Competition in vitro among fungi acquired from the exoskeleton of the giant Madagascar hissing-cockroach, Gromphadorhina portentosa, and its relevance to human health

Abstract

Abstract Few insects have received as much public attention as the Madagascar hissing-cockroach, Gromphadorhina portentosa , coveted by children as a household pet. Recently, this insect has been linked to allergies and asthma triggered by an antagonistic external mycoflora, and some of these fungi are known to cause serious illnesses, namely Aspergillus spp., Mucor spp. and Rhizopus spp. Other cockroach species serve as carriers of medically important fungi, but G. portentosa is of special concern because it is deliberately reared in captivity. The aim of this study was to ascertain why certain types of fungi dominate the exoskeleton’s mycoflora whereas others do not. Six ascomycetes ( Alternaria alternata , Aspergillus flavus , Aspergillus niger , Cladosporium cladosporioides , Penicillium glabrum , Trichoderma viride ), and two zygomycetes ( Mucor racemosus , Rhizopus stolonifer ) initially acquired from the exoskeleton of G. portentosa , were tested for their competitive ability to inhibit one another in vitro , using the cockroach as the primary substratum. R. stolonifer , M. racemosus , and mycoparasitic T. viride grew at a significantly faster rate compared to the other taxa, and growth rates correlated positively with conidia output. Slower colony growth rates were noted prior to physical contact, suggesting that the fungi engaged in chemical antagonism. Three genera emerged that best fit the profile of exploitative competitors – Rhizopus , Mucor , Trichoderma . The latter two genera completely inhibited the growth of all fungal taxa tested and also had some capacity to grow over the mycelium of another fungus. We conclude that M. racemosus and T. viride in particular, are successful agents of G. portentosa ’s mycoflora because they utilize both exploitation and interference competition to their advantage. Eliminating or reducing the simple sugar load entering the cockroach colony may be one way to tilt the composition of the mycoflora to favour strains that are less medically significant. Thus, we emphasize reduction of the antagonistic fungi at the colony (food entry) level and less so on the exoskeleton level.