Theory of antimicrobial combinations: biocide mixtures - synergy or addition?

Hormesis and synergistic effects of disinfectants chloroxylenol and benzethonium chloride on highly efficient heterotrophic nitrification-aerobic denitrification functional strain: From performance to mechanism

The heterotrophic nitrification-aerobic denitrification (HNAD) strain Exiguobacterium H1 (H1) was isolated in this study. The changes in nitrogen metabolism functions of H1 strain were discussed in presence of disinfectants chloroxylenol (PCMX) and benzethonium chloride (BEC) alone and combined pollution (PCMX+BEC). The H1 strain could use NH4+-N, NO2--N and NO3--N as nitrogen sources and had good nitrogen removal performance under conditions of C/N ratio 25, pH 5-8, 25-35 oC and sodium acetate as carbon. PCMX and BEC alone exhibited hormesis effects on H1 strain which promoted the growth of H1 strain at low concentrations but inhibited it at high concentrations, and combined pollution showed synergistic inhibitory on H1 strain. H1 strain owned a full nitrogen metabolic pathway according to functional genes quantification. PCMX encouraged nitrification process of H1, while BEC and combined pollution mostly blocked nitrogen removal. PCMX, but not BEC, mainly led to the enrichment of resistance genes. These findings will aid in systematic assessment of contaminant tolerance characteristics of HNAD strain and its application prospects.

Antimicrobial adhesive films by plasma-enabled polymerisation of m-cresol

This work reveals a versatile new method to produce films with antimicrobial properties that can also bond materials together with robust tensile adhesive strength. Specifically, we demonstrate the formation of coatings by using a dielectric barrier discharge (DBD) plasma to convert a liquid small-molecule precursor, m-cresol, to a solid film via plasma-assisted on-surface polymerisation. The films are quite appealing from a sustainability perspective: they are produced using a low-energy process and from a molecule produced in abundance as a by-product of coal tar processing. This process consumes only 1.5 Wh of electricity to create a 1 cm² film, which is much lower than other methods commonly used for film deposition, such as chemical vapour deposition (CVD). Plasma treatments were performed in plain air without the need for any carrier or precursor gas, with a variety of exposure durations. By varying the plasma parameters, it is possible to modify both the adhesive property of the film, which is at a maximum at a 1 min plasma exposure, and the antimicrobial property of the film against Escherichia coli, which is at a maximum at a 30 s exposure.

Synergism versus Additivity: Defining the Interactions between Common Disinfectants

Many of the most common disinfectant and sanitizer products are formulations of multiple antimicrobial compounds. Products claiming to contain synergistic formulations are common, although there is often little supporting evidence. The antimicrobial interactions of all pairwise combinations of common disinfectants (benzalkonium chloride, didecyldimethylammonium chloride, polyhexamethylene biguanide, chlorocresol, and bronopol) were classified via checkerboard assay and validated by time-kill analyses. Combinations were tested against Acinetobacter baumannii NCTC 12156, Enterococcus faecalis NCTC 13379, Klebsiella pneumoniae NCTC 13443, and Staphylococcus aureus NCTC 13143. Synergistic interactions were identified only for the combinations of chlorocresol with benzalkonium chloride and chlorocresol with polyhexamethylene biguanide. Synergism was not ubiquitously demonstrated against all species tested and was on the borderline of the synergism threshold. These data demonstrate that synergism between disinfectants is uncommon and circumstantial. Most of the antimicrobial interactions tested were characterized as additive. We suggest that this is due to the broad, nonspecific mechanisms associated with disinfectants not providing an opportunity for the combined activities of these compounds to exceed the sum of their parts.

The silent pandemic: Emergent antibiotic resistances following the global response to SARS-CoV-2

The ongoing SARS-CoV-2 pandemic has highlighted the importance of the rapid development of vaccines and antivirals. However, the potential for the emergence of antibiotic resistances due to the increased use of antibacterial cleaning products and therapeutics presents an additional, underreported threat. Most antibacterial cleaners contain simple quaternary ammonium compounds (QACs); however, these compounds are steadily becoming less effective as antibacterial agents. QACs are extensively used in SARS-CoV-2-related sanitization in clinical and household settings. Similarly, due to the danger of secondary infections, antibiotic therapeutics are increasingly used as a component of COVID-19 treatment regimens, even in the absence of a bacterial infection diagnosis. The increased use of antibacterial agents as cleaners and therapeutics is anticipated to lead to novel resistances in the coming years.

