Population Dynamics

Population dynamics

This Focus issue on bacterial growth, highlights the versatility and adaptability with which bacterial cells respond to their environmental and community context.

Bacteria have an immense capacity to grow. As men- antagonizing each other; for example, by the secretion of tioned by Megan Bergkessel, David Basta and Dianne toxins or through the type VI secretion system. By con- Newman on page 549, if Escherichia coli were to con- trast, mutualistic clonemates growing next to each other tinue exponential growth, a single bacterial “cell would often cooperate; for example, through the secretion of grow to a population with the mass of the Earth within 2 public goods. days”. However, bacteria rarely encounter perfect growth To regulate cooperative behaviour, bacteria use quo- conditions outside of the laboratory: nutrients are lim- rum sensing, whereby the concentrations of secreted sig- ited, the bacteria have to compete with other cells for nalling molecules inform bacteria about the surrounding resources or they are under attack by other bacterial population density. On page 576, Bonnie Bassler and Kai species, host defences or antimicrobial therapy. Thus, Papenfort review quorum sensing systems in Gram- bacteria have developed a wide variety of mechanisms negative bacteria, highlighting the different signalling that enable them to optimize their growth patterns molecules, receptors and response networks. They also according to the surrounding conditions. This Focus describe the broad effects that quorum sensing can have issue explores factors that influence bacterial growth by not only enabling communication between members dynamics and how bacterial populations respond to of one bacterial species but also between species, gen- them; for example, by forming biofilms and produc- era and even kingdoms; for example, between the gut ing a structured extracellular matrix, by executing microbiota and the mammalian host. population-wide behaviours based on growth density Finally, Megan Bergkessel, David Basta and Dianne or by arresting growth when nutrients are scarce. Much Newman introduce the concept of growth arrest, which progress has been made in understanding the complex occurs when fast growing bacteria encounter nutri- interdependencies that govern bacterial growth. ent and energy limitation. Cells drastically slow their Since 1987, when Bill Costerton and colleagues metabolism and enter into a resting state, from which first defined biofilms1 as a community of bacterial cells they can be awoken when nutrients become avail- encased in a polymeric matrix and attached to a surface, able again. Environmental bacteria probably spend the original definition of biofilms has evolved, includ- a large part of their existence in growth arrest, and ing the realization that non-bacterial microorganisms recent research has elucidated some of the underlying also form biofilms and that free-floating biofilms exist. molecular mechanisms. However, the importance of extracellular polymeric This Focus issue highlights the importance of the substances has remained one of the key properties ecological context for bacterial growth, be it in a bio- of biofilms. On page 563, Hans-Curt Flemming and film, in the microbiota or in a low-energy state. What colleagues examine the role of the biofilm matrix for governs bacterial growth is one of the central questions bacterial growth. This matrix provides properties that of microbiology and it has been at the heart of the field emerge when bacteria grow within a biofilm, such as for decades. Despite a long history of research and the ability to capture nutrients and metabolites, or to many ground-breaking discoveries, much remains to bacteria exclude antimicrobials. be studied. The simple questions of what makes a bac- have developed Another core component of the original definition terial population grow or stops it from growing have no of biofilms, the growth of a community of cells, also simple answers. We hope that the Reviews in this Focus a wide variety of remains an important avenue of research. A rich body issue will guide and inspire future research, open new mechanisms that of theoretical and experimental work studies commu- perspectives and possible answers to long-standing ques- enable them to nity dynamics in biofilms. Two opposing interactions tions. As Roberto Kolter stated in a comment2 on his optimize their — competition and cooperation — can occur in bio- research on stationary phase growth several years ago: growth patterns films. As Kevin Foster, Carey Nadell and Knut Drescher “in order to be able to look freshly at the challenges of explain on page 589, these social phenotypes often the day, it is always an excellent idea to stop and see what according to depend on the spatial distribution of genetic lineages in others saw long before one opened one’s eyes.” the surrounding the biofilm. Mixed populations that consist of different 1. Costerton, J. W. et al. Annu. Rev. Microbiol. 41, 435–464 (1987). conditions lineages tend to compete with different clones or species 2. Kolter, R. J. Bacteriol. 181, 697–699 (1999).