School science laboratory classes and hands-on public engagement activities share many common aims and objectives in terms of science learning and literacy. This article describes the development and evaluation of a microbiology public engagement activity, "The Good, the Bad and the Algae", from a school laboratory activity. The school activity was developed as part of an educational resource which aimed to promote practical microbiology in the classroom. The public engagement activity was derived locally for National Science and Engineering Week 2011 and was subsequently adapted for a national science and engineering fair (The Big Bang 2012). The aim of the session was to raise awareness of the importance of algae and to encourage hands-on laboratory examination in a fun and informal manner. Evaluation of the first event, delivered in a workshop format, helped shape the educational resource before publication. The second event was modified to enable delivery to a larger audience. Both events were successful in terms of enjoyment and engagement. Over 2200 people participated in the Big Bang activity over three days, with evaluation indicating 80% of participants had increased awareness/knowledge of algae after the event. The success of both iterations of the activity demonstrates that it is possible to transform a simple school activity into an exciting and effective public engagement activity.