The effect of EDTA in combination with some antibiotics against clinical isolates of gram negative bacteria in Mansoura, Egypt

Extensive use of antibiotics in clinical practice has been associated with increasing frequency of resistant microorganisms. So new strategy is needed to treat the resistant pathogens. Hence this study was conducted to determine the effect of Ethylenediaminetetraacetic acid (EDTA) in increasing the inhibition effect of some antibiotics on multi-drug resistant (MDR) gram-negative bacteria. For this purpose, 40 E. coli isolates, 40 K. pneumoniae isolates and 50 P. aeruginosa isolates were collected from different University's hospitals in Mansoura, Egypt. Antibacterial susceptibility pattern against 9 different antimicrobials were studied by disc diffusion method. Also the effect of two sub-inhibitory concentrations of EDTA (1 and 2 mM) on the inhibition zones of antibiotic discs against the highly multidrug resistant (MDR) isolates was determined. Checkerboard method was used for testing the activity of gentamicin/EDTA and cefotaxime/EDTA combinations on the highly MDR isolates. Additionally, the effect of EDTA on the expression of efflux pump genes was tested by real time-PCR. Most of the clinical isolates were found to be resistant to the tested antibiotics except imipenem and high prevalence of MDR isolates was recorded. 34 isolates were selected as those showed the highest multi-drug resistance and were tested to specify their MIC for EDTA as EDTA showed strong antibacterial activity with MIC ranging 4-8 mM. The addition of sub-MIC of EDTA (1or 2 mM) to the agar plate resulted in changing the 11 tested E. coli isolates from resistant to sensitive to ceftazidime, gentamicin, rifampin, ampicillin, erythromycin and vancomycin, the tested K. pneumoniae isolates were turned also from resistant to sensitive to gentamicin and ceftazidime, additionally the tested P. aeruginosa isolates became sensitive to gentamicin, ceftazidime and ciprofloxacin. Indifference to additive activity was observed for tested combinations and MIC value of cefotaxime or gentamicin in combination with EDTA was less than antibiotic alone in the most tested isolates. Moreover, significant reduction (P < 0.01) in the expression of all tested efflux pump genes in treated E. coli, K. pneumoniae and P. aeruginosa isolates with EDTA compared to untreated isolates was observed. In conclusion, these results suggest that the combination of antibiotic especially gentamicin with EDTA may be fruitful for management of resistant gram negative infections.

Ways to improve biocides for metalworking fluid

Metalworking fluids (MWF) are mainly emulsions of oil in water containing additives such as corrosion inhibitors, emulsifiers, defoamers, and biocides. Microbial contamination of MWF is almost systematic, and some of their constituents serve as nutrients for contaminating microorganisms. Biocides for MWF are protection products used to counter microbial contaminations and growth. Ideally, a biocide for MWF should have the following non-exhaustive criteria: have a broad-spectrum activity, be usable at low concentrations, be compatible with the formulation and the physical-chemical properties of MWF, be stable over time, retain its effectiveness in the presence of soiling, have no corrosive action on metals, present no danger to humans and the environment, be inexpensive. The future lies in the development of new molecules with biocidal activity corresponding to these ideal specifications, but in the meantime, it is possible to improve the performance of existing molecules currently on the market. Different strategies for potentiation of the activity of existing biocides are possible. The compatibility of the potentiation strategies with their use in metal working fluids is discussed.

Inactivation of Prions by Low-Temperature Sterilization Technology Using Vaporized Gas Derived from a Hydrogen Peroxide-Peracetic Acid Mixture

