Diagnostic Challenges and Classification of Pathogenic Mycolata

The new order Corynebacteriales (the mycolata), previously the suborder Corynebacterineae, is renowned for containing a number of taxa that are significant as causal agents of disease, with Mycobacterium tuberculosis being the most frequently recognised. Out of the sixteen genera in the order, ten genera contain species that are pathogens or opportunistic pathogens of plants, animals and humans. Recent increases in the reported infections caused by the mycolata, in particular those due to Rhodococcus equi in humans, can be attributed to the increased numbers of immunocompromised patients as well as improved clinical microbiological awareness. Nevertheless, mycolic acid containing pathogens are still often missed or misidentified, as it remains difficult to assign unknown, clinically significant, non-sporing actinomycetes to the correct genus, let alone to the right species. This in turn can lead to incorrect chemotherapy resulting in relapse or even fatalities. The application of modern taxonomic procedures has led to marked improvements in the classification of the genus Rhodococcus and members of the order Corynebacteriales but reliable, practical and cost-effective methods are still needed to identify clinically significant rhodococci and related mycolic acid containing actinomycetes to the rank of species.