Yeast susceptibility testing: reference methods and commercial test systems

Synergistic antimicrobial activity by combining an allylamine with benzoyl peroxide with expanded coverage against yeast and bacterial species

BACKGROUND

Dermatophyte infections can be polymicrobial. Topical antifungal therapies offer limited coverage of yeasts and Gram-positive and Gram-negative bacteria. Moreover, the increased usage of these topical antimicrobial agents has resulted in the development of resistant cases. Benzoyl peroxide (BP), used in concert with antimicrobial agents containing an accessible tertiary amine, has previously been shown to increase radical activity and biological effect.

OBJECTIVES

To determine the applicability of using the tertiary amine terbinafine in concert with BP in dermatophyte and mixed skin infections by means of in vitro testing.

METHODS

In this preliminary in vitro study, the effect of BP, alone and in combination with terbinafine, was tested against Candida albicans, Pseudomonas aeruginosa and Staphylococcus aureus isolates following a checkerboard modification of the National Committee for Clinical Laboratory Standards M27-A2 and M7-A6. The individual minimum inhibitory concentrations of terbinafine, BP, and the combination, were determined against each isolate.

RESULTS

The combination of BP with terbinafine led to additive activities against the majority of Candida albicans isolates tested and additionally expanded the bacterial coverage of terbinafine.

CONCLUSIONS

The combination of antifungal agents bearing a tertiary amine with BP may have benefit in polymicrobial infections, given its wider antimicrobial coverage. Further appreciation of this mechanism of catalysis of BP radical formation by certain antimicrobials and other tertiary amine-containing compounds may lead to the discovery of improved treatments for several dermatological conditions.

Proficiency testing program for clinical laboratories performing antifungal susceptibility testing of pathogenic yeast species

Antifungal susceptibility testing is expected to facilitate the selection of adequate therapy for fungal infections. The general availability of antifungal susceptibility testing in clinical laboratories is low, even though a number of standard methods are now available. The objective of the present study was to develop and evaluate a proficiency testing program (PTP) for the antifungal susceptibility testing of pathogenic yeasts in laboratories licensed by the New York State Department of Health. A number of quality control standards, and methods for documenting laboratory performance, were developed in consultation with the laboratory directors. The participating laboratories were provided with five American Type Culture Collection strains of pathogenic yeasts for which the minimum inhibitory concentrations (MICs) of amphotericin B and fluconazole were well defined. A majority of laboratories (14 of 17) used broth microdilution, and these were evenly split between the NCCLS M-27A protocol and the Sensititre YeastOne method. The other three laboratories performed susceptibility testing with Etest. Overall, the levels of agreement between MIC reference ranges and the reported MICs were 85 and 74% for amphotericin B and for fluconazole, respectively. All laboratories except one successfully detected fluconazole resistance in a Candida krusei strain. However, amphotericin B resistance in a Candida lusitaniae strain was not detected by any of the participating labs. It is concluded that a suitably designed PTP could adequately monitor the competence of clinical laboratories performing antifungal susceptibility testing.