[ml_thoughts]

The DNA/RNA that encodes the proteins can itself be structured in a way that might be disrupted by synonymous amino acid changes. In particular, recent work in the field has shown that changing codons near the start of the gene can disrupt transcriptional/translational machinery.

These effects are often minor but this bacteria has undergone hundreds of millions of years of optimization via natural selection and some researchers have come along and disrupted 18,000 sites. Probably the slower growth and length abnormalities are just that the bacteria is a little miscalibrated and displaying minor symptoms of malaise.