As with studies on other acidophilic methanotrophs, culture purity was KU-60019 clinical trial rigorously proven using a variety of microscopic and molecular analyses. Growth was greater on methane than on acetate (maximum OD410 nm of 0.8–1.0 and 0.25–0.30, respectively), as was the growth rate (μ=0.06 and 0.006 h−1, respectively). These data would suggest that methane is the preferred substrate of this strain. However, when both acetate and methane were used simultaneously, overall growth was enhanced, as first noted by Whittenbury et al. (1970) for other methanotrophs. Interestingly,
strain H2s was not found to grow significantly on any other organic acid or sugar (Table 1). With the finding of a facultative Methylocystis strain, Belova
et al. (2011) screened validly described Methylocystis species for facultative methanotrophic growth, and found that another acidophilic species with an optimal pH range of 5.8–6.2, Methylocystis heyeri H2, also grew significantly on acetate. Most mesophilic Methylocystis species (i.e. growth pH of 6.8) did not grow on acetate, with the exception of Methylocystis echinoides IMET10491 which grew in the presence of acetate see more from an initial OD410 nm of ∼0.03 to a final OD410 nm of 0.09 after 200 h of incubation. A second recent study supports the finding of facultative mesophilic Methylocystis species, with the characterization of Methylocystis strain SB2, a novel methanotroph that can only express pMMO (Im et al., 2011). This isolate, collected from a spring bog with an optimal growth pH of 6.8, was able to utilize methane, ethanol, or acetate as growth substrates. Growth was highest on methane followed by ethanol and acetate (maximum OD600 nm of 0.83, 0.45, and 0.26, respectively). Interestingly, growth on methane and ethanol followed standard exponential kinetics (μ=0.052 and 0.022 h−1, respectively), but growth on acetate
could be modeled as either exponential or linear growth. Such a finding supports the hypothesis that acetate is transported into Methylocystis strain SB2 as the undissociated acid, and at this growth pH, the proton-motive force is dissipated for acetate uptake (Axe & Bailey, 1995). Finally, as Immune system with other investigations of facultative methanotrophy, culture purity was verified using a variety of microscopic and molecular techniques. The recent findings of facultative methanotrophy raises some very interesting questions. Particularly, is the MMO expressed when these strains are grown on multicarbon compounds in the absence of methane? Interestingly, acetate has been shown to repress MMO expression in some facultative methanotrophs, while others constitutively express MMO regardless of the growth substrate. Specifically, when using acetate as an alternative substrate, M. silvestris was clearly shown to repress expression of the sMMO (the only form of MMO it expresses) in either the absence or presence of methane (Theisen et al., 2005).