# The Viable but Non-Culturable (VBNC) State, a Poorly Explored Aspect of Beneficial Bacteria
## 3 The VBNC State in Beneficial Bacteria
### Gammaproteobacteria and the VBNC State  
One of the groups with the most beneficial species in which the VBNC status has been demonstrated is Gammaproteobacteria. _Methylomonas methanica_, _Methyloscarina fibrate_, _Methyloculum gracile_, _Methylomicobium alcaliphium_, and _Methylococcus capsulatus_ are methylotrophs. This type of bacteria uses methanol and methane as the only carbon source [71]. Methylotrophs can inhabit soil, water, and plants, powering the carbon cycle [72]. When _M. fibrate_ was co-inoculated with species of _Methylomonas_ and _Cupiravidus taiwanensis_ LMG 19424, its growth was highly stimulated [73]. Moreover, methane oxidation was elevated when methanotrophs interacted with algae and moss [74]. Cryopreservation is one method that induces the VBNC state in methanotrophs (Table 1). The study of the VBNC state in methanotrophs is essential due to its role in the carbon cycle and therefore in the planet's life. It is relevant to consider that interaction with other microorganisms or with plants could be an alternative strategy for these microorganisms to return to the cultivable state, being able to continue exerting their beneficial effects. _Microbulbifer aggregans_, another Gammaproteobacteria species, is a halophilic, Gram-negative bacterium isolated from sediment in the Matang mangrove forest, Malaysia [75]; its importance is centered on its capability to reduce sulfur due to the presence of genes involved in this process [50]. Sulfur is an essential element for life and is present in amino acids, proteins, enzymes, vitamins, and other biological molecules [76]. _M. aggregans_ is a rarely explored bacterium. Recently, it was observed that non-culturable cells exhibited a change in cell shape from rod to coccus, and the genes responsible for sulfate reduction were upregulated in the VBNC state [50]. These findings demonstrate this species' importance in the environment in which it lives, because the effective reduction of sulfur in the non-culturable state may occur in response to changes in the sulfur concentration in the environment, playing a relevant role in the sulfur cycle in marine environments. Within the Gammaproteobacteria, there is also the genus _Pseudomonas_. The species of this genus have a great capacity to use different nutrients as carbon sources, which explains their ubiquity. Their enzymatic activity makes them an important group of microorganisms responsible for the aerobic degradation of many compounds in different ecosystems [77]. Some species of _Pseudomonas_ can promote plant growth by suppressing pathogenic microorganisms, synthesizing growth-stimulating plant hormones, and promoting increased plant disease resistance [78]. _Pseudomonas fluorescens_ and _Pseudomonas putida_ KT2440 are bacteria able to colonize plants' roots and promote their growth. Some strains of _P. fluorescens_ have been shown to degrade a variety of organic compounds, thus being important in bioremediation [79]. This bacterium is used for biocontrol to protect plants against soilborne fungal pathogens. One mechanism for the biocontrol by _P. fluorescens_ is the ability to produce antibiotics [80]. It also stimulates induced systemic resistance (ISR) [81] and the production of volatile compounds [82]. On the other hand, _P. putida_ KT2440 is a bacterium capable of using different aromatic compounds as a carbon source [83]. Metabolizing xenobiotic compounds, it can colonize the roots of plants such as corn, wheat, strawberry, sugarcane, and spinach [84] and is capable of promoting the growth and health of plants [85; 86]. _P. putida_ KT2440 has been used in various bacterial formulations to enhance plant growth [86; 87]. The potential of _P. putida_ KT2440 and _P. fluorescens_ to exert beneficial effects can be affected by their exposure to different stresses that induce the VBNC state, such as saline environments for _P. fluorescens_ and desiccation for _P. putida_ KT2440 (Table 1).