The term ‘bacterial cytokine’ was coined by Mukamolova et al. (1998) for the resuscitation-promoting factor (Rpf), a protein that revived dormant Micrococcus luteus cells and increased the growth rate of vegetative cells. Rpf also stimulated the growth of other members of the Actinobacteria including Mycobacterium tuberculosis, and a family of related growth factors was identified (Kell & Young, 2000). A family of proteins with a similar function in the Firmicutes was subsequently discovered (Ravagnani et al., 2005). Rpf was later demonstrated to have a lysozyme-like structure and muralytic

activity (Cohen-Gonsaud et al., 2005). How Rpf stimulates the growth of dormant see more cells remains to be determined, but it is possible that remodelling of the peptidoglycan in the cell walls of dormant cells is required before growth can resume. Interestingly, it has been demonstrated recently that peptidoglycan fragments bind to PrkC, a serine/threonine protein kinase, in Bacillus subtilis to stimulate spore germination (Shah et al., 2008). Muropeptides generated by Rpf degradation of peptidoglycan may selleck chemicals interact with PknB, a homologue of PrkC in M. tuberculosis, and thereby initiate resuscitation and stimulate growth (Kana & Mizrahi, 2009). Signalling molecules present only within the natural habitat are thought to be essential for the growth of many bacteria (Lewis, 2007; Nichols et al., 2008).

In the absence of these beneficial interactions and signals, some bacteria may struggle to grow in monoculture. Furthermore, faced with an unfamiliar environment devoid of essential factors, bacteria may, as a survival strategy, enter into a temporary state of low metabolic activity accompanied by the inability to proliferate or

to form colonies on culture media (Barcina et al., 1990; Colwell, 2000; Lewis, 2007; Nichols FER et al., 2008), which may be mistaken for a constitutional resistance to culture. Significant efforts have been made in recent years to devise culturing methods for as-yet-uncultivated species. Developments in the last decade, particularly in the field of environmental microbiology, have led to the recovery of unculturables from diversely populated habitats including soil and aquatic (marine and freshwater) environments. The majority of culture media used to date have been nutrient-rich. It is now thought that these conditions may favour the growth of faster-growing bacteria at the expense of slow-growing species, some of which thrive in nutrient-poor environments (Koch, 1997; Connon & Giovannoni, 2002), and may be inhibited by substrate-rich conventional media. Consequently, the use of dilute nutrient media has led to the successful cultivation of previously unculturable bacteria from various aquatic and terrestrial habitats (Watve et al., 2000; Connon & Giovannoni, 2002; Rappe et al., 2002; Zengler et al., 2002).