Group size dependent benefits in microbe cooperation

Thesis

Microbes produce many molecules that are important for their growth and development, and the production and exploitation of these public goods has become an important paradigm in the field of microbial social evolution. Here I explore this type of microbial sociality in two primary chapters.

In the first chapter I focus on the relationship between the shape of the benefit curve and cellular density with a model assuming three types of benefit functions: diminishing, accelerating, and sigmoidal (accelerating then diminishing). I classify the latter two as being synergistic and argue that sigmoidal curves are common in microbial systems. Synergistic benefit curves interact with group sizes to give very different expected evolutionary dynamics. In particular, whether or not and to what extent microbes evolve to produce public goods depends strongly on group size. This synergy can create an ``evolutionary trap'' which can stymie the establishment and maintenance of cooperation. By allowing density dependent regulation of production (quorum sensing), this trap may be avoided.

In the second chapter I focus again on group size benefits, but with a particular focus on pathogen risk assessment. Many pathogens are thought to behave collectively, and yet the models used in assessing microbial risk assume otherwise. In particular the dominant paradigm, articulated in the Independent Action Hypothesis, is that infecting microorganisms do not interact with each other and that each cell has an independent likelihood of causing infection. Initial data from a bacteria-insect system suggest that indeed the Independent Action Hypothesis may be incorrect and leads to poor risk assessment at low doses. I argue that more attention to mechanisms of infection is essential for accurate risk assessment.

Authors: Cornforth, D

• Publication date: 2013
• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: cooperative infection; Group size; synergy; risk assessment
• Subjects: Disease (zoology); Evolution (zoology)