Cell surface acid–base properties of Escherichia coli and Bacillus brevis and variation as a function of growth phase, nitrogen source and C:N ratio

Potentiometric titration has been conducted to systematically examine the acid–base properties of the cell surfaces of Escherichia coli K-12 and Bacillus brevis as a function of growth phase, nitrogen source (ammonium or nitrate), and carbon to nitrogen (C:N) ratio of the growth substrate. The two bacterial species revealed four distinct proton binding sites, with pKa values in the range of 3.08–4.05 (pK1), 4.62–5.57 (pK2), 6.47–7.30 (pK3), and 9.68–10.89 (pK4) corresponding to phosphoric/carboxylic, carboxylic, phosphoric, and hydroxyl/amine groups, respectively. Two general observations in the data are that for B. brevis the first site concentration (N1), corresponding to phosphoric/carboxylic groups (pK1), varied as a function of nitrogen source, while for E. coli the fourth site concentration (N4), corresponding to hydroxyl/amine groups (pK4), varied as a function of C:N ratio. Correspondingly, it was found that N1 was the highest of the four site concentrations for B. brevis and N4 was the highest for E. coli. The concentrations of the remaining sites showed little variation. Finally, comparison between the titration data and a number of cell surface compositional studies in the literature indicates one distinct difference between the two bacteria is that pK4 of the Gram-negative E. coli can be attributed to hydroxyl groups while that of the Gram-positive B. brevis can be attributed to amine groups.