On 2015 May 02, Martine Crasnier-Mednansky commented:

The role of cAMP in the glucose-acetate diauxie was not taken into account by the authors however if it is taken into account, interesting possibilities arise.

Acetate utilization by the fast-switchers is suppressed during growth on glucose as shown by a biphasic growth curve indicating preferential use of glucose over acetate (Figure 1A, blue circles), and a nearly constant concentration of acetate in the medium to the switch point (Figure 1C, blue circles). In contrast, an extended diauxic lag is observed with the slow-switchers (Figure 1A, red circles). Both switchers however show identical growth rate during the first growth phase (red and blue circles in Figure 1A), same switching point in time, and same glucose use (red and blue circles in Figure 1B).

A diauxic lag disappearance and biphasic growth are typically associated with addition of cAMP to the growth medium (Ullmann A, 1968). It was established that, during a downshift from glucose to acetate, the CRP-cAMP complex peaked during the first hour of transition, which correlated with an increase in cAMP (Kao KC, 2004). Therefore addition of cAMP to a typical glucose-acetate diauxie most likely will affect the diauxic lag. Moreover, a cAMP-dependent regulation of the aceBAK operon was previously inferred by the presence of CRP-binding sites within the operon and a strong glucose repression, as indicated by transcriptome data (Table 1 in Zhang Z, 2005).

So, what if the fast-switchers are no longer 'dependent' on the cAMP signal upon entry in the second phase of growth on acetate (in other words there is no need for CRP-cAMP in the absence of IclR for the transcription of the aceBAK operon). It is then expected that the fast-switchers with an 'ancestral' iclR will exhibit a typical diauxic growth, which is what is observed (Figure 4A).

As regards the slow-switchers, their growth pattern does not significantly vary from the growth pattern of the ancestor. They both resume growth on acetate after an extended lag, but very poorly as compared to the fast-switchers. It is therefore not surprising that introduction of the iclR^IS1 in the ancestor bares no consequences on the growth pattern.

A final note; micromolar concentrations of glucose were used for growth in the presence of both glucose and acetate (as indicated in M&M under Growth curve assay). The increased concentration of acetate in the medium from the slow-switchers (Figure 1C, red circles) should not occur at glucose concentrations used in Figure 1A as dissimilation of acetate occurs under aerobiosis in excess glucose.

On 2015 May 02, Martine Crasnier-Mednansky commented:

The role of cAMP in the glucose-acetate diauxie was not taken into account by the authors however if it is taken into account, interesting possibilities arise.

Acetate utilization by the fast-switchers is suppressed during growth on glucose as shown by a biphasic growth curve indicating preferential use of glucose over acetate (Figure 1A, blue circles), and a nearly constant concentration of acetate in the medium to the switch point (Figure 1C, blue circles). In contrast, an extended diauxic lag is observed with the slow-switchers (Figure 1A, red circles). Both switchers however show identical growth rate during the first growth phase (red and blue circles in Figure 1A), same switching point in time, and same glucose use (red and blue circles in Figure 1B).

A diauxic lag disappearance and biphasic growth are typically associated with addition of cAMP to the growth medium (Ullmann A, 1968). It was established that, during a downshift from glucose to acetate, the CRP-cAMP complex peaked during the first hour of transition, which correlated with an increase in cAMP (Kao KC, 2004). Therefore addition of cAMP to a typical glucose-acetate diauxie most likely will affect the diauxic lag. Moreover, a cAMP-dependent regulation of the aceBAK operon was previously inferred by the presence of CRP-binding sites within the operon and a strong glucose repression, as indicated by transcriptome data (Table 1 in Zhang Z, 2005).

So, what if the fast-switchers are no longer 'dependent' on the cAMP signal upon entry in the second phase of growth on acetate (in other words there is no need for CRP-cAMP in the absence of IclR for the transcription of the aceBAK operon). It is then expected that the fast-switchers with an 'ancestral' iclR will exhibit a typical diauxic growth, which is what is observed (Figure 4A).

As regards the slow-switchers, their growth pattern does not significantly vary from the growth pattern of the ancestor. They both resume growth on acetate after an extended lag, but very poorly as compared to the fast-switchers. It is therefore not surprising that introduction of the iclR^IS1 in the ancestor bares no consequences on the growth pattern.

A final note; micromolar concentrations of glucose were used for growth in the presence of both glucose and acetate (as indicated in M&M under Growth curve assay). The increased concentration of acetate in the medium from the slow-switchers (Figure 1C, red circles) should not occur at glucose concentrations used in Figure 1A as dissimilation of acetate occurs under aerobiosis in excess glucose.