.

Evaluation of peracid formation as the basis for resistance to infection in plants transformed with haloperoxidase

Nonheme haloperoxidase (HPO-P) isolated from Pseudomonas pyrrocinia catalyzed the peroxidation of alkyl acids to peracids. Among acids tested as substrates, acetic acid was most readily peroxidized. The reaction product peracetate possessed potent antifungal activity: 50% death (LD(50)) of Aspergillus flavus occurred at 25 microM peracetate. Viability of A. flavus was inhibited by up to 80% by leaf extracts of tobacco plants transformed with the HPO-P gene from P. pyrrocinia compared to viability of fungi exposed to extracts from controls. To elucidate if peracid formation by HPO-P was the basis for antifungal activity in transgenic leaf tissues, lethalities of hydrogen peroxide-acetate-HPO-P combinations against A. flavus were examined in vitro. LD(50) of A. flavus exposed to the combinations occurred at 30 mM acetate when concentrations of hydrogen peroxide and HPO-P were held constant. This value was identical to the LD(50) produced by 30 mM acetate in the absence of hydrogen peroxide-HPO-P and therefore did not account for enhanced antifungal activity in transgenic plants. For clarification, kinetics of the enzymic reaction were examined. According to the concentration of acetate needed for enzyme saturation (K(m) = 250 mM), acetate was lethal prior to its oxidation to peracetate. Results indicate that peracid generation by HPO-P was not the basis for enhanced antifungal activity in transgenic plants expressing the HPO-P gene.

All-D-cecropin B: synthesis, conformation, lipopolysaccharide binding, and antibacterial activity

Cecropin B (LCB) is a natural peptide with antibacterial and antifungal properties. The enantiomer of LCB, containing all-D amino acids (DCB), was synthesized to examine its antibacterial and binding properties. The conformation of DCB was compared to its enantiomer by circular dichroism. Both the L- and D-peptides showed an identical induction of alpha-helical secondary structure. However, binding studies between Lipopolysaccharide (LPS) and DCB or LCB were studied with a dimethylmethylene blue spectrophotometric assay, showing the two enantiomeric peptides differed in their interaction with LPS. Antibacterial activity of DCB was determined against three Gram-negative bacteria, Pantoea agglomerans (ATCC 27996), Escherichia coli (ATCC 8739), and Pseudomonas aeruginosa (ATCC 17648), giving comparable results to LCB.

Apolipophorin-III and the interactions of lipoteichoic acids with the immediate immune responses of Galleria mellonella

We investigated the effects of lipoteichoic acids, surface components of Gram-positive bacteria, on the hemocytes and phenoloxidase activity in last instar Galleria mellonella larvae, as well as the binding of apolipophorin-III, an insect lipid-binding protein, to lipoteichoic acids. Binding of apolipophorin-III to lipoteichoic acid was studied using an assay based on 1,9-dimethylmethylene blue. Apolipophorin-III bound the lipoteichoic acids from Bacillus subtilis, Enterococcus hirae, and Streptococcus pyogenes and to intact cells of E. hirae. E. hirae lipoteichoic acid promoted the binding of apolipophorin-III to the cells of this species. All lipoteichoic acids tested caused a dose- and time-dependent drop in the total counts of hemocytes and, depending on the species of lipoteichoic acid, partial or complete depletion of plasmatocytes. Granulocyte counts were not affected. Apolipophorin-III prevented partially the loss of plasmatocytes due to B. subtilis lipoteichoic acid. All three lipoteichoic acids studied activated phenoloxidase in vitro; injections of B. subtilis lipoteichoic acid into the larvae elevated the phenoloxidase activity, whereas injections of E. hirae or S. pyogenes lipoteichoic acid, or apolipophorin-III alone, suppressed it. Apolipophorin-III decreased the activation of phenoloxidase by B. subtilis lipoteichoic acid.

Antifungal and peroxidative activities of nonheme chloroperoxidase in relation to transgenic plant protection

Nonheme chloroperoxidase (CPO-P) of Pseudomonas pyrrocinia catalyzes the oxidation of alkyl acids to peracids by hydrogen peroxide. Alkyl peracids possess potent antifungal activity as found with peracetate: 50% killing (LD(50)) of Aspergillus flavus occurred at 25 microM compared to 3.0 mM for the hydrogen peroxide substrate. To evaluate whether CPO-P could protect plants from fungal infection, tobacco was transformed with a gene for CPO-P from P. pyrrocinia and assayed for antifungal activity. Leaf extracts from transformed plants inhibited growth of A. flavus by up to 100%, and levels of inhibition were quantitatively correlated to the amounts of CPO-P activity expressed in leaves. To clarify if the peroxidative activity of CPO-P could be the basis for the increased resistance, the antifungal activity of the purified enzyme was investigated. The LD(50) of hydrogen peroxide combined with CPO-P occurred at 2.0 mM against A. flavus. Because this value was too small to account for the enhanced antifungal activity of transgenic plants, the kinetics of the enzyme reaction was examined and it was found that the concentration of hydrogen peroxide needed for enzyme saturation (K(m) = 5.9 mM) was already lethal. Thus, the peroxidative activity of CPO-P is not the basis for antifungal activity or enhanced resistance in transgenic plants expressing the gene.