Prion diseases are proteopathies that cause neurodegenerative disorders in humans and animals. Prion is highly resistant to both chemical and physical inactivation. Here, vaporized gas derived from a hydrogen peroxide–peracetic acid mixture (VHPPA) was evaluated for its ability to inactivate prion using a STERIACE 100 instrument (Saraya Co., Ltd.). Brain homogenates of scrapie (Chandler strain) prion-infected mice were placed on a cover glass, air-dried, sealed in a Tyvek package, and subjected to VHPPA treatment at 50–55 °C using 8% hydrogen peroxide and <10% peracetic acid for 47 min (standard mode, SD) or 30 min (quick mode, QC). Untreated control samples were prepared in the same way but without VHPPA. The resulting samples were treated with proteinase K (PK) to separate PK-resistant prion protein (PrPres), as a marker of the abnormal isoform (PrP^Sc). Immunoblotting showed that PrPres was reduced by both SD and QC VHPPA treatments. PrPres bands were detected after protein misfolding cyclic amplification of control but not VHPPA-treated samples. In mice injected with prion samples, VHPPA treatment of prion significantly prolonged survival relative to untreated samples, suggesting that it decreases prion infectivity. Taken together, the results show that VHPPA inactivates prions and might be applied to the sterilization of contaminated heat-sensitive medical devices.

Tungsten carbide nanoparticles show a broad spectrum virucidal activity against enveloped and nonenveloped model viruses using a guideline-standardized in vitro test

Five tungsten carbide nanoparticle preparations (denoted WC1-WC5) were investigated for broad spectrum virucidal activity against four recommended model viruses. These are modified vaccinia virus Ankara (MVA), human adenovirus type 5 (HAdV-5), poliovirus type 1 (PV-1) and murine norovirus (MNV). All virucidal tests were performed two to five times using the quantitative suspension test, which is a highly standardized test method to evaluate the virucidal efficacy of disinfectants in accordance with the European norm EN 14476+A1 and the German DVV/RKI guidelines. Quantitative detection of viruses was conducted by endpoint titration and quantitative real-time PCR. Results showed that three of the five tested compounds (WC1-WC3) were able to reduce the infectivity of all model viruses by at least four log₁₀ of tissue culture infective dose 50% per ml after 15 min, whereas the other two compounds exhibited only limited efficacy (WC4) or showed cytotoxicity (WC5). Virucidal activity of nanoparticles increased with incubation time and a dose-effect curve showed dependence of virucidal activity with particle concentration. Whereas WC1-WC4 showed little cytotoxicity, WC5 which was doped with copper exhibited a significant cytotoxic effect. These findings propose tungsten carbide nanoparticles to be very promising in terms of new disinfection techniques. SIGNIFICANCE AND IMPACT OF THE STUDY: The present study investigates the virucidal activity of tungsten carbide nanoparticles using the quantitative suspension test in accordance with the European norm EN 14476+A1 and the German DVV/RKI guidelines. Due to highly standardized assay conditions, results of this test are considered very reliable for evaluation of the virucidal activity of disinfectants. Broad-spectrum activity and high efficacy of three different tungsten carbide nanoparticles preparations is concluded.

Synergistic reaction of silver nitrate, silver nanoparticles, and methylene blue against bacteria

In this paper we describe the antibacterial effect of methylene blue, MB, and silver nitrate reacting alone and in combination against five bacterial strains including Serratia marcescens and Escherichia coli bacteria. The data presented suggest that when the two components are combined and react together against bacteria, the effects can be up to three orders of magnitude greater than that of the sum of the two components reacting alone against bacteria. Analysis of the experimental data provides proof that a synergistic mechanism is operative within a dose range when the two components react together, and additive when reacting alone against bacteria.

An antibacterial copper composite more bioactive than metallic silver

Although known for its biocidal activity, copper is still not considered as a viable alternative to silver in many of its biocidal applications, mainly because it is generally considered to be a milder antibacterial metal. As copper is much cheaper than silver (1/100), it is potentially more accessible to the health and hygiene needs of third-world countries, to large volume consumer products, and to large-scale agricultural and water treatment needs. Therefore, enhancing the biocidal efficacy of copper is a sought-after goal. We report a method for achieving this goal: by entrapping molecules of the biocidal agent chlorhexidine (CH) within a metallic copper metal powder, using a new materials methodology, the antibacterial efficacy of copper towards two model nosocomial opportunistic bacteria - the Gram-negative Pseudomonas aeruginosa and the Gram-positive Staphylococcus epidermidis- is enhanced to provide a powerful antibacterial agent exceeding the activity of silver. ICP-MS elemental analysis and UV-spectroscopy indicated that the enhanced bactericidal effects of the synthesized composite, CH@Cu, are associated with the sustained release of both copper ions and CH, giving rise to synergistically enhanced activity.

