Black coloured colonies on Tinsdale medium: A challenge in resource-limited settings

The upper respiratory tract harbours diverse microbial flora that contribute to homeostasis and immunity but may occasionally become pathogenic. The upper respiratory tract acts as a point of entry for various pathogenic bacteria, which can have local or systemic manifestations. Many non-pathogenic Corynebacterium species exist as commensals, but a few species can produce toxins and can have systemic manifestations. Identifying such pathogenic Corynebacterium at the earliest is a diagnostic challenge faced by microbiological laboratories. To shorten diagnostic turnaround time, study adopted primary inoculation on selective Tinsdale medium and Loeffler's Serum Slope (LSS) for rapid growth. As Tinsdale medium is known to produce standout colony characteristics for rapid identification, we exploited the ability of Tinsdale medium to form characteristic grey to black colonies of Corynebacterium genus.

In this cross-sectional study, 50 throat swab specimens collected from patients presenting with clinical features suggestive of diphtheria were cultured on Tinsdale medium and Loeffler's Serum Slope. Grey to black-pigmented colonies were identified using conventional microbiological methods and the VITEK 2 Compact system.

Of the 50 specimens processed, 35 (70.0%) yielded colonies with grey-black pigmentation on Tinsdale medium; none of these isolates was finally identified as Corynebacterium. Out of the organisms identified by conventional methods as well as by VITEK, Enterococcus faecalis was the most prevalent organism (25.7%), followed by Lactobacillus acidophilus (14.3%) and Streptococcus mitis (14.2%) and Lactobacillus gasseri (11.4%). The remaining isolates comprised Actinomyces naeslundii, Clostridium spp., Candida albicans, Schaalia odontolytica, and Pseudopropionibacterium propionicum.

Black colonies on Tinsdale medium were not specific for Corynebacterium, as several other organisms produced similar colonies. These findings reinforce the need for confirmatory identification methods, as no Corynebacterium species were isolated from the throat swab specimens processed in this study.