Polycyclic aromatic hydrocarbons (PAHs) degradation potential, surfactant production, metal resistance and enzymatic activity of two novel cellulose-degrading bacteria isolated from koala faeces

Abstract

Cellulose-degrading bacterial strains, KC3 and 5, capable also of degrading both low and high molecular weight PAHs were isolated from koala faeces for the first time. KC3 and 5 belong to the genera Bacillus and Pseudomonas, respectively. These strains completely degraded the model 3-(phenanthrene), 4-(pyrene) and 5-(benzo-a-pyrene) ring PAHs at 6, 7 and 40–50 days, correspondingly. They were shown to be able to produce a rhamnolipid type of biosurfactant during PAH degradation. The biosurfactants produced from both the strains showed good pH (2–12) as well as thermal (up to 80 °C) stability and were able to tolerate up to 20 g L−1 salinity. The strains also had resistance towards heavy metals, attributed to the amount of biosurfactant produced. The Bacillus strain in particular showed excellent metal resistance; the minimum inhibitory concentrations were 5 (Cd2+, Cu2+) and 7 (Pb2+, Zn2+) mg L−1 of relatively bioavailable metal ions, but >15 mg L−1 metal concentrations were lethal to the microbe. Additionally, both strains possessed activity of more than one extracellular enzyme (cellulase, lipase and protease). The limiting factors in PAH biodegradation are low PAH bioavailability and microbial intolerance towards HMW PAHs and co-contaminants (heavy metals). The novel strains identified thus had (a) potential to biodegrade both LMW and HMW PAHs, (b) pH, thermal and saline-tolerant biosurfactant production that aids PAH solubility enhancement, and importantly, (c) heavy metal resistance. Both Bacillus and Pseudomonas strains are appropriate candidates in field-scale PAH bioremediation at mixed contamination sites and for several industrial applications due to their enzymatic activities.