Mechanism of Sporicidal Activity for the Synergistic Combination of Peracetic Acid and Hydrogen Peroxide

There is still great interest in controlling bacterial endospores. The use of chemical disinfectants and, notably, oxidizing agents to sterilize medical devices is increasing. With this in mind, hydrogen peroxide (H2O2) and peracetic acid (PAA) have been used in combination, but until now there has been no explanation for the observed increase in sporicidal activity. This study provides information on the mechanism of synergistic interaction of PAA and H2O2 against bacterial spores. We performed investigations of the efficacies of different combinations, including pretreatments with the two oxidizers, against wild-type spores and a range of spore mutants deficient in the spore coat or small acid-soluble spore proteins. The concentrations of the two biocides were also measured in the reaction vessels, enabling the assessment of any shift from H2O2 to PAA formation. This study confirmed the synergistic activity of the combination of H2O2 and PAA. However, we observed that the sporicidal activity of the combination is largely due to PAA and not H2O2. Furthermore, we observed that the synergistic combination was based on H2O2 compromising the spore coat, which was the main spore resistance factor, likely allowing better penetration of PAA and resulting in the increased sporicidal activity.

Polybiguanide (PHMB) loaded in PLA scaffolds displaying high hydrophobic, biocompatibility and antibacterial properties

Polyhexamethylenebiguanide hydrochloride (PHMB), a low molecular weight polymer related to chlorohexidine (CHX), is a well-known antibacterial agent. In this study, polylactide (PLA) nanofibers loaded with PHMB were produced by electrospinning to obtain 3D biodegradable scaffolds with antibacterial properties. PLA fibers loaded with CHX were used as control. The electrospun fibers were studied and analyzed by SEM, FTIR, DSC and contact angle measurements. PHMB and CHX release from loaded scaffolds was evaluated, as well as their antibacterial activity and biocompatibility. The results showed that the nanofibers became smoother and their diameter smaller with increasing the amount of loaded PHMB. This feature led to an increase of both surface roughness and hydrophobicity of the scaffold. PHMB release was highly dependent on the hydrophilicity of the medium and differed from that determined for CHX. Lastly, PHMB-loaded PLA scaffolds showed antibacterial properties since they inhibited adhesion and bacterial growth, and exhibited biocompatible characteristics for the adhesion and proliferation of both fibroblast and epithelial cell lines.

What should be considered in the treatment of bacterial infections by multi-drug therapies: a mathematical perspective?

Bacterial infections are a global health concern with high levels of mortality and morbidity associated. The resistance of pathogens to drugs is one leading cause of this problem, being common the administration of multiple drugs to improve the therapeutic effects. This review critically explores diverse aspects involved in the treatment of bacterial infections through multi-drug therapies, from a mathematical and within-host perspectives. Five recent models were selected and are reviewed. These models fall into the following question: which drugs to select, the respective dose, the administration period to effectively eradicate the infection in the shortest period of time and with reduced side effects? In this analysis, three groups of variables were considered: pharmacokinetics, pharmacodynamics and disturbance variables. To date, there is no model that fully answers to this issue for a living organism and it is questionable whether this would be possible for any case of infection.

Evaluation of the best method to assess antibiotic potentiation by phytochemicals against Staphylococcus aureus

The increasing occurrence of bacterial resistance to antibiotics has now reached a critical level. Finding antibiotic coadjuvants capable to inhibit the bacterial resistance mechanisms would be a valuable mid-term solution, until new classes of antibiotics are discovered. Selected plant alkaloids were combined with 5 antibiotics against 10 Staphylococcus aureus strains, including strains expressing distinct efflux pumps and methicillin-resistant S. aureus strains. The efficacy of each combination was assessed using the microdilution checkerboard, time-kill, Etest, and disc diffusion methods. The cytotoxicity of the alkaloids was evaluated in a mouse fibroblast cell line. Potentiation was obtained in 6% of all 190 combinations, especially with the combination of: ciprofloxacin with reserpine (RES), pyrrolidine (PYR), and quinine (QUIN); tetracycline with RES; and erythromycin with PYR. The highest cytotoxicity values were found for QUIN (half maximal inhibitory concentration [IC50] = 25 ± 2.2 mg/L) and theophylline (IC50 = 100 ± 4.7 mg/L).

