Impact of growth conditions on the abundance and diversity of cultivable bacteria recovered from Pheronema carpenteri and investigation of their antimicrobial potential.

The deep sea is a largely unexplored extreme environment supporting a diverse biological community adapted to low temperatures and high pressures. Such environments support microbial life that may be a source of novel antibiotics and other drugs. Whilst this is often the case, many species with bioactive capabilities may be missed with traditional culturing methods. In this study, a total of 16 different concentrations and types of media were employed, to culture 389 bacterial isolates using Dilution to Extinction methods and Actinobacteria Directed Cultivation techniques. This generated 72 (18.6%) isolates with narrow and broad-spectrum activity against ESKAPE pathogens including Escherichia coli (E. coli), methicillin-resistant Staphylococcus aureus, and vancomycin-resistant Enterococci. We also report that an early-stage 'One Strain Many Compounds' approach can reveal a greater number of bioactive isolates that otherwise would have been missed; 12 isolates initially deemed 'inactive' were seen to exhibit activity towards S. aureus and/or E. coli. We emphasize the importance of a thorough initial screening method to capture bioactive isolates and show how selecting only morphologically distinct isolates for screening may result in species with promising bioactivity being overlooked. Our findings justify on-going investigation of Pheronema sponges for bioactive microbiota.