Diversity of antimicrobial peptides and their mechanisms of action

Antimicrobial peptides encompass a wide variety of structural motifs. Many peptides have alpha-helical structures. The majority of these peptides are cationic and amphipathic but there are also hydrophobic alpha-helical peptides which possess antimicrobial activity. In addition, some beta-sheet peptides have antimicrobial activity and even antimicrobial alpha-helical peptides which have been modified to possess a beta-structure retain part of their antimicrobial activity. There are also antimicrobial peptides which are rich in a certain specific amino acid such as Trp or His. In addition, antimicrobial peptides exist with thio-ether rings, which are lipopeptides or which have macrocyclic Cys knots. In spite of the structural diversity, a common feature of the cationic antimicrobial peptides is that they all have an amphipathic structure which allows them to bind to the membrane interface. Indeed, most antimicrobial peptides interact with membranes and may be cytotoxic as a result of disturbance of the bacterial inner or outer membranes. Alternatively, a necessary but not sufficient property of these peptides may be to be able to pass through the membrane to reach a target inside the cell. The interaction of these peptides with biological membranes is not just a function of the peptide but is also modulated by the lipid components of the membrane. It is not likely that this diverse group of peptides has a single mechanism of action, but interaction of the peptides with membranes is an important requirement for most, if not all, antimicrobial peptides.

The origin of cecropins; implications from synthetic peptides derived from ribosomal protein L1

We recently showed that Helicobacter pylori grown on plates produce cecropin-like antibacterial peptides to which H. pylori is resistant. This antibacterial activity was traced to fragments from the N-terminus of ribosomal protein L1 (Pütsep et al., Nature, April 22, 1999). The evolutionary suggestion from this finding has now been extended by the synthesis of eight peptides with sequences taken from the N-terminus of ribosomal protein L1 (RpL1) of five different species. Two peptides of different length derived from H. pylori RpL1 showed a potent antibacterial activity, while a peptide with the sequence from Escherichia coli was 20 times less active. Like cecropins the H. pylori peptides were not cytolytic. We suggest that the cecropins have evolved from ribosomal protein L1 of an ancestral intracellular pathogen that developed to a symbiont ending as an organelle. When the R1 gene moved into the host nucleus, a duplication provided a copy from which today cecropins could have evolved.

Effects of chloroperoxidase and hydrogen peroxide on the viabilities of Aspergillus flavus conidiospores

The effects of chloroperoxidase [EC 1.1.1.10] and hydrogen peroxide on the viabilities of quiescent and germinating conidiospores of an aflatoxigenic fungus, Aspergillus flavus, were determined. Hydrogen peroxide was found moderately lethal and chloroperoxidase produced a 30-fold increase in the lethality of hydrogen peroxide to germinating conidia, which were 75-fold more susceptible to chloroperoxidase than were quiescent conidia. According to infrared examinations of fungal corpses, mortality occurred by oxidation rather than peroxidative chlorination.

Haemolymph proteins of larvae of Galleria mellonella detoxify endotoxins of the insect pathogenic bacteria Xenorhabdus nematophilus (Enterobacteriaceae)

Haemolymph of non-vaccinated Galleria mellonella larvae contains two proteins, LBP-1 (17.2kDa) and LBP-2 (26.0kDa) that:bond to the surfaces of the insect pathogenic bacteria, Xenorhabdus nematophilus;prevented lipid A-binding dye attaching to the lipid A of X. nematophilus endotoxin; andreduced endotoxin activity on the haemocytes.Protein LBP-1 also blocked the inhibition of prophenoloxidase activation by the endotoxins. It is proposed that proteins LBP-1 and LBP-2 are part of the containment responses of the insects to bacteria.

Fungicidal activity of cecropin A

Cecropin A (CA) fungicidal properties were explored. Nongerminated and germinated Aspergillus spp. and Fusarium spp. conidia were treated with CA. CA achieved complete lethality at < or = 25 microM (99 micrograms/ml) for germinating, but not nongerminating, conidia of Aspergillus spp. CA achieved total lethality for nongerminated and germinating conidia of Fusarium spp at 1.5 microM (6 micrograms/ml). MIC and minimal lethal concentration assays in buffered RPMI medium gave similar results.