When the most potent combination of antibiotics selects for the greatest bacterial load: the smile-frown transition

Conventional wisdom holds that the best way to treat infection with antibiotics is to 'hit early and hit hard'. A favoured strategy is to deploy two antibiotics that produce a stronger effect in combination than if either drug were used alone. But are such synergistic combinations necessarily optimal? We combine mathematical modelling, evolution experiments, whole genome sequencing and genetic manipulation of a resistance mechanism to demonstrate that deploying synergistic antibiotics can, in practice, be the worst strategy if bacterial clearance is not achieved after the first treatment phase. As treatment proceeds, it is only to be expected that the strength of antibiotic synergy will diminish as the frequency of drug-resistant bacteria increases. Indeed, antibiotic efficacy decays exponentially in our five-day evolution experiments. However, as the theory of competitive release predicts, drug-resistant bacteria replicate fastest when their drug-susceptible competitors are eliminated by overly-aggressive treatment. Here, synergy exerts such strong selection for resistance that an antagonism consistently emerges by day 1 and the initially most aggressive treatment produces the greatest bacterial load, a fortiori greater than if just one drug were given. Whole genome sequencing reveals that such rapid evolution is the result of the amplification of a genomic region containing four drug-resistance mechanisms, including the acrAB efflux operon. When this operon is deleted in genetically manipulated mutants and the evolution experiment repeated, antagonism fails to emerge in five days and antibiotic synergy is maintained for longer. We therefore conclude that unless super-inhibitory doses are achieved and maintained until the pathogen is successfully cleared, synergistic antibiotics can have the opposite effect to that intended by helping to increase pathogen load where, and when, the drugs are found at sub-inhibitory concentrations.

Activity and synergistic antimicrobial activity between diketopiperazines against bacteria in vitro

The aim of the present study was to determine the synergistic effects of diketopiperazines [cyclo-(L-Pro-L-Leu) (1), cyclo-(D-Pro-L-Leu) (2), and cyclo-(D-Pro-L-Tyr) (3)] purified from a Bacillus sp. N strain associated with entomopathogenic nematode Rhabditis (Oscheius) sp. on the growth of bacteria. The minimum inhibitory concentration and minimum bactericidal concentration of the diketopiperazines was compared with that of the standard antibiotics. The synergistic antibacterial activities of the combination of diketopiperazines against pathogenic bacteria were assessed using the checkerboard assay and time-kill methods. The results of the present study showed that the combination effects of diketopiperazines were predominately synergistic (FIC index <0.5). Furthermore, time-kill study showed that the growth of the tested bacteria was completely attenuated with 4-12 h of treatment with 50:50 ratios of diketopiperazines. These results suggest that the combination of diketopiperazines may be microbiologically beneficial. The three diketopiperazines are nontoxic to normal human cell line (L231 lung epithelial) up to 200 m μg/ml. The in vitro synergistic activity of cyclo-(L-Pro-L-Leu), cyclo-(D-Pro-L-Leu), and cyclo-(D-Pro-L-Tyr) against bacteria is reported here for the first time. These findings have potential implications in delaying the development of resistance as the antibacterial effect is achieved with lower concentrations of both drugs (diketopiperazines).

Bactericidal action of binary and ternary mixtures of carvacrol, thymol, and eugenol against Listeria innocua

The bactericidal effect of 3 natural agents (carvacrol, thymol, and eugenol) was evaluated as well as their binary and ternary mixtures on Listeria innocua inactivation in liquid model systems. Minimal bactericidal concentrations (MBC) of these agents were determined, and then binary and ternary mixtures were evaluated. Culture media were inoculated with L. innocua and incubated for 72 h at 35 °C. Turbidity of studied systems were determined every 24 h. The most effective individual antimicrobial agent was carvacrol, followed by thymol and then eugenol with MBCs of 150, 250, and 450 mg kg(-1), respectively. It was observed that the most effective binary mixture was 75 mg kg(-1) carvacrol and 62.5 mg kg(-1) thymol. Furthermore, the ternary mixture carvacrol-thymol-eugenol in concentrations of 75, 31.25, and 56.25 mg kg(-1), correspondingly, was the most effective for L. innocua inactivation. Several binary and ternary mixtures of these 3 natural antimicrobial agents worked adequately to inactivate L. innocua.

