The following question was why, among all studied compounds (food dyes, test dyes (Figure ), derivatives of small molecules (Figure and Figure , Supporting Information)) only BB and Congo red behaved differently when applied to Gram-positive and Gram-negative bacteria. The comparison of the structures of major outer proteins of bacteriophages and bacteria included in this study suggested that Gram-negative bacteria contained less basic amino acids than phages and Gram-positive bacteria proteins. The content of basic amino acids of T4 bacteriophage major capsid protein gp23 (P04535 (CAPSH_BPT4)), Salmonella P22 phage major capsid protein (P26747 (CAPSD_BPP22)), Escherichia phage MS2 capsid protein (J9QBW2 (J9QBW2_BPMS2)), Enterobacteria phage M13 capsid protein G8P (J7I0P6 (J7I0P6_BPM13)), and S. aureus surface protein G, A-domain (Q2G2B2 (SASG_STAA8)) are 9.6%, 10.2%, 9.8%, 12%, and 11.2%, respectively. The value for E. coli outer membrane protein A (P0A910 (OMPA_ECOLI)) is only 5.9% (Figure ). It was previously reported that sulfonic groups interact with the -NH3 + group in the side chain of the amino acids .