Agarolytic bacterium Persicobacter sp. CCB-QB2 exhibited a diauxic growth involving galactose utilization pathway

Abstract

The agarolytic bacterium Persicobacter sp. CCB-QB2 was isolated from seaweed (genus Ulva) collected from a coastal area of Malaysia. Here, we report a high-quality draft genome sequence for QB2. The Rapid Annotation using Subsystem Technology (RAST) annotation server identified four β-agarases (PdAgaA, PdAgaB, PdAgaC, and PdAgaD) as well as galK, galE, and phosphoglucomutase, which are related to the Leloir pathway. Interestingly, QB2 exhibited a diauxic growth in the presence of two kinds of nutrients, such as tryptone and agar. In cells grown with agar, the profiles of agarase activity and growth rate were very similar. galK, galE, and phosphoglucomutase genes were highly expressed in the second growth phase of diauxic growth, indicating that QB2 cells use galactose hydrolyzed from agar by its agarases and exhibit nutrient prioritization. This is the first report describing diauxic growth for agarolytic bacteria. QB2 is a potential novel model organism for studying diauxic growth in environmental bacteria.

Keywords: Persicobacter; agarase; agarolytic bacterium; diauxic growth; leloir pathway.

Figure 1

Characterization and phylogenetic analysis of QB2. (A) Colony morphology of QB2 grown on a H‐ASWM agar plate was observed using a Colony Doc‐It Imaging Station. (B) Agarolytic activity on the same plate was detected using Lugol's solution. (C) Phylogenetic tree based on 16S rRNA sequence from QB2 and related members of the Bacteroidetes phylum. The tree was constructed using the neighbor‐joining method (Saitou & Nei, 1987) of the ClustalW (v.2.1) program (Thompson, Higgins & Gibson, 1994). The numbers at the nodes indicate bootstrap confidence values obtained after 1000 replications. The scale bar indicates genetic distance

Figure 2

Cell growth (colony‐forming units) and agarase activity of QB2. QB2 cells cultured in H‐ASWM (0.5% tryptone) with or without 0.2% agarose (A) or L‐ASWM (0.05% tryptone) with or without 0.2% agarose (B). Gray and black solid lines indicate cell numbers in the sample without agar and the sample with agar, respectively. Gray and black dash lines indicate agarase activities in the sample without agar and the sample with agar, respectively. All data shown are mean values from three replicate experiments. Error bars denote the standard deviation of triplicate samples

Figure 3

Phylogenetic analysis of QB2 agarases PdAgaA, PdAgaB, PdAgaC, and PdAgaD with known β‐agarases classified by the CAZy database. The tree was constructed with the neighbor‐joining method from the ClustalW (v.2.1) program. The numbers indicate the bootstrap confidence values obtained after 1,000 replications. The scale bar indicates the genetic distance

Figure 4

Relative expression levels of four agarases, pdAgaA, pdAgaB, pdAgaC, and pdAgaD (A) and galK and phosphoglucomutase gene (B) during lag (6 hr), first growth (9 hr), and second growth (30 hr) phases from cells cultured in same medium in Fig. 2. The 16S rRNA gene was used as an endogenous control. Data are the mean of two replicate experiments. Errors indicate the standard deviation of duplicate samples