Enhancement of the antimicrobial performance of biocidal formulations used for the preservation of white mineral dispersions

Biocides play an important role in the preservation of white mineral dispersions (WMD). Due to the occurrence of biocide-resistant bacteria and technical limitations in the use of biocides, new preservation strategies are required-like the enhancement of biocides by non-biocidal compounds. The aim of this study was to evaluate the biocide enhancement performance of lithium against various biocide-resistant bacteria in WMD. Subsequently, the minimal enhancing concentration (MEC) of lithium and the bioavailability of lithium in respect to the mode of introduction into WMD were investigated. The antimicrobial performance of biocidal formulations comprising isothiazolinones and formaldehyde releasers or isothiazolinones and glutaraldehyde has been evaluated against the related resistant bacterial spectrum in the presence of lithium. The MEC of lithium ranged from 1,350 to 1,500 ppm (based on the liquid phase weight of a WMD with 75% solids) for formaldehyde releasers and glutaraldehyde-based biocidal formulations, respectively. The biocide enhancing property of lithium was independent of whether lithium was introduced into WMD via a lithium-neutralised dispersant, added during the calcium carbonate grinding step, or dosed into the final product. Lithium is a non-biocidal compound which has been discovered to be a potent and universal biocide enhancer. Lithium boosts the biocidal activity of various biocides and provides a novel technique to overcome biocide resistance in WMD. Such a biocide enhancer represents a breakthrough that offers a potential tool to revolutionise the consumption of biocidal agents in the WMD producing industry.

In vitro evidence of phyto-synergy for plant part combinations of Croton gratissimus (Euphorbiaceae) used in African traditional healing

AIM OF THE STUDY

Despite the extensive traditional use of Croton gratissimus Burch. var. gratissimus for medicinal purposes, scientific studies validating the therapeutic properties of this indigenous plant are lacking. As the bark, roots and leaves of C. gratissimus are used separately as well as in combination, this study focused on determining antimicrobial efficacies of the plant parts independently and in combination to assess possible pharmacological interactions (e.g. synergy, antagonism).

MATERIAL AND METHODS

The hydro-distilled leaf essential oil and extracts of bark, root and leaf were comparatively assessed for antimicrobial activity by means of microdilution minimum inhibitory concentration (MIC). The fractional inhibitory concentrations (FIC) were determined for the leaf and root (1:1), bark and root (1:1), leaf and bark (1:1) combination. Isobolograms were plotted to demonstrate interactions between various ratios of the roots and leaves.

RESULTS

The MIC and FIC results indicated variable efficacies for the various plant part combinations, the greatest of which was noted for Cryptococcus neoformans in the root and leaf combination (MIC 0.4 mg/ml and FIC of 0.4). Isobolograms indicated the greatest synergy for Bacillus cereus, Candida albicans and Cryptococcus neoformans.

CONCLUSION

The observed synergistic interactions clearly indicate that the reductionist approach may often be short-sighted and that biological activity may be improved through combination therapy, where different complex metabolic pools collectively contribute to the enhanced effect.

An investigation of the Gamma hypothesis: A predictive modelling study of the effect of combined inhibitors (salt, pH and weak acids) on the growth of Aeromonas hydrophila

The Gamma hypothesis, that multiple inhibitory factors combine independently, is the underlying hypothesis for the quantification of the Hurdle concept used in food manufacture. The literature, however, is confused as to whether interactive effects exist and under which circumstances they occur, if at all. Using the method of time to detection (TTD), the inhibitory effect of pH, salt and specific weak acids (acetic, propionic, sorbic and benzoic) and combinations of these with respect to the growth of Aeromonas hydrophila (ATCC 7966) were analysed. A model based on the relative rate to detection described all combinations analysed as having independent effects on the TTD. No synergistic interactions were found between pH and salt, between pH and individual weak acids or between combinations of weak acids and pH for any of the systems under study. This study supports the validity of the Gamma concept -- that individual environmental effects act independently and should, in turn, facilitate attempts to model the growth of other microorganisms under a variety of conditions.

Cyclic dipeptides exhibit synergistic, broad spectrum antimicrobial effects and have anti-mutagenic properties

Cyclic dipeptides are known to have antiviral, antibiotic and antitumour properties. The aim of this study was to determine the combined effects of cyclo(L-leucyl-L-prolyl) and cyclo(L-phenylalanyl-L-prolyl) on the growth of vancomycin-resistant enterococci (VRE) and pathogenic yeasts, as well as determining their anti-mutagenic effects. This drug combination was especially effective against five VRE strains: Enterococcus faecium (K-99-38), E. faecalis (K-99-17), E. faecalis (K-99-258), E. faecium (K-01-312) and E. faecalis (K-01-511) with MIC values of 0.25-1 mg/l. It was also effective against Escherichia coli, Staphylococcus aureus, Micrococcus luteus, Candida albicans and Cryptococcus neoformans with MIC values of 0.25-0.5 mg/l. In addition, the cyclic dipeptides exerted anti-mutagenic activity against Salmonella typhimurium TA98 and TA100 strains in a Salmonella mutation assay. The number of mutant colonies of S. typhimurium strains TA98 and TA100 induced by exposure to AF-2 (0.2 microg/plate) decreased in a concentration-dependent manner in the presence of the two cyclic dipeptides (correlation 0.72 and 0.78, respectively). Here, for the first time, we report synergistic effects of the cyclic dipeptides [cyclo(L-leu-L-pro) and cyclo(L-phe-L-pro)] in inhibiting the growth of pathogenic microorganisms, as well as their anti-mutagenic effects in Salmonella strains.

The synergistic effect of EDTA/antimicrobial combinations on Pseudomonas aeruginosa

AIMS

To demonstrate that the nonlinear concentration-dependent inhibition of Pseudomonas aeruginosa to EDTA can be used to successfully model and predict the potentiation of antimicrobials by EDTA.

METHODS AND RESULTS

A model used successfully to describe the concentration-dependent inhibition of bacterial growth caused by many antimicrobials was unable to describe the inhibition of P. aeruginosa by EDTA. Examination of the inhibition profiles for EDTA against P. aeruginosa revealed a biphasic inhibitory pattern suggesting different mechanisms of action at different concentrations. A modelled, two-stage inhibitory process was shown to fit the observations. This model was then used to examine the effect of combining EDTA with other antimicrobials. The apparent synergy of mixtures of EDTA with quaternary ammonium surfactants (QAC) and specific antibiotics was successfully modelled. Minimum inhibitory concentrations (MIC) of the QAC and that of oxacillin and cefamandole were reduced by a factor of 3-10, whereas ampicillin was reduced by a factor of 70 from an MIC of 1524 to 21 mg l(-1) in the presence of 500 mg l(-1) of EDTA.

CONCLUSIONS

A nonlinear concentration-dependent inhibition of P. aeruginosa by EDTA gives rise to apparent observation of synergy with other antimicrobials.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is a further example where the current methodology for the examination of antimicrobial synergy (the summed fractional inhibitory concentrations) leads to false conclusions.

Membrane damage to bacteria caused by single and combined biocides

AIMS

To examine the effect on the leakage of low molecular weight cytoplasmic constituents from Staphylococcus aureus using phenolics singly and in combination, and to see if the observations could be modelled using a non-linear dose response.

METHODS AND RESULTS

The rate of potassium, phosphate and adenosine triphosphate leakage was examined in the presence of chlorocresol and m-cresol. Individually, leakage was observed only at long contact times or high concentrations. Combined at these ineffective concentrations, the cytoplasmic pool of all constituents studied was released within minutes. Both chlorocresol and m-cresol were shown to have non-linear dose responses. A rate model for the combinations, which takes account of these non-linear responses, accurately predicted the observations.

CONCLUSIONS

Antimicrobials, which when used alone exhibit a non-linear dose response, will also give a non-linear dose response in combination. The simple linear-additive model ignores the concept of the dilution coefficient and will always describe the phenomenon of synergy for combinations where one or more of the components has a dilution coefficient greater than one. This has been borne out by examination of the purported prime lesion of chlorocresol and m-cresol, alone and in combination.

SIGNIFICANCE AND IMPACT OF THE STUDY

Studies aimed at producing synergistic mixtures of antimicrobials, which ignore the non-linear additive effect, may waste valuable research effort looking for a physiological explanation for an apparent synergy, where none, in-fact, exists. Patents granted on the basis of analyses using the linear-additive model for combinations of compounds with non-linear dose responses may no longer be supportable.