How Microbes Evolved to Tolerate Oxygen.

Ancient microbes invented biochemical mechanisms and assembled core metabolic pathways on an anoxic Earth. Molecular oxygen appeared far later, forcing microbes to devise layers of defensive tactics that fend off the destructive actions of both reactive oxygen species (ROS) and oxygen itself. Recent work has pinpointed the enzymes that ROS attack, plus an array of clever protective strategies that abet the well known scavenging systems.

Do reactive oxygen species or does oxygen itself confer obligate anaerobiosis? The case of Bacteroides thetaiotaomicron.

Bacteroides thetaiotaomicron was examined to determine whether its obligate anaerobiosis is imposed by endogenous reactive oxygen species or by molecular oxygen itself. Previous analyses established that aerated B. thetaiotaomicron loses some enzyme activities due to a high rate of endogenous superoxide formation.

cytochrome peroxidase is a respiratory oxidase that enables the use of hydrogen peroxide as a terminal electron acceptor.

Microbial cytochrome peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Intriguingly, transcription requires both the presence of HO and the absence of O Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous HO However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors.