An investigation into an in vitro model of post-death metabolism and the cryptic growth phase.

Abstract

Bacteria in nature are normally found in the long-term stationary phase due to the nutrient poor environments they grow in. This fifth phase has traditionally been neglected by microbiologists despite its importance in understanding how wild bacteria behave. It is known that the long-term stationary phase depends on cryptic growth – a process where material from dead bacteria is recycled as nutrients for the remaining living bacteria. It allows a colony to survive despite the lack of available nutrients. Whilst cryptic growth has been accepted in the literature since the late 1950’s, no previous attempt has been made at understanding the precise mechanisms which underpin it. To start to understand this essential process, an Escherichia coli mutant with the Lon protease knocked-out was investigated through short and long-term growth assays. The Lon gene had previously been shown to cause a boost in the growth of a culture when present in its extracellular environment. This thesis presents evidence for the post-death activity of the Lon protease.
The absence of the Lon protease was observed to alter the growth phases of the E. coli population. The Lon-deficient cultures failed to enter the stationary phase promptly and, significantly, no evidence was seen of cryptic growth or entry to the long-term stationary phase. Additionally, the results show that Lon knock-outs have a temporary fitness advantage when in a mixed wild-type mutant culture, especially when at a low initial concentration, although this is unlikely to be selected for. In conclusion, Lon plays a fundamental role in post-death metabolism and also potentially its regulation. This puts the role Lon plays in the cell into a new perspective and may lead other catabolic enzymes to be considered for complementary roles in post-death metabolism.