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Yeasts are able to grow on routine bacteriological media, such as blood agar and chocolate agar plates. If yeasts are present together with a mixed bacterial population, it is, however, possible that bacteria will suppress yeast growth. It is therefore generally accepted that a selective medium such as Sabouraud agar should be used for the cultivation of yeasts from clinical specimens (6). Microbiological laboratories include such media if a fungal or yeast culture has been requested by the clinician, but if such a request has not been made, selective media are often omitted.

In order to assess the importance of including a selective fungal medium, we compared the efficiency of yeast recovery from routine bacteriological media and from Sabouraud agar. The design of the study was quite simple, as we only wanted to investigate the impact of selective fungal medium on the recovery of yeasts from clinical specimens. No attempt was made to study the clinical importance of the yeast isolates. In addition, we evaluated the methods used and the ability of Norwegian microbiological laboratories to detect yeast in mixed cultures by including specimens containing both bacteria and yeasts in two recent distributions in the Norwegian external quality assessment program for medical bacteriology, mycology, and parasitology (10).

Comparative studies. A total of 558 specimens, submitted as pus specimens to the microbiological laboratory at the Norwegian Radium Hospital, were included in the study. The origins of the specimens were quite heterogeneous, including specimens from abdomens, abscesses, wound and drain secretions, etc. The specimens were cultivated on blood, chocolate, and lactose agar plates and on Sabouraud agar (Acumedia Manufacturers, Inc., Baltimore, Md.). Streptomycin (40 µg/ml) and penicillin (13 µg/ml) were added to the Sabouraud agar to inhibit bacterial growth. Anaerobic media were included if appropriate. The bacteriological media were always inoculated before the Sabouraud agar plate. All media were incubated at 37°C for 48 h, and the growth of yeasts was reported semiquantitatively as light, moderate, or heavy.

External quality assessment specimens. Twenty-three laboratories participate in the Norwegian external quality assessment program. In 1998, a simulated abdominal pus specimen consisting of a mixture of C. albicans, Proteus mirabilis, and Bacteroides fragilis from a patient with peritonitis following an intra-abdominal perforation was included in the program. Fungal culture was not specifically requested, and in this case, only two laboratories included a selective fungal medium. These two laboratories detected C. albicans. Of the remaining 21 laboratories, which used no selective media, only eight (38%) detected the yeast. In 1999, a similar specimen (C. albicans, E. coli, and Enterococcus faecalis) was distributed. Twelve laboratories used a selective fungal medium to assess that sample. All of these, but only three (27%) of the 11 laboratories which did not use selective media, recovered C. albicans.

Discussion. Yeasts in pure cultures are consistently isolated on bacteriological media. However, yeasts are frequently present in mixed cultures with different bacteria, and on such occasions, it is possible that bacteria will suppress yeast growth. Yeasts might, on the other hand, suppress growth of slow-growing bacteria. In this study, yeast was recovered on bacteriological media from only 50% of 44 yeast-positive pus specimens and from 22.5% of 22 yeast-positive throat specimens consisting of mixed cultures. These results show that yeasts in specimens submitted to the microbiological laboratory often are suppressed by bacteria and are, therefore, not detected on the routine bacteriological media used. The reason for this might be that yeasts are overgrown by bacteria or are killed by bacterial toxins (4, 5). It is therefore important that clinicians specifically request yeast isolation when clinically indicated and that microbiological laboratories employ a selective fungal medium when clinically significant yeasts are likely to be encountered. In our opinion, a selective fungal medium should, for instance, always be included when processing abdominal pus specimens from patients with intra-abdominal perforations. The results of several studies indicate that yeasts are important pathogens in such situations (1, 7, 12).