Spore formation in Bacillus subtilis

Abstract

Although prokaryotes ordinarily undergo binary fission to produce two identical daughter cells, some are able to undergo alternative developmental pathways that produce daughter cells of distinct cell morphology and fate. One such example is a developmental programme called sporulation in the bacterium Bacillus subtilis, which occurs under conditions of environmental stress. Sporulation has long been used as a model system to help elucidate basic processes of developmental biology including transcription regulation, intercellular signalling, membrane remodelling, protein localization and cell fate determination. This review highlights some of the recent work that has been done to further understand prokaryotic cell differentiation during sporulation and its potential applications.

Figure 1

Schematic representation of morphological changes that occur during sporulation in Bacillus subtilis. Distinct stages of sporulation are denoted with a Roman numeral, according to the numbering scheme proposed by Ryter (Ryter, 1965). Peptidoglycan is depicted in gray, membranes are depicted in yellow, DNA is depicted in black, the position of the origin of replication of the chromosomes is shown as a red dot at stage 0 and I, and the spore coat is depicted in green. At stage 0, chromosomes are replicated, but no obvious morphological landmarks of sporulation are yet present. Stage I is defined by chromosome condensation and the anchoring of the origins of replication to the extreme poles of the cell. In stage II, the polar septum is elaborated, followed by engulfment of the forespore in stage III. Stage IV and V represent cortex and coat assembly, respectively. Stage VI refers to “spore maturation”; a particularly obvious morphological feature elaborated at this stage is the tightly condensed, toroidal structure of the forespore chromosome. In stage VII, the mother cell lyses, releasing the mature, largely dormant spore into the environment.

Figure 2

Genetic circuitry that governs the entry into sporulation. Arrows indicate activation; repression is denoted by a bar. Developmental events are depicted in the color corresponding to Spo0A levels or phosphorylation states that govern that event. Thus, unphosphorylated Spo0A corresponds to active DNA replication; low levels of phosphorylated Spo0A (SpoA~P) leads to biofilm formation and cannabilistic behavior; and high levels of phosphorylated Spo0A drives the entry into sporulation. Proteins other than Spo0A that participate in each activation or repression step are depicted in gray.