Is E. coli an absolute negative for Sperbers Modified Agar?

In the paper, 'Molecular Characterization of Mineral Phosphate Solubilization In Serratia marcescens and Methylobacterium sp.' by A. Kumari, it's stated that E. coli lacks mineral phosphate solubilizing ability because it does not possess the PQQ cofactor.

However, when spotting both E. coli strains 51813 and 8739 (ATCC) onto Modified Sperber's Media (MSM), I found small clearing zones of the Tricalcium Phosphate after 48 hrs.

A colony of E. coli was spotted on each segment of the triplate and the plates were incubated for 48 - 72 hrs.

This same method was used with Pseudomonas aeruginosa as a positive control/reference and images of those plates are also attached.

In addition, Bromothymol Blue (BTB) was added to the MSM media to assist in visualizing the zones and the color change of media suggests the E. coli was fermenting the glucose within the media to produce acetic acid. Since one proposed method of microbial phosphate solubilization is through the secretion of organic acids, is it possible that this could explain the clearings observed?

Are there any other instances of E. coli strains exhibiting phosphate solubilizing activity? Or is there likely an error in my methods?

Modified Sperber's Media used:

For 1L of H20

10g Glucose, 0.5g Yeast Extract, 0.1g CaCl2, 0.25g MgSO4, 18g Agar

10% CaCl2 w/v @ 3ml & 10% K2HPO w/v @ 2ml/ 100ml was added to the media before pH adjustment & pouring.

The pH achieved was 7.114

Any assistance or guidance is appreciated!