Chapter 7: How Bacteria & Archaea Grow

Microbial growth is characterized by an increase in cellular mass and population size, typically achieved through binary fission in most bacteria and archaea, though variant reproductive strategies like budding and spore formation are also observed. The bacterial cell cycle progresses through a growth phase and overlapping periods of chromosome replication, partitioning, and cytokinesis (septum formation). Key to these processes are cytoskeletal proteins: FtsZ, a tubulin homologue, forms the Z ring scaffold for division, while MreB, an actin homologue, determines the rod shape and guides peptidoglycan synthesis during cell elongation. Archaeal growth cycles show diversity; for instance, Sulfolobus exhibits phases analogous to the eukaryotic G1, S, and G2, culminating in division mediated by ESCRT protein homologues. When cultivated in a closed batch culture, a population displays five distinct stages—lag, exponential (log), stationary, death, and long-term stationary—characterized quantitatively by the generation time and growth rate constant. Survival in nature depends heavily on adaptations to environmental factors, classifying microbes based on their optimal needs for temperature (psychrophiles, mesophiles, hyperthermophiles), pH (acidophiles, alkaliphiles), and solute concentrations (osmophiles, halophiles). Furthermore, organisms tolerant of oxygen must possess protective enzymes, such as superoxide dismutase and catalase, to neutralize toxic Reactive Oxygen Species (ROS) produced during metabolism. In their natural habitats, many microbes enter growth arrest or dormancy, often forming surface-attached communities called biofilms, which are protected by an EPS matrix. Within these dense communities, cells coordinate group behaviors, like virulence or light production, using quorum sensing—a density-dependent communication mechanism involving secreted chemical signals. Laboratory study relies on various media—including defined, complex, selective, and differential media—and techniques like the streak plate method to obtain pure cultures. Population size is measured using direct counts (e.g., flow cytometry, counting chambers) or viable counts (e.g., plate counts, reported as CFU). For continuous study under nutrient-limited conditions mimicking natural environments, open systems like the chemostat are employed, where growth rate is controlled by the dilution rate of the limiting nutrient.