Nancy B. McClenny

Molecular strain typing by restriction fragment length polymorphism analysis was used to demonstrate that two clusters of Mycobacterium tuberculosis cultures involving six patients resulted from cross-contamination in the mycobacteriology laboratory. Contaminated cultures were processed by the decontamination procedure and were read on the BACTEC instrument following acid-fast bacillus smear-positive specimens from patients with active tuberculosis. Investigation of these episodes suggested opportunities for modification of laboratory procedures to minimize cross-contamination and confirmed the adverse medical and public health consequences of false-positive cultures. Strain-typing results were used in decisions regarding patient care, including the curtailment of unnecessary treatment in one patient. Molecular strain typing appears to be a valuable means of identifying false-positive cultures of M. tuberculosis in selected settings.

Molecular and immunologic tests promise better, faster laboratory diagnosis of aspergillosis, but microscopy and culture remain commonly used and essential tools. Procedural changes, as well as adequate training of laboratory professionals, can enhance the value of these traditional tools. Using Blankophor or Calcofluor for microscopic examinations; improving recognition of morphologic characteristics of opportunistic fungi in stained smears of specimens; maximizing the growth rate and production of conidia by Aspergillus spp. in culture; and recognizing atypical variants of common aspergilli can improve the laboratory's contribution to rapid diagnosis. Surveys indicate that the number of laboratory professionals is declining as the demand for healthcare is rising. Effective recruitment, retention, and training of personnel must be concurrent with advances in technology.

It is not uncommon to see amphotericin B treatment failure in patients with systemic infection caused by Candida lusitaniae. We report a patient with stage IV ovarian carcinoma and C. lusitaniae sepsis whose treatment with amphotericin B failed. The initial blood isolate was susceptible to amphotericin B in vitro; however, the MIC for a blood isolate recovered 7 weeks after treatment began showed a fourfold increase. Direct subculture of two positive blood samples obtained within a week of the patient's death showed the coexistence of two distinct colony color variants on CHROMagar Candida (CAC). One variant was susceptible to amphotericin B, and one was resistant. These results emphasize the importance of repeat amphotericin B susceptibility testing for patients with persistent C. lusitaniae infection. The presence of colony variants on CAC may signal the emergence of amphotericin B resistance in C. lusitaniae and should